ndt_manual.pdf

Central Railway

Non Destructive Testing and Inspection Manual

February 2006

Rambøll Denmark A/S Bredevej 2 DK-2830 Virum Denmark Phone +45 4598 6000 www.ramboll.com

February 2006 Ref 5721063-07_L002_Ver4_NDT_manual.doc Date 2006-02-24 Prepared by LTP / SVE Checked by FNJ Approved by FNJ

Central Railway

Non Destructive Testing and Inspection Manual

Statement of copyright and liability

Copyright Copyright 2006 Ramboll and its sub consultants. All rights reserved. This publication must not be copied, reproduced, translated into any other language, in any way, manu-ally or otherwise, or exhibited in its entire form or partly without the expressed written consent of Ramboll Denmark A/S, Bredevej 2, 2830 Virum, Denmark. Liability Ramboll Denmark and its sub consultants assume no warranty with regard to accuracy, completeness, or usefulness of the information contained in this publication, and spe-cifically no liability with regard to the product described in the publication, or to the use, or usefulness of the product for specific purposes.

Ref. 5721063-07_L002_Ver4_NDT_manual.doc

I

Table of contents

1. Introduction 1-1 1.1 Non Destructive Testing 1-1 1.2 Non Destructive Evaluation 1-1 1.3 Scope of the present manual 1-2 1.4 Purpose 1-3 1.5 Applications – NDT-methods 1-4

2. Extended principal inspection – overview 2-1 2.1 Primary Planning 2-1 2.1.1 Requisition 2-1 2.1.2 List of bridges 2-1 2.1.3 Planning travel route 2-2 2.1.4 Information retrieval 2-2 2.1.5 Check of equipment 2-2 2.1.6 Making appointments 2-2 2.1.7 Safety considerations 2-3 2.2 Detailed planning of tests 2-3 2.3 Execution of visual inspection 2-3 2.4 Execution of tests 2-4 2.5 Assessment of damage cause and extent 2-4 2.6 General considerations regarding future maintenance activities 2-4 2.7 Reporting 2-4

3. Special inspection - overview 3-1 3.1 Primary Planning 3-1 3.2 Detailed planning of tests 3-1 3.3 Execution of tests 3-1 3.4 Assessment of damage cause and extent 3-1 3.5 Setting up of relevant repair strategies 3-2 3.6 Economic analysis of the strategies 3-2 3.7 Reporting 3-2

4. Planning of inspections using NDT-measurements 4-1 4.1 Visual inspection 4-2 4.2 Areas requiring investigation 4-2 4.3 Homogeneous areas 4-2 4.4 Evaluation of test results 4-3

5. Types of damage 5-1 5.1 General structural damage 5-1 5.2 Damage due to water 5-1 5.3 Damage on surface treatment systems 5-1 5.4 Damage on concrete structures 5-1 5.5 Damage on steel structures 5-2 5.6 Damage at masonry structures 5-2 5.7 Damage at wooden structures 5-2


II

5.8 Registration of damage 5-2 5.8.1 Concrete structures 5-2 5.8.2 Steel structures 5-4

6. Damage mechanisms 6-1 6.1 General 6-1 6.2 Structural Deficiencies 6-1 6.2.1 I. Structural cracks (load induced cracks) in concrete members 6-1 6.2.2 II. Excessive/unintended deflections and movements 6-1 6.2.3 III. Fracture/crushing 6-2 6.2.4 IV. Structural Problems, Steel Components 6-2 6.2.5 Structural Cracks in Concrete Members 6-2 6.2.6 Flexural Cracks 6-4 6.2.7 Longitudinal cracks 6-5 6.2.8 Shear Cracks 6-5 6.2.9 Bending and Shear 6-7 6.2.10 Torsion 6-8 6.2.11 Bearing Cracks 6-8 6.2.12 Splitting Cracks 6-10 6.2.13 Structural Cracks, Examples 6-12 6.2.14 Structural Problems, Steel Components 6-23 6.3 Non-Structural Cracks in Concrete 6-26 6.3.1 Shrinkage cracks (due to drying) 6-26 6.3.2 Thermal Cracks (due to hydration) 6-28 6.3.3 Cracks due to plastic shrinkage 6-28 6.3.4 Cracks due to plastic settlement 6-29 6.3.5 Initiation of Corrosion 6-31 6.3.6 Carbonation 6-32 6.3.7 Chlorides 6-34 6.3.8 Carbonation and Chlorides 6-38 6.3.9 Propagation of Corrosion 6-39 6.3.10 Corrosion products and corrosion rate 6-40 6.3.11 Local/general corrosion 6-42 6.3.12 Bridge Deck 6-43 6.3.13 Pier column 6-44 6.3.14 Wing walls / retaining walls 6-45 6.4 Alkali-aggregate Reactions 6-46 6.4.1 Crack Pattern 6-47 6.5 Chemical Attack of Concrete and Masonry 6-49 6.5.1 Acid Attack 6-50 6.5.2 Sulphate Attack 6-51 6.5.3 Seawater Attack 6-53 6.6 Erosion / Scour 6-54 6.6.1 Aggradation / degradation 6-54 6.6.2 General scour 6-55 6.6.3 Local scour 6-55 6.7 Corrosion of steel structures 6-56 6.7.1 Electrochemical corrosion 6-56 6.7.2 Chink Corrosion 6-60 6.7.3 Galvanic Corrosion 6-60


III

6.7.4 Stress Corrosion 6-60 6.7.5 Corrosion and Fatigue 6-60 6.7.6 Atmospheric Corrosion 6-61 6.8 Ageing of Steel 6-62 6.9 Erosion of masonry structures 6-63

7. NDT-methods 7-1 7.1 General 7-1 7.2 Visual inspection 7-2 7.3 Crack measuring gauge 7-3 7.4 Crack detection microscope 7-3 7.5 Boroscope 7-4 7.6 Half-cell potential 7-5 7.7 Corrosion rate meter 7-8 7.8 Cover meter Measurements 7-9 7.9 Spraying indicators (pH) 7-10 7.10 Impact-Echo equipment 7-10 7.11 Impulse Response equipment 7-12 7.12 Capo-Test (concrete strength) 7-14 7.13 Pull-off/Bond-Test 7-16 7.14 Schmidt hammer 7-17 7.15 Ground penetration radar 7-17 7.16 Chloride content 7-19 7.17 Coring equipment 7-20 7.18 Evaluation of concrete cores 7-22 7.18.1 Macro analysis on cores and plane sections 7-23 7.18.2 Crack detection on impregnated plane sections 7-24 7.18.3 Micro analysis on thin sections 7-25 7.18.4 Air void analyse on plane section 7-28 7.18.5 Moisture analysis 7-28 7.18.6 Residual Reactivity Test 7-29 7.19 Acoustic emission monitoring 7-31 7.20 Structural testing system 7-33 7.21 Structural scan equipment 7-34 7.22 Ultrasonic testing 7-36 7.22.1 Definition of ultrasound 7-36 7.22.2 Through transmission technique 7-36 7.22.3 The pulse echo technique 7-37 7.22.4 Definitions and general terms 7-40 7.22.5 Refraction and reflection of ultrasonic waves 7-42 7.22.6 Probes 7-44 7.22.7 Examination of rolled, cast and forged objects 7-50 7.22.8 Examination of welds 7-58 7.22.9 Determination of defect size 7-65 7.22.10 References 7-71 7.23 Ultrasonic thickness gauge 7-74 7.23.1 Introduction 7-74 7.23.2 Thickness measurements of steel plates 7-75 7.23.3 Special equipment 7-77 7.23.4 Thickness measurements of hot steel plates 7-78


IV

7.23.5 Thickness measurements of other materials than steel 7-79 7.24 Coating Thickness Measurement 7-81 7.24.1 Magnetic Film Thickness Gages 7-81 7.24.2 Eddy Current 7-83 7.24.3 Ultrasonic 7-84 7.24.4 Micrometer 7-84 7.24.5 Destructive Tests 7-85 7.24.6 Gravimetric 7-85 7.24.7 Thickness Measurements in Practice 7-86 7.24.8 Thickness Standards 7-87 7.25 Dye penetrant 7-88 7.25.1 Introduction and History of Penetrant Testing 7-88 7.25.2 Improving Detection 7-90 7.25.3 Basic Processing of a Dye Penetrant Testing 7-91 7.25.4 Common Uses of Dye Penetrant Inspection 7-93 7.25.5 Advantages and Disadvantages of Dye Penetrant Testing 7-94 7.25.6 Dye Penetrant Testing Materials 7-96 7.25.7 Penetrants 7-98 7.25.8 Emulsifiers 7-105 7.25.9 Developers 7-106 7.25.10 Preparation of Part 7-109 7.25.11 Selection of a Penetrant Technique 7-110 7.25.12 Penetrant Application and Dwell Time 7-113 7.25.13 Penetrant Removal Process 7-115 7.25.14 Use and Selection of a Developer 7-119 7.25.15 Quality Control 7-123 7.25.16 System Performance Check 7-129 7.25.17 Nature of the Defect 7-130 7.25.18 Health & Safety Precautions in Dye Penetrant Inspection 7-131 7.25.19 References and Resources 7-133 7.26 Magnetic Particle Flow Test 7-135 7.26.1 Introduction to Magnetic Particle Inspection (MPI) 7-135 7.26.2 Basic Principles 7-136 7.26.3 History of Magnetic Particle Inspection 7-137 7.26.4 Magnetism 7-138 7.26.5 Magnetic Materials 7-139 7.26.6 Magnetic Domains 7-140 7.26.7 Magnetic Field Characteristics 7-141 7.26.8 Electromagnetic Fields 7-143 7.26.9 Magnetic Field Produced by a Coil 7-144 7.26.10 Quantifying Magnetic Properties (Magnetic Field Strength, Flux Density, Total

Flux and Magnetization) 7-145 7.26.11 The Hysteresis Loop and Magnetic Properties 7-146 7.26.12 Permeability 7-148 7.26.13 Magnetic Field Orientation and Flaw Detectability 7-149 7.26.14 Magnetization of Ferromagnetic Materials 7-151 7.26.15 Magnetizing Current 7-153 7.26.16 Longitudinal Magnetic Fields, Distribution and Intensity 7-155 7.26.17 Circular Magnetic Fields, Distribution and Intensity 7-157 7.26.18 Demagnetization 7-161


V

7.26.19 Measuring Magnetic Fields 7-162 7.26.20 Portable Magnetizing Equipment 7-164 7.26.21 Stationary Magnetizing Equipment 7-168 7.26.22 Multidirectional Magnetizing Equipment 7-169 7.26.23 Lights 7-170 7.26.24 Magnetic Field Indicators 7-173 7.26.25 Magnetic Particles 7-176 7.26.26 Suspension Liquids 7-178 7.26.27 Testing Practices 7-179 7.26.28 Inspection using Magnetic Rubber 7-181 7.26.29 Continuous and Residual Magnetization Techniques 7-181 7.26.30 Field Direction and Intensity 7-183 7.26.31 Particle Concentration and Condition 7-186 7.26.32 Lighting 7-187 7.26.33 Eye Consideration 7-189 7.26.34 Examples of Visible Dry Indications 7-189 7.26.35 Examples of Fluorescent Wet Indications 7-192 7.27 Strain gauging 7-195 7.27.1 Measurement Principle 7-195 7.27.2 Gauge Construction 7-195 7.27.3 Applications 7-196 7.27.4 Structural design 7-199 7.27.5 Fitness for purpose 7-200 7.27.6 Testing and documentation 7-201 7.27.7 Mechanical Strain Gauge 7-203 7.28 Electromagnetic Testing (ET) or Eddy Current Testing 7-204 7.29 Radiography (RT) 7-205 7.30 Sonic Methods 7-206 7.31 Accelerometers 7-207

8. Economic analysis 8-1 8.1 General 8-1 8.2 Present Value Method 8-2 8.2.1 Repair Strategies 8-3 8.2.2 Service Life 8-4 8.2.3 Time Frame 8-5 8.2.4 Time of Repair 8-5 8.2.5 Residual Value 8-6 8.2.6 Discount Rate 8-6 8.2.7 Sensitivity Analysis 8-6 8.2.8 Optimum solution – special inspection 8-7

9. Reporting of Extended Principal Inspection 9-1 9.1 General 9-1 9.2 Text Section 9-1 9.2.1 Cover Page 9-1 9.2.2 Front Page 9-1 9.2.3 Summary 9-2 9.2.4 Motivation of the extended principal inspection 9-2 9.2.5 Background documents 9-2


VI

9.2.6 Registrations 9-2 9.2.7 Evaluation of registrations 9-2 9.2.8 General considerations regarding future maintenance activities 9-3 9.3 Appendices 9-3 9.3.1 A: Background Material 9-3 9.3.2 B: Selected Drawings 9-4 9.3.3 C: Visual Inspection 9-4 9.3.4 D: NDT-method no. 1 9-4 9.3.5 E - ?: NDT-method no. 2 - ? 9-5 9.3.6 F.. Other 9-5

10. Reporting of Special Inspection 10-1 10.1 General 10-1 10.2 Text Section 10-1 10.2.1 Cover Page 10-1 10.2.2 Front Page 10-1 10.2.3 Summary 10-2 10.2.4 Motivation of the special inspection 10-2 10.2.5 Background documents 10-2 10.2.6 Registrations 10-2 10.2.7 Evaluation of registrations 10-2 10.2.8 Repair strategies 10-3 10.2.9 Recommendation of follow-up activities 10-4 10.3 Appendices 10-4 10.3.1 A: Background Material 10-4 10.3.2 B: Selected Drawings 10-4 10.3.3 C: Visual Inspection 10-4 10.3.4 D: NDT-method No. 1 10-5 10.3.5 E - ?: NDT-method No. 2 - ? 10-5 10.3.6 F: Economic analysis 10-5 10.3.7 G.. Other 10-5

11. References 11-1


VII

Appendices A: Handout of slides from classroom training in NDT-methods A1: Introduction to the Classroom Training in NDT- and UWI A2: General Introduction to Deterioration Mechanism A3: General Introduction to Systematic Operation and Maintenance A4: Special Inspection A5: Structural Assessment - Case A6: Crack Measuring Gauge and Crack Detection Microscope A7: Boroscope A8: Half Cell Potential Measurements A9: Corrosion Rate Meter A10: Covermeter A11: Spraying Indicators A12: Impact-Echo A13: Impulse Response (s’MASH) A14: CAPO-test A15: Pull off / Bond test A16: Schmidt Hammer A17: Ground Penetration Radar A18: Chloride Content A19: Coring Equipment A20: Evaluation of Concrete Cores A21: Acoustic Emission Monitoring A22: Structural Testing System A23: Structural Scan Equipment A24: Introduction to Non Destructive Testing of Steel Structures A25: Ultrasonic Testing A26: Ultrasonic Thickness Gauge A27: Magnetic Thickness Gauge A28: Dye Penetrant Inspection System A29: Magnetic Particle Testing A30: Strain Gauging A31: Introduction to Rehabilitation of Concrete, Steel and Masonry Bridges A32: Introduction to Laboratory Tests of Steel B: Template for Extended Principal Inspection Report C: Template for Special Inspection Report

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1. Introduction

The present manual is addressed to Central Railway and covers the subjects of Non Destructive Testing (NDT) and inspections including NDT. The manual has been pre-pared as part of a pilot project in the area of Non Destructive Testing of railway bridges on Central Railway. The manual describes the basic issues regarding plan-ning, execution and reporting of bridge inspections including NDT-investigations. Selected types of damage and damage mechanisms are also described in this man-ual as an extensive knowledge of the possible damage mechanisms and signs of damage are very important for selecting the right NDT-method to apply in each indi-vidual case. Selected NDT-methods to be used on concrete, steel and masonry bridges are described in the manual. The methods described in the present manual are primarily the NDT-methods included in the pilot project program.

1.1 Non Destructive Testing The field of Non Destructive Testing (NDT) is a very broad, interdisciplinary field that plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion. NDT technicians and engineers de-fine and implement tests that locate and characterize material conditions and flaws that might otherwise cause trains to derail. These tests are performed in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and materials to be inspected and measured without damaging them or with only little damage compared to the knowledge gained by the test. Be-cause it allows inspection without interfering with a product's final use, NDT provides an excellent balance between quality control and cost-effectiveness.

The number of NDT methods that can be used to inspect components and make measurements is large and continues to grow. Researchers continue to find new ways of applying physics and other scientific disciplines to develop better NDT meth-ods.

1.2 Non Destructive Evaluation Non Destructive Evaluation (NDE) is a term that is often used interchangeably with NDT. However, technically, NDE is used to describe measurements that are more quantitative in nature. For example, a NDE method would not only locate a defect, but it would also be used to measure something about that defect such as its size, shape, and orientation. NDE may be used to determine material properties such as fracture toughness, formability, and other physical characteristics.


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1.3 Scope of the present manual An inspection of a bridge by the use of NDT/NDE is a detailed investigation of dam-age and/or material properties of a number of specified components. Depending on the level of execution and reporting of the results from the inspection such an in-spection is denoted “an extended principal inspection” or “a special inspection”. The special inspection is the most detailed inspection focusing on a specific bridge com-ponent or a specific area and includes an economic evaluation of different repair strategies. The extended principal inspection includes condition rating of the bridge components based on a visual inspection as well as an evaluation of the damage type and extent based on Non Destructive Testing of selected components.

This manual describes the issues regarding both extended principal inspections and the special inspections.

The extended principal inspection or the special inspection could be initiated as a consequence of the recommendations from a visual inspection. The damage to be investigated may be due to environmental impact (climate, saline soil etc.), wear (insufficient maintenance), design and construction errors, overload or similar. The purpose of performing inspections using NDT-methods is to determine:

− the type of damage

− the extent of damage

− the cause of damage

The above mentioned evaluations are determined in both the extended principal in-

spection as well as in the special inspection. When performing a special inspection

the expected development in time of damage is determined in addition to the type,

extent and cause. The information from the NDT-investigations provides the basis for decisions con-cerning the selection of the optimum repair strategy. Extended principal inspections and special inspections include field measurements and field testing. For this purpose, special equipment and tools such as NDT-methods are required. The report from the inspections is an important part of the necessary background material for the planning and specification of repair and rehabilitation works. Performing inspections using NDT-equipment involves making evaluations and taking decisions on-site, based on knowledge and experience. As a consequence, it is not possible to prescribe a full step-by-step set of instructions in performing an extended principal inspection or a special inspection and in using


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NDT-methods. The descriptions in this manual must only be regarded as guidelines, hints and examples.

1.4 Purpose The purpose of this manual is to give guidelines for carrying out inspections using NDT-methods and to describe some of the most common damage mechanisms. For each of the NDT-methods included in the pilot project descriptions are given with regards to the following subjects:

• Theory – Technical Method Description

• Applications and Limitations

• Test Planning and Execution of Field Tests

• Interpretation and Reporting of Results

As appendices to this manual the handouts of the presentations from the classroom

training are enclosed. All the above mentioned subjects are described in the hand-

outs.

Chapters on reporting of extended principal inspections and special inspections are

also included as well as a chapter of economic analysis using the present value

method.


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1.5 Applications – NDT-methods The manual covers descriptions of the NDT-methods mentioned in Table 1-1, which

are the methods included in this pilot project. The table also includes methods and procedures used for calibration of various NDT measurements and includes methods for analysing material samples. In addition to the test methods in Table 1-1, few

additional test methods are described in section 7.

NDT-method Used for structures made of:

Crack measuring gauge Concrete, steel and masonry

Crack detection microscope Concrete, steel and masonry

Boroscope Concrete, steel and masonry

Half cell potential measurements Concrete

Corrosion rate meter Concrete

Cover meter Concrete

Spraying indicators (pH) Concrete

Impact-Echo equipment Concrete

Impulse response equipment Concrete

CAPO test Concrete

Pull-off/Bond test Concrete

Schmidt Hammer Concrete and masonry

Ground Penetration Radar Concrete and masonry

Chloride content Concrete

Coring equipment Concrete and masonry

Evaluation of concrete cores Concrete

Acoustic emission monitoring Steel

Structural testing system Concrete, steel and masonry

Structural scan equipment Concrete, steel and masonry

Ultrasonic Thickness gauge Steel

Ultrasonic testing Steel

Magnetic thickness gauge Steel

Dye penetrant Steel

Magnetic particle testing Steel

Strain gauging Steel

Table 1-1: Test methods included in this pilot project.


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2. Extended principal inspection – overview

An extended principal inspection is carried out to determine the condition of the bridge components and to determine in detail the type, extent and cause of damage by the use of NDT-methods on specific components. In this way, the extended prin-cipal inspection forms the necessary basis for the detailed assessment of the damage and overall thoughts of the future need for major rehabilitations.

The extended principal inspection activities comprise:

• Primary planning.

• Detailed planning of tests of selected components.

• Execution of visual inspection (condition rating of the bridge components).

• Execution of tests (using NDT-methods).

• Assessment of damage cause and extent based on the test results.

• General considerations regarding future maintenance activities.

• Reporting.

2.1 Primary Planning 2.1.1 Requisition In the requisition of the extended principal inspection some information is required which the inspector needs in order to make a proper planning, i.e. the proper selec-tion of the required NDT-methods. The owner of the bridge must state the reason for initiation of the extended principal inspection and which components to include in the NDT-testing. Future expected changes to the road and bridge usage, such as load assumptions or possible road widenings must be stated as well. 'As built drawings' and all relevant reports including the inventory report should be enclosed with the requisition. Before execution of the inspection, the owner of the bridge and the company carry-ing out the inspection should agree on a time schedule and a budget.

2.1.2 List of bridges The primary planning often covers a series of extended principal inspections involv-ing the same NDT-inspections performed by the same inspection team.

In general, the bridges with the most serious damage should be inspected first, but other factors may influence the ranking list, such as the traffic volume or a conven-ient travel route to remote bridges.

A series of bridges of the same design and with the same type of damage should preferably be inspected together.


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For railway bridges an important issue in planning the inspections is the need for shutting down the power lines and to gain access to working on or close to the rails. To minimize the traffic interference extended principal inspections might be carried out on several bridges on the same railway line at the same time. During the planning phase it is important to contact the local railway authority as soon as possible to schedule the work on or close to the rails. The need for traffic blocks, traffic regulations and safety arrangement for railway traffic must be clarified in cooperation with the local railway authority. Typically the work has to be carried out in intervals with the lowest traffic intensity if possible.

2.1.3 Planning travel route

• Travel route to the selected bridges.

• Accommodation of inspection team.

2.1.4 Information retrieval The major part of the primary planning consists of retrieving information on the bridge(s) in question. This information includes:

• All relevant previous reports concerning the bridge(s) (including inventory).

• "As built" drawings.

• Any previous calculations done on the bridge – e.g. structural capacity assess-

ments, damage evaluations and so forth.

• Information about power cables or other utilities buried or located within the

bridge that might be damaged during the NDT-inspection (particularly when

breaking up roadway surface) if no precautions are taken.

• Information on major events during maintenance.

2.1.5 Check of equipment

• What kinds of tests must be done on each bridge?

• Which equipment in the inspection vehicle must be used? Check that the equip-

ment (and necessary consumables) is present and is functioning.

• Which equipment (e.g. traffic signs, ladders, scaffolding) and personnel should

be provided by the local Technical Unit?

2.1.6 Making appointments

• With the Bridge Engineer (concerning closure of railway line, traffic signs, inspec-

tion, equipment, possible reestablishment after break-ups etc.).

• With other authorities – e.g. the road owner if the bridge is crossing a road.

• About accommodations for inspection personnel.


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2.1.7 Safety considerations During the planning it is important to consider the safety factors. The inspector must always contact the bridge owner and other relevant authorities to gain knowledge of the safety rules submitted by the authorities. Some authorities may for instance re-quire all inspectors to attend a specific safety course. When working on or nearby the railway line some standard precautions have to be taken as for instance:

• Contact the railway authority to get information regarding demands for minimum distances to the railway line of scaffolding, material, etc.

• Make sure that the railway authority knows in what exact position of the railway line you are working on.

• Make sure that the power cables are shut off.

• Do not use any metal equipment close to the power lines.

• Make sure that there is always someone watching for trains to warn you if a train should pass.

2.2 Detailed planning of tests The planning of which tests to perform on the bridge is based on a preliminary hy-pothesis regarding the type, cause and extent of damage.

The planning consists of selecting the appropriate test methods and deciding on which parts of the bridge to apply the tests. The number of tests carried out must be sufficient to confirm or reject the hypothesis, and to determine the type and extent of necessary repair works.

Very often the planning of tests is performed in two or more steps: The testing may start with an initial survey comprising a limited number of test samples. The result of this is used to determine the final extent of the testing.

See also chapter 4 for a more detailed description of planning inspections including

NDT-methods.

2.3 Execution of visual inspection Typically the visual inspection is carried out on the entire bridge. During the planning phase it is important to clarify with the bridge owner in which distance the visual inspection is made. For instance in touching distance using scaffolding in selected parts of the bridge and by use of the common access facilities for the rest of the bridge e.g. by using binoculars. The visual inspection should be carried out by an experienced bridge engineer and the result of the visual inspection is a condition rating of all the bridge components included in the inspection. See section 9.2.7 for

more details regarding condition rating.


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2.4 Execution of tests The tests are carried out according to the test plan and following the 'directions for use' of the test equipment. See also chapter 7 for more detailed description of the

NDT-methods included in this pilot project.

It is important to follow the test plan and not draw premature conclusions from the first test results obtained.

2.5 Assessment of damage cause and extent If the test methods have been thoroughly selected and applied, the results should give a reliable picture of the condition of the bridge as well as of the cause and ex-tent of each damage type.

However, it is not possible to set up a set of rules that give an unambiguous answer as to the type of damage. For this reason it is essential that the assessment (as well as the planning of tests) is carried out by experienced engineers with a thorough knowledge of the relevant damage mechanisms and test methods.

2.6 General considerations regarding future maintenance activities Based on the test results general considerations regarding future maintenance activi-ties are described. The need for major rehabilitation jobs and further inspections is described but no economic analyse is made.

2.7 Reporting In order to facilitate comparison of extended principal inspection reports, and in or-der not to forget important aspects of the inspection, the reporting is made using a fixed table of contents:

• Summary.

• Motivation of (reason for) the extended principal inspection.

• Background documents (list of the background material that has been available for the inspection).

• Registrations (extent and location of tests, and a summary of the results).

• Evaluation of registrations (condition rating of the bridge components, what do the NDT-registrations indicate regarding cause and extent of damage).

• General considerations regarding future maintenance activities.

The inspection report will include those relevant of the following appendices:

• Background materials (inventory and principal inspection printouts with possible revisions).

• Selected drawings (relevant extracts from 'as built' drawings).

• Visual inspection.


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• NDT-method no. 1.

• NDT-method no. 2.

• etc.

The content of each section in the extended principal inspection report is described in more details in chapter 9.


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3. Special inspection - overview

A special inspection is carried out to determine in detail the type, extent and cause of damage. Furthermore the special inspection should evaluate the future develop-ment of damage. In this way, the special inspection forms the necessary basis for the detailed assessment of the damage and the preparation of the rehabilitation de-sign.

The special inspection activities comprise:

• Primary planning.

• Detailed planning of tests.

• Execution of tests.

• Assessment of damage cause and extent based on the test results.

• Evaluation of future damage development.

• Setting up of relevant repair strategies.

• Economic analysis of the strategies (incl. selection of the optimum strategy).

• Reporting.

3.1 Primary Planning The primary planning of the special inspection is similar to the one described for the extended principal inspection – see section 2.1.

3.2 Detailed planning of tests The detailed planning of tests included in the special inspection is similar to the one described for the extended principal inspection – see section 2.2.

3.3 Execution of tests The detailed planning of tests included in the special inspection is similar to the one described for the extended principal inspection – see section 2.4.

3.4 Assessment of damage cause and extent The assessment of the damage type, cause and extent is similar to the one of the extended principal inspection – see section 2.5.

During the execution of the special inspection a more accurate picture of the condi-tion of the bridge is obtained; therefore, a new principal inspection should be carried out as part of the special inspection activity.


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3.5 Setting up of relevant repair strategies Based on the test results and the subsequent assessment, relevant repair strategies are set up. In most cases, two or three different strategies will be relevant:

• Thorough repair.

• Bringing the bridge up to a wanted or required standard.

• Provisional repair.

• Repairs carried out in order to postpone a major repair/reconstruction.

• “Doing nothing”- and letting the bridge deteriorate until a major reconstruction or total replacement is carried out.

A repair strategy does not only include a choice of repair method, it also describes the optimum year of repair, the traffic situation before, during and after the repair, the estimated repair cost and possible maintenance costs before and after the repair.

3.6 Economic analysis of the strategies As help in choosing the optimum repair strategy, an economic analysis should be carried out for each of the relevant repair strategies. The 'Present Value Method' is used.

The postponement of a repair work may lead to increased repair and maintenance costs as the amount of damage will increase.

In order to compare costs that occur at different times, all amounts are discounted back to the same year (compensating for interest and inflation). The sum of the dis-counted values of the costs of a strategy is the present value of the strategy. The strategy with the smallest present value is the most profitable and is the optimum strategy for the rehabilitation.

The principle of the economic analysis is more detailed described in chapter 8.

3.7 Reporting In order to facilitate comparison of special inspection reports, and in order not to forget important aspects of the inspection, the reporting is made using a fixed table of contents:

• Summary

• Motivation of (reason for) the special inspection

• Background documents (list of the background material that has been available for the inspection)

• Registrations (extent and location of tests, and a summary of the results)

• Evaluation of registrations (what do the registrations indicate regarding cause and extent of damage, including the risk of further deterioration)


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• Relevant repair strategies with cost estimates (client's estimate) and economic analyses

• Recommendation of follow-up activities (such as: further investigations, monitor-ing, repair works, doing nothing).

The inspection report will include those relevant of the following appendices:

• Background materials (inventory and principal inspection printouts with possible revisions).

• Selected drawings (relevant extracts from 'as built' drawings).

• Visual inspection.

• NDT-method no 1.

• NDT-method no 2.

• etc.

• Economic analysis.

The content of each section in the special inspection report is described in more de-tail in chapter 10.


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4. Planning of inspections using NDT-measurements

To create a proper background for the planning and specification of repair and reha-bilitation works, the following information is required from an extended principal in-spection or from a special inspection:

• Identification and evaluation of the condition in the damaged areas.

• Identification and assessment of the total damaged area.

• Assessment of the cause of the damage.

• Evaluation of future damage development - development rate of existing damage

and risk of future damage in apparently undamaged areas. However, this is in-

cluded only in the special inspection. To fulfil these requirements, a certain minimum number of tests must be carried out. The tests are planned using all available information from the as-built drawings, pre-vious inspections of the bridge, inspections of similar bridges and the knowledge and experience of the persons performing the inspection. On this basis a hypothesis con-cerning the cause of damage, the total damaged area and the condition of the dam-aged area may be formulated. The hypothesis serves as a basis for the selection of the type and number of measurements to be performed - including the type and number of NDT-measurements. As a general rule in the planning of an inspection comprising NDT-measurements, it must be remembered that no single NDT-method in itself will give a complete as-sessment leading to the final assessment of the structure; a number of tests will be required to obtain an accurate overview of the damage. A checklist of available standard tests and optional tests will facilitate the planning of the inspection. Every single test method has to be supplemented with other test methods to confirm and complete the results of the measurements. If e.g. the potential measurements indicate corrosion, a break-up is required to confirm the statement. Furthermore, a measurement of the carbonation depth and/or the content of chloride will usually also be required to evaluate the cause of damage and the future development of damage. Otherwise, if the supplementary measurements cannot confirm the results of the potential measurement, one or more of the single measurements may be incorrect.


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4.1 Visual inspection

A detailed visual inspection is always carried out as the first activity in the field. The visual inspection must include the entire structure. On paper sketches the cracks (length, width, direction) are marked as well as areas with spalling, rust stains, dis-integration and other relevant observations. The result of the visual inspection is part of the basis for determining the NDT-methods to be used, and the extent of the tests.

If the visual inspection reveals significant damage to other components than those for which a special inspection was required, the inspector must consult the owner of the bridge in order to revise the requisition. If the inspection engineer is not the same person as the rehabilitation engineer, the test plan must be discussed with the rehabilitation engineer before the inspection is carried out.

4.2 Areas requiring investigation The determination of areas requiring investigation depends on:

• The extent of visible damage (first impression of the condition).

• The size of the structure.

• The hypothesis for the damage mechanism. As a rule-of-thumb:

• In determining the areas requiring investigation, just as much attention should

be paid to the areas without visible signs of deterioration as to areas with visible

signs (especially in case of corrosion).

• Evaluation of the condition in areas with no visible signs of deterioration is more

difficult, but very important, when estimating the remaining service life of the

bridge and the optimum repair strategy.

4.3 Homogeneous areas

On the basis of the visual inspection and prior knowledge the structure may be di-vided into homogeneous areas. A homogenous area is defined as an area where there the present level of deterioration and parameters affecting the deterioration of the structure exhibits only a random variation. Consider for example a bridge pier in saline water. The chloride surface concentra-tion will be large in the tidal and splash zones. The chloride surface concentration will decrease with increasing distance from the mean water level. In this case it makes no sense to compare results from different piers if the tests are not performed at the same distance from the mean water level. To overcome this problem the piers may be divided into homogenous areas. Tests originating from the same homogenous


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area may be treated as coming from the same population e.g. in conjunction with a statistical analysis. The division of the structure into homogenous areas also depends on the material parameters. If for example two different concrete compositions have been used for two piers in saline water results of chloride measurements from these two different structures cannot be treated as a whole even though the measurements have been performed at the same distance from the mean water level.

4.4 Evaluation of test results

When all planned tests are completed, the visual registrations and test results must be evaluated to see if they form a sufficient basis for concluding the type, cause, extent, and possible development of the damage. Otherwise, supplementary tests must be selected and performed. If the test results do not confirm the hypothesis regarding the cause of damage, the hypothesis must be revised. It may be necessary to perform supplementary tests to confirm the revised hypothesis.


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5. Types of damage

This section includes a short description of the different types of damage observed for the different causes of damage and for the different types of materials. The bridge owner is advised to compile and update a database of the most typical dam-age observed at the principal inspections. The content of this database will depend on the type of bridges, the climate, etc.

5.1 General structural damage • Permanent deformations (deflections/displacement) • Tilt/settlement • Abnormal vibration (too slender structures/insufficient supports) • Water leakage • Loss of friction

5.2 Damage due to water • Scour • Ponding of water • Deposition • Debris and vegetation • Blocked drainage • No pipe/inadequate pipe length • Difference in level • Erosion • Material loss/disintegration • Silting at culvert • Inadequate size

5.3 Damage on surface treatment systems • Mechanical damage • Material deterioration • Weathering • Loss of adherence between layers

5.4 Damage on concrete structures • Cracks • Spalling • Corrosion of reinforcement/tensioning bars or cables • Wear and abrasion • Material deterioration • Impact damage • Fracture


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• Weathering • honeycombing

5.5 Damage on steel structures • Corrosion • Cracks • Loose connections (loose bolts) • Unintended eccentricities • Impact damage • Fracture

5.6 Damage at masonry structures • Deteriorated stones • Deteriorated joints • Cracks • Unintended eccentricities • Overloading • Moisture movement • Thermal movement • Impact damage • Fracture

5.7 Damage at wooden structures • Fungous growth • Rot • Noxious animals or insect pests

5.8 Registration of damage When inspecting the inspectors should pay particular attention to the following com-ponents:

5.8.1 Concrete structures • Reinforced concrete girders may present a crack pattern as shown in Figure

5-1. This is not necessarily dangerous – however if the cracks are due to over-

loading then a structural assessment should be made. The crack widths can be measured using a “crack measuring gauge” – see section 7.1.

Reinforced cantilevered structures show a crack pattern, see Figure 5-2, slightly

modified. However the same principles apply.

• The influence of cracks on the bearing capacity may be harmless at the time of inspection but some cracks may initiate corrosion that later may be critical. Fine cracks in reinforced structures may be harmless unless the structure is exposed


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to very aggressive environment, e.g. positioned in the splash zone of saline wa-ter. Fine cracks in pre-stressed structures are more critical.

• Bearings often need a close inspection. The stresses at the bearings are high, therefore there is a danger of concrete crushing, in particular if the bearings are misplaced or badly designed.

• Common reinforced concrete structures will not fail without an early warning such as coarse cracks and visible deflections.

• Pre-stressed concrete structures are much more sensitive to damage and corro-sion of cables or failure of anchorage may lead to sudden failures of a structure.

Figure 5-1: Crack pattern on reinforced concrete beam.


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Figure 5-2: Crack pattern in cap beam/corbel.

5.8.2 Steel structures • Connections in steel structures are very likely to collect dust and water at cor-

ners or poorly drained surfaces. Therefore the risk of corrosion is high. The inspector should pay attention to such areas that it might be necessary to clean to see if there is corrosion.

• If a welding in high-stress areas of steel structures is not executed correctly, fatigue cracks may occur at the edge of the welding. A close inspection is neces-sary to find such cracks. If the inspector detects more cracks at the same type of weld particular attention should be paid to extend the random samples to cover larger samples of components with the same positions.

• Connections in steel structures exposed to repetitive loads may fail in fatigue without any other warning than very fine cracks. Therefore, potentially “danger-ous” details of steel structures should be pinpointed in advance of the inspection in order to give these details a closer inspection.

Surface treatment:

• The quality of the surface treatment relies in general on the quality of manual application of the paint. Poor climate conditions during application and hardening and also the manual performance failures, e.g. an edge that is forgotten, a blocked spray nozzle, erroneous distance or angle for spraying, etc. Therefore the position and extent of damage will very often mirror the cause of the dam-age.

• Particular attention should be drawn to inspect areas where the access for appli-cation has been poor or where the number of construction damage due to tem-porary attachments have been higher than at normal sections.


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6. Damage mechanisms

6.1 General This chapter contains a description of the most commonly occurring damage mecha-nisms encountered. The intention is that the description should be sufficient as a guideline to recognise the damage when it occurs on a bridge, and to evaluate how significant it is. However, the descriptions in this relatively short manual cannot be exhaustive, and it is essential that investigations are carried out by experienced engineers with a thorough knowledge of bridges and damage mechanisms, and a good amount of common sense.

6.2 Structural Deficiencies Structural deficiencies may be a danger to the structural safety. Therefore, identify-ing such problems is very important. Structural deficiencies can be divided into the following four types, which can be dis-tinguished by their appearance:

6.2.1 I. Structural cracks (load induced cracks) in concrete members Structural cracks can be recognised as cracks with well defined orientation and with specific crack patterns related to each type of internal forces (bending, shear). Structural cracks might be a sign of a structural deficiency. For reinforced concrete (RC) structures, cracks in most cases are not serious. RC is allowed to crack. The crack width and spacing will indicate whether there is some-thing wrong or not, taking the specific type of load and type of reinforcement into consideration. Coarse cracks are an indication of over-load and/or under-design. For pre-stressed concrete, cracking is an indication of a potentially serious problem (overload, inadequate initial load bearing capacity, or plain design error). Normally no cracks should be visible.

6.2.2 II. Excessive/unintended deflections and movements Examples:

• Settling of foundation (possible causes: poor soil condition, scour).


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• Deflection of girders (possible causes: low stiffness, creep, poor design or im-

proper formwork).

• Horizontal movements of retaining walls and wing walls (possible causes: low

stiffness, creep, compaction of back fill, soil condition, under-design).

• Bearings out of position (possible causes: wrong positioning, unforeseen move-

ments, shrinkage, creep, deterioration and temperature).

6.2.3 III. Fracture/crushing Examples:

• Local crushing at supports/bearings (possible causes: honeycombs, wrong posi-

tioning of bearings and/or reinforcement, overload, inadequate initial load bear-

ing capacity).

• Columns (possible causes: impact during flooding).

• Superstructure (possible causes: impact from vehicle (vertical clearance)).

• Local crushing at expansion joints (possible causes: inadequate joint system,

wear, movements restrained).

6.2.4 IV. Structural Problems, Steel Components Examples:

• Fatigue cracks at welded connections.

• Brittleness due to ageing and cold brittleness.

• Buckling of compression members.

• Eccentricities in welded connections. In the following sections, selected examples from the four groups of structural defi-ciencies are shown.

6.2.5 Structural Cracks in Concrete Members Types of Structural Cracks From a structural point of view, it is important to distinguish structural cracks from the non-structural cracks caused by concrete shrinkage etc.


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Pure Tension All cracking in concrete members is caused by tensile stresses (concrete has a low tensile strength, but high compressive strength). Therefore it is obvious to consider pure tension as the basic case.

In a reinforced prismatic concrete beam subjected to pure tension, cracks formed will cross the whole cross section. An increase/jump in the steel stress will suddenly arise at the crack, when the cracks are formed. This affects the bond between the concrete and the bar in a certain zone (slip distance l0) around the cracked section, so that no shear stresses can be trans-ferred, see Figure 6-1.

Between the cracks, the steel stresses will be lower due to the effect of the sur-rounding uncracked concrete (this is called tension stiffening).

Figure 6-1: Tension crack / bond slip.

The crack width has its minimum at the rebar and increases with the distance from the bar, see Figure 6-1 and Figure 6-2.

This variation is one of the reasons for the statistical scattering of measured crack widths, which for an example is greater for slabs with larger rebar spacing than for beams with close rebar spacing.


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Figure 6-2: Variation in crack width.

6.2.6 Flexural Cracks For a beam subjected to bending, two different types of cracks will occur, see Figure 6-3. The first cracks to be formed emerge from the face in tension and extend to the

neutral axis. They are called bending cracks. When the bending moment is increased, new cracks will emerge from the face in tension to just beyond the main bars. These cracks are tension cracks. In a heavily reinforced beam with a depth (height) more than 0.4 m, these usually closer spaced tension cracks tend to join the bending cracks in the web forming a fork like crack pattern. The crack width of the bending cracks in the web above the main bars can be very high, if there is not sufficient longitudinal reinforcement in the web between the main bars and the compressive flange.

Figure 6-3: Flexural cracks.


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It must be noted that the bending cracks indicate the position of the neutral axis. This means that it is possible to compare this position with that found from calcula-tions. Since the position of the neutral axis depends on the amount of reinforcement, measurements of the actual position can be used as a check of the reinforcement. In the same way, the measured crack widths indicate the stress level in the bars, bearing in mind that normally the crack width measured neither corresponds to the maximum load nor the dead load, but lies somewhere in between. Note that a systematic crack pattern in the surfacing on top of the bridge may be closely related to structural crack in the superstructure (for example bending cracks caused by a negative bending moment above an intermediate support for a continu-ous bridge).

Figure 6-4: Longitudinal cracks, cross section.

6.2.7 Longitudinal cracks Longitudinal cracks may be formed in girders as a consequence of the stresses in the main bars giving local compression stresses in the concrete around the bars: The tension strain in a deformed main bar produces inclined compressive stresses be-tween the concrete and the ribs of the bar. These stresses tend to split the cross-section transversely. See Figure 6-4. This kind of longitudinal cracks may occur in

cases of high stresses in deformed main bars or in cases of anchorage failure at the end of a reinforcement bar (at the cross section where the number of bars change).

6.2.8 Shear Cracks In beams and slabs subjected to shear (and bending), inclined shear cracks will oc-cur in areas at the supports, see Figure 6-5.


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Figure 6-5: Shear cracks.

Close to a simple support, the angle between the inclined shear cracks and the beam axis will be approximately 45 degrees (maximum shear, bending moment low). Of-ten, some of the usually fine tension cracks from the bending moment tend to join the shear cracks, see Figure 6-5.

In the area between mid-span and the support, the bending cracks will be more or less inclined by the shear force depending on the ratio between bending and shear, refer Figure 6-5 and Figure 6-6.

Figure 6-6: Shear and bending cracks.


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For T-girders, the stringer force in the compression flange will change to a tensile force above the support. This means, that possible tension cracks tend to join the shear cracks resulting in more vertical cracks just above simple supports, see Figure 6-5.

The ordinary inclined shear cracks in a girder web may in some cases be connected to horizontal shear cracks, see Figure 6-7.

Figure 6-7: Longitudinal shear crack.

Shear cracks cross the total web thickness of a girder. But differently from tension cracks, shear cracks have limited strain development due to the main longitudinal rebars and the compression zone. This means, that even if the yield stress may be reached in the stirrups when the cracks are formed, a new state of equilibrium is established between stringer forces, stirrups and concrete struts, which leads to lower stresses in the stirrups after crack-ing. Therefore, the crack width can be rather high if the web is wide and/or the shear forces are high.

6.2.9 Bending and Shear Bending and shear cracks in girders: Shear cracks may not only appear at the support. Many older bridges (typically slab-girder bridges with two girders) have shear problems in the mid-span zone also, which are closely connected to low bending capacity in this area combined with a low shear capacity as well (few stirrups) in this area. The start of a typical "shear-bending" failure is characterised by a steep shear crack connected to a horizontal crack along the bridge deck and a horizontal crack just above the main reinforcement, forming an S-curve, refer Figure 6-8.


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Figure 6-8: “Shear bending”.

The next step may be a fatigue failure in a stirrup or in one of the main bars (if there is a local splicing in the critical zone). The final step is a complete "shear-bending" failure. This type of cracks may be seen in girders in the mid-span zone as well as in the areas close to the supports. This type of cracks is serious and should immediately initiate an assessment of the load carrying capacity, which may lead to possible weight restrictions until a rehabili-tation and strengthening project is prepared in order to prevent a real failure in or-der to save money in the repair phase.

6.2.10 Torsion Torsion causes inclined cracks similar to the shear cracks, but different from the or-dinary shear cracks they are spiral and are crossing the whole depth (not only the area between the stringers) of all faces of prismatic members, refer Figure 6-9.

Figure 6-9: Torsion cracks.

6.2.11 Bearing Cracks Bearing cracks are defined as cracks, which occur in connection with bearings. Some typical examples of bearing cracks are shown in Figure 6-10.


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Figure 6-10: Bearing cracks.

In (a), the elastomeric pad is placed too close to the end of the girder and a kind of a splitting crack is formed between the cover and the bent main bars. Generally, bent main bars in large dimensions give rise to bearing cracks, because the large bar size requires larger bending diameters. This means, that the edge of the end of the beam is not sufficiently reinforced, especially if horizontal forces due to e.g. temperature may occur too. This case is shown in (b). A special variant of case (b) is shown in (c). Wrong concreting or un-removed poly-styrene causes friction between girder and cap beam, which results in cracks behind the bearing. In (d), a bearing crack in a cap beam is shown. This type of bearing usually occurs in connection with slab bridges with only asphaltic paper as bearing. The cracking is caused by friction due to horizontal forces (temperature) and the angle of rotation, which tend to move the reaction force to the outer edge. Many problems are connected to the concrete block rocker bearing type – see Figure 6-11. Often the lead is pressed out and bearings out of position may cause

either spalling or/and splitting or/and crushing of the concrete block itself, the bear-ing zone of the girder or/and the top of the pier. This increases the risk of unin-tended settlements.


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Figure 6-11: Concrete rocker.

Honeycombs in the bottom of the girders due to concentration of reinforcement bars above the bearings increases the risk of local crushing and thereby unintended set-tlements. Narrow supports increase the risk of local spalling, local crushing, and especially of loosing the support during an earthquake.

6.2.12 Splitting Cracks Splitting cracks are related to highly concentrated loads, for an example at bearings and in the anchorage zones for the pre-stressing cables. Usually two types of splitting are considered. The first type of splitting is located very close/just below the acting concentrated force, which tries to split the concrete sec-tion locally. This is normally prevented by a "fretwork" of reinforcing bars. The second type is caused by the necessary distribution of the concentrated force to the whole cross-section, which normally will take place over a certain distance de-pending on the geometrical conditions, see Figure 6-12.


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Figure 6-12: Splitting cracks.


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6.2.13 Structural Cracks, Examples In the following pages, some further examples are shown.

Slabs

Figure 6-13: Slab.

Columns and Piers Eccentricities between piles in pile bents - out of plane - may introduce considerable bending moments and under extreme circumstances lead to collapse.


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Figure 6-14: Eccentricity in pile bent.

Compression failure in a column will have the same appearance as a compression failure in a test cylinder in the laboratory.

Figure 6-15: Compression failure.

Figure 6-16: Cap beam.

Pier Caps, Cap Beams


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Figure 6-17: Pier cap.

Corbels A special type of corbel is considered in Figure 6-19 and Figure 6-20 which is a

projecting beam with reduced depth often seen in bridges with suspended spans. For this type, attention should be paid to the very important "lifting" reinforcement, which transfers the load to the top of the beam. In Figure 6-21, a beam with a cor-

bel is shown.

Figure 6-18: Cracks in corbels.


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Figure 6-19: Beam with reduced depth.

Figure 6-20: Corbel with wrongly placed reinforcement.


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Figure 6-21: Beam with corbel.

Diaphragms Special problems are related to the connection between the diaphragms and the main girders. Especially the connection between an outer main girder and a dia-phragm.

The problem is related to a very weak anchorage of the main bars of the diaphragm in the very often thin web of the main girder. The problem appears as spalling and cracking at the connection, and influences the load transfer between the diaphragm and the girders (lateral distribution and load transfer from the bridge deck slab to girders).

Figure 6-22: Cracks in/at diaphragms.

In case of more than two main girders, large bending cracks are seen in the areas close to the intermediate main girders. These cracks indicate a high stress level and that the bending reinforcement is not sufficient compared to the actual load effects.

Bridge Deck Panels Quite often, deck panels with small depths are heavily cracked and potholes are de-veloped if measures are not taken. The reasons for this damage mechanism are first


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of all heavy loads, which lead to high tensile stresses in the reinforcement. But also too little thickness of the deck (some times less than 20 cm with 4 crossing layers of bars and varying accuracy of the position of the bars) and poor concrete quality are contributing to the problem. In thin slab panels the steel stresses are very sensitive to the thickness of the actual concrete cover.

The problem starts as map cracking with cracks in both directions, which normally follow the bars because the rebars act as crack "guidance", refer Figure 6-23, sec-

tion A-A and B-B. As mentioned previously, cracking is followed by loss of bond be-tween rebars and concrete on a certain distance from the crack. When the high load-ings are repeated several times, this slip distance will increase. And a mechanism which may be called "bond fatigue" will take place, which eventually leads to the separation of the concrete cover. When the concrete cover is lost, a punching shear failure may develop and finally a big pothole may develop, where only the bars remain. However, it will often be pos-sible to detect the problem in an early stage, as cracks will show in the bridge sur-face above the girders.


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Figure 6-23: Cracks in deck panel.


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Pre-stressed and post-tensioned concrete

Figure 6-24: Horizontally curved cables.

Figure 6-25: Anchorage zone, pre-stressed beam.


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Unintended deflection and movements Deflections, girders

Figure 6-26: Deflections caused by large span lengths in RC concrete (creep) and

improper formwork.

Retaining walls

Figure 6-27: Deflections caused by low stiffness (creep), compaction or soil condi-

tions.


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Bearings

Figure 6-28: Unforeseen movements (shrinkage, creep, temperature), wrong posi-

tioning.

Fracture/crushing, Concrete At supports/bearings

Figure 6-29: Crushing caused by honeycombs, wring type of bearing or poor work-

manship.

At expansion joints


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Figure 6-30: Local crushing (possible loose bolts).

Vertical clearance

Figure 6-31: Impact from vehicle (concrete beam).


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Figure 6-32: Impact from vehicle (steel girder).

6.2.14 Structural Problems, Steel Components Truss Bridges The most common damage to truss bridges with interior passage are impacts to the lateral braces from trucks either because of too low vertical clearance or because the total height of the vehicle exceeds the allowed height.

The consequences may vary from almost harmless if only secondary braces are damaged to very harmful/collapse if the primary components are damaged. Even though only the braces are hit, it may introduce problems for the main compo-nents. If for an example the braces are connected to vertical compression members, the impact may create a permanent deformation of the compression members mak-ing these components more or less useless (and introducing a redistribution of the load effects through the structure), refer to Figure 6-33.


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Figure 6-33: Impact damage to truss bridges

Stability problems (buckling) in compression members - especially vertical - may also occur due to overload introducing a similar redistribution of the load effects as mentioned above. In both cases, a load capacity evaluation should be carried out to evaluate the risk of a possible collapse. Special attention should be paid to the connections/nodes between the different components (tension/compression components and horizontal beams components), since they are disposed for corrosion due to accumulation of dust and humidity. Plate Girders (including plate girders in steel-concrete composite structures) A common type of damage to plate girders is impact from vehicles with heights ex-ceeding the actual vertical clearance. Girders may be seriously affected by such an impact, which means that the load effects for some of the remaining girders will be increased. This is not an acceptable situation and remedial actions should be carried out.


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Figure 6-34: Fatigue problems in welded steel members. Many plate girders are welded. It is normal to find stress peaks at welds. The stress level depends of the type of weld and the quality of the welding. If the stress range from the live load is considerable at a poor welding, there may be a risk of fatigue cracks. Therefore welded details, which may be critical in fatigue should be inspected carefully. This may for example be at a welding in the bottom flange of a girder, re-fer Figure 6-34. Or at a welded connection between the horizontal wind truss and

the bottom of the girders. Such details may at the same time also be critical to cor-rosion. Arch Bridges Attention should be paid to the hanger connections to the main girders and the arch. Look for corrosion of the hanger cables, which normally have a high stress level and therefore may be exposed to stress corrosion with the consequence of a brittle be-haviour. Look also for other connections for an example between the longitudinal girders and the transverse beams.


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6.3 Non-Structural Cracks in Concrete These cracks are divided into four groups:

• Shrinkage cracks (due to drying) (see below)

• Thermal cracks (due to hydration) (see page 6-28)

• Cracks due to plastic shrinkage (see page 6-28)

• Cracks due to plastic settlement (see page 6-29)

6.3.1 Shrinkage cracks (due to drying) The appearance and development of shrinkage cracks depend on the geometry, the size of the member and possible restraints. The crack orientation is normally well defined and depends on the geometric conditions (e.g. restraints caused by other parts of the structure). Drying shrinkage cracks pass through the whole cross sec-tion. Normally, these cracks are harmless from a structural point of view. But they may be harmful with regard to durability. Some examples are shown below.

Figure 6-35: Geometric conditions.

Figure 6-36: Shrinkage cracks, abutments.


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Figure 6-37: Shrinkage cracks, slabs.

Figure 6-38: Shrinkage cracks, bridge deck panel.

Figure 6-39: Shrinkage cracks, I-girder web.


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6.3.2 Thermal Cracks (due to hydration) The appearance of thermal cracks caused by thermal stresses due to temperature differences in the hardening concrete is very similar to the appearance of ordinary shrinkage cracks (geometric conditions). However, different from ordinary shrinkage cracks and structural cracks, thermal cracks are young cracks (developed in the young concrete). This means that the cracks will follow the surface of the coarse aggregates and stones, and not go through them.

6.3.3 Cracks due to plastic shrinkage Plastic shrinkage cracks are caused by rapid drying of the concrete surface (low hu-midity, wind, high temperature) in its plastic state (e.g. caused by improper curing). Similarly to the temperature cracks, the cracks will follow the surface of the stones, not go through them. The cracks are normally wide and shallow and may form a definite pattern. In cases of a wide surface (concrete wearing course or similar), a state of hydrostatic tension will arise (no shear stresses). If there is no crack "guidance", the crack will be formed "at random" and the appearance will be a net crack pattern, in most cases with hexagonal meshes. Typical crack patterns caused by plastic shrinkage are shown in Figure 6-40-Figure 6-42.

Plastic shrinkage cracks are normally harmless from a structural point of view (al-though wide cracks may influence the load carrying capacity and the behaviour un-der service load), but may be harmful to durability.

Figure 6-40: Parallel at random.


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Figure 6-41: Skewed at random.

Figure 6-42: Hexagonal mesh.

6.3.4 Cracks due to plastic settlement Normally, these cracks are due to a high concrete slump when cast. The appearance and position of these cracks is normally above the reinforcement at the surface, refer Figure 6-43, or at changes in the cross section, refer Figure

6-44. Plastic settlements are also seen in slabs with voids.

Normally, the cracks are harmless from a structural point of view, but may be very harmful with regard to durability. This is because the bars are not sufficiently pro-tected against environmental effects (the cracks often reach the bars).


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Figure 6-43: Plastic settlement at rebars.

Figure 6-44: Plastic settlement in slab with voids and slab/girder.


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6.3.5 Initiation of Corrosion In the highly alkaline environment in concrete (pH-value close to 13) the anodic steel surface becomes coated with a very thin grey passive layer of ferric oxide. The ferric oxide is stable over a wide range of potentials and so acts as a protective coat-ing. Thus, reinforcement is protected against corrosion when embedded in a concrete of a good quality and with a sufficient cover. But, the protection against corrosion is not everlasting. The surroundings will always affect the concrete and finally lead to a breakdown of the passive layer. The break-down of the passive layer may be caused by free chlorides at the reinforcement or by carbonation of the concrete cover. These mechanisms are described in the follow-ing sections. However, corrosion depends on moisture content and the availability of oxygen and therefore of the rate at which oxygen diffuses through the concrete. The period during which the passive layer breaks down is normally called the period of initiation. The duration of the initiation period depends on:

• The thickness of the concrete cover; the thinner the cover, the shorter is the

period of initiation.

• The quality of the concrete cover (primarily water/cement ratio dependent); i.e.

the initiation period decreases when the concrete quality gets poorer (the wa-

ter/cement ratio increases). In special cases (honeycombs, "cold joints", too

small cover), poor workmanship can lead to corrosion immediately after casting.

• The aggressiveness of the environment, the temperature and the humidity.

• The kind of mechanism causing deterioration. Chloride penetration is by far the

most aggressive mechanism, leading to a much shorter initiation period than the

mechanism of carbonation (chloride ions facilitate the corrosion process). During the period of initiation there is no actual corrosion going on. The protection of the reinforcement is being broken down with no visible signs of deterioration, neither on the surface of the concrete nor on the reinforcement. Therefore: The risk of future corrosion damage can only be assessed by performing special in-vestigations.

Four steps of corrosion of steel in concrete may be defined as:

1. The passive state.

2. Pitting corrosion.


6-32/311 6-32

3. General corrosion.

4. Active low potential corrosion.

These relate to corrosion as an electrochemical process which requires a potential difference between two connected electrodes in an electrolyte. In concrete the elec-trodes may be neighbouring points on the same reinforcing bar, or separate bars or groups of bars which have a potential difference between them.

6.3.6 Carbonation Carbonation is caused by the carbon dioxide (CO2) in the air. The CO2 reacts with the calcium hydroxide, Ca(OH)2, in the cement paste eventually leading to a critical de-crease of the alkalinity. The pH-value decreases to less than 9, which normally is insufficient to protect the reinforcement against corrosion. The reaction in which calcium hydroxide is converted to calcium carbonate is as fol-lows: Ca(OH)2 + H2CO3 → CaCO3 + 2H2O 3CaO•2SiO2•3H2O + 3CO2 → 3CaCO3•2SiO2•3H2O These reactions consist of the following elementary steps: 1. The diffusion of atmospheric CO2 in the gaseous phase of the concrete pores. 2. The dissolution of solid Ca(OH)2 from cement gel into the pore water and the

diffusion of dissolved Ca(OH)2 from regions of highly alkalinity to those of low. 3. The reaction of dissolved CO2 with dissolved Ca(OH)2 in the pore water. 4. The reaction of dissolved CO2 with CSH. The effective diffusivity, De, CO2, of CO2 in concrete is given by the following empiri-cal expression:

2.26

, 1001101.2

2 ⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛ −⋅= − RHpD COe

Where p is the porosity of the hardened cement paste and RH is the ambient relative humidity. The speed of the reaction will depend on the rate of removal of water formed. In other words, carbonation depends on a drying atmosphere and are impeded in the presence of water. On the other hand dry CO2 does not react with dry Ca(OH)2 so the presence of moisture is essential to the carbonation process. The optimum mois-ture content for carbonation is intermediate between 40 and 70 % relative humidity. An increase in temperature with 10 oC will approximately double the speed of the reaction. Carbonation of concrete results in increased strength and reduced perme-ability, possible because water released by carbonation aids the process of hydration and CaCO3 reduces the voids within the cement paste.


6-33/311 6-33

The carbonation depth in concrete, x in mm, is given by the following empirical equation:

time = Tconstant = K

depth ncarbonatio = xwhere

T K = x

When x is measured in mm and T in years, the value of the constant K can be esti-mated as the following in air at 50 % relative humidity:

⎟⎟⎠

⎞⎜⎜⎝

⎛−= 126.0172

cfK , where fc is the concrete strength in MPa.

If the relative humidity in the pores of the concrete is different from 50%, K must be multiplied by a factor < 1, dependent of the humidity. See Figure 6-45.

Figure 6-45: Depth of carbonation in relation to humidity.

Example: Compressive strength: 20 MN/m2 Age of structure: 25 years Relative humidity: 65 % RH

7.03 = 0.126-201 72 = K ⎟

⎠

⎞⎜⎝

⎛


6-34/311 6-34

K must be multiplied by 0.95 (Figure 6-45, RH = 65%). An estimate of the car-

bonation depth will be:

(mm) 33 25 7.03 x 0.95 = x ≈

If the actual carbonation depth and the concrete cover are measured on site, a pre-diction of the time until initiation of corrosion can be made:

⎟⎠⎞

⎜⎝⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠⎞

⎜⎝⎛

xc T =

xTc =

Kc = T

T K = c

T / x = K

T K = x

222

1

1

where: x = the actual depth of carbonation (mm, measured on site) c = cover (mm) K = constant T = age of the concrete (years) T1 = period of initiation (years) Example: Carbonation depth, measured: x = 25 (mm) Concrete cover, measured: c = 35 (mm) Age of the concrete: T = 20 (years) Initiation of corrosion is estimated to begin when the structure is T1 years old:

39(years) 2535 20 = T

2

1 ≈⎟⎠⎞

⎜⎝⎛

6.3.7 Chlorides


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Chloride induced reinforcement corrosion is in many areas considered the main du-rability problem for reinforced concrete structures. The amount of chloride necessary to initiate reinforcement corrosion (the critical chloride concentration or the threshold value) depends, among others, on the composition of the concrete and the moisture content of the concrete, see Figure 6-46.

Figure 6-46: Critical chloride content as a function of relative humidity.

Chlorides in the concrete may originate from various sources:

• the mix water,

• the aggregates,

• admixtures

• curing water,

• surrounding soil (from which chlorides are washed out in wet periods),

• de-icing salt (in cold areas),

• in coastal areas from the seawater (reaching the concrete directly or air-borne in

windy periods). In general, most of the chlorides contained in fresh concrete ("initial chlorides", i.e. chlorides from mix water, aggregates and some from the curing water) will be chemically bonded during the hardening of the concrete. Bonded chlorides are not regarded as harmful while remaining bonded, as they are not breaking down the passive layer on the steel. Threshold Value


6-36/311 6-36

The threshold value is the chloride value where the chloride concentration is so large that corrosion occurs in the solution can be approximated as follows:

61.0=OH

Cl

CC

Where CCl and COH is the concentrations of hydroxide and chloride ions in equivalents per litre, respectively. The hydroxide concentration can be calculated as follows:

( ) ( )1003923 ⋅

⋅+

⋅

=P

KcNac

COH

Where c is the cement content (kg/m3), (Na), (K) is the weight share Na and K re-spectively in cement, and P is the porosity of the concrete in % by volume. Normally, there are strict requirements to the maximum chloride content of fresh concrete. Further more sufficient protective properties of the cover (denseness and cover thickness) should be selected considering the environmental exposure. If the desired protective properties of the cover cannot be obtained, additional protective means need to be applied. Diffusion of chlorides into concrete Ingress of chlorides may take place by: • Capillary suction or permeation of water containing chloride. • Diffusion of chloride ions in the pore liquid. Based on Fick’s 2nd law diffusion of chlorides into concrete can roughly be described by the following two equations:

TKx x= (1)

( ) ⎟⎠

⎞⎜⎝

⎛⋅

⋅−−=TD

xerfCCCC issx 2 (2)

Where: x = depth below concrete surface (mm) Kx = constant T = age of concrete (years) Cx = chloride content at depth x Cs = chloride content at the concrete surface Ci = initial chloride content Erf = Error function D = diffusion coefficient (mm2)


6-37/311 6-37

The chloride diffusion coefficient of ordinary Portland cement concrete (w/c > 0.4) in a Danish environment may be obtained form the following equation:

D = 2000 (w/c – 0.35) [mm2/year] The Error function takes the following values.

x

erf(x)

x

erf(x)

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7

0

0.122 0.223 0.329 0.428 0.521 0.604 0.678

0.8 0.9 1.0 1.2 1.4 1.6 2.0 2.4

0.742 0.797 0.843 0.910 0.952 0.976 0.995 0.999

Based on measurements of the chloride profiles in the concrete, the different pa-rameters can be calculated giving a prediction of the time until corrosion occurs. The calculations will be similar to the calculations regarding carbonation. A typical chloride content profile looks as follows:

Figure 6-47: Chloride content profile. In this example, the initial chloride content has been approximately 0.02 %.


6-38/311 6-38

A prediction of the service life depends on the knowledge of a critical chloride level. Each structure is supposed to have its own critical limits, because the limit is de-pendent on various factors such as humidity and concrete quality. As a preliminary assumption, 0.05 % of dry concrete weight can be used as the criti-cal limit in normal concrete. For piers in seawater a value of 0.10 % of dry concrete in normally used.

6.3.8 Carbonation and Chlorides For concrete with a high initial content of chlorides, the chemically bonded chlorides in front of the carbonation front will be freed. Since carbonated concrete provides almost no resistance against chloride penetra-tion, the chlorides will be break through the carbonation front (diffusion). Therefore, the chloride content will increase constantly behind the carbonation front when the carbonation front is moving into the concrete, leading to corrosion when the critical limit of chloride content is reached at the reinforcement. A typical chloride content profile in a concrete with a carbonation depth of approxi-mately 30 mm is as follows:

Figure 6-48: Chloride content profile with carbonation.

If this mechanism takes place together with alternating wetting and drying in a chlo-ride contaminated environment, the corrosion process can run very fast (e.g. in tidal waters).


6-39/311 6-39

A simple method for evaluating the chloride penetration from outside is, in this case, to ignore the carbonated concrete layer. The thickness of the carbonated layer can be calculated from the previous mentioned equations.

6.3.9 Propagation of Corrosion When the chloride content at the reinforcement level reaches the critical limit or the carbonation front reaches the reinforcement, the passive layer is broken down and the corrosion process starts. A corrosion process is an electrochemical process, where a current runs between corroding areas (the anodes, where the passive layer has been broken down) and non-corroding areas (the cathodes, where the passive layer is complete). The current runs due to the theoretical fact of the behaviour of metals in liquids and concrete, see Figure 6-49. If two metals with different electrochemical potential are

electrically connected, corrosion is likely to take place on the metal with the lowest potential. Therefore, the risk of corrosion can be evaluated by measuring the potentials (HCP-measurements, see Chapter 7.6).

Figure 6-49: The electrochemical series. When the passive layer breaks down locally, the area changes its place in the elec-trochemical series becoming more negative. This creates a potential difference, which generates the corrosion current. Where the passive layer is broken, the following "anodic reaction" takes place.

Anodic reaction:

Fe -> Fe++ + 2e-


6-40/311 6-40

If water and oxygen is present at the steel surface, the "cathodic reaction" takes place.

Cathodic reaction:

O2 + 2H2O + 4e- -> 4OH- The anode and the cathode may be far apart, as long as there is an electric connec-tion between them. The anodic process produces electrons, and the cathodic process consumes elec-trons. If there is an electric (through the reinforcement) and electrolytic (through the moist concrete) connection between the anode and cathode, an electric current will flow. The corrosion process is illustrated in Figure 6-50.

Figure 6-50: The corrosion process.

6.3.10 Corrosion products and corrosion rate The Fe++ reacts with oxygen, OH- and water, forming corrosion products. The type of corrosion products produced primarily depends upon the available amount of oxygen and water. If there is little available oxygen, the first products will be white Fe(OH)2. These white products may be transformed into black FeO and water. This type of corrosion product is typical for local, chloride initiated corrosion.


6-41/311 6-41

If oxygen is added to FeO, black Fe3O4 may be formed. Fe3O4 is not expansive, so there are not necessarily any exterior signs of corrosion. If water and oxygen become available to the white Fe(OH)2, the products turn via a green intermediate stage into brown Fe(OH)3. If additional water is available, expansive yellow/red/brown Fe(OH)3,nH2O (rust) is formed. If oxygen is plentiful, the expansive yellow/red/brown Fe(OH)3,nH2O (rust) is formed without any intermediate stages. This type of corrosion product is typical of carbona-tion initiated corrosion in porous concrete.

The development of the corrosion attack and the velocity of the process primarily depend upon:

• The temperature.

• The ratio between corroding and non-corroding areas.

• The moisture content. As for most chemical reactions the corrosion rate increases with increasing tempera-ture. If the area of the anode is small compared to the area of the cathode (Aa/Ac << 1) the corrosion rate will be high, because the corrosion takes place in a small area (local corrosion). The process of local corrosion normally leads to corrosion products, which are black, non expansive and liquid-like. This means that local corrosion cannot be expected to give visible signs of corrosion on the surface. Local corrosion is hidden corrosion where serious attacks can be developed without visible signs, increasing the risk of unexpected collapse.

In case of chloride attack, the chloride ions facilitate the formation of Fe++, thus increasing the corrosion rate. As chloride initiated corrosion normally starts as local corrosion, a considerable reduction of cross section of a steel bar may take place within a short time and without visible signs on the concrete surface. A ratio between anode area and cathode area, Aa/Ac approx. 1, gives what is called general corrosion. This type of corrosion leads to the well-known yellow/red/ brown corrosion product, which quickly gives rise to visible signs of corrosion (spalling of the concrete, cracks, rust stains).

The corrosion rate depends on the moisture content and the electrolytic contact be-tween anode and cathode. The RH % below refers to the moisture content in the pores of the concrete at the anode area (the electric resistance at the anode is deci-sive for the corrosion current, and the resistance drops with increasing moisture con-


6-42/311 6-42

tent. In outdoor concrete there will normally be sufficient water for the cathode process). The moisture content in the concrete in average over a year can be dif-ferent from the RH % in the air, especially in case of the presence of chlorides in the concrete. At RH % less than 80, the corrosion rate is negligible. Normally the corrosion rate drops at RH above 95% and reaches 0 at RH = 100%. See. But if there is electric contact to areas with less moisture content, a cathode may be formed in this area and the corrosion rate will increase.

Figure 6-51: Corrosion rate in relation to humidity.

This may be the case when a column is partly submerged in saturated soil. Corrosion may take place at an anode area below ground (RH=100%), because the cathode process takes place above ground at a lower humidity. The two types of corrosion can normally be located at different places in the struc-ture.

6.3.11 Local/general corrosion Local Corrosion (giving no visible signs on the surface) is typically located:

• at restrained cross-sections (max. negative bending moment), e.g. brackets and

cantilever decks,


6-43/311 6-43

• at construction joints and cracks in concrete with high content of chloride and

moisture,

• in concrete with a high initial content of chloride. General Corrosion (giving visible signs at the surface) is typically located:

• in concrete of poor quality, especially in the cover concrete;

• in concrete with insufficient thickness of the cover;

• in concrete, where alternating wetting and drying takes place, such as horizontal

surfaces (especially soffits) and vertical surfaces above sea- or ground level. In practice, most structures (except structures submerged in water or saturated soil) will be exposed to both wetting and drying. In the early phase, corrosion will normally develop as local corrosion. In the later phase, general corrosion follows with evident signs of corrosion at the surface. In structures with corrosion caused by chlorides, the corrosion (i.e. the reduction of cross section) will normally be critical in the later phase. Three typical cases of corrosion (bridge deck, column and wing wall) are described in the following.

Figure 6-52: Example, bridge deck.

6.3.12 Bridge Deck

1) Areas with cracks caused by negative bending moment: The corrosion will de-

velop as local corrosion with a high corrosion rate and consequently reduction

of cross section.

2) Areas in which the corrosion in the first phase will develop as local corrosion

and in later phases will lead to more general corrosion followed by spalling of

the concrete.


6-44/311 6-44

The development of corrosion depends on the moisture content and the dura-

tion of wet and dry periods. In decks with no overlay/wearing course in hot

climate and with only occasional water influence (alternating wetting and dry-

ing), the corrosion will quickly develop as general corrosion with expanding

corrosion products. The time until corrosion attack becomes visible on the sur-

face is normally shorter but the damage to the reinforcement less than in

decks with a constantly high water content (e.g. in case of an overlay which is

not waterproofed, allowing water to penetrate but not to evaporate again).

In decks with an overlay, which is not waterproofed, the corrosion conditions

are at optimum. In this case, visible signs of corrosion normally occur at a

very late phase when the overlay cracks (or when the overlay for other rea-

sons is replaced).

A high rate of corrosion can be seen, even if there is a lack of the necessary

oxygen at the upper layer. This is because the oxygen-consuming cathodic re-

action can take place on the lower reinforcement layer.

3) Normally, the risk of corrosion in the soffit is limited except from areas with

water leakages, especially with chloride contaminated water. If larger areas suffer from water (and chloride) influence, the mechanism of damage will be equal to case 2) giving rise to spalling of the cover.

Figure 6-53: Example 2, pier column.

6.3.13 Pier column

1) Areas with the risk of local corrosion especially in constantly wet and chloride

contaminated concrete. In concrete exposed to alternating wetting and drying,

the corrosion product will expand causing cracks and spalling of the cover.

2) General corrosion, mainly caused by carbonation. The corrosion will develop

slowly, but visible damage will appear at an early stage.


6-45/311 6-45

3) Areas at risk of local corrosion. The lack of oxygen does not prevent corrosion

as the cathodic process will take place at the steel areas above ground level.

The corrosion rate can be very high.

In general, most of the chlorides contained in fresh concrete ("initial chlorides", i.e. chlorides from mix water, aggregates and some from the curing water) will be chemically bonded during the hardening of the concrete. Bonded chlorides are not regarded as harmful whilst remaining bonded, as they are not breaking down the passive layer on the steel. Diffusion of chlorides into concrete can roughly be described by the following two equations:

( ) ( ) (2) TD / x 0.5 erf C - C - C = C

(1) T K = x

issx

x

Figure 6-54: Example 3, Wing walls.

6.3.14 Wing walls / retaining walls Wing walls may suffer from several types of corrosion attack:


6-46/311 6-46

1) At ground level and below, the conditions are similar to those of a column (ex-

ample 1).

2) At the top, there is a risk of very high chloride content originating from surface

water running from the overpassing road and the slopes (through chloride

contaminated soil) to the top and the upper part of the back side of the wall.

Due to alternating wetting and drying, visible signs of corrosion of the rein-

forcement are seen as vertical splitting at the top. This splitting causes easier

access to water, which increases the rate of the corrosion process, etc.

3) On the backside of the wall, there may be a constantly high moisture and chlo-

ride content level. Even if there is a lack of oxygen, severe corrosion may oc-

cur. (The situation is similar to that of a column footing).

4) At the front of the wall, the chloride content may be very high, because saline

water penetrating from the back side evaporates, leaving chlorides in the con-

crete.

5) At all surfaces exposed to the air, carbonation initiated general corrosion may

occur.

Note Any investigation of corrosion problems on out-door concrete structures has to in-clude an evaluation of:

• The actual corrosion level and -activity

• The future risk of corrosion Both in areas with evident corrosion and in areas without visible signs of corrosion (especially in areas where corrosion normally is expected to take place), the investi-gation should be carried out as described in Chapter 4.2.

6.4 Alkali-aggregate Reactions In humid concrete, alkalis from the cement (or from the surroundings) may react with siliceous aggregate. The process may briefly be described as follows: Alkali ions (Na+ and K+) from cement, mix water or from the surroundings (on bridges, typically seawater or de-icing chemicals) lead to an increase of the concen-tration of OH- in the water in the pores of the concrete. OH- dissolves amorphous silicon (SiO2) which may be present e.g. in flint aggregate, producing a gel.


6-47/311 6-47

This gel absorbs possible water from the surroundings and expands. Under certain circumstances the expansion pressure leads to cracks in the concrete, forming a crack pattern on the concrete surface. If the reactions take place in coarse aggregate near the concrete surface, the result may be 'pop-outs'. See Figure 6-55.

6.4.1 Crack Pattern In general, concrete attacked by alkali silica reactions (or some other deterioration processes) will expand and crack in 'the easiest way'. This means that:

• The coarsest cracks will form parallel to the section with the weakest reinforce-

ment.

• Weaknesses in the design of the reinforcement will be disclosed. E.g. very coarse

cracks may appear if a cross section is not (or very weakly) reinforced.

• Normally, a system of internal cracks parallel to the concrete surface is formed.

This kind of cracks may be detected by drilling out cores, or by more sophisti-cated techniques, such as the 'Impact-Echo' method (see Chapter 7 regarding

test methods).

• The interaction between internal tension stresses from the alkali-aggregate re-

actions and from the load induced forces reveals the path of the internal forces

in the structure: The cracks will follow the direction of the internal stresses, par-ticularly clear at concentrated loads. See Figure 6-12.

The conclusion on the above damage pattern is that serious carrying capacity prob-lems may occur if the design of the reinforcement is insufficient. (E.g. un-reinforced sections or too small overlap at splices). If the reinforcement design is good (closed stirrups, sufficient overlaps, U-stirrups at beam ends, transverse reinforcement at anchorage zones etc.) alkali-aggregate reactions do not reduce the carrying capacity considerably.


6-48/311 6-48

Figure 6-55: Alkali silica reactions. The gel may form drops emerging from the cracks. The gel is soluble in water, there-fore — on outdoor structures — it is washed away by rain. But on indoor structures it may be seen. Alkali-aggregate reactions can only take place if the concrete contains alkalis, cal-cium hydroxide, silica and water – see Figure 6-56. It has to be noted that even if

the concrete does not contain enough alkalis the reactions may take place anyway if alkalis are supplied from the surroundings.

Figure 6-56: Illustration of the necessary components for alkali-aggregate reac-

tions. To prevent damage, one or more of the four components must be shut out or limited to an acceptable level. On existing structures with alkali-aggregate problems, the only practicable way is to keep water away from the concrete, either by putting up a shield, or by applying a surface protection (waterproofing, but permitting diffusion of vaporised water).

AAR

Reactive Silica (e.g. SiO2·H2O)

Alkali (Na+, K+)

Ca(OH)2

Water (H2O)


6-49/311 6-49

It is not possible to establish exact threshold values, but the following may serve as a guide regarding the humidity: If the relative humidity in the pores of the concrete is about 80-95% RH, the gel is solid, and the expansion pressure may cause cracks and pop outs. If the humidity is higher, the gel is liquid. It will fill pores and cracks and it may penetrate to the surface, but it will not cause serious damage. If the hu-midity is lower than about 80%, the reactions are not expected to take place. On new structures (and on rehabilitation works on existing structures), alkali-aggregate reactions must be prevented by using aggregate which does not react with alkalis, and/or using cement with a low alkali content and at the same time pro-tect the structure from alkalis from external sources. If no knowledge is available regarding the reactivity of the aggregates, it must be determined by tests. It is very important in preventing alkali-aggregate reactions that a great effort is made to avoid initial cracks in the concrete (see Chapter 6.3).

The alkali-aggregate reactions may take place for a considerable time (years) before visible signs of damage occur. In time, the internal stresses have grown to a magni-tude that causes expansion and cracking of the concrete. The cracking and expan-sion go on for some time (years) after which they die out – typically caused by lack of alkalis or reactive material. To evaluate the risk of future development of damage due to alkali silica reactions a “residual reactivity test” can be performed – see sec-tion 7.18.

The deterioration rate may rise considerably if frost/thaw action takes place at the same time, because freezing water creates internal stresses in the cracks initiated by the alkali silica reactions. The only reliable way to identify alkali-aggregate reactions with certainty is by means of specific microscope techniques in the laboratory (microscope inspection of thin slices of a drilled out concrete core) – see section 7.18.

6.5 Chemical Attack of Concrete and Masonry Water containing sulphates attacks the concrete or masonry chemically. Further more dissolved carbon dioxide in the rain water will attack masonry chemically. Both the ground water and the surface water may contain sulphates. The mix water may contain sulphates too, if not checked for. Seawater contains sulphates, and sewage water may contain sulphates too. Further, sulphates may contaminate the aggregates.


6-50/311 6-50

6.5.1 Acid Attack Pollutant gases are increasing drastically in the atmosphere. These air contaminants are usually found in gas form (SO3, HNO3, HCl, O3 and organic acids), in particle form (H2SO4, NH4SO4, (NH4)2SO4) or dissolved in water as droplets (CH+, NH4

+, HSO3

-, SO42-, NO2

-, NO3-). They are fairly reactive and may cause degradation on

concrete and masonry. Measurements in Gothenburg in Sweden have shown that pH-value varied between 3.0 – 4.5.

Degradation of concrete may occur as the result of the dissolution of the cement hydration products causing a more porous weaker matrix as a result of internal ex-pansion associated with gypsum formation and its subsequent reactions.

Pure water can dissolve the calcium products (calcium hydroxide) in concrete. Theo-retically the hydrolysis of cement paste can continue until most of the calcium hy-droxide has been leached away. This exposes other cementitious constituents to chemical decomposition. The process leaves behind silica and alumina gels with little or no strength.

The hydrated calcium silicate, which is alkaline, is neutralised by contact with hydro-chloric acid water:

2HCl + Ca(OH)2 → CaCl2 + 2H2O

Calcium chloride made by the reaction of hydrated calcium silicate and hydrochloric acid is very soluble to water. All acids attack concrete, and the cement paste is de-graded generally as in the example with hydrochloric acid:

2HX + Ca(OH)2 → CaX2 + 2H2O

where X is the acid residue.

Exposure to NOX, while also lowering pH, results in the formation of calcium nitrate hydrates. The calcium nitrate hydrates volumes significantly exceed those of the solid reactants; however, these nitrates are highly soluble and readily leached. As a consequence, the principal durability issue in this regard is the development of a more porous matrix, in turn, more susceptible to other forms of attack.

Nitric acid will react with the calcium hydroxide and form calcium nitrate:

Ca(OH)2 + 2HNO3 + 2H2O → Ca(NO3)2•4H2O

Exposure to SO2, either alone or in combination with elevated levels of CO2, is likely to lead to severe disruption. pH is lowered and the formation of gypsum results in expansion in cement. In addition the subsequent reactions of gypsum with the hy-dration products of cement result in additional expansion.


6-51/311 6-51

Sulphuric acid will attack calcium hydroxide with the formation of gypsum

Ca(OH)2 + H2SO4 → CaSO4•2H2O

Water containing free CO2 attacks concrete and produces Ca(HCO3)2 which is very soluble. Without considerations of stoichiometry the following may be described:

H2O + CO2 + Ca(OH)2 → Ca(HCO3)2

Or attack on carbonated calcium hydroxide

H2O + CO2 + CaCO3 → Ca(HCO3)2

Acid Reactant Product Volume change (%)

Carbonic

Sulphuric

Nitric

Ca(OH)2

(OH)2

Ca(OH)2

CaCO3

CaSO4•2H2O

Ca(NO3)2•4H2O

11

123

274

Table 6-1: The changes in the volumes of the solid phases associated with the at-

tack of calcium hydroxide and calcium silicate hydrate by nitric, sulphuric or carbonic

acid.

Rainwater contains dissolved carbon dioxide forming a very mild acid which dissolves calcium carbonate by producing soluble bicarbonate. Lime mortars will eventually be destroyed by percolating rainwater because calcium carbonate is their main binding agent.

6.5.2 Sulphate Attack Water containing sulphates attacks the concrete in different ways. When sulphate ions come in contact with the hydration products, a chemical reaction starts. In most cases, this reaction results in volume expansion. 1. High concentrations of sulphate ions (SO4

- -): SO4

- - + Ca(OH)2 + 2H2O -> CaSO42H2O (gypsum) + 2OH- + volume expansion 2. Lower SO4

- - concentrations: Calcium Aluminate Hydrate + CaSO42H2O -> 3CaOAl2O3CaSO432H2O (ettringite) + volume expansion 3. Most serious: Magnesium and Ammonium Sulphate: MgSO4 reacts with CaO3Al2O3's hydrate products


6-52/311 6-52

and MgSO4 + Ca(OH)2 -> CaSO4 + Mg(OH)2 + volume expansion In case 1 and 2, hydrated calcium silicates will be transformed into gypsum and et-tringite, which means that the concrete will lose its strength. Further, the cement paste will expand, which causes cracking and disintegration of the concrete surface (scaling). Risk of Sulphate Attack The risk of sulphate attack depends on the following parameters:

• The content of sulphate in the surroundings (seawater, ground water, sewage, soil). The aggressiveness of water and soil can be divided into three groups:

o Moderate: water with less than 300 mg SO3/l or soil with less than 0.2 % SO3.

o Aggressive: water with 300 - 1000 mg SO3/l or soil with 0.2 - 0.5 % SO3.

o Very aggressive: water with more than 1000 mg SO3 or soil with more than 0.5 % SO3.

• The moisture content of the concrete. The reaction needs the presence of water.

• The type of cement. Especially the C3A content of the cement.

• Sulphate contaminated aggregates.

• The permeability and/or the ability of capillary suction (e.g. from the buried part of a column to the free part). The sulphate concentration will constantly increase in the evaporation zone lead-ing to a rapid deterioration. Capillary suction may also lead to a general sulphate attack on the concrete in the whole cross section.

• Possible surface protection of the concrete.

A special case of both sulphate attack and corrosion is seen on partly submerged structures, especially structures with large dimensions. Partly submerged structures normally have the biggest problems in the splash zone. However, if the cover below sea level disintegrates due to sulphate attack, one big anode is formed under the sea level. The cathode will be the part above sea level. In structures with large dimensions, a large amount of current flows, leading to se-vere corrosion attack on the submerged part.


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In masonry sulphate attacks are the most common problem and is due to the reac-tion between sulphate ions in water solution and the tricalcium aluminate (C3A) phase in mortars to form calcium sulphoaluminate or ‘ettringite’. The commoner sulphates are the sodium, potassium and magnesium salts, which are all freely soluble, and calcium sulphate, which is less soluble but will leach in persis-tently wet conditions. The sulphates may be present in groundwater and can effect masonry below the waterproofing and masonry in contact with the ground such as retaining walls, bridges and tunnels. Sulphates are also present in some types of clay bricks and will be transported to the mortar in wet conditions. Sulphates do not attack pure lime mortars as there is no calcium aluminate present but may have some effect on hydraulic lime mortars.

6.5.3 Seawater Attack Concretes exposed to marine environment may deteriorate as a result of the com-bined effects of chemical action of sea water constituents on cement hydration prod-ucts, alkali silica expansion, crystallisation of salts in concrete, corrosion of embed-ded steel and physical erosion due to wave action and floating objects.

Direct chemical attack comes from the magnesium salts in seawater. The concentra-tions may be low but they are sufficient to produce calcium chloride, gypsum (both soluble) and ettringite.

In the following the action of magnesium and calcium salts are given.

Action of sulphate:

MgSO4 + Ca(OH)2 → CaSO4 + Mg(OH)2

Where calcium sulphate can be soluble or solid and may act in a secondary reaction and produce ettringite:

CaSO4 + C3A + 32H2O → C3A•3CaSO4•32H2O

Action of chloride, MgCl2:

MgCl2 + Ca(OH)2 → CaCl2 + Mg(OH)2

The action of calcium chloride can result in producing chloroaluminate, ettringite and thaumasite with large expansions as result:

Chloroaluminate:

CaCl2 + C3A + 10H2O → C3A•CaCl2•10H2O


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Ettringite:

+ SO3 → C3A•3CaSO4•32H2O

Thaumasite:

+ CO2 + SiO2 → CaCO3•CaSO4•CaSiO3•15H2O

Interestingly, in spite of the high sulphate content of seawater, and even with high C3A Portland cement and large amounts of ettringite from sulphate attack, little ex-pansion is usually present. The deterioration is usually characterised by erosion or loss of solid constituents form the mass. It has been suggested that the ettringite expansion is suppressed in environments where OH- ions have essential been re-placed by Cl- ions.

Concrete between the tide marks, subjected to alternating wetting and drying, is severely attacked, while permanently immersed concrete is attacked less. The actual progress of attack by seawater varies, and is slowed down by the blocking of the pores in the concrete through deposition of magnesium hydroxide. In warmer envi-ronments the attack is more rapid.

6.6 Erosion / Scour Scour is the erosive action of running water, excavating and carrying away material from the bed and banks of waterways.

Scour is one of the most frequent causes of bridge failures, mainly because it may develop to a very large extent within a short time.

If the level of the riverbed has changed significantly — in general or around piers/abutments — there is always reason to carry out closer investigations. Note that there may very well be problems, even if the erosion has not reached the level of the underside of the foundation. In many cases the load carrying capacity of a direct foundation is dependent on the pressure (the weight) from the surrounding soil. And particularly pile foundation depends on the surrounding soil.

Scour problems may be divided into three groups:

• Aggradation / degradation

• General scour

• Local scour

6.6.1 Aggradation / degradation Aggradation and degradation are long term changes in the level of the riverbed.

Aggradation is deposition of material, elevating the riverbed, while degradation is the lowering of the bed caused by erosion.


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Aggradation reduces the cross section of the waterway. This will cause the surface of the water to rise, and during a flood, the superstructure may be affected by stream-ing water and debris, causing unintended horizontal forces to the bridge.

Degradation may lead to undermining of the foundation, eventually leading to failure of piers / abutments and thus of the whole bridge.

6.6.2 General scour General scour is characterised by the removal (erosion) of material from the whole width of the waterway. Generally, it is caused by increased water speed. There is no strict distinction between general scour and degradation, but in general, degradation is a slow erosion of material over a long period of time (years), while general scour may take place over a shorter period.

General scour often occurs because of a contraction of the flow of water. It may be a result of the construction of the bridge, as piers, abutments and embankment slopes reduce the cross section of the waterway channel. However, it may also be caused by obstructions or other changes in the waterway, upstream or downstream.

Another possible cause of general scour is mining in the riverbed, i.e. excavation of sand and gravel.

6.6.3 Local scour Local scour is scour that only affects a minor part of the width of the waterway. Gen-erally, it occurs where obstructions (natural or artificial) change the flow of water, creating accelerations and vortex systems.

The occurrence of local scour very much depends on the design of the obstructions to the water flow (Piers, abutments).

If a scour protection only covers part of the riverbed, local scour may occur at the edges of the protection. Particularly if the protection is of a solid type like concrete or asphalt. Open, flexible types of protection (wire mattresses, riprap) are less vulner-able to local scour.

Scour is prevented by:

• Minimising the reduction of the waterway cross section caused by the bridge.

• Not making regulations of the stream (upstream or downstream) that cause in-creasing water speed at the bridge.

• Giving structures in the stream (piers and abutments, scour protection) designs that minimise the formation of vortexes.

• Protecting riverbed and slopes.


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• Preventing mining (excavation of sand and gravel) from the riverbed.

6.7 Corrosion of steel structures In this part the mechanical corrosion such as abrasion, cavitations, erosion, etc. and chemical corrosion such as gas corrosion is not described.

This part will describe the type of corrosion that takes place when moisture is a part of the corrosion process.

6.7.1 Electrochemical corrosion If a metal is exposed to water or a solution of water a certain part of the metal is dissolved by metal ions leaving the surface and making the solution positive while the electron stays in the metal and makes it negative:

Me → Me++ + 2e-

As the process increases, the metal negativity increase, and it is more and more difficult for the positive ions to leave the metal surface and dissolve. At the end the process stops and a equilibrium is achieved. In this condition of equilibrium ions are send from the metal into the solution and back again into the metal as:

Me++ + 2e- → Me

The potential difference between metal and solution when the same amount of metal atoms is dissolved and ions precipitated per time unit is called the equilibrium poten-tial. The metals mutual inclination to reaction is described in electrochemical series, see Table 6-2.

The metals ability to corrosion in a practical situation depends on the metals ability to create dense oxide layers and of the solution they are exposed against. To give an overview of the corrosion tendencies in a specific environment galvanic series is sat up. In Table 6-2 metal exposed to seawater are put in a galvanic series.

The potential difference between two metals in a series shows how dangerous it is to connect the metal to each other. The metal with the lowest potential will corrode.

The metal will stop dissolve when equilibrium is reached. However if the electron is removed from the metal gradually the dissolution of the metal will continue and the metal corrodes. The recipient of the electron is called an acceptor or calls it an oxida-tion or depolarisation.

The normal electron acceptor is oxygen that causes the formation of hydroxyl ions. In acid solutions is H+ acceptor. Seldom more cathodic metal acts as electron accep-tor.


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Oxygen acts as electron acceptor by creation of hydroxyl ions with the free electron and water. The formed OH- reacts with the free Me++ and will create rust products. For instance if we look at corrosion of iron in water:

The iron reacts with the following process:

Fe → Fe++ + 2e- (anode process)

From the atmosphere the water uptake oxygen and together with the free electron is created hydroxyl ions:

2H2O + O2 + 4e- → 4OH- (cathode process)

Fe++ from the anode process and OH- from the cathode process creates Fe(OH)2. This product is not stabile in oxidise atmosphere; it is oxidised further to the read-brown Fe(OH)3 which is able to uptake water and create rust Fe(OH)3nH2O. It is the last created product that result in the typical read-brown colour of the rust.

Iron is stable as long as the air relative humidity is below 65 %. Over 65 % will the water film, which is on the surface, be so thick that it is able to act as an electrolyte.


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Figure 6-57: As example on the rust creation we will look on a water drop on a steel

surface. The water drop will result in visible rust after a few hours. The rust will be

formed ring shaped around blank steel. This due to the following:

The water drop will dissolve oxygen for the atmosphere, however in the marginal zone where the water layer is thin the oxygen reach the steel very fast and act as an electron acceptor according to the cathode process. Inhomogeneities in the steel surface result in a certain area to be come anode and dissolve Fe++ while the re-maining e- runs though the steel and leaves it near the marginal zone where it is up taken by the acceptor. The circuit is working and the steel corrodes anodic in the centre and a rust ring of iron hydroxide is created around the anode area. The rust ring will grow to a rust hillock that will cover the blank anode. Be aware that oxygen is necessary for the process to continue but no corrosion takes place where the oxy-gen is supplied.

Water Drop

Steel


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Electrochemical series

Thermodynamic series

Galvanic series Metals in sea water

Metal/Cation Normal potential in volt at 25 °C against hydrogen electrode

Metal Potential in volt at 20-25 °C against hydrogen electrode

An

od

ic

Mg/Mg++

Al/Al++

Zn/Zn++

Cr/Cr++

Fe/Fe++

Cd/Cd++

Ni/Ni++

Sn/Sn++

Pb/Pb++

H/H+

-2,34

-1,67

-0,76

-0,74

-0,44

-0,40

-0,25

-0,14

-0,13

0,00

Magnesium

Zinc

Al alloys

Cadmium

Steel, cast iron

Stainless steel, active

Copper

Tin

Lead

H/H+

-1,4

-0,8

-0,8 ∼ -0,5

-0,5

-0,5 ∼ -0,4

-0,3 ∼ -0,1

-0,1

-0,1

0,0

0,00

Cath

od

ic

Cu/Cu++

Cu/Cu+

Ag/Ag+

Pt/Pt++

Au/Au+

+0,34

+0,52

+0,80

+1,12

+1,68

Ni-Al-Bronze

Stainless steel, passive

Silver

Platinum

Graphite

0,0

-0,1 ∼ +0,3

+0,1

+0,4

+0,4 ∼ +0,5

Table 6-2: Electrochemical and galvanic series.


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6.7.2 Chink Corrosion A normal situation for electrochemical corrosion is where oxygen concentration cells create cathode areas on free steel surfaces while the corresponding anode areas with the dangerous corrosion is hidden in pores, cracks and connections. The attack is often called chink corrosion or crack corrosion.

Figure 6-58: The principle in chink corrosion is as follows:

Two plates overlap each other, and there is water in the overlaps (in the chink). The

free water surface uptake oxygen and the metal surfaces in the marginal zones be-

come cathodes. In the chink the oxygen have difficulties to penetrate and the metal

surfaces become anodes and is corroded. This corrosion is not visible from the out-

side and dangerous.

6.7.3 Galvanic Corrosion If two different metals are in electrical contact in a moist environment galvanic cor-rosion is created. The more anodic metal releases ions and is corroded.

An important factor in evaluation of the danger of this type corrosion is the ratio be-tween anode and cathode area. If the anode area is small compared to the cathode area the current density over the anode will be large and the corrosion will be se-vere. The galvanic series is shown in Table 6-2.

6.7.4 Stress Corrosion Stress corrosion occurs in corrosive environment when the steel is exposed to ten-sion stresses. Often the attacks are not visible and thereby very dangerous.

The stresses may be introduced when the steel melt is solidified, or due to cold de-formation of the steel, or due to outer static forces.

The corrosion takes place where the stresses and thereby the energy level is highest.

6.7.5 Corrosion and Fatigue If the steel is exposed to alternating stresses there are a risk of fatigue fracture of the steel. This risk is increased when corrosion occurs together with the alternating stresses.

Water


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6.7.6 Atmospheric Corrosion Atmospheric corrosion is corrosion on un-protected steel surfaces exposed to the atmosphere. The air humidity is normally always higher than 65 % relative humidity that is the limit for adsorption of connected water film and thereby electrochemical corrosion.

The water films is only a few molecule layers thick, however when there are small amounts of different salts on the surface, the water film becomes a powerful electro-lyte. Some salts, for instance calcium hydroxide is very hygroscope and creates a water film already at 30 % relative humidity. Different factors influence the risk and velocity of the corrosion:

• Temperature The corrosion velocity is doubled for every 10 oC increase in temperature.

• Air pollution Both NaCl and other salts from the sea and the content of the SO2 play an im-portant role. The sulphur dioxide originates the volcanic activity and from burn-ing of fossils fuels like coal and oil. The sulphur dioxide creates H2SO3 which is oxidized to sulphur acid (H2SO4) which increases the corrosion velocity. In industrial environments the creation of soot is high. The soot contains sulphur and carbon and due to the hygroscopic properties the soot will be changed to sulphur acid. Between carbon and steel the sulphur acid is a strong electrolyte and a powerful corrosion cell is created. In marine environments the large amount of salts in the air may cause stronger corrosion than in the inner parts of the country. The salts originate from the sea where fog and moisture is airborn.


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Corro-sion class

The aggres-siveness of the environ-ment

In-door environment Out-door environment

0 None Rooms with relative humidity lower than 60 %, dehumidi-fied rooms

1 Insignificant Non heated well ventilated rooms without condense, steel buildings with natural ventilation

Heated rooms with relative humidity more than 60 % without condense

2 Medium Changing exposure of mois-ture with brief condense

Non polluted land at-mosphere and similar environment with low sulphate acid-base and chloride pollution

3 Large Alternating humidity, severe condense

Chemical exposure

Polluted atmosphere, sulphate and other pollu-tion from industry occurs

Not polluted marine at-mosphere

4 Very large Constant moist

Chemical exposure

Submerged

Polluted marine atmos-phere

Chemical exposure

In water and in earth

Table 6-3: Corrosion classes defined in Denmark.

6.8 Ageing of Steel Impact on steel at very low temperatures may result in fracture without any large deformations as seen at normal temperatures. The brittle fracture form may also be seen on very old steels at normal temperatures. This phenomenon is called brittle fracture due to ageing of the steel.

Steels ability to uptake dynamic loading is measured in Charpy’ s impact-notch sen-sitivity measuring test. The tendency to brittle fracture is evaluated by carrying out Charpy tests over a specified temperature interval. The ability is characterised by the transition temperature. It normally vary from –85 oC for the best steel types to +55 oC for the poorest steel types. However it should be mentioned that the transition


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temperature is not similar to the lowest operation temperature of a structure con-structed with the said steel. The operation temperature depends on the loading, thickness of the members and the notch sensitivity in the structural connections.

The steel may be aged and hereby the transition temperature is changed. This is due to dissolved nitrogen in the steel immediately after the rolling which is precipitated after some time as very small nitride crystals that make the steel brittle. The change in the transition temperature is approximately 15 oC for the best steels and 80 oC for the poorest steels. Steel with a low transition temperature have normally lesser ten-dency to ageing.

Figure 6-59: Example on evaluation of a steel’s ability to brittle fracture. At normal

temperatures the steel is ductile. Around 0 oC the impact ductility drops to very low

values. This property is described by the transition temperature which may be de-

fined as the temperature where the impact ductility is reduced by 27 Nm.

6.9 Erosion of masonry structures Erosion of masonry structures is mainly caused by particles in flowing water and wind, frost attacks salt crystallization and plant root action

Frost Frost is the principal eroding agent of masonry exposed to normal exterior condi-tions. Its effect is due to the stresses created by the expansion of water on freezing in the pore system of materials and thus only occurs in water-saturated or near-saturated conditions in porous materials. Typical effects are the spalling of small ar-eas of the mortar to form a layer of detrius at the foot of the wall or just general softening and erosion of the mortar indistinguishable from chemical erosion.

Brittle Transition Ductile

Ageing

Transition temperature

Impact ductility according to Charpy-V test


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Salt crystallization damage Salt crystallization damage is a analogous process to frost attack and is due to ex-pansive crystallization of hydrated salts in the pore structure. Salt crystallization damage often occurs in warm conditions where the rapid drying of water is causing the salts to crystallize out below the surface.

Abrasion Abrasion by particles in wind and water mostly acts together with other processes. The appearance will normally be of loss of surface and change of colour and texture.


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7. NDT-methods

7.1 General The philosophy of a special inspection is to combine a superior visual assessment of a structure with appropriate test methods to obtain sufficient information on the condition of the structure. Location and selection of representative samples are im-portant for giving accurate conclusions for the entire structural component consid-ered. The extent of tests must be sufficient for determining the right repair strategy, and for giving a good estimate of the total area requiring repair. The personnel performing the special bridge inspection and using NDT-methods should be experienced and competent in three ways. They should be knowledgeable on the subjects stated below:

• How to carry out the available methods of testing in practice, including how to

operate the equipment.

• How to select the right type of test method and the right test locations for differ-

ent types of damage.

• How to interpret the results of the measurements. The test methods can be divided into three categories:

• The non-destructive survey methods, which are suitable for mapping damage on

large areas of the structure.

• The detailed non-destructive and destructive sampling and measurements on

small areas.

• The laboratory analysis, which when applied on the samples, provides very de-

tailed and precise information about a specific location. Normally, the combination of all three categories leads to very reliable conclusions on mechanisms of deterioration (deterioration types), causes and the extent of dam-age. However in most cases, the two first categories of test are satisfactory to con-clude upon. Recording of registrations must refer to a unique numbering system for the struc-ture. The numbering system must be indicated on a sketch (normally a plan view). One way to select a numbering system is to use the compass directions, e.g. column N1, E1 etc. The road destinations in the two directions should appear from the plan.


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7.2 Visual inspection Visual inspection involves using an inspector's eyes to look for defects. The inspector may also use special tools such as magnifying glasses, mirrors, endoscopes, or borescopes to gain access and more closely inspect the subject area. Visual examiners follow procedures that range from simple to very complex. Accessories for visual inspection

Mirrors

Borescope Endoscope

Magnifying glasses


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7.3 Crack measuring gauge A crack measuring gauge is used to measure the crack width of a visible crack. This method can be used for both concrete, steel and masonry structures. The method is most commonly used on concrete and masonry structures. Crack measuring gauges comes in a variety of different types ranging from a sheet of transparent plastic with lines of different thickness to electronic callipers with an accuracy of 0.01 mm. When using the crack measuring gauge one must bear in mind that some cracks have broken edges that make the crack look wider than it is.

Some cracks are measured continuously at specified intervals in order to monitor a possible crack growth. When the crack width is measured using a calliper it is neces-sary to establish permanent measuring points on both sides of the crack. At each measurement the distance between these two points is measured. Naturally, the line connecting the two points should be perpendicular to the crack. By each measurement the crack width (the distance between the two points) is measured together with the air temperature in order to remove temperature de-pendencies from the measurements. Normally, a series of measurements spanning several years must be made in order to determine whether the crack is expanding. If a large number of measurements have been performed the temperature-induced changes of the crack width may be filtered out. The results are reported in terms of a graph showing the crack width as a function of time.

7.4 Crack detection microscope A crack detection microscope is a portable lightweight microscope. The magnification of the crack detection microscope may e.g. be 25 times. This method can be used for both concrete, steel and masonry structures.


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A crack detection microscope is used to locate cracks and to measure the width of cracks. Some crack detection microscopes may also be used to measure the depth of the crack. The crack detection microscope is placed over the crack line and the focus is ad-justed to get a sharp picture. Using the built-in scale the crack width is measured. Using a crack detection microscope the crack width may be determined with an accu-racy of 0,01 mm. Based on the findings using the crack detection microscope and a visual inspection a map of the detected cracks may be produced.

7.5 Boroscope This method can be used for both concrete, steel and masonry structures. The method is most commonly used on concrete and masonry structures. A boroscope is used to look inside inaccessible or small voids. For example, if cable ducts are not injected, it is possible to inspect the strands by means of an endoscope through a contact drilling (here a drilled hole from the surface to the cable duct). For steel structures the method is usually used for investigation of closed profiles to gain information regarding the condition of the interior surfaces of the closed pro-files. For masonry structures the boroscope can be used to gain information of the depth of the outer layer of bricks or natural stones and it can provide information of the filling material in between the arches. It may also be used to examine the mortar between bricks or natural stone. The boroscope equipment includes a lighting source and a fibre optic cable to trans-fer the light to the boroscope. A system of lenses enables the boroscope to be used as a monocular. A camera or video camera can also be mounted on the boroscope for photo documentation. Generally speaking, the method is appropriate and may also be used for inspections of structural components such as expansion joints, honeycombs and cracks/slots.

The many variations and features which can be obtained for borescopes make them an almost universal tool for internal inspections. These include a wide range of lengths and diameters, solid tubular or flexible bodies, lenses for forward, sideways or retro viewing, still and video camera attachments, and mains or battery power supplies.


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7.6 Half-cell potential This method can be used for concrete structures. The potential difference between a standard half-cell (normally a copper/copper sul-phate reference electrode) placed on the surface of the concrete and the reinforce-ment underneath may be measured using the principle shown in the figure below.

Voltmeter

Reinforcement Concrete

+-

Reference electrode

The potential difference is associated with the rate of corrosion of the reinforcement. The purpose of potential measurements is to map the electrochemical potentials in order to locate areas with risk of corrosion – see section 6.3.9.

In the field the following steps have to be followed:

1) Exposure of a rebar for the electrical connection.

Normally go for:

• Stirrups

• The most convenient areas: The cover often varies, find the areas with the smallest cover on bridge decks. With an asphaltic overlay, the connection can be made to rebars in the edge beams.

2) Check the circuit of the reinforcement:

• On columns, make a contact to the reinforcement of another column to

check the circuit by using the multimeter. The resistance must be zero to

have the circuit required. If that is the case, use the same connection

during the required series of measurements.


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If the resistance is not zero, first of all check the connection to the rebar.

If this connection is good, the internal connection of the reinforcement is

not sufficient and it is necessary to make contact to every column. Look

for joints in the bridge deck and check columns on both sides of the

joints.

• On decks, use the above described procedure.

3) Make a measuring grid (columns and rows) on each part to be measured, not-

ing the following:

When making survey measurements on large areas, a mesh size of 500 x 500

mm may be chosen.

Prior to making the grid, survey measurements at (more or less) random loca-

tions may help locating the areas to be mapped.

When making measurements in areas where corrosion is likely to occur (se-

lected as a result of survey measurements, experience or other test types),

the mesh size should be 250 x 250 mm or less.

The grid size, location and orientation must be marked on sketches of the

structure.

4) Check the stability of the potential measurements:

• wet a single measuring point

• place the electrode and note the potential and time

• wait until the potential is stable. NOTE the potential and time. This time

difference is the necessary time required between wetting and measuring.

In very dry concrete, it is normally necessary to wet continuously for a

longer period. This means that one person is constantly wetting the struc-

ture in front of the person doing the measuring.

5) Start the Measuring

It should be noted that the potential measurement itself does not lead to a fi-nal assessment of the condition. Supplementary testing has to be carried out. As a first guide to an evaluation of the reliability of the measured potential values, the measurements are normally divided into groups. Immediately after completing the measurements, the results are printed out and then evaluated


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according to a scale based on experience, e.g. (when using a copper/copper sulphate reference electrode): Group 1: potential > -200 mV: 90 % probability of no corrosion Group 2: -200 mV > potential > -350 mV: An increasing probability of corrosion

Group 3: -350 mV > potential 90 % probability of corrosion

The probabilities of corrosion given above are also given in ASTM C876. It is important to notice that the probability of corrosion also depends on many other factors such as:

− The oxygen concentration

− Moisture content

− Carbonation

− Chloride concentration

− Temperature

− Use of corrosion inhibitors

− Concrete resistance

− Coatings and sealers

− Cathodic protection systems All the above mentioned factors must be taken into account when assessing the probability of corrosion. Hence, the results of half-cell measurements must always be calibrated on the basis of break-ups. Make break-ups to confirm the first evaluation and to evaluate the reduction of cross-sections. Note that the potential measurement is meant only for the de-tection of areas with corrosion activity. The reduction in cross sections cannot be assessed by half-cell measurements. Break-ups must be carried out for each group. As a rule of thumb, the break-ups are placed in the most negative areas in each group. Breaks-up should therefore be performed in groups 1, 2 and 3. Start making a break-up in gro-up 1 and group 2. If the rebars are without corrosion in group 2 then no break-up is necessary in group 1. When the connection between the potential values and the actual corrosion condition has been established through break-ups, the half-cell measurements can be used to assess the size of the damaged areas as a basis for rehabilita-tion design.


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If the first evaluation does not confirm the results (if e.g. severe corrosion is found in group 2) limits must be changed for the three groups accordingly. These new limits must be confirmed by new break-ups.

1 2 3 4 5 6 7 8 9 10 11 12

4.0

3.0

2.0

1.0

0.3

0.0

Avstånd till fog [m]

Hö

jd [m

]

100-15050-1000-50-50-0-100--50-150--100-200--150-250--200-300--250-350--300-400--350-450--400-500--450-550--500-600--550-650--600-700--650

0,05 - 0,10 % CL- at reinforcemnt

< 0,05 % CL- at reinforcement

Break up, corrosion has been initiated

Break up, surface corrosion

Break up, no corrosion

Break up, severe corrosionCore, corrosion has been initiated

Core, surface corrosion

Core, no corrosion

Core, severe corrison

> 0,10 % CL- at reinforcement

Potential [mV]

The results of the half-cell measurements may be reported in terms of a sur-face graph as shown above. In the surface graph all relevant measurements of the chloride concentration, observations from break-ups and cores may also be given.

7.7 Corrosion rate meter This method can be used for concrete structures. The corrosion rate is often expressed in terms of the density of the corrosion current expressed in μA/cm2. On the basis of Faraday’s law the corrosion current may be converted to section loss. 1 μA/cm2 corresponds to 11,5 μm/year. Unfortunately, it is not possible to perform direct measurements of the corrosion rate. The corrosion rate may be determined on the basis of the polarisation resis-tance. The relation between the polarisation resistance and the corrosion current is usually assumed to be linear. The measurement of the polarisation resistance is performed using a commercially available test equipment.

Heig

ht [m

]

Distance to joint [m]


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The tests are conducted and reported in the same manner as the half-cell measure-ments. Unfortunately, the results of the corrosion rate measurements may vary with an order of magnitude depending on the instrument used for the measurements. Furthermore, the corrosion rate exhibits a much larger variation with factors such as humidity and temperature than the potential. Hence, the results of the corrosion rate measurements should be used with caution and should always be compared with half-cell measurements and calibrated on the basis of break-ups.

7.8 Cover meter Measurements This method can be used for concrete structures. The cover meter is used to locate the reinforcement in the concrete and to measure the depth of the concrete cover. The cover meter is often used to locate the rebars before starting other investigations such as HCP-measurements, core drilling, Capo-tests, inspection of cables etc. The cover meter measurement is based on changes in the magnetic field lines/eddy current. The presence of nearby magnetic rebars will cause changes, which can be measured by passing the measuring head over the surface above the rebars. The measuring head is an encapsulated unit containing the search coil. As the coil windings are directional, the head should always be used with its longitudinal axis parallel to the expected line of the reinforcing bars. A lead from the head is plugged into the battery-operated cover meter. The measurements are performed by performing a vertical and horizontal sweep of the considered area, see figure below.

The method is generally suitable. Tests have shown that the inaccuracy increases from 5-10% at approx. 35 mm depth to approx. 15-25% at 60-70 mm depth. The findings may be reported in terms of the maximum and minimum values as well as the mean and standard deviation of the cover. The findings may also be pre-sented as a surface graph.


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7.9 Spraying indicators (pH)

This method can be used for concrete structures. This test is performed by applying an indicator solution to concrete surfaces just fractured. The colour of the solution will change with corresponding changes in pH of the concrete. The carbonation depth is then measured by means of a scale.

The indicator normally used is 'phenolphthalein' (1% solution in alcohol) - After ap-plication, the colour of the alkaline concrete surfaces will immediately turn red-violet indicating a pH > 9.5, and the carbonated surfaces will remain colourless.

7.10 Impact-Echo equipment This method can be used for concrete structures. Impact-echo equipment introduces a short-duration stress pulse into the considered member by a mechanical impact.

The impact introduces three types of waves:

− P-waves (compressional wave)

− S-wave (shear wave)

− R-wave (surface wave) The P-wave will be reflected when it reaches a surface or a material with another acoustic impedance. The successive arrivals of the P-waves to the surface are regis-tered by a displacement transducer. On the basis of a spectral analysis of the reflected P-waves, the frequency of the reflected wave is determined. The thickness of the material or the depth to the de-fect, d, may be determined by:


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fvd

2=

where v denotes the wave speed and f the frequency of the reflected wave. The principle behind the Impact-Echo method is shown in the figure below. It is seen that the frequency of the measured response is higher when a void is present than when no void is present. This is due to the fact that the wave reflected from the void reaches the transducer faster than the wave reflected from the bottom of the test specimen.

FrequencyFrequency

Amplitude Amplitude

Impact ImpactTransducer Transducer

Concrete slap

Void

The wave speed may be determined by testing a specimen with known thickness containing no defects. Alternatively the wave speed may be measured on the surface using two transducers. The equipment may also be used to measure the depth of a crack. Using the setup indicated below, the crack depth may be determined on the basis of a measurement of the time it takes the P-wave to reach the two transducers.

ImpactTransducer

Concrete slap

Transducer

Crack

In general the equipment may be used to determine:

− Thickness of members

− Presence and depth of cracks, voids and honeycombing

− Missing bond between concrete and asphalt overlay/waterproofing

− Injection quality of cable ducts


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The measurements are performed by producing an impact on the concrete surface e.g. with steel balls (“impactors”) with diameters ranging between 2 to 15 mm. The size of the steel ball should be selected on the basis of the thickness of the consid-ered test specimen. For each measurement the spectrum of the measured response is shown on a laptop PC connected to the transducer. On the basis of the response spectrum the operator estimates whether defects are present. The results obtainable from using the Im-pact-Echo equipment to a great extent depend on the experience of the operator. The method is fast and an experienced operator is able to test relatively large areas (e.g. a bridge deck with the dimensions 12 x 50 m) for defects within a working day. The measurements should always be calibrated on the basis of independent tests. Usually the inspection of concrete structures is calibrated on the basis of cores, break-ups or a visual inspection using a boroscope. The results of the measurements are used to report the general condition of the con-sidered component as well as a detailed mapping of the detected defects.

7.11 Impulse Response equipment This method can be used for concrete structures. Impulse response equipment is used to produce a stress wave in the considered component. The stress wave may e.g. be produced by an impact with an instru-mented rubber tipped hammer. The impact causes the component to act in bending mode. A velocity transducer placed adjacent to the impact point measures the re-sponse of the component. In contrast to the Impact-Echo method the impulse response equipment does not measure the reflection of the impact. Furthermore, the impact used to produce the response is considerably larger than the impulse used for the Impact-Echo method. The hammer used to produce the impact and the transducer used to measure the response of the component are both connected to a laptop PC. The laptop performs a spectral analysis of the impact as well as the response. Dividing the resultant veloc-ity spectrum by the force spectrum then derives the “mobility”. An example of a mo-bility graph is shown below.


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TransducerImpact

Hammer

Frequency

Mobility

For each measurement the resulting mobility graph is shown. On the basis of the mobility graph the following parameters are determined:

−Average mobility: The average mobility is shown as the green line in the fig-

ure above. The average mobility depends on the thickness of the material. If

the thickness is reduced the average mobility is increased. This implies that

laminated concrete has a higher average mobility than non-laminated con-

crete.

−Stiffness: The stiffness is determined as the inverse of the inclination of the

part of the mobility graph below 80 Mz, the red line in the figure above.

The stiffness depends on the stiffness of the material, the thickness of the ma-

terial and it depends on how the component is supported. Based on a com-

parison of the stiffness at a number of different locations potential “weak” ar-

eas may be located.

−Mobility slope: The presence of honeycombs in the concrete will reduce the

damping of the signal. This implies that the mobility graph will be increasing

within the considered frequency range, see figure below.

−Voids index: The voids index is defined as the ratio between the initial maxi-

mum of the mobility and the average mobility.

If the component is laminated the initial maximum of the mobility will be con-


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siderably higher than the average mobility. If the voids index is higher than 2

– 4 it indicates a potentially “weak” area, see figure below.

The impulse response method is a fast method which may be used to screen a rela-tively large area within a short period of time. The equipment delivers surface graphs of the measured parameters. In the figure below a surface graph of the average mo-bility of a bridge deck is shown.

The results of the impulse response testing shall always be calibrated on the basis of e.g. cores, break-ups or a visual inspection using a boroscope. The locations of these tests are selected on the basis of the surface graphs of the measured parameters.

7.12 Capo-Test (concrete strength) This method can be used for concrete structures. The theoretical background of the Capo-test is that the compression strength is cor-related to the force necessary to pull out a bolt from the concrete, if the fracture shape is a cone with a specific angle, see the figure below. This correlation is inde-pendent of aggregate type and strength, as long as the correct fracture shape is achieved.


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To ensure the correct geometry of the fracture, a special bolt and a circular counter-pressure device is used. The bolt is extended in a recess milled at the bottom of a 25 mm hole in the concrete and pulled out by means of a small hydraulic jack. Reading the hydraulic pressure gives the pullout force, from which the compression strength of the concrete can be found by means of a calibration chart. Special attention must be paid to:

− Selection of representative areas of the damage in question.

− Milling of the recess with sharp edges (by keeping the miller at a right angle

to the surface all the time).

− Greasing the insert before inserting.

− Assembling and tightening the various parts of the expansion bolt and jack in

the right sequence.

− Fastening the insert without rotating it. If the fracture surface is not conical, the measurement is not valid and a new test must be made. The test measures the strength in a very small area. The presence of coarse aggre-gates or minor deficiencies in the concrete at the test location may affect the meas-ured strength. To compensate for this, at least three tests are made. It is relatively time-consuming to perform CAPO-tests. It is necessary to repair the concrete surface after the tests have been made. However, it is less expensive to perform CAPO-tests than cutting cores and deter-mine the strength in the laboratory.


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If the purpose of the tests is to determine the characteristic value of the compressive strength of the concrete at least five tests must be made. However, the rules for determining the characteristic values from CAPO-test is determined my national codes and guidelines. Alternatively, some prior information or some information from other test methods must be used in order to determine the characteristic value of the compressive strength of the concrete.

7.13 Pull-off/Bond-Test This method can be used for concrete structures. Normally, the method is primarily used for testing the bond strength of a newly cast concrete layer to the existing concrete. Further it can be used for testing the tensile strength of the existing concrete surface when certain strength is required before the new concrete layer and/or waterproofing can be placed. Finally, it can be used for testing the bond of a membrane to a concrete surface. In the bond–test a disc is glued on a prepared surface. The disc is pulled off after a partial core has been cut around the disc. On the basis of the pull-off force the ten-sile strength of the material may be determined, see the figure below.

When testing the strength of a newly cast concrete layer to the existing concrete one of the following failure types may be observed:

− Failure in the substrate

− Failure in the adhesion layer

− Failure in the overlay Failure in the substrate is the preferred, as it proves the adhesion strength of the overlay to be higher than the tensile strength of the substrate. The results are usually reported in terms of acceptance or non-acceptance of the strength of the considered material. The decision is made on the basis of a decision


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rule formulated prior to the tests. If a sufficient number of tests are performed the results may be used to estimate the characteristic value of the tensile strength of the concrete.

7.14 Schmidt hammer This method can be used for concrete and masonry structures. It is most commonly used for concrete structures. The Schmidt hammer is used for testing the strength of hardened concrete. The device consists of a spring loaded steel mass that is automatically released against a plunger when the hammer is pressed against a concrete surface. Part of the energy is absorbed by the concrete through plastic deformation and part of the energy causes a rebound of the hammer. The rebound of the hammer depends on the hardness and thereby the strength of the concrete. In order to estimate the strength of concrete at least 20 measurements should be made. The measurements shall be performed at locations where the concrete surface is smooth. The distance between the individual measurements should be at least 0,5 – 1,0 m. All measurement shall naturally be performed within a homogenous area. The actual measurements are made by pressing the Schmidt hammer with the plunger extended slowly against the concrete surface until the hammer is released. At the moment of impact the hammer must be held perpendicular to the surface. The Schmidt hammer should not be used to measure the strength of weak concrete, fractured concrete and concrete with an uneven surface. The results obtained using a Schmidt hammer are not as accurate as CAPO-test or strength testing of concrete cores drilled from the structure. The method is best suited for scanning large areas in order to divide the structure into homogenous ar-eas, i.e. areas with different values (levels) of the concrete strength. The compres-sive strength of the concrete in the poorest areas may then be estimated on the ba-sis of a more accurate method such as e.g. CAPO-tests.

7.15 Ground penetration radar This method can be used for concrete and masonry structures. A ground penetration radar makes use of high frequency electromagnetic pulses which are directed by a transceiver towards the surface. Waves are reflected back to a receiver. The waves received indicate the composition of the considered compo-nent. As the wave propagates through an component and encounters an interface between two materials with different dielectric constants, a portion of the energy is reflected back. The remaining energy continues through the component.


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The investigation depth depends on the selected frequency of the ground penetration radar. For investigations of bridge decks a frequency of 1,5 MHz may be selected. The investigation depth is then 500 mm. Ground penetration radar may be used to locate reinforcement in concrete struc-tures. In particular, the ground penetration radar may be used to locate pre-stressed reinforcement, which due to the large cover cannot be located using an ordinary cover meter. The ground penetration radar may also be used to identify areas with high humidity as well as voids. The distance from the surface to the given defect may be measured with an accuracy of ± 10 – 15 %. Investigations of bridge decks may be performed using a truck-mounted ground penetration radar. The investigation of a bridge deck may be conducted at a speed of 5 km/h. Hence, the method is highly efficient. The results of the test must be interpreted by a trained professional. Further, the results must be calibrated on the basis of independent tests such as cores, break-ups and a visual inspection.

The results of the inspection may e.g. be reported in terms of 3D-graphs showing the location of areas containing a given defect (e.g. high humidity) or the location of e.g. pre-stresed reinforcement as shown above.


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7.16 Chloride content This method can be used for concrete structures. The chloride content in concrete may be determined on the basis of:

− Cores

− Dust samples Cores are obtained as described in the section “Coring equipment”. The diameter of the cores should be at least 75 mm. Dust samples are obtained using a power drill. The power drill should preferably be mounted with a unit for automatic collection of the concrete dust. The dust sample should weigh at least 15 g. The dust samples are usually obtained at different depths at the same location – usually in steps varying between 10 – 20 mm (in depth). The number of holes necessary to obtain 15 g dust is shown below as a function of the diameter or the drill. The holes should be located within a circle with a diameter of 75 mm.

It is recommended to measure the chloride content at the following depths from the surface:

− 0 – 10 mm

− 10 – 20 mm

− 20 – 30 mm

− 30 – 50 mm

− 50 – 70 mm The chloride concentration may be determined by the Rapid Chloride Test (RCT) or by Volhard titration. The Rapid Chloride Test, RCT, is a fast method of determining the acid soluble amount of chlorides of concrete in-situ. Pulverised concrete obtained by hammer drilling of hardened concrete or from a con-crete core is mixed with a chloride extraction liquid and shaken for 5 minutes. The amount of acid soluble chlorides - expressed as weight percent of concrete weight -


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is determined directly by means of a calibrated chloride sensitive electrode con-nected to the RCT-electrometer. Volhard titration must be conducted in accordance with a given code. Both methods measures bonded as well as free chlorides. Using the dust samples from different depths - the chloride profile is determined by testing each depth interval. Examining the profile, the probable source of the chlorides and mechanism of pene-tration can be detected (curing water, saline soil, seawater, freed chlorides from ag-gregates, air-borne chlorides etc.). On the basis of the chloride profile and a mathematical model of chloride ingress (e.g. Fick’s second law – see section 6.3.7) the time to initiation of corrosion may be

determined. The critical chloride concentration for initiation of corrosion must be known. This value may e.g. be estimated on the basis of chloride measurements performed at break-ups where corrosion has been initiated. The results may e.g. be reported in terms of charts showing the chloride concentra-tion as a function of the distance from the surface. The results may also be shown on the surface graphs of the registered half-cell potentials.

7.17 Coring equipment This method can be used for concrete and masonry structures. A qualitative assessment of the concrete quality may be obtained by scrutiny of drilled-out cores. The right place to take the cores depends on the structure geometry, the condition of the concrete or masonry, and what information is required in order to determine the type and extent of damage. Prior to the drilling out of cores the condition of the concrete or masonry has usually been investigated on the basis of a visual inspection or some NDT-measurement such as e.g. half-cell potential, Impact-Echo or impulse response (for concrete struc-tures). In areas where the previous investigations with a high degree of accuracy have shown that the structure is either damaged or undamaged only few cores should be drilled out. The majority of the cores should be drilled out at locations where the results of the previous measurements are inconclusive. The cores will then provide a basis for an interpretation of the results of the NDT-measurements in these areas, thereby assuring that the degree of deterioration of the structure is es-timated with the highest possible degree of accuracy.


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The number of cores depends on the size of the considered area. Normally, about 2 -4 concrete cores are drilled out within each area investigated by a given NDT-method. Avoid cutting reinforcement bars. To ensure this, locate the reinforcement by means of the cover meter before drilling. Cores are usually drilled out using a portable electric concrete core drill as shown below.

Usually, the diameter of the concrete cores is 100 mm. Once the core has been drilled out a photograph of the cores is taken and the loca-tion where the core was taken is registered. The location should be registered in terms of the grid used for the NDT-measurements (HCP-measurements, Impact-Echo or Impulse Response), see the figure below. Also registration of the hole left in the structure (where the core has been taken out) is to be made. Especially signs of cracking are to be registered.


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Once the surface of the core has dried out the core is wrapped in saran wrap and put into an air-tight plastic bag. The concrete cores provide very accurate information about the quality of the struc-ture from which the cores were taken. However, it is time-consuming to drill out cores. Furthermore, a core leaves a defect in the structure from which it was taken.

7.18 Evaluation of concrete cores This method is naturally used for concrete structures. However it has to be noted that cores of other materials can also be examined in the laboratory both as macro-scopic and microscopic evaluations. Besides serving as calibration for specific NDT-methods (e.g. Impact-Echo and im-pulse response measurements) the evaluation of concrete cores can be used to de-termine the concrete quality and composition and to evaluate the cause of damage. By laboratory investigations of concrete cores the information of the composition, condition and damage cause can be utilised to estimate the future development of damage. This information can be used to define the optimum time of repair. Some of the results from evaluation of concrete cores are: • Macro analysis on cores and plane sections.

• Carbonation depth measurements.

• Crack detection on impregnated plane sections.

• Micro analysis on thin sections.

• Air void analysis on plane sections.

• Moisture analysis.

• Residual reactivity (AAR/ASR – Alkali-Aggregate / Alkali-Silica Reactivity).

• SEM-analysis (SEM – Scanning Electron Microscopy).


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7.18.1 Macro analysis on cores and plane sections Close macroscopic inspection of concrete cores (possibly using a magnifying glass or a stereo microscope) can give information about the concrete mix (the aggregate type, aggregate content, encapsulated air voids), and it may uncover internal defi-ciencies such as cracks and inhomogeneities. Casting defects and the condition of joints can be determined by the macroscopic evaluation as well. In particular the core can tell how deep cracks reach into the concrete, thus giving an indication of the cause of the cracks. If reinforcement is included in the cores the condition of the reinforcement can also be evaluated. An example of a concrete core is shown in Figure 7-1.

Figure 7-1: Concrete core for macroscopic evaluation.

The results from the macroscopic evaluations are usually registered by filling in a standard form. The registrations are always supplemented by one or more photos of the core.

By making a fresh cut in the concrete core the carbonation depth can be determined by using phenolphthalein – see also section 7.9.


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Figure 7-2: Measurement of carbonation depth by use of phenolphthalein. The red

part of the core is not carbonated.

7.18.2 Crack detection on impregnated plane sections The crack pattern can give valuable information regarding the cause of damage. To evaluate the crack pattern impregnation of a plane section of the concrete core is a great tool. The impregnation of the plane section is performed in two steps as illus-trated in Figure 7-3.

a)

b)

Figure 7-3: Illustration of the two steps in impregnation of a plane section for de-tection of cracks. a) Vacuum-impregnation of full core with fluorescent epoxy resin. Cracks, voids and porous paste connected to the core surface will be filled with fluo-rescent epoxy resin. b) Impregnation of plane section with fluorescent epoxy resin. Cracks, voids and porous paste near the cut surface will be filled with fluorescent epoxy resin.

By use of ultra-violet light on the impregnated plane section all cracks, voids and porous paste near the cut surface will be shown clearly. An example of a fluorescent impregnated plane section is shown in Figure 7-4. The crack pattern is very clear and

the extent and distribution of cracks can be determined.


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Figure 7-4: Fluorescent impregnated plane section from a bridge deck under ultra

violet light. The cause of damage could be AAR or freeze-thaw.

7.18.3 Micro analysis on thin sections More information may be obtained from the cores by performing 'thin section petro-graphy' which is a technique using a microscope in combination with various optical filters and epoxy resin impregnation to investigate very thin slices of the concrete. This technique requires sophisticated laboratory equipment and extensive experi-ence. A thin section is a 20-micron thick slice of concrete, which has been impregnated with a fluorescent epoxy resin. The thin section is typically 35 mm x 45 mm in size. The semi-transparency of the concrete slice allows the examination of the concrete by transmitted light microscopy. The impregnation with the fluorescent epoxy resin makes it possible to determine the water-cement ratio and the homogeneity of the cement paste. Further more the air voids, cracks (including micro cracks) and porous materials are clearly shown in the thin section.


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Figure 7-5: To the left a thin section is shown. To the right a thin section is exam-

ined in a microscope.

When performing a thin section analyse it is possible to determine the following pa-rameters: • concrete composition • cement type and content • aggregate type and mineralogy • w/c-ratio • air void content and void structure • defects (cracks and inhomogeneities) • aggressive environment (e.g. acid) • moisture conditions and effects • signs of deterioration (e.g. AAR) • strength level • initial defects (casting, curing etc.)


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Figure 7-6: Part of thin section. To the left the thin section is shown in ordinary

light with parallel polarizers and to the right the thin section is shown in fluorescent

light. To the right the homogeneity of the cement paste is shown by the colour in-

tensity – the darker colour the more dense cement paste (low w/c).

Signs of deterioration can also be identified in a thin section. In Figure 7-7 an illus-

tration of alkali silica reaction is shown.

Figure 7-7: Sand aggregate of reactive porous flint with interior and exterior crack-

ing.

The results of the thin section analyse is very precise when performed by an experi-enced engineer or geologist. It is however very important to keep in mind that the results from the microscopic analyse is only valid for the part of the structure repre-sented by the thin section. Thus, selection of the position for the thin section is very important – remembering that the thin section is only 35 mm x 45 mm large. Typi-cally the thin section is placed so it includes one or more cracks if any. Also intact areas of the concrete should be included in the thin section. It might be necessary to make two thin sections of one core to represent the concrete of the entire core.

Air void

Sand

Ettringite in air void


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7.18.4 Air void analyse on plane section By preparing a plane section of the concrete core, the air content and air void distri-bution can be determined.

Figure 7-8: Plane section prepared for determination of the air content and distri-

bution. All air voids are white and all paste and aggregate are grey or black.

7.18.5 Moisture analysis By slicing the concrete core into several slices a moisture profile through the core can be determined.

If the moisture profile is to be determined it is very important that the core is sealed in an air tight bag right after drilling out the core. The core must then be stored cold (e.g. in a refrigerator) untill the measurements starts.

Figure 7-9: Slicing the concrete core makes it possible to determine the moisture

profile.

The mass of each of the concrete slices is measured (m0) and the concrete slices are then stored in water. The initial mass is used to determine the actual water content of the concrete. The mass is measured regularly and the measurements continue till the mass is constant (mcap) – this constant mass is used to determine the degree of capillary saturation. By placing the concrete slices in a pressure camber more water


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can be pressed in to the concrete and by measuring the mass again (mpressure), the degree of pressure saturation can be determined. Finally the concrete slices are stored at 105 oC until the mass is constant (mdry) – this provides the dry mass of the concrete. The formulas for determining the actual water content (U), the degree of capillary saturation (Scap) and the degree of pressure saturation (Spresure) are given by:

%1000 ⋅−

=dry

dry

mmm

U

%1000 ⋅−

−=

drycap

drycap mm

mmS

%1000 ⋅−

−=

drypressure

drypressure mm

mmS

Figure 7-10: Moisture profile trough concrete core.

7.18.6 Residual Reactivity Test If the registrations from the macroscopic and microscopic evaluations indicate alkali-aggregate reactions (AAR) as the cause of damage a residual reactivity test can be preformed on one or more concrete cores. The purpose of the test is to evaluated the potential risk of development of AAR damage and to estimate the residual poten-tial for further reactions under the following conditions: - unlimited access for moisture (the test specimen is wrapped in a wet towel and kept in a plastic container by the humidity of 100%)

- unlimited access for moisture and sodium chloride (the test specimen is kept in a container filled with concentrated NaCl-solvent)

0.0 2.0 4.0 6.0 8.0

0-20

20-40

40-60

60-80

80-100

100-120

120-140

140-160

160-180

180-200

200-220

220-230

Kerne 2

Moisture content [U%]

Dep

th fro

m s

urf

ace

[mm

]


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The test is performed by storing two specimens cut out of the concrete core in the conditions mentioned above. The size of the test specimens could be app. 4x4x15 cm. To accelerate the chemical reactions the specimens are stored at 50 oC. By regu-lar measurements of the expansion of the test specimens the development of AAR can be evaluated in the case of unlimited access for moisture respectively unlimited access for moisture and sodium chloride (alkalis). The time of storing and there by the time of performing the test depends on the type of reactive aggregate. If the expansion exceed 1 0/00 harmful cracking of the structure could occur in the future. In Figure 7-11 an example of test results from residual reactivity tests is shown.

Figure 7-11: Example of test results for “residual reactivity tests”.

In the example shown in Figure 7-11 there is a small risk of future harmful cracking

due to AAR if alkalis are provided from the surroundings.

Storing time in weeks at 50oC in sodium chloride solution and at 100% relative humidity

Exp

an

sio

n 0

/0

0

Specimens stored in sodium chloride

Specimens stored at 100% RH


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7.19 Acoustic emission monitoring This method can be used for steel, masonry and concrete structures. Acoustic emission monitoring is based on measuring acoustic waves that are emitted during the growth of microscopic defects such as fatigue cracks and stress corrosion cracks. When a solid material is stressed, imperfections within the material emit short bursts of acoustic energy called "emissions." As in ultrasonic testing, acoustic emissions can be detected by special receivers. Emission sources can be evaluated through the study of their intensity, rate, and location. The acoustic emission is an elastic wave generated by the rapid release of energy accumulated in stressed materials. Sources of acoustic emission include fracture of materials, material corrosion, surface rubbing and micro earthquakes. Within the field of bridge inspection, the acoustic emission monitoring has been used to:

− detect fracture of pre-stressed reinforcement in concrete structures

− detect reinforcement corrosion in concrete structures

− detect fatigue cracks in steel structures The acoustic emission monitoring system consists of a number of sensors coupled to the material surface, a data acquisition system and a pattern recognition system which is able to distinguish between acoustic emissions from different sources and noise.

Portable Acoustic Emission test system


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The system provides a long-term continuous monitoring of flaws and early warning of e.g. crack growth. Further, the method may be used to locate flaws at locations where it is difficult or impossible to conduct a measurement. Acoustic emission monitoring is costly. The method is usually only used if prior in-spections and analyses have shown that there is a high risk of severe deterioration of the bridge. In such cases the acoustic emission monitoring may be applied in or-der to monitor the progress of the deterioration. This provides a basis for decision-making concerning the repair of the structure.


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7.20 Structural testing system This method can be used for concrete, masonry and steel structures. Inspections of a bridge may reveal one or more problems which must be monitored continuously. Instead of performing a large number of inspections with a small time-interval it is often more practical to install a monitoring system. The system may be on-line, i.e. the measurements are registered and sent to the relevant persons automatically. Alternatively, the actual measurements are con-ducted with a fixed time-interval using the sensors which have been installed in the bridge. The design of a monitoring system follows four steps:

1. Identification of needs and problems

For existing structures the needs and problems are usually identified on

the basis of a detailed inspection of the structure using NDT-methods.

2. Clarify objective and outline layout

Firstly, it is necessary to clarify how monitoring will assist in handling

the needs and problems which have been identified. Once this has been

done the designer of the system must choose what to measure, where

the measurements should be performed and what kind of instrumenta-

tion should be used. In most cases one or more of the following types of

instruments will suffice:

− Accelometres

− Strain gauges

− Wind speed and direction

− Vehicle control sensors

− Temperature transducer

− Displacement transducer

− Deflection/tiltmeter

− High precision differential GPS

− Moisture probes

− Corrosion probes

− Audio and video

3. Design of system

The design of the system consists of selecting the proper sensors de-

pending on the required accuracy, sampling frequency etc. Further, the

designer must select the system for data acquisition, cabling, communi-

cation, user interface and operation.


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4. Installation, commissioning and hand-over

Installation of the system should be performed by a hired professional

with experience in the installation of structural monitoring systems. The monitoring system provides a tool for gathering continuous information on criti-cal items. This information may e.g. be used in order to determine the optimal time for initiation of a given repair strategy. The system may be designed such that an alarm is triggered if a given event occurs. The alarm may e.g. be triggered by a rise in humidity or potential. The major advantage of a monitoring system is that it may provide a large quantity of information and thereby allows for a very accurate assessment of the condition of the structure and a very accurate planning of repair and maintenance of the consid-ered structure. Unfortunately, the use of structural monitoring is often expensive.

7.21 Structural scan equipment This method can be used for concrete, masonry and steel structures. A structural scan equipment uses x-rays to penetrate thick concrete and steel com-ponents, and may reveal flaws inside the concrete or steel structure.

The system may be used for:

− Mapping of reinforcement

− Studying inhomogeneities in concrete

− Inspection of pre-stressed cables and cable ducts In order to use the method both sides of the considered component must be accessi-ble. Further, the method is slow. The major advantage of the method is the high degree of accuracy. It is e.g. possible to detect a 20 mm porosity in 1000 mm thick concrete.


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The method provides a direct and precise illustration of the inner structure of the considered component. Further, the method is one of the best NDT-methods with respect to the amount of information, quality and reliability. On the other hand, the equipment is very heavy, it is very expensive and the use of the equipment requires special safety arrangements due to the radiation dangers.


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7.22 Ultrasonic testing This method can be used for steel structures.

7.22.1 Definition of ultrasound Mechanical vibrations of different kinds can travel through solids due to their elastic properties. A good example is a spring, which is tightened at one end. The other end is able to expand up and down. If it makes enough oscillations per second, you will be able to hear a sound. This is due to the fact that the air also starts vibrating as compression waves. The human ear can hear these compression waves, if the fre-quency is higher than the lowest audible range, which is about 12 oscillations/sec.

The faster the spring oscillates the higher the sound. Over a certain number of oscil-lations, we are not able to hear anything. We have then reached the upper audible level, which is about 20.000 oscillations per second.

Sound waves with a higher frequency are called ultrasound waves.

After changing to the use of ultrasound the method became useful in a greater scale. Ultrasonic waves gives due to their higher frequency and smaller wave length a much better possibility of finding defects and determine their size and their position. The vibrations are normally generated by the use of a piezoelectric crystal, which can be excited by an electrical pulse. See section 7.22.6 for more information about

transducers.

We are going through the two most common test methods, the through transmission technique and the pulse echo technique.

7.22.2 Through transmission technique When using this technique you have a transmitter on one side and a receiver on the other side of the object to be tested. The transmitter sends out ultrasonic waves either as continuous oscillations or as short pulses, each consisting of a few oscilla-tions. In the last case the pulses are send out with an interval, which is long com-pared to the duration of the pulse itself. The wave travels through the object and is then received by the receiver. The signal from the receiver shows the sound energy, which has travelled from the transmitter to the receiver. If the sound beam hits a discontinuity in the object, the received sound energy will be less. The signal from the receiver will then be smaller. This signal can be registered and used in different ways. For example the signal can automatically activate an alarm, if the sound beam hits a defect over a certain size, by which the received sound energy goes down un-der an equivalent fixed level.

The signal can also be registered with ordinary ultrasonic equipment, which contains an oscilloscope. It is seen as vertical reflections of the signal on the screen at a dis-tance to the right of the deflection on the left side of the screen. This reflection to the left is called the initial pulse.


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The ultrasonic equipment's way of working will be discussed later. If there is a defect between the transmitter and the receiver, it will prevent a larger or smaller part of the sound beam from reaching the receiver, which will weaken the signal. This is seen on the screen as a smaller deflection as seen on Figure 7-12.

A ABB

Figure 7-12: Principles of through transmission technique.

7.22.3 The pulse echo technique This is the most common used technique. The principle in this technique is almost the same as used in an echo sounder. A transmitter sends out a short pulse consisting of a few oscillations into the object to be tested. The sound wave travels through the object with a constant speed, the sound veloc-ity, which is always the same in the same material, regardless of the frequency. If the object is without defects, the pulse continues until it hits the back wall of the object, from where it is reflected like light beam from a mirror. The pulse then trav-els back through the object - still with the same velocity - and is received by a re-


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ceiver. As the pulse travels with a constant speed, the time the sound pulse has travelled from the transmitter till it returns is equivalent to twice the thickness of the object. After a while a new pulse is send out, which travels exactly like the first one. In order to measure the very short time from sending out one pulse till it is received again, the ultrasonic equipment is provided with an oscilloscope or a digital display. An electron beam makes a bright spot to travel horizontally across the screen with a constant velocity from left to right. The movement begins at the same time as the pulse is send out from the crystal. The initial pulse gives a vertical deflection on the left side of the screen. After that the bright spot continues to the right with a speed that can vary from

about 2001 to 5 times the velocity of sound in steel. See Figure 7-13.

A A A

F

F

FB

B

Figure 7-13: Principles of pulse echo technique.

If you adjust the velocity of the bright spot on the screen, to be the same as the sound velocity in the test piece, it will travel to the right on the screen with the same speed as the pulse travels inside the object. When it returns to the receiver, the bright spot has travelled a distance, which is twice the thickness of the test piece. The moment the pulse hits the receiver, it sends an electrical signal to the ultrasonic equipment. On the screen it is seen as a brief, vertical reflection of the bright spot. This is called a bottom echo. The distance on the screen between the initial pulse and the bottom echo is in this case twice the thickness of object.


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Changing the speed of the bright spot on the screen, the distance between the initial pulse and the bottom echo can be adjusted. You can change it in such a way that the thickness of the steel object between approximately 2 mm and 10 m can be read off on the screen. If the sound wave hits a reflecting surface during its way through the object for ex-ample a crack, a part of the sound will reflect back and will be seen as a vertical re-flection before the bottom echo. This deflection is called the defect echo. By its posi-tion on the screen you can determine the distance from the surface of object quite accurate, see Figure 7-13. The height and shape of the flaw echoes might give you

some information about the size and type of the defect. The sending out of a pulse and the movement on the screen is repeated many times a second. The single instant pictures appear on the screen as constantly shining lines, which only moves when the probe is moved across the surface of the object. However, there is a distance between the pulses, which allows the first to die out before a new pulse is send out. In most ultrasonic equipment it means that a pulse can move backwards and forwards in a 10 m long steel bar, before a new pulse is send out. Figure 7-14 and Figure 7-15 show some common ultrasonic equipment.

Figure 7-14: USM 35 and EPOCH IV.


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Figure 7-15: USK 7 D.

7.22.4 Definitions and general terms Sound beams are mechanical vibrations of every single particle within an object. If you imagine the object split up into many small particles, which are mutual con-nected with elastic power, you will generate a wave motion by getting one or several particles out of balance for example by giving them a shock. Because of the elastic power the neighbour particles will after a while get out of balance too. The shock will spread in the object as a wave motion.

It is characteristic for a wave motion that a transport of energy is taking place, but it does not result in a transport of substance. Each particle oscillates with larger or smaller oscillation around their equilibrium, but keeps their position in relation to the other particles. It can be shown that the oscillations are sinus oscillations.

Wave motions can appear in many different shapes. In the following we will mention some important definitions and general terms concerning ultrasonic oscillations and the most important types of oscillations.

Frequency (number of oscillations) The frequency is the number of oscillations per second.

One oscillation is a movement from a mean position to a maximum through a mean position to a maximum and back to a mean position again.

The term for a cycle is Hertz or c/s.

And the time used for one oscillation is called a period.


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In ultrasonic testing frequencies between 1-6 MHz (megahertz, mega = million) is often used. For example is the time for one single oscillation at 1 MHz equivalent to

000.000.11 sec.

The sound direction is the direction of the wave propagation. It does not have to merge with the direction of the particle movement. The particles can move in the direction of propagation or at right angles to the direction of propagation.

Wavelength The wavelength is the distance measured in the sound direction from one particle to the next particle in the same mode.

The wavelength is inversely proportional to the frequency that is to high frequencies you will have small wavelengths and conversely.

The sound velocity If you call the wavelength λ and frequency f this equation applies for a wave motion f x λ = V = constant The constant is the velocity of sound in the material and not of the single particle itself. It tells you how many wavelengths, that is how long a distance, the wave propagates per second. The velocity is a quality of the object and for a certain object, the velocity of sound is constant for all frequencies and wavelengths. Types of oscillations Sound can propagate under various forms: Longitudinal waves Longitudinal waves are characterised by the fact that the particle motion is in the direction of propagation of the sound Longitudinal waves can propagate in solids as well as in gases and liquids. The audi-ble sound is for example longitudinal waves in air. Transverse waves Transverse waves are characterised by the fact that the particle motion is at right angles to the direction of propagation. The elastic forces, which make the particles oscillate, are displacement forces. These forces are not found in liquids and gases, so transverse waves can only be transmit-ted in solids.


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Pure longitudinal- or transverse waves can only be generated inside an object if the extent is great compared to the wavelength.

7.22.5 Refraction and reflection of ultrasonic waves Refraction If a sound beam hits an interface between two different materials in an oblique di-rection, a part of the wave will be refracted in the surface and continue into the other material in a diverging direction. Another part of the incident wave will be re-flected from the interface. This refraction and reflection happens due to the laws of optics for the lights refraction and reflection. See Figure 7-16.

ri

Material 1

Material 2

V

V

1

2

Figure 7-16: Sound wave passing through an interface.

Reflection at right angles If a sound beam hits an interface at right angles a part of the sound will travel through the interface and continue in the same direction, but with another velocity. No refraction happens in this case of course. The other part of the sound will be reflected back from the interface at a right angle. At the interface between steel and air the reflections coefficient is almost -1 that means that the incident sound wave is completely reflected and that there is no sound wave transmitted into the air. By ultrasonic testing of materials for internal defects the reflected signal will always come from an interface of this type. Most discontinuities in a material will have an interface towards the part without a defect, which has the character of an air gap.

Such an interface is a great reflector at air cracks with a thickness down to 000.101

mm.


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Reflection at an oblique angle of incidence If a sound wave hits an interface with an angle of incidence “i”, a part of the wave will be reflected with the angle of reflection “r”, as already illustrated in Figure 7-16.

As for a light beam reflecting in a mirror the same rule applies that the angle of inci-dence and the angle of reflection are equal, that is i = r. Besides this similarity be-tween the laws of light and those of sound waves, there is an important difference due to the fact that sound beams in solids can be either transverse or longitudinal and change from one form to the other under certain conditions.

Longitudinal wave Longitudinal wave Transversal wave

i

r

r

T

L

Material 1(solid)

Material 2 e.g. air

Figure 7-17: Reflection of a longitudinal wave at an interface.

When a longitudinal wave in a solid material hits a plane interface with an angle of incidence “i”, a part of the beam will be reflected as a longitudinal wave with an an-gle of reflection equal to “i”. Furthermore a part of the beam will change into a transverse wave, which is reflected with a smaller angle of reflection (Ut) due to the smaller velocity for transverse waves (Figure 7-17).

Steel is one of the most common used materials in construction and therefore the material most often tested with ultrasound. Below the condition at an interface between steel and air is further discussed. As mentioned earlier such an interface will reflect a sound beam completely which means that it is totally reflected. Longitudinal waves in steel propagate with the velocity VL ≅ 5900 m/sec. and trans-verse waves with the velocity VT ≅ 3230 m/sec. Decibel On most equipment you have a control, with which you can regulate the gain. This control is divided and adjusted in decibel (dB). In this way it is possible to measure


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and compare the height of different echoes. Such measurements are necessary when comparing defects, testing for absorption and estimating the size of defects.

7.22.6 Probes Normal probes A normal probe generates longitudinal waves, which leaves the probe at a right an-gle to its contact surface. If the probe is in contact with a specimen, the sound wave penetrate into it. It travels in straight lines, with a certain beam spread. See Figure 7-18.

Figure 7-18: Normal probes.

Construction A normal probe is constructed as shown in Figure 7-19 below.

Figure 7-19: Normal probe.


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The crystal must be damped in order to quickly stop the oscillations after it has been excited, either by an electrical pulse or by a reflected sound wave. In this way the initial pulse and the echoes on the screen of the equipment are prevented from being too wide

Dual probe (TR-probes) The near resolution can be increased considerably by using a probe with two sepa-rate crystals one for transmission and one for receiving. Figure 7-20 shows the in-

side of a TR- probe.

Figure 7-20: TR-probes.

The piezoelectric crystal is glued to perspex blocks, which works as a delay line for

the sound. The crystals are placed in a slight angle to the surface of the object, and turned against one another. Due to that you can detect defects right under the sur-face. Unfortunately this construction may give spurious echoes from surface waves.

This can be avoided using a probe where the crystals are parallel to the surface of the object. In return you have a minor sensitivity for defects right under the surface. Beyond these common types you have special probes developed for specific tasks e.g. waterproof types and heat resistant types. Choice of probe Normal probes differ from one another as regards to the type of crystal, its diameter and frequency. When choosing a probe you have to evaluate the influence of these variables to the quality of the probe. Below is summarised what to consider when choosing a probe. Choosing a higher frequency gives you: 1. The possibility of detecting smaller defects. 2. Less sensitivity, shorter penetration into the material.

Transmitter Receiver

Perspex

Transmitter Receiver


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3. A longer near field. 4. Smaller angle of divergence. 5. A better resolution.

Choosing a crystal with a bigger diameter gives you: 1. A better sensitivity, longer penetration into the material. 2. A longer near field. 3. A smaller angle of divergence. Choosing a crystal of another material you can obtain another combination of sensi-tivity and resolution.

Angle probes Angle probes are normally manufactured with frequencies between 2 and 5 MHz and the angles 35°, 45°, 60°, 70° and 80° for testing in steel. Other frequencies and angles are available. The angles are always stated in proportion to the normal. See Figure 7-21.

The probe index is marked on the side of the probe with a line.

Figure 7-21: Angle probes.


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Construction An angle probe is constructed as shown on Figure 7-22 below.

Figure 7-22: Angle probe. Between the piezoelectric crystal and the exit point a wedge-shaped middle piece is placed. When the probe is brought into contact with an object, the longitudinal waves, generated from the crystal, will travel at an oblique angle to the interface between the wedge and the object. Here they will be refracted and continue into the object with a different angle. If the wedge angle is small, a part of the longitudinal waves in the wedge will con-tinue into the object as longitudinal waves and a part as transverse waves. These two wave types will penetrate into the object in different directions, which means that it will be difficult to decide, where possible echoes comes from. Due to that the wedge is fabricated with an angle, which is larger than the 1. critical angle. The longitudinal waves are totally reflected inside the wedge. The final result is a refracted transverse wave in the specimen. The shape of the wedge results in quite a lot of spurious echoes on the screen, be-cause a part of the sound beam being reflected at the interface and inside the wedge will be reflected back and forth and finally hit the crystal. As shown on Figure 7-22 you can provide the wedge with a crystal backing for ab-

sorption of the reflected sound beams inside the wedge. Another solution is to pro-vide the wedge with different saw cuts in order to make the reflected sound beams not hit the crystal. If you place an angle probe on a plate the sound wave will travel between the two surfaces as shown in the Figure 7-23.

Damping

Perspex wedgeConnector

Crystal

Lead


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Figure 7-23: Sound beam from an angle probe.

The distance between the place, where the ultrasonic wave enters the plate and the point where it hits the top again is called the skip distance P. If you place a probe on other materials than steel with another sound velocity, the angle α will change. Therefore the above mentioned factors can only be used when testing steel. Table 7-1 shows how the angle (α) changes from testing steel to aluminium, copper

and cast iron.

Steel α° Aluminium α° Copper α° Cast iron α°

35° 33 23.6 23

45° 42,4 29.7 28

60° 55.5 37.3 35

70° 63.4 41 39

80° 69.6 43.4 41

Table 7-1: Angles in different materials.

Field of application Angle probes are normally used for testing welds and for testing of parent material in pipes. Welds are tested for internal weld defects e.g. slag inclusions, porosity, cracks, lack of sidewall fusion and lack of penetration. Pipes are tested for material defects and for transverse and longitudinal cracks Common angle probes can be used on hot surfaces with a temperature up to 70-80° C. You should also be aware of the fact that the refracted angle may change, due to the sound velocity in the probe and in the object changes with temperature. The sensitivity of the probe will decrease with temperature, because the attenuation in the probe will increase.

P

α s t


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Check of probes A probe is characterised by a series of qualities, which is of importance for its func-tion. Before you start using a complete new probe or when you have used a probe a while, it might be of interest to check that it fulfils the specifications, which are listed by the manufacturer. It can be necessary to check the following qualities: Frequency - sensitivity - resolution - the width of the initial pulse - the width of the flaw echo and the shape of the sound field. For angle probes you must check the probe index and the direction of sound. Most probes have a “wear plate” called the sole between the crystal and the object in order to avoid wear of the crystal. When checking the probe you should inspect and maybe have the sole replaced.

Coupling media At the interface between steel and air or between air and another solid material or liquid, you will have an almost 100% reflection of the sound. In practice it is not possible to produce a direct contact without air gap between the probe and the ob-ject under test, so you will have to place a coupling media between the probe and the object. As coupling media are used oil, grease, water, glycerine or wallpaper paste. Oil or grease is normally used when testing machine elements. By continues testing of welded seams, you will of financially reasons often use water as coupling. Angle probes can be provided with special water coupling so the water comes out in the middle of the contact surface. Wallpaper paste is rather thick and therefore ideal when testing on sloping or vertical surfaces. After finishing the examination you can easily remove it with cold water. Using water as a coupling in frosty weather can be difficult because it freezes. Add-ing spirit is an easy and cheap solution. Better, but more expensive is the use of glycerine, which at the same time spares the hands of the user.


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7.22.7 Examination of rolled, cast and forged objects Lamination examination of rolled plates and profiles Lamination examination is carried out with a normal probe or a TR-probe. If you place a normal probe on a plate with no defects you get a row of echoes, where the mutual distance corresponds to the thickness of the plate. (Figure 7-24a).

If there is lamination present (Figure 7-24b) the pulse is reflected from the lamina-

tion and the distance between the echoes is reduced corresponding to this. The size of the lamination can be estimated by moving the probe along the edge of the lami-nation. Normally a lamination is extensive slag inclusions in the middle of a plate, but sometimes it consists of small slag inclusions, which can be either in the same depth or distributed in the entire thickness of the plate. From this type of defect you still have echoes corresponding to the normal thickness of the plate, but between those you see small echoes from the slag inclusions (Figure 7-24c).

Figure 7-24: Lamination examination of plates.

a

b

c


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Laminated plate material is rather common. It is very dangerous to use such plates in welded constructions in places, where pull occurs at right angles to the surface of the plate. Before welding a fitting onto a plate or a profile, you should carry out a lamination examination to make sure that the material is not laminated. Figure 7-25 shows what can happen if a fitting is welded on to a laminated plate. Figure 7-25: Fitting welded onto plate with lamination.

The laminations arise when inclusions and hollowness from the chill mould are im-perfect rolled. A plate will therefore mostly contain laminations in the areas shown on Figure 7-26.

Figure 7-26: Areas where lamination often occurs in plates.


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Testing a plate can be done manually with a plate tester as shown on Figure 7-27

or in an automatic ultrasonic testing installation, where the plate is moved past a row of probes for example 40-80 probes according to the width of the plate. Each probe scans the plate along a line and the results are registered on a paper slip.

Figure 7-27: Plate tester with mounted ultrasonic equipment.

The results can be stored in a computer with the specifications from the client as regards for dimensions and allowable content of laminations and slag inclusions. The computer then cuts out the plate and distributes these according to order. This sys-tem often causes that clients who do not specify permissible content of laminations and slag inclusions may have screened out plates, which may contain defects. Testing of rolled profiles for laminations in body or flanges is done in the same way as for lamination testing of plates. Another situation where it is important to check for lamination is e.g. where 2 plates are welded together. If a lamination is present in the area where you would scan with angle probes the sound beam will do as shown in Figure 7-28.


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Figure 7-28: Sound beam reflecting from lamination.

The lamination testing of plates are normally carried out with a nominal frequency of 2 or 4 MHz. Plate thicknesses down to about 5 mm can be tested quite accurate. For testing thinner plates you must use a higher frequency or TR-probes. When using a TR-probe you normally look for defect echoes showing up before 1st bottom echo. You must also be aware that the maximum sensitivity from TR-probes can be in different depths depending on the angle between the transmitting and re-ceiving crystal. The relationship between defect and bottom echo will vary with this angle and is different compared to a normal probe. The permissibly extent of lamination in a plate varies of course with application. If the plate is to be used to transmit great tractive forces at right angle to the surface, you must ask for complete lamination free material. For other purposes you may tolerate smaller areas with lamination. In order to carry out a lamination examination you need an agreement between the parties involved. This agreement concerns the extent of examination, the size of defects and the number of defects according to a normative reference.

Examination of castings In the production of casting defects like cracks, porosity, and big gas bubbles, shrinkage and sand inclusions may occur. Ultrasonic examination of such defects can be done in most cases depending on the following conditions:

The penetrating power of material structures Certain materials like steel and aluminium are easy to penetrate for ultrasonic oscil-lations. Others are more difficult or impossible to penetrate for example grey iron, bronze and stainless steel. The attenuation of the sound beam is due to the reflec-tion from graphite flakes in cast iron, and from segregation's in grain boundaries in the other materials. SG-iron is easy to penetrate for ultrasonic oscillation because the graphite flakes is found in the shape of small balls.


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For some materials the penetration power varies with the cooling rate the material has been exposed to after the casting. By quick cooling you get a fine-grained struc-ture with small segregation in the grain boundaries and with a good penetration for ultra sound. The penetration power of a specimen depends on frequency of the sound wave. Sometimes you get sufficient penetration power to carry out the examination by choosing a low frequency e.g. 0.5 MHz. At the same time the lower limit is raised for the size of the defect, which can be detected.

The reflection ability of defect surfaces In castings you can find defects with so uneven surfaces that all of the sound is re-flected away, which means you do not get a flaw echo. See Figure 7-29.

Figure 7-29: Casting with irregular casting defect.

Such defects can only be found using the through transmission technique with a separated transmitter and receiver probe. As long as the receiver detects pulses from the transmitter the subject is accepted. If the receiver detects nothing, there must be a defect in the intervening material.

Examination of rivet joints In rivet joints on steam boilers you can have stress corrosion or caustic breaking cracks in the plates. The cracks start from the rivet holes and propagate from one rivet to another (Figure 7-30).


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Figure 7-30: Examination of rivet joints for cracks in plates and rivets.

The cracks can be found with angle probes as shown in Figure 7-30. Cracks with

the same character can arise in the rivet shank itself. They can be found with a nor-mal probe from the end of the rivet. Spurious echoes


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Besides the direct and expected form echoes e.g. echoes from a back wall, an edge in a plate, a welding cap or a recess and defect echoes from internal defects in the object, you can have other echoes as well - the so called spurious echoes. These echoes arise when the sound beam can be reflected in more different ways inside the object, making the pulses return to the probe in different sound paths. These sound paths normally have different length and the pulses do not return at the same time, which means that equipment registers several echoes after one another. A delay in time of a longitudinal wave can also be due to the fact, that it has been transformed into a transverse wave on a part of the sound path, where it has travelled at the smaller velocity of the transverse wave. If the sound beam is spread out more than expected or if the subject has another form than expected, it can also give unexpected echoes on the screen. Below some common occurring spurious echoes are mentioned.

Examination of long objects with a normal probe A normal probe is placed on the top or the edge of a long object, which is narrow, compared to the sound beam. A part of the scattered sound beam will be reflected from the sides and travel in paths, which are longer than the direct sound path. This is seen as a row of successive echoes, as shown in Figure 7-31.

When reflecting against the side, the longitudinal wave is split up. A part continues as a transverse wave, which again is transformed into longitudinal waves after a later reflection. Finally the sound beam returns to the probe as a longitudinal wave and you can see an echo on the screen. Figure 7-31 shows some of the possible sound paths in a long

object. Sound wave 1 gives 1st bottom echo. No. 2 gives 1st spurious echo after a single reflection of the longitudinal wave on the side. No. 3 gives 2nd spurious echo after a transformation into a transverse wave on a part of the sound path. No. 4 gives the 3rd spurious echo after repeated reflections of the transverse wave. All spurious echoes shows up after the 1st bottom echo - and will not interfere with the examination, if the object has the same cross section over the whole length. If you on the other hand have recesses or shrinked on bushing, the spurious echoes will be able to cover possible echoes from defects in more distant areas of the ob-ject.


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Figure 7-31: Spurious echoes after the bottom echo when examining long ob-

jects.

L = logitudinal waves T = transversal waves L = longitudinal wavesT = transversal waves


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7.22.8 Examination of welds Application and purpose The extensive use of ultrasound for examination of welds is due to the fact that it is one of the most reliable and cheapest ways of getting information about the quality of a weld. The method has also shown that it is suitable for automation and it is used for continuous control of welding plants, where large amounts of identical welds are produced under factory-like circumstances. For examination of manual welds the ultrasound method is suitable as well. In this case manual examination is more common and will be described in the following and which still is a model for the more simplified automatic examination. When examining a weld it is all about getting a detailed and accurate picture of the existing defects, in order to record their position and size in a report, which is inde-pendent of the used equipment and the team using the equipment. The report should give the client a satisfactory foundation in order to make his decision on the quality of the weld.

The ability of a defect to reflect Because the method is build on the ability of the defects in the weld to reflect the used sound waves, we will first go through the welding defects common in practice, seen from this point. The defects can be split into planar defects, meaning defects which are very small in one direction, but has a certain size in the two other direc-tions as e.g. cracks, lack of penetration and lack of side wall fusion see Figure 7-32,

and volumetric defects such as defects, which has a certain extension on all 3 sides as e.g. slag lines, slags in lack of penetration, gas porosity and slag inclusions, see Figure 7-33.

Figure 7-32: Planar defects.

A + B: Longitudinal, oblique. E.g. lack of sidewall fusion cracks C: Longitudinal, right angle to the surface. E.g. lack of penetration and cracks D: Longitudinal, parallel to surface. E.g. thin layer of slags E: Transverse, right angle to the surface. E.g. transverse cracks


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The planar defects will reflect the sound waves well, if the extension in both direc-tions across the beam direction is larger than about ½ a wavelength. The reflection will then happen according to the laws of the optics and a necessary condition for the defect to be found is the sound beam being either directly or via a close by surface reflected back to the probe. The volumetric defects will reflect sound beams more or less scattered. The best reflections you will get, is from a defect with an almost plane/level surface, which is at right angles to the wave. That is found on e.g. slag lines and lack of penetration (Figure 7-33F + G). Less good reflections you get from wormholes, even though

they are lengthy in the direction of the weld (Figure 7-33 H). The worst reflection is

from porosity and scattered slag inclusion (Figure 7-33 J + K).

Figure 7-33: Volumetric defects.

F: Slag lines G: Lack of penetration H: Wormhole J: Porosity K: Slaginclusions

Examination technique Because of the above mentioned possible location and orientation of welding defects the method of testing must be carried out in such a way that as many defects as possible are found. In a lot of cases where you know the welding method, you can exclude some defects and simplify the method of testing. For instance is testing for


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transverse flaws not necessary in welding in plain carbon steel with low yield point, as transverse flaws are very rare. Butt welds in thinner plates For this you should use an angle probe placed on the surface of the plate besides the welding as shown on Figure 7-34. The sound beam travels obliquely into the plate

and is reflected alternately from the lower and higher surface of the plate, so the beam describes a zigzag path in the plate. An important condition for this method to work correctly is that the plate is free of lamination, which can reflect the beam, be-fore it reaches the opposite side.

Figure 7-34: Examination of a butt weld with an angle probe.

An effective test for the previous mentioned types of defects needs the following movements of the probe (Figure 7-34).

Figure 7-35: Scanning directions of angle probe when examining a butt

weld.

Pos. 2 Pos. 1


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Calibrating the range and measuring the beam angle is done in the easiest way by using an IIW-block. You find it in two types, type 1 for angle probes of almost all common sizes and type 2 for miniature angle probes. The shape of these calibration blocks are shown in Figure 7-36 + Figure 7-37.

Figure 7-36: Calibration block 1


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Figure 7-37: Calibration block 2.

Using a normal probe on the 91 mm distance does the calibration of the range on type 1, see Figure 7-36. This distance for a longitudinal wave corresponds to a trans-

verse wave having travelled 50 mm sound path. The successive bottom echoes from the 91 mm distance are used to calibrate the range. The normal probe is replaced with the actual angle probe. It is directed towards the 100 mm arc, see Figure 7-38 and the maximum echo is found. The probe index point

will be on a level with the mark showing the centre of the arc. The probe index is marked on the side of the probe. The echo from the 100 mm arc is displaced with the delay or zero control, until it is placed on 100 mm on the actual range.

Figure 7-38: Determination of probe index.

Maximising the echo from the 25 mm or 50 mm arc does the calibration of the range on type 2. Using the 25 mm arc the first echo is placed on 25 mm with the delay or zero control and the second echo is placed on 100 mm with the material velocity control, and the probe in an unchanged position on the block. By using the 50 mm arc the first echo is placed on 50 mm with the delay control and the second echo is placed on 125 mm with the material velocity control (Figure 7-38).

Measuring the beam angle is done by directing the sound beam towards the cylindri-cal drilled hole in the block as shown in Figure 7-39 and find the position of the

probe, where maximum echo is obtained. On the grade scale, which is engraved on the side of the block, you can read the beam angle on a level with the earlier marked probe index.


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Figure 7-39: Determination of beam angle.

The sensitivity calibration is partly done by adjusting the pulser which - depending of the type of equipment, - can be regulated stepwise in up to 5 different values - or partly by adjusting the gain of the receiver in dB steps. Which sensitivity you choose, depend on how big defects are allowed in the welding. This information you must have and you should have a specification made by the constructor for the specific welding, which indicates the maximum allowed defects and how many minor defects is allowed pr. m weld and what types of defects you can tolerate. These tolerances can be found in national and international standards and codes or in specific procedures. In practice you manage by using a sensitivity calibration corresponding to that of a well defined, artificial defect like a cylindrical drilled hole parallel to the contact sur-face and in the same distance as the actual defects, gives an echo of a certain size. (Figure 7-40).

It is mandatory that the test block is made of a type of steel, which has the same attenuation as the test piece and that the surface is similar to the test piece as well.

Figure 7-40: Calibration of sensitivity on a side drilled hole parallel to the sur-

face.


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The test is done with this calibration, so that all defect indications equal to or bigger than the indication from the drilled hole is noted in the report. It must be noticed that if a defect lays in a considerable other distance (sound path) from the probe than the drilled hole, you must adjust the sensitivity again on another drilled hole in the same distance as the defect. On basis of the above mentioned calibrations and measurements of the used equipment the location of defect and the marking can be done. Moving the probe in the previous mentioned ways tests the welding and the screen is monitored all the time. All indications, above the fixed maximum, are noted and in each case you must decide, if it is reflections from excess weld metal, undercut, backing or other outer limitations. If this is not the case the position of the defect in the horizontal direction from the probe and in depth is calculated. If a defect is big compared to the cross section of the sound beam, the limitations of the defect shall be laid down.

When calculating the horizontal distance and a vertical depth you use different remedies. a) You can mathematically calculate the horizontal and the vertical projection of the

read off sound path (s) see Figure 7-41, because the horizontal projection a is

a = s x sin β

Figure 7-41: Locating a defect by calculating horizontal distance "a" and verti-

cal depth "d" according to the read off sound path.

and the vertical projection (d) is

d = s x cos β

a

ds

αβ


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7.22.9 Determination of defect size Introduction Ultrasonic examination of welds and materials normally involves that the found de-fects are estimated partly to make sure where the place of reflection is and partly to get information about the size and type of the defect.

Determination of the size of large defects In the cases, where the extension of the defect in one or in both directions, meas-ured at right angles to the sound beam, is large compared to the sound beam, you estimate the size by an acoustic scanning, determining the boundaries of the area, from where you have the defect indicated. To get precise information from where to where the defect runs, you use the so-called “half value boundaries” or 6 dB-drop method as the positions of the probe, where the echo height has just reduced to half the normal height from a place on the defect. The limits are determined by displacing the probe either laterally parallel to the weld direction in order to determine the length (Figure 7-42), or forward and

beckwards at right angles to the weld direction to determine the extension of the depth.

Figure 7-42: The length is measured between the half value limits (6 dB-drop).

The accuracy of the defect size based on the half values can be rather good, but sometimes you get large deviations from the actual sizes. Decisive for that is both the surface of the defect and the characteristca of the soundbeam. The method has the advantage of being quick and gives in many cases adequate accurate and unambiguous results


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Determination of the width of defects is not always necessary, but can be used to identify the places of reflection close the opposite surface. It is determined by meas-uring the horizontal distance to the defect from both sides of the weld. As shown in Figure 7-43 an incomplete penetration will show a relatively wide defect, a crack or a

lack of penetration a quit narrow defect, and large excess penetration as a defect with a negative width.

Figure 7-43: A: Incomplete penetration gives great defect width

B: Lack of penetration and cracks in the root give a small width

C: Excess penetration gives negative defect width

a a1 2

a1

a1 a 2

a2

Positive defect width

Defect width

Negative defect width


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Determination of the size of small defects When a reflective surface becomes small compared to the cross section of the sound beam, you will not be able to limit it by moving the sound beam. On the other hand the amount of reflected energy and with that the echo height will give information about the size of the defect. There is however a number of factors, which plays a role, so it is not possible to give a simple connection between the size of a defect and the echo height you get on the screen. Factors having an influence on the size of the flaw echo are:

1. Pulse energy 2. Amplification 3. Coupling between probe and object 4. Type of defect, shape of defect (plane, spherical etc. ) 5. The reflecting surface (roughness) 6. Orientation of defects in relation to the direction of sound 7. The position of the defect in the sound field, the distance between defect and

probe 8. The acoustic properties of the object.

One of the most important factors is the type and shape of the defect, which makes the ability of reflection vary a lot from defect to defect. Also the orientation of the defect in relation to the direction of sound is of great importance for the size of the echo.

The significance of the position of the defect in the sound field. Near field and far field The size of an echo from a small reflector will generally decrease with increasing distance from the probe. It will happen in a regular way at larger distances, but if the defect gets closer to the probe, you will see large variations in echo height for small changes of distances. That is due to interference, which can occur in the so-called near field (Figure 7-44).


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Figure 7-44: Schlieren image of sound field, showing near field and far field.

Comparison with artificial defects As seen from the previous situation, it can be rather complicated to determine the size of a defect only on basis of the echo of the defect. It is therefore reasonable to try to simplify the methods to determine the size of small defects. In practice it has been proved very suitable to use a comparison of the actual indications of defects with the indications, you can get from artificial defects of known size. Such defects can be worked out with a well-defined shape e.g. as a cylindrical drilled hole, flat-bottomed cylindrical holes or right-angled grooves. They must be placed in reference blocks with such dimensions that the same distance from the probe can be reached as the actual defect distance in the test piece. An example of such a refer-ence block with cylindrical drilled holes is shown in Figure 7-45. It is also important,

that the reference block is of a material, which attenuates the sound just as much as the actual material and that the surface is of the same nature, so the same degree of coupling is reached.

Figure 7-45: Reference block with cylindrical bored holes for comparing echo sizes.

N F


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Such artificial defects are used to calibrate the echo height to a certain height. With unchanged calibration you carry out the examination and all indications, which reaches a certain level are noted. The method of comparison is normally satisfactory for approval control, because you rarely will have difficulties concerning characteristics, sizes and surfaces of the refer-ence blocks. One must not forget that such artificial defects normally will represent optimum con-ditions of reflections and you will in the principle from such a comparison only be able to find the smallest value of defects.

DGS diagram If reference blocks with artificial defects cannot be produced or if it appears to be awkward to use a large number of different reference blocks, you can use the DGS diagram. (D = distance, G = gain, S = size) Such a diagram valid for 4 MHz angle probes of the type Krautkrämer MWB70 - 4 is shown in Figure 7-46. Here you can read the size of a reflecting surface on basis of

the sound pressure in the reflected sound beam in relation to the distance of sound, which means the distance between the reflector and the probe index on the probe.


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Figure 7-46: DGS diagram for a 4 MHz angle probe.


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7.22.10 References Journals:

Non Destructive Testing. Applied Materials Research. The British Journal of Non-Destructive Testing. Magnafacts. Materials Evaluation. Ultrasonics. INFO, NDT. Materials Research and Standards. Das Echo. Materialprüfung. Technische Überwachung. Schweissen + Schneiden.

Books:

L. Filipozynski and others "Ultrasonic Methods of Testing Materials". Butter-worths. England. 1966. B. Banks and others "Ultrasonic Flaw Detection in Metals. Theory and Prac-tice". Lliffe Books Ltd. London. 1962. Benson Carlin. "Ultrasonics". McGraw-Hill Book Company, Inc. 1949. J. F. Hinsley. "Non-Destructive Testing". MacDonald & Evans Ltd. London. 1959. W. J. McGonnagle. "Non-Destructive Testing". McGraw-Hill Book Company, Inc. London. 1961. J. Blitz. "Fundamentals of Ultrasonics". Butterworths. England. 1963. Non-Destructive Testing Handbook. Vol. I-II. The Ronald Press Co. 1959. Non-Destructive Testing, Programmed Instruction Handbooks, General Dy-namics. Krautkrämer. "Werkstoffprüfung mit Ultraschall". 2. edition. Springer-Verlag. 1966. Vaupel. Bild-Atlas für die zerstörungsfreie Materialprüfung. I-II-III. J. Matauscheck. "Einführung in die Ultraschalltechnik". VEB Verlag Technik. Berlin. 1957. E. A. W. Müller. "Handbuch der zerstörungsfreien Materialprüfung". R. Ol-denburg. 1963. Ludwig Bergmann. Der Ultraschall. S. Hirzel Verlag, Stuttgart. 1954.


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Norms and directions:

Stahl-Eisen Lieferbedingungen 072-69. (1. edition December 1969): Ultra-schallgeprüftes Grobblech. Schweissen + Schneiden, Heft 6/66. Rudolf Trumpfheller, Technischer Ü-berwachungs-Verein, Essen a. V.: Abnahmeprüfungen an Schweissnähten nach dem Ultraschallprüfverfahren.

Deutsche Industrie Normen:

DIN 54119. Vornorm Zerststörungsfreie Prüfung; Ultraschallprüfung, Begrif-fe. DIN 54120. Vornorm Zerstörungsfreie Werkstoffprüfung; Kontrol1körper 1 und seine Verwendung zur Justierung u. Kontrolle von Ultraschall-Impulsecho-Geräten. DIN 54122. Entwurf Zerstörungsfreie Werkstoffprüfung; Kontrol1körper 2 und seine Verwendung zur Justierung u. Kontrolle von Ultraschall -Impulsecho-Geräten.

ASTM Standards:

A 435-67. Standard Method and Specification for Longitudinal-Wave Ultra-sonic Inspection of Steel Plates for Pressure Vessels. E 317, Part 31. Evaluating performance characteristics of pulseecho ultra-sonic testing systems. Rec. practice for. E 127, Part 31. Fabricating and checking aluminium alloy ultrasonic standard reference blocks. Rec. practice for. E 214, Part 31. Immersed ultrasonic testing by the reflection method using pulsed longitudinal waves. Rec. practice for. A 578, Part 4. Longitudinal wave ultrasonic testing and inspection of plain and clad steel plates for special applications. Spec. for. A 435, Part 4. Longitudinal wave ultrasonic inspection of steel plates for pressure vessels. E 164, Part 31. Ultrasonic contact inspection of weldments. A 503, Part 4. Ultrasonic examination of large forged crankshafts. Rec. prac-tice for. E 273, Part 31. Ultrasonic inspection of longitudinal and spiral welds of welded pipe and tubing. E 213, Part 31. Ultrasonic inspection of metal pipe and tubing for longitudi-nal discontinuities. A 531, Part 4. Ultrasonic inspection of turbine-generator steel retaining rings. Rec. practice for.


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A 577, Part 4. Ultrasonic, shear wave inspection of steel plates. Spec. for. A 388, Part 4. Ultrasonic testing and inspection of heavy steel forgings. Rec. practices for. A 418, Part 4. Ultrasonic testing and inspection of turbine and generator steel rotor forgings. E 114, Part 31. Ultrasonic testing by the reflection method, using pulsed longitudinal waves induced by direct contact. Rec. practice for. E 113, Part 31. Ultrasonic testing by the resonance method. Rec. practice for.

British Standards:

BS 2704. Specification for calibration blocks and recommendations for their use in ultrasonic flaw detection. BS 3683. Glossary of terms used in non-destructive testing. BS 3923. Methods for ultrasonic examination of welds. BS 4331. Methods for assessing the performance characteristics of ultraso-nic flaw detection equipment.

Ultrasonic standards: DS/EN 12062 General rules for metallic materials DS/EN 1712 UT of welded joints. Acceptance levels DS/EN 1713 UT examination. Characterization of

indications in welds DS/EN 1714 UT of welded joints (technique) DS/EN 10160 UT of steel flat product (lamination) DS/EN 10308 UT of steel bars DS/EN 583-2 Sensitivity and range setting DS/ENV 583-6 TOFD-technique (sizing of defects) DS/EN 12680-1 Steel castings for general purposes DS/EN 12680-2 Steel castings for highly stressed components DS/EN 12680-3 Spheroidal graphite cast iron castings DS/EN 14127 Ultrasonic thickness measurement ASME V Art. 4 UT for inservice inspection “ Art. 5 UT methods for materials and fabrication “ VIII Art. 9-3 UT examination of welds ADM HP 5/3 Zerstörungsfreie Prüfung der Schweisverbindungen


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7.23 Ultrasonic thickness gauge This method can be used for steel structures.

7.23.1 Introduction An ultrasonic equipment is suitable for measuring thicknesses and it can be done just with one side accessible and if the material can generate ultrasonic waves. For un-even or corroded surfaces, it may be necessary to grind the surface at the test posi-tions to make proper contact. Possible rust on the opposite surface does not disturb the reflections. If steel plates are laminated, the measured thickness will only be the depth of the first layer. Bringing a normal probe in contact with the object does the measurement. The dis-tance between two successive bottom echoes on the screen indicates the thickness. If the range is calibrated using a calibration block with a known thickness you can read off the thickness of the specimen on the screen. To achieve the best possible accuracy when measuring you not only read the dis-tance between two successive bottom echoes, but the distance between 0 and the last readable bottom echo. The last echo shall preferably be placed to the right on the screen, in order to get the best accuracy, when reading the screen. This means that the scale must have a good linearity, also to the very right of the screen. Then you count the number of echoes you see and divide the reading with that number. The thickness of the object can be measured with 1-2% accuracy. On new digital equipments one or two gates are used where the measurement is done at the intersection between the echo and the gate.

If the measurements are used in order to determine the thickness of a number of components the results are reported in terms of the mean and standard deviation. Relevant percentiles may also be reported. Such statistical analysis is only possible if the measurements are independent and if the measurements form a homogenous population (if a single measurement is performed at each of a number of identical components).

The major advantages of the method are that it is easy to use and that it produces instantaneous results. Normally, a relatively large number of measurements are performed. The measure-ments may be used to map the thickness of the considered component. The results may be reported in the form of a surface graph.


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7.23.2 Thickness measurements of steel plates The thickness of a steel plate can be measured with a normal probe or a TR-probe, which is placed on the surface. See Figure 7-47.

Figure 7-47: Thickness measurement of plate.

Before starting the measurements, a contact liquid is applied to the test locations. Further, the equipment must be calibrated. For common steel alloys, the calibration is performed by means of test blocks. For unknown alloys (or if you are not sure), the calibration is performed by adjusting the sound velocity setting of the equipment until the equipment shows the same thickness as can be measured by a slide calliper at a free edge. After calibrating the range of the ultrasonic equipment using a suitable calibration block for example a 5, 10 or 25 mm thick steel block, you can read off the thickness of the plate in different ways. Reading off the position of 1st bottom echo This method is often used, if the plate has a very uneven surface, which only gives you the 1st bottom echo. You would normally use a TR-probe. See Figure 7-48.

Figure 7-48: Thickness measurement of uneven steel plate.

t

TR-probe

t

t


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At plate thicknesses below 20 mm, the range shall preferably be calibrated to 25 or 10 mm sound path in steel to achieve the best possible reading accuracy. Another option is to use waterproof equipment. That means that you can bring both the transducer and the apparatus with you into the water.

Reading off the position of one of the successive bottom echoes If more successive bottom echoes turns up on the screen you choose one, which is placed to the right of the screen. You count the number by counting echoes from the left, and you read off its position on the screen, that is the equivalent sound path, see Figure 7-49.

Figure 7-49: Thickness measurement with successive echoes.

The equipment time base range should be calibrated in order to give you as many bottom echoes as possible on the screen. The gain should not be greater than corre-sponding to 1st bottom echo from the calibration block in scale height. This will give you the most accurate reading.

Digitalised equipment does not need successive echoes because the reading is done where the gate intersects with the echo you want to use. Reading off the position of one of the successive bottom echoes, with 1st or 2nd bot-tom echo on 0 on the scale If the surface of the plate is covered with paint or scale or if it has just become rusty or rough, this method should be used. Doing this you avoid measuring the thickness of the layer covering the surface, no matter if it is stuck to the surface or it is cou-pling agent on a corroded surface. Such coatings can result in rather considerable errors. The velocity in for example water is about ¼ of the velocity in steel. A 0.5 mm thick layer of water will therefore give an error of 2 mm, if 1st bottom echo is read off. The reading of the thickness is done exactly in the same way as for the previous method.

t

1 2 3 4 5

t = s/4

Soundpath s

Bottom echoes


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You should be aware when using this method that it is necessary to place 1st or 2nd bottom echo on 0 on the scale before each measurement. See Figure 7-50.

Figure 7-50: Thickness measurement of plate with a coating.

Todays' digital equipments are using so-called "gates" which means that you do not have to move 1st or 2nd echo to zero. You place the 2 gates in order to measure between 2 successive echoes. See Figure 7-51.

Figure 7-51: Measuring through a layer with digital equipment.

7.23.3 Special equipment Thickness measurements can also be done with special equipment, which normally only are intended for this purpose. They are called thickness gauges and they can operate with TR-probes or normal probes. The gauges normally give you the thick-ness in digits but some gauges have both digits and an oscilloscope. Some of these gauges are capable of compensating for layers. See Figure 7-52.

Layer

SteelSteel

Steel

t

1 2 3 4 5

t = s/4

Soundpath s

Bottom echoes


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Figure 7-52: Thickness gauge for measuring through coating (left), and (right)

for use without coating.

These gauges are normally calibrated in steel. When measuring on other materials the results must be corrected as shown later.

7.23.4 Thickness measurements of hot steel plates With normal equipment you can carry out thickness measurements on objects with a temperature up to about 70° C. Using a special heat resistant probe it is possible to measure hot objects with a temperature as high as 800° C, but you must be aware of the following:

a. The probes are normally TR-probes, where the delay line is made from heat-insulated material. The probe should be cooled in water frequently

b. The velocity in the material in the delay line drops significant with temperature. The echoes on the screen will due to this move towards the right of the screen. Often the screen is photographed to get a fast and accurate reading or you could use the freeze function.

c. The attenuation in the insulating material increases with temperature. Due to that the reading must be done fast and you should have enough gain reserve in the ultrasonic equipment.

d. The coupling is often silicone, which evaporates in a few seconds at 400° C. Therefore there is no time to adjust the probe on the object to get the clearest picture on the screen. You must use a fixture, to make sure, that the contact sur-face is parallel with the object surface during the movement towards the object, so that you immediately have a good contact.


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e. The velocity in steel decreases with increasing temperature. In the area 0-400° C

it decreases with approximately 1% per 100° C. A 400° C hot object is measured approximately 4% too thick. The results must therefore be corrected.

7.23.5 Thickness measurements of other materials than steel Most metals, porcelain, glass and some plastic materials can be measured in the same way as steel. The equipment time base range should be calibrated on a cali-bration block of the material concerned. If this cannot be produced, but you know the velocity of the material, you can cali-brate the equipment on a steel block and then read off the thickness as usual. The results have to be corrected by multiplication with a correction factor. This factor depends on the velocity in the material compared to the velocity in steel. If the sound for example only travels 0.8 times as fast in copper than in steel, the read thickness of a copper plate has to be multiplied by 0.8 to get the real thickness. Normally applies that the real thickness = the read thickness multiplied by the pro-portion between the velocity of the material and the velocity in steel. For velocities see Table 7-2.

steel

materialreadreal v

vtt x = ; V is the velocity for longitudinal waves

Measuring the velocity for longitudinal waves You can measure the velocity of longitudinal waves in an object with ordinary ultra-sonic equipment and probe in the following way: Measure the real thickness mechanically using a gauge. Then measure the thickness in the same place with the ultrasonic equipment. The unknown velocity can be found using this formula:

steelread

realmaterial v

ttv x =


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Material

Density ρ

Velocities in m/sec.

Acoustic im-pedance Z = ρ x VL

103 kg/m3 VL longitudinal- waves

VT transverse waves

sec.m

kg10 26

M E T A L S

Aluminium Lead White metal Hard metal Copper Constantan Mercury Magnesium Manganin Brass Nickel Cast iron Steel Tin Titanium Bismuth Wolfram Zinc

2.7 11.4 7.3 11-15 8.9 8.8 13.6 1.7 8.4 8.1 8.8 7.2 7.7 7.3 4.5 9.8 19.1 7.1

6320 2160 3300 6800-7300 4700 5240 1450 5770 4660 3830 5630 3500-5600 5900 3320 6070 2180 5460 4170

3130 700 4000-4700 2260 2640 3050 2350 2050 2960 2200-3200 3230 1670 3110 1100 2620 2410

17 25 24 77-102 42 46 20 10 39 31 50 25-40 45 24 27 21 104 30

N O N M E T A L S

Aluminium oxide Araldit Rubber, soft Glass, flint Rubber, vulcanised Paraffin wax Ice Glass, crown Quartz glass Nylon, perlon Perspex Polystyrene Porcelain Teflon

3.9 1.18 0.9 3.6 1.2 0.83 0.9 2.5 2.6 1.1-1.2 1.18 1.06 2.4 2.2

1000 2500 1480 4260 2300 2200 3980 5660 5570 1800-2200 2730 2670 5600-6200 1350

1100 2560 1990 3420 3520 1430 3500-3700

39 1.4 15 2.8 1.8 3.6 14 14.5 2.0-2.7 3.2 2.8 13 3.0

L I Q U I D S

Diesel oil Glycerine Motor oil Water (20° C)

0.80 1.26 0.87 1.0

1250 1920 1740 1483

1.0 2.5 1.5 1.5

Table 7-2: Density, velocities and acoustic impedance for different materials.


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7.24 Coating Thickness Measurement This method can be used for steel structures.

Coating thickness is an important variable that plays a role in product quality, proc-ess control, and cost control. Measurement of film thickness can be done with many different instruments. Understanding the equipment that is available for film thick-ness measurement and how to use it is useful to every coating operation.

The issues that determine what method is best for a given coating measurement include the type of coating, the substrate material, the thickness range of the coat-ing, the size and shape of the part, and the cost of the equipment. Commonly used measuring techniques for cured organic films include nondestructive dry film meth-ods such as magnetic, eddy current, ultrasonic, or micrometer measurement and also destructive dry film methods such as cross-sectioning or gravimetric (mass) measurement. Methods are also available for powder and liquid coatings to measure the film before it is cured.

7.24.1 Magnetic Film Thickness Gages

Magnetic film gages are used to nondestructively measure the thickness of a non-magnetic coating on ferrous substrates. Most coatings on steel and iron are meas-ured this way. Magnetic gages use one of two principles of operation: magnetic pull-off or magnetic/electromagnetic induction.

Magnetic Pull-off Magnetic pull-off gages use a permanent magnet, a calibrated spring, and a gradu-ated scale. The attraction between the magnet and magnetic steel pulls the two to-gether. As the coating thickness separating the two increases, it becomes easier to pull the magnet away. Coating thickness is determined by measuring this pull-off force. Thinner coatings will have stronger magnetic attraction while thicker films will have comparatively less magnetic attraction. Testing with magnetic gages is sensi-tive to surface roughness, curvature, substrate thickness, and the make up of the metal alloy.

Figure 7-53: Pencil-type magnetic pull-off thickness gauge.

Magnetic pull-off gages are rugged, simple, inexpensive, portable, and usually do not require any calibration adjustment. They are a good, low-cost alternative in situa-tions where quality goals require only a few readings during production. Pull-off

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gages are typically pencil-type or rollback dial models. Pencil-type models (PosiPen shown in Fig 1) use a magnet that is mounted to a helical spring that works perpen-dicularly to the coated surface. Most pencil-type pull-off gages have large magnets and are designed to work in only one or two positions, which partially compensate for gravity. A more accurate version is available, which has a tiny, precise magnet to measure on small, hot, or hard-to-reach surfaces. A triple indicator ensures accurate measurements when the gage is pointed down, up, or horizontally with a tolerance of ±10%.

Rollback dial models (PosiTest shown in Figure 7-54) are the most common form of

magnetic pull-off gage. A magnet is attached to one end of a pivoting balanced arm and connected to a calibrated hairspring. By rotating the dial with a finger, the spring increases the force on the magnet and pulls it from the surface. These gages are easy to use and have a balanced arm that allows them to work in any position, independent of gravity. They are safe in explosive environments and are commonly used by painting contractors and small powder coating operations. Typical tolerance is ±5%.

Figure 7-54: Roll-back dial magnetic pull-off thickness gauge.

Magnetic and Electromagnetic Induction Magnetic induction instruments use a permanent magnet as the source of the mag-netic field. A Hall-effect generator or magneto-resistor is used to sense the magnetic flux density at a pole of the magnet. Electromagnetic induction instruments use an alternating magnetic field. A soft, ferromagnetic rod wound with a coil of fine wire is used to produce a magnetic field. A second coil of wire is used to detect changes in magnetic flux.

These electronic instruments measure the change in magnetic flux density at the surface of a magnetic probe as it nears a steel surface. The magnitude of the flux density at the probe surface is directly related to the distance from the steel sub-strate. By measuring flux density the coating thickness can be determined.

Electronic magnetic gages (e.g. PosiTector 6000 F Series shown in Figure 7-55,

PosiTest DFT Ferrous) come in many shapes and sizes. They commonly use a con-stant pressure probe to provide consistent readings that are not influenced by differ-ent operators. Readings are shown on a liquid crystal display (LCD). They can have options to store measurement results, perform instant analysis of readings, and out-

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put results to a printer or computer for further examination. Typical tolerance is ±1%.

The manufacturer’s instructions should be carefully followed for most accurate re-sults. Standard test methods are available in ASTM D 1186, D 7091-05, ISO 2178 and ISO 2808.

Figure 7-55: Electronic magnetic induction thickness.

7.24.2 Eddy Current

Eddy current techniques are used to nondestructively measure the thickness of non-conductive coatings on nonferrous metal substrates. A coil of fine wire conducting a high-frequency alternating current (above 1 MHz) is used to set up an alternating magnetic field at the surface of the instrument's probe. When the probe is brought near a conductive surface, the alternating magnetic field will set up eddy currents on the surface. The substrate characteristics and the distance of the probe from the substrate (the coating thickness) affect the magnitude of the eddy currents. The eddy currents create their own opposing electromagnetic field that can be sensed by the exciting coil or by a second, adjacent coil.

Eddy current coating thickness gages (e.g. PosiTector 6000 N Series) look and oper-ate like electronic magnetic gages (see Figure 7-55). They are used to measure coat-

ing thickness over all nonferrous metals. Like magnetic electronic gages, they com-monly use a constant pressure probe and display results on an LCD. They can also have options to store measurement results or perform instant analysis of readings and output to a printer or computer for further examination. The typical tolerance is ±1%. Testing is sensitive to surface roughness, curvature, substrate thickness, type of metal substrate and distance from an edge.

Standard methods for the application and performance of this test are available in ASTM B 244, ASTM D 1400, D 7091-05 and ISO 2360.

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It is now common for gauges to incorporate both magnetic and eddy current princi-ples into one unit (e.g. PosiTector 6000 FN, PosiTest DFT Combo). Some simplify the task of measuring most coatings over any metal by switching automatically from one principle of operation to the other, depending upon the substrate. These combination units are popular with painters and powder coaters.

7.24.3 Ultrasonic

The ultrasonic pulse-echo technique of ultrasonic gages (e.g. PosiTector 100 and PosiTector 200 shown in Fig 4) is used to measure the thickness of coatings on non-metal substrates (plastic, wood, etc.) without damaging the coating.

The probe of the instrument contains an ultrasonic transducer that sends a pulse through the coating. The pulse reflects back from the substrate to the transducer and is converted into a high frequency electrical signal. The echo waveform is digi-tized and analyzed to determine coating thickness. In some circumstances, individual layers in a multi-layer system can be measured.

Typical tolerance for this device is ±3%. Standard methods for the application and performance of this test are available in ASTM D 6132.

Figure 7-56: Ultrasonic gauge can measure the thickness of coatings on non-

metallic substrates:

7.24.4 Micrometer

Micrometers are sometimes used to check coating thickness. They have the advan-tage of measuring any coating/substrate combination but the disadvantage of requir-ing access to the bare substrate. The requirement to touch both the surface of the coating and the underside of the substrate can be limiting and they are often not sensitive enough to measure thin coatings.

Two measurements must be taken: one with the coating in place and the other with-out. The difference between the two readings, the height variation, is taken to be the coating thickness. On rough surfaces, micrometers measure coating thickness above the highest peak.

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Figure 7-57: The Micrometer Screw Gauge.

7.24.5 Destructive Tests

One destructive technique is to cut the coated part in a cross section and measure the film thickness by viewing the cut microscopically. Another cross sectioning tech-nique uses a scaled microscope to view a geometric incision through the dry-film coating. A special cutting tool is used to make a small, precise V-groove through the coating and into the substrate. Gages are available that come complete with cutting tips and illuminated scaled magnifier.

While the principles of this destructive method are easy to understand, there are opportunities for measuring error. It takes skill to prepare the sample and interpret the results. Adjusting the measurement reticule to a jagged or indistinct interface may create inaccuracy, particularly between different operators. This method is used when inexpensive, nondestructive methods are not possible, or as a way of confirm-ing nondestructive results. ASTM D 4138 outlines a standard method for this meas-urement system.

7.24.6 Gravimetric

By measuring the mass and area of the coating, thickness can be determined. The simplest method is to weigh the part before and after coating. Once the mass and area have been determined, the thickness is calculated using the following equation:

T = m x 10 A x d

where T is the thickness in micrometers, m is the mass of the coating in milligrams, A is the area tested in square centimeters, and d is the density in grams per cubic centimeter.


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It is difficult to relate the mass of the coating to thickness when the substrate is rough or the coating uneven. Laboratories are best equipped to handle this time-consuming and often destructive method.

Figure 7-58: Weight for measuring the mass of coating.

7.24.7 Thickness Measurements in Practice

As mentioned before, one of the most used methods to measure the thickness of a nonmagnetic dry coating on ferrous substrates is Magnetic Film Thickness Gages using the principle: Electromagnetic induction

The measurements on greater steel-constructions can be done by the so called “80/20 rule. This measurement method specifies the number of measurements and the acceptcriterias:

Figure 7-59: Electronic magnetic induction thickness gauge.


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80/20-rule (example):

1. On the construction areas of 10 m2 is selected (minimum 5%

of the surface has to be covered). Each selected area shall be

connected.

2. Minimum 5 fields of 50 cm2 are selected in each area.

3. In each field measure 3 thickness measurements. Calculate

the means of these 3 measurements and look at them as one

single measurement.

4. Accept criteria: Only 20% of the total number of single meas-

urements is allowed to be lower than the nominal dry coating

thickness, and the lowest value from a single measurement

shall be at least 80% of the nominal coating thickness.

7.24.8 Thickness Standards

Coating thickness gages are calibrated to known thickness standards. There are many sources of thickness standards but it is best to ensure they are traceable to a national measurement institute such as NIST (National Institute of Standards & Technology). It is also important to verify the standards are at least four times as accurate as the gage they will be used to calibrate. A regular check against these standards verifies the gage is operating properly. When readings do not meet the accuracy specification of the gage, the gage must be adjusted or repaired and then calibrated again.

Figure 7-60: Coating thickness calibration standard.

http://www.defelsko.com/standards.html


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Summary Film thickness in coatings can have a big impact on cost and quality. Measurement of film thickness should be a routine event for all coaters.

The measurements may e.g. be performed at random locations in a homogenous area where the coating thickness may be assumed to exhibit only a random varia-tion. These measurements may be used to determine the mean and standard devia-tion of the coating thickness. On the basis of these measurements it may be deter-mined whether the coating thickness complies with the required thickness.

The measurements may also be performed at a number of known “problem areas” such as corners, edges and inaccessible areas. Based on these measurements it may be decided to perform patch repairs of the coating.

The correct gage to use depends on the thickness range of the coating, the shape and type of substrate, the cost of the gage, and how critical it is to get an accurate measurement.

7.25 Dye penetrant This method can be used for steel structures.

7.25.1 Introduction and History of Penetrant Testing Dye penetrant inspection is a method that is used to reveal surface breaking flaws by bleed out of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a "blotter." It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultravio-let "black light". A very early surface inspection technique involved the rubbing of carbon black on glazed pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later it became the practice in railway workshops to examine iron and steel components by the "oil and whiting" method. In this method, a heavy oil com-monly available in railway workshops was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being stroked with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century through to approximately 1940, when the mag-netic particle method was introduced and found to be more sensitive for the ferromagnetic iron and steels.


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Figure 7-61: Bleed out of a colored dye from the defect.

A different (though related) method was introduced in the 1940's, where the surface under examination is coated with a lacquer, and after drying the surface is vibrated by hitting with a hammer, for example. This causes the brittle lacquer layer to crack generally around surface defects. The brittle lacquer (stress coat) has been used primarily to show the distribution of stresses in a part and not finding defects. Many of these early developments were carried out by Magnaflux in Chicago, IL, USA in association with the Switzer Bros., Cleveland, OH, USA. More affective penetrating oils containing highly visible (usually red) dyes were developed by Magnaflux to en-hance flaw detection capability. This method, known as the visible or colour contrast dye penetrant method, is still used quite extensively today. In 1942, Magnaflux in-troduced the Zyglo system of penetrant inspection where fluorescent dyes were added to the liquid penetrant. These dyes would then fluoresce when exposed to ultraviolet light (sometimes referred to as "black light") rendering indications from cracks and other surface flaws more readily visible to the inspectors' eyes.

Figure 7-62: The Zyglo system.


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7.25.2 Improving Detection

Figure 7-63: Crack indication.

The advantage that a dye penetrant inspection offers over an unaided visual inspec-tion is that it makes defects easier to see for the inspector. There are basically two ways that a penetrant inspection process makes flaws more easily seen. First, LPI produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself. Many flaws are so small or narrow that they are undetectable by the unaided eye. Due to the physical features of the eye, there is a threshold below which objects cannot be resolved. This threshold of visual acuity is around 0.003 inch for a person with 20/20 vision. The second way that LPI improves the detect ability of a flaw is that it produces a flaw indication with a high level of contrast between the indication and the back-ground which also helps to make the indication more easily seen. When a visible dye penetrant inspection is performed, the penetrant materials are formulated using a bright red dye that provides for a high level of contrast between the white developer that serves as a background as well as to pull the trapped penetrant from the flaw. When a fluorescent penetrant inspection is performed, the penetrant materials are formulated to glow brightly and to give off light at a wavelength that the eye is most sensitive to under dim lighting conditions.

.

Figure 7-64: Example of different contrasts.


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7.25.3 Basic Processing of a Dye Penetrant Testing

1. Surface Preparation: One of the most critical steps of a dye penetrant in-spection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects.

Figure 7-65: Example of pre-cleaning a part with high-pressure steam.

2. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the parts in a penetrant bath.

Figure 7-66: Penetrant application.

3. Penetrant Dwell: The penetrant is left on the surface for sufficient time to allow as much penetrant as possible to be drawn from or to seep into a de-


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fect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the pene-trant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being in-spected. Minimum dwell times typically range from 5 to 60 minutes. Gener-ally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and is often very specific to a particular application.

4. Excess Penetrant Removal: This is a most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water.

Figure 7-67: Excess Penetrant Removal.

5. Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).


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Figure 7-68: Developer.

6. Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a mini-mum of 10 minutes and significantly longer times may be necessary for tight cracks.

7. Inspection: Inspection is then performed under appropriate lighting to de-tect indications from any flaws which may be present.

8. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be ac-ceptable.

7.25.4 Common Uses of Dye Penetrant Inspection

Dye penetrant inspection is one of the most widely used nonde-structive evaluation (NDE) methods. Its popularity can be attrib-uted to two main factors, which are its relative ease of use and its flexibility. LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous. Mate-rials that are commonly inspected using LPI include the follow-ing:

• Metals (aluminum, copper, steel, titanium, etc.)

• Glass

• Many ceramic materials

• Rubber

• Plastics

LPI offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft compo-nents. Penetrant material can be applied with a spray can or a cotton swab to inspect for flaws known to occur in a specific area or it can be applied by dipping or spraying to quickly inspect large areas. The picture to the right above shows, visible dye penetrant being locally applied to a highly loaded connecting point to check for fatigue cracking.


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Penetrant inspection systems have been developed to inspect some very large com-ponents. In this picture, DC-10 banjo fittings are being moved into a penetrant in-spection system at what used to be the Douglas Aircraft Company's Long Beach, California facility. These large machined aluminum forgings are used to support the number 3 engine in the tail of a DC-10 aircraft.

Dye penetrant inspection is used to inspect for flaws that breaks the surface of the sample. Some of these flaws are listed below:

• Cracks

• Overload and impact fractures

• Porosity

• Laps

• Seams

• Pin holes in welds

• Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that flaws must be open to the surface.

7.25.5 Advantages and Disadvantages of Dye Penetrant Testing

Like all nondestructive inspection methods, dye penetrant inspection has both ad-vantages and disadvantages. The primary advantages and disadvantages when com-pared to other NDE methods are summarized below.


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Primary Advantages

• The method has high sensitive to small surface discontinuities.

• The method has few material limitations, i.e. metallic and nonmetallic, mag-netic and nonmagnetic, and conductive and nonconductive materials may be inspected.

• Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.

• Parts with complex geometric shapes are routinely inspected.

• Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.

• Aerosol spray cans make penetrant materials very portable.

• Penetrant materials and associated equipment are relatively inexpensive.

Primary Disadvantages

• Only surface breaking defects can be detected.

• Only materials with a relative nonporous surface can be inspected.

• Precleaning is critical as contaminants can mask defects.

• Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI.

• The inspector must have direct access to the surface being inspected.

• Surface finish and roughness can affect inspection sensitivity.

• Multiple process operations must be performed and controlled.

• Post cleaning of acceptable parts or materials is required.

• Chemical handling and proper disposal is required.


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7.25.6 Dye Penetrant Testing Materials

The penetrant materials used today are much more sophisticated than the kerosene and whiting first used by railroad inspectors near the turn of the 20th century. To-day's penetrants are carefully formulated to produce the level of sensitivity desired by the inspector. To perform well, a penetrant must possess a number of important characteristics. A penetrant must

• spread easily over the surface of the material being inspected to provide complete and even coverage.

• be drawn into surface breaking defects by capillary action.

• remain in the defect but remove easily from the surface of the part.

• remain fluid so it can be drawn back to the surface of the part through the drying and developing steps.

• be highly visible or fluoresce brightly to produce easy to see indications.

• must not be harmful to the material being tested or the inspector.

All penetrant materials do not perform the same and are not designed to perform the same. Penetrant manufactures have developed different formulations to address a variety of inspection applications. Some applications call for the detection of the smallest defects possible and have smooth surface where the penetrant is easy to remove. In other applications the rejectable defect size may be larger and a pene-trant formulated to find larger flaws can be used. The penetrants that are used to detect the smallest defect will also produce the largest amount of irrelevant indica-tions.

Figure 7-69: Example of capillary action.


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Penetrant materials are classified in the various industry and government specifica-tions by their physical characteristics and their performance. Aerospace Material Specification (AMS) 2644, Inspection Material, Penetrant, is now the primary specifi-cation used in the USA to control penetrant materials. Historically, Military Standard 25135, Inspection Materials, Penetrants, has been the primary document for specify-ing penetrants but this document is slowly being phased out and replaced by AMS 2644. Other specifications such as ASTM 1417, Standard Practice for Dye Penetrant Examinations, may also contain information on the classification of penetrant mate-rials but they are generally referred back to MIL-I-25135 or AMS 2644.

Penetrant materials come in two basic types. These types are listed below:

• Type 1 - Fluorescent Penetrants • Type 2 - Visible Penetrants

Fluorescent penetrants contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation. Visible penetrants contain a red dye that provides high contrast against the white developer background. Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication. However, visible penetrants do not require a darkened area and an ultraviolet light in order to make an inspection. Visible penetrants are also less vulnerable to contamination from things such as cleaning fluid that can signifi-cantly reduce the strength of a fluorescent indication.

Figure 7-70: Inspection under ultraviolet light.

Penetrants are then classified by the method used to remove the excess penetrant from the part. The four methods are listed below:

• Method A - Water Washable

• Method B - Post Emulsifiable, Lipophilic

• Method C - Solvent Removable

• Method D - Post Emulsifiable, Hydrophilic


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Water washable (Method A) penetrants can be removed from the part by rinsing with water alone. These penetrants contain some emulsifying agent (detergent) that makes it possible to wash the penetrant from the part surface with water alone. Wa-ter washable penetrants are sometimes referred to as self-emulsifying systems. Post emulsifiable penetrants come in two varieties, lipophilic and hydrophilic. In post emulsifiers, lipophilic systems (Method B), the penetrant is oil soluble and interacts with the oil-based emulsifier to make removal possible. Post emulsifiable, hydrophilic systems (Method D), use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash. Solvent remov-able penetrants (Method C) require the use of a solvent to remove the penetrant from the part.

Penetrants are then classified based on the strength or detectability of the indication that is produced for a number of very small and tight fatigue cracks. The five sensi-tivity levels are shown below:

• Level ½ - Ultra Low Sensitivity

• Level 1 - Low Sensitivity

• Level 2 - Medium Sensitivity

• Level 3 - High Sensitivity

• Level 4 - Ultra-High Sensitivity

The major US government and industry specifications currently rely on the US Air Force Materials Laboratory at Wright-Patterson Air Force Base to classify penetrants into one of the five sensitivity levels. This procedure uses titanium and Inconel specimens with small surface cracks produced in low cycle fatigue bending to classify penetrant systems. The brightness of the indication produced is measured using a photometer. The sensitivity levels and the test procedure used can be found in Mili-tary Specification MIL-I-25135 and Aerospace Material Specification 2644, Penetrant Inspection Materials.

An interesting note about the sensitivity levels is that only four levels were originally planned but when some penetrants were judged to have sensitivities significantly less than most others in the level 1 category, the ½ level was created.

7.25.7 Penetrants The industry and military specification that control the penetrant materials and their use all stipulate certain physical properties of the penetrant materials that must be met. Some of these requirements address the safe use of the materials, such as tox-icity, flash point, and corrosiveness, and other requirements address storage and contamination issues. Still others delineate properties that are thought to be primar-ily responsible for the performance or sensitivity of the penetrants. The properties of


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penetrant materials that are controlled by AMS 2644 and MIL-I-25135E include flash point, surface wetting capability, viscosity, contact angle, color, brightness, ultravio-let stability, thermal stability, water tolerance, and removability. How some of these properties can affect penetrant testing are described next.

Some properties of a penetrant Capillary Action Capillary action is the tendency of certain liquids to travel or climb when exposed to small openings. In nature there are many examples of capillary action. Plants and trees have a network similar to capillary tubes that draw water upward supplying nourishment. The earth brings water to the surface through the capillary action of the earth's exterior. Dye Penetrant Inspection Capillary action is the phenomena that makes dye penetrant inspection possible. All of the steps that are taken in the process of conducting a penetrant test, from pre-cleaning through the actual evaluation of the results, is done to promote capillary action. Precleaning When a part is precleaned, everything is removed that will prevent the penetrant from entering discontinuities and therefore, interfere with capillary action. Once the surface is clean and dry, the penetrant is applied. The penetrant is then drawn into the discontinuities through capillary action, see figure below.

Figure 7-71: Penetrant entering a discontinuity through capillary action.

If the part is not clean and dry, less penetrant and possibly none, will be drawn into the discontinuities. Discontinuities that would have been revealed may be over-looked.


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Applying Developer Following the removal of the excess penetrant, a developer is applied. The developer induces reverse capillary action to take place. Penetrant is drawn from the disconti-nuities into the developer in the same way that the fibers of a paper towel absorb or blot a liquid, see figure below.

Figure 7-72: Blotting action of developer draws penetrant from discontinuity.

Surface Tension There are many factors in capillary action; among these are surface tension, cohe-sion, wetting ability, adhesion and contact angle. Each of these factors has a strong influence in the performance of capillary action. Of these, surface tension is one of the two most important factors. Water in a pond exhibits surface tension when it supports the weight of an insect - a spider or mosquito for example. The insect is supported by a molecular membrane created by the attraction (cohesiveness) of one water molecule to another. Each water molecule is attracted laterally and vertically (above and below) to adjacent molecules. The molecules on the surface are attracted only laterally and below because of the absence of molecules above them. This change in attraction between surface molecules creates the effect of a stretched membrane on the surface of the water strong enough to support small objects. Wa-ter has high surface tension because of the strong cohesive attraction between the molecules of water. The amount of surface tension will vary between different liquids depending upon how cohesive the molecules are. Wetting Ability and Contact Angle The second most important factor in capillary action is wetting ability. How well a liquid wets the surface of a specimen is referred to as its wetting ability. The wetting ability of a liquid is determined by the contact angle produced when a liquid meets a surface. The cohesive force that determines surface tension competes with the adhe-sive properties of a liquid producing a specific degree of contact angle. Adhesion


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Adhesion is how strongly the molecules of a liquid are attracted to a particular sur-face. If a capillary tube is placed in a beaker of water, the water will rise in the tube to a level higher than the water surrounding the tube. The water climbs in the tube because the molecules of water are more strongly attracted to the inside surface of the tube than they are to each other. The stronger the attraction between the mole-cules of a liquid and a surface, the smaller will be the contact angle and the higher a liquid will rise in a capillary tube. Adhesive Properties of A Liquid When the adhesive properties of a liquid are stronger than the cohesive properties the contact angle will be less than 90° and the liquid will rise in a capillary tube. With a contact angle of less than 90°, the liquid is said to have good wetting ability (Figure 7-73 A). The smaller the contact angle, the more the liquid will wet the sur-

face and exhibit greater capillary rise. If the contact angle of a liquid is 90 °, it will have poor wetting ability and will not rise in a capillary tube (Figure 7-73 B).


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Droplet

Capillary tube

Liquid

A. Good wetting ability is obtained when the

contact angle is less than 90° A. A contact angle less than 90° will cause

capillary rise

Capillary tube

Liquid

Droplet

B. Poor wetting ability is obtained when the contact angle is 90°

B. A contact angle of 90° provides no capil-lary rise or depression

Capillary tube

Liquid

Droplet

C. No wetting ability is obtained when the contact angle is greater than 90°

C. A contact angle greater than 90° will cause capillary depression

Figure 7-73: Relationship of contact angle to wetting ability.

Cohesive Properties of A Liquid When the cohesive properties of a liquid are stronger than the adhesive properties the contact angle will be greater than 90° (Figure 7-73 C) and the liquid will de-

scend in a capillary tube. Mercury, as an example, has a large contact angle and will descend in a capillary tube to a level below that of the surrounding liquid. The mer-cury will not wet the surface because of the large contact angle.


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Penetrant Contact Angle Penetrant materials must have a very small contact angle in order to make a good penetrant. Some have contact angles close to 0°. Water by itself does not make a good penetrant material, because in addition to having high surface tension, it also has a large contact angle when compared to the oil base that is used in most pene-trants. If a wetting agent is added to water, reducing surface tension and the contact angle, it becomes a good penetrant base material that is used in some test applica-tions. Surface Condition Surface condition of the material - roughness, cleanliness, etc., will have an effect on the size of the contact angle. It will change the adhesive properties of the liquid and as a result wetting ability. A surface layer of film will lower adhesion, increase the contact angle, and reduce wetting ability. Size of Opening Another consideration in capillary action is the size of the opening. The narrower the opening, the stronger the capillary action. A liquid with good wetting ability will rise further in a capillary tube that has a small diameter than it will in a tube that has a large diameter (Figure 7-74 A). If a liquid exhibits capillary depression (lack of wet-

ting ability) it will be depressed further in a small tube than a large tube (Figure 7-74, below).

(A) Capillary Rise (B) Capillary Depression

Figure 7-74: Capillary action in different size openings.

Color and Fluorescent Brightness

The color of the penetrant material is of obvious importance in visible dye penetrant inspection, as the dye must provide good contrast against the developer or part be-ing inspected. Remember from our earlier discussion of contrast sensitivity that gen-erally the higher the contrast, the easier objects are to see. The dye used in visible dye penetrant is usually vibrant red but other colors can be purchased for special applications.


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Figure 7-75: Flaw filled with red visible penetrant.

When fluorescent materials are involved, the effect of color and fluorescence is not so straightforward. LPI materials fluoresce because they contain one or more dyes that absorb electromagnetic radiation over a particular wavelength and the absorp-tion of photons leads to changes in the electronic configuration of the molecules. Since the molecules are not stable at this higher energy state, they almost immedi-ately re-emit the energy. There is some energy loss in the process causing the pho-tons to be re-emitted at a slightly longer wavelength, which is in the visible range. The radiation absorption and emission could take place a number of times until the desired color and brightness is achieved. Two different fluorescent colors can be mixed to interact by a mechanism called cascading. The emission of visible light by this process involves one dye absorbing ultraviolet radiation to emit a band of radia-tion that makes a second dye glow. Since the human eye is the most commonly used sensing device, most penetrants are designed to fluoresce as close as possible to the eyes' peak response.

Penetrant Brightness

Fluorescent brightness was erroneously once thought to be the controlling factor with respect to flaw detection sensitivity. Measurements have been made to evaluate the intrinsic brightness of virtually all commercially available penetrants and they all have about the same brightness. Intrinsic brightness values are determined for thick liquid films and the dimensional threshold of fluorescence is a more important prop-erty. The measurement of fluorescent brightness is detailed in ASTM E-1135, "Stan-dard Test Method for Comparing the Brightness of Fluorescent Penetrants."


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Figure 7-76: Fluorescent brightness.

7.25.8 Emulsifiers

When removal of the penetrant from the defect due to over-washing of the part is a concern, a post emulsifiable penetrant system can be used. Post emulsifiable pene-trants require a separate emulsifier to break the penetrant down and make it water washable. Most penetrant inspection specifications classify penetrant systems into four methods of excess penetrant removal. These are listed below:

1. Method A: Water-Washable

2. Method B: Post Emulsifiable, Lipophilic

3. Method C: Solvent Removable

4. Method D: Post Emulsifiable, Hydrophilic

Method C relies on a solvent cleaner to remove the penetrant from the part being inspected. Method A has emulsifiers built into the penetrant liquid that makes it pos-sible to remove the excess penetrant with a simple water wash. Method B and D penetrants require an additional processing step where a separate emulsification agent is applied to make the excess penetrant more removable with a water wash. Lipophilic emulsification systems are oil-based materials that are supplied in ready-to-use form. Hydrophilic systems are water-based and supplied as a concentrate that must be diluted with water prior to use.

Lipophilic emulsifiers (Method B) were introduced in the late 1950's and work with both a chemical and mechanical action. After the emulsifier has coated the surface of the object, mechanical action starts to remove some of the excess penetrant as the mixture drains from the part. During the emulsification time, the emulsifier diffuses into the remaining penetrant and the resulting mixture is easily removed with a wa-ter spray.

Hydrophilic emulsifiers (Method D) also remove the excess penetrant with mechani-cal and chemical action but the action is different because no diffusion takes place. Hydrophilic emulsifiers are basically detergents that contain solvents and surfactants. The hydrophilic emulsifier breaks up the penetrant into small quantities and prevents


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these pieces from recombining or reattaching to the surface of the part. The me-chanical action of the rinse water removes the displaced penetrant from the part and causes fresh remover to contact and lift newly exposed penetrant from the surface.

The hydrophilic post emulsifiable method (Method D) was introduced in the mid 1970's and since it is more sensitive than the lipophilic post emulsifiable method it has made the later method virtually obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time. While emulsification time should be controlled as closely as possible, a varia-tion of one minute or more in the contact time will have little effect on flaw detect-ability when a hydrophilic emulsifier is used. However, a variation of as little as 15 to 30 seconds can have a significant effect when a lipophilic system is used.

Figure 7-77: Emulsifier.

7.25.9 Developers

The role of the developer is to pull the trapped penetrant material out of defects and to spread the developer out on the surface of the part so it can be seen by an in-spector. The fine developer particles both reflect and refract the incident ultraviolet light, allowing more of it to interact with the penetrant, causing more efficient fluo-rescence. The developer also allows more light to be emitted through the same mechanism. This is why indications are brighter than the penetrant itself under UV light. Another function that some developers performs is to create a white back-ground so there is a greater degree of contrast between the indication and the sur-rounding background.


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Developer Forms

The AMS 2644 and Mil-I-25135 classify developers into six standard forms. These forms are listed below:

1. Form a - Dry Powder

2. Form b - Water Soluble

3. Form c - Water Suspendible

4. Form d - Nonaqueous Type 1 Fluorescent (Solvent Based)

5. Form e - Nonaqueous Type 2 Visible Dye (Solvent Based)

6. Form f - Special Applications

The developer classifications are based on the method that the developer is applied. The developer can be applied as a dry powder, or dissolved or suspended in a liquid carrier. Each of the developer forms has advantages and disadvantages.

Dry Powder Dry powder developer is generally considered to be the least sensitive but it is inexpensive to use and easy to apply. Dry developers are white, fluffy powders that can be applied to a thoroughly dry surface in a number of ways. The developer can be applied by dipping parts in a container of de-veloper, or by using a puffer to dust parts with the developer. Parts can also be placed in a dust cabinet where the developer is blown around and allowed to settle on the part. Electrostatic powder spray guns are also available to apply the devel-oper. The goal is to allow the developer to come in contact with the whole inspection area.

Unless the part is electrostatically charged, the powder will only adhere to areas where trapped penetrant has wet the surface of the part. The penetrant will try to wet the surface of the penetrant particle and fill the voids between the particles, which brings more penetrant to the surface of the part where it can be seen. Since dry powder developers only stick to the part where penetrant is present, the dry developer does not provide a uniform white background as the other forms of developers do. Having a uniform light background is very im-portant for a visible inspection to be effective and since dry developers do not pro-vide one, they are seldom used for visible inspections. When a dry developer is used,


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indications tend to stay bright and sharp since the penetrant has a limited amount of room to spread.

Water Soluble As the name implies, water soluble devel-opers consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away. The best method for applying water soluble developers is by spraying it on the part. The part can be wet or dry. Dipping, pouring, or brushing the solution on to the surface is sometimes used but these methods are less desirable. Aqueous developers contain wetting agents that cause the solution to function much like dilute hydrophilic emulsifier and can lead to additional removal of entrapped penetrant. Drying is achieved by placing the wet but well drained part in a recalculating warm air dryer with the temperature held be-tween 70 and 75°F. If the parts are not dried quickly, the indications will will be blurred and indistinct. Properly developed parts will have an even, pale white coating over the entire surface. Water Suspendible

Water suspendible developers consist of insoluble developer particles suspended in water. Water suspendible developers require frequent stirring or agitation to keep the particles from settling out of suspension. Water suspendible developers are ap-plied to parts in the same manner as water soluble developers. Parts coated with a water suspendible developer must be forced dried just as parts coated with a water soluble developer are forced dried. The surface of a part coated with a water sus-pendible developer will have a slightly translucent white coating.

Nonaqueous Nonaqueous developers suspend the developer in a volatile solvent and are typically applied with a spray gun. Nonaque-ous developers are commonly distributed in aerosol spray cans for portability. The solvent tends to pull penetrant from the indications by solvent action. Since the solvent is highly volatile, forced drying is not required. A nonaqueous devel-oper should be applied to a thoroughly dried part to form a slightly translucent white coating.


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Special Applications

Plastic or lacquer developers are a special developers that are primarily used when a permanent record of the inspection is required.

7.25.10 Preparation of Part One of the most critical steps in the penetrant inspection process is preparing the part for inspection. All coatings, such as paints, varnishes, plat-ing, and heavy oxides must be removed to ensure that defects are open the surface of the part. If the parts have been ma-chined, sanded, or blasted prior to the penetrant inspection, it is possible that a thin layer of metal may have smeared across the surface and closed off defects. It is even possible for metal smearing to occur as a result of cleaning operations such as grit or vapor blasting. This layer of metal smearing must be removed before inspection. Contaminants Coatings, such as paint, are much more elastic than metal and will not fracture even though a large defect may be present just below the coating. The part must be thoroughly cleaned as sur-face contaminates can prevent the penetrant from entering a de-fect. Surface contaminants can also lead to a higher level of background noise since the excess penetrant may be more diffi-cult to remove. Common coatings and contaminates that must be removed include: paint, dirt, flux, scale, varnish, oil, etchant, smut, plating, grease, oxide, wax, decals, machining fluid, rust, and residue from previous penetrant inspections. Some of these contaminants would obviously prevent penetrant from entering de-fects and it is, therefore, clear that they must be removed. However, the impact of other contaminants such as the residue from previous penetrant inspections is less clear, but they can have a disastrous affect on the inspection. Take the link below to review some of the research that has been done to evaluate the effects of contami-nants on LPI sensitivity. A good cleaning procedure will remove all contamination from the part and not leave any residue that may interfere with the inspection process. It has been found that some alkaline cleaners can be detrimental to the penetrant inspection process if they have silicates in concentrations above 0.5 percent. Sodium metasilicate, sodium sili-cate, and related compounds can adhere to the surface of parts and form a coating that prevents penetrant entry into cracks. Researchers in Russia have also found that some domestic soaps and commercial detergents can clog flaw cavities and re-duce the wettability of the metal surface, thus, reducing the sensitivity of the pene-


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trant. Conrad and Caudill found that media from plastic media blasting was partially responsible for loss of LPI indication strength. Microphotographs of cracks after plas-tic media blasting showed media entrapment in addition to metal smearing.

Figure 7-78: A good cleaning procedure is important.

It is very important that the material being inspected has not been smeared across its own surface during machining or cleaning operations. It is well recognized that machining, honing, lapping, hand sanding, hand scraping, shot peening, grit blast-ing, tumble deburring, and peening operations can cause a small amount of the ma-terial to smear on the surface of some materials. It is perhaps less recognized that some cleaning operations, such as steam cleaning, can also cause metal smearing in the softer materials. Take the link below to learn more about metal smearing and its affects on LPI.

7.25.11 Selection of a Penetrant Technique The selection of a dye penetrant system is not a straightforward task. There are a variety of penetrant systems and developer types that are available for use, and one set of penetrant materials will not work for all applications. Many factors must be considered when selecting the penetrant materials for a particular application. These factors include the sensitivity required, materials cost, number of parts and size of area requiring inspection, and portability.


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Figure 7-79: Penetrant System.

When sensitivity is the primary consideration for choosing a penetrant system, the first decision that must be made is whether to use fluorescent dye penetrant, or visi-ble dye penetrant. Fluorescent penetrants are generally more capable of producing a detectable indication from a small defect because the human eye is more sensitive to a light indication on a dark background and the eye is naturally drawn to a fluores-cent indication. The graph below presents a series of curves that show the contrast ratio required for a spot of a certain diameter to be seen.

The curves in Figure 7-80 show that for indications spots larger than 0.076 mm

(0.003 inch) in diameter, it does not really matter if it is a dark spot on a light back-ground or a light spot on a dark background. However, when a dark indication on a light background is further reduced in size, it is no longer detectable even though contrast is increased. Furthermore, with a light indication on a dark background, indications down to 0.003 mm (0.0001 inch) were detectable when the contrast be-tween the flaw and the background was high enough.

From this data, it can be seen why a fluorescent penetrant offers an advantage over visible penetrant for finding very small defects. Data presented by De Graaf and De Rijk supports this statement. They inspected "Identical" fatigue cracked specimens using a red dye penetrant and a fluorescent dye penetrant. The fluorescent pene-trant found 60 defects while the visible dye was only able to find 39 of the defects.


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Figure 7-80: Contrast ratio, Brightness of spot / brightness of background

versus spot diameter.

Under certain conditions, the visible penetrant may be a better choice. When fairly large defects are the subject of the inspection, a high sensitivity system may not be warranted and may result in a large number of irrelevant indications. Visible dye penetrants have also been found to give better results when surface roughness is high or when flaws are located in areas such as weldments.

Since visible dye penetrants do not require a darkened area for the use of an ultra-violet light, visible systems are more easy to use in the field. Solvent removable penetrants, when properly applied can have the highest sensitivity and are very con-venient to use but are usually not practical for large area inspection or in high-volume production settings.

Another consideration in the selection of a penetrant system is whether water wash-able, post-emulsifiable or solvent removable penetrants will be used. Post-emulsifia-ble systems are designed to reduce the possibility of over-washing, which is one of the factors known to reduce sensitivity. However, these systems add another step, and thus cost, to the inspection process.

Penetrants are evaluated by the US Air Force according to the requirements in MIL-I-25135 and each penetrant system is classified into one of five sensitivity levels. This procedure uses titanium and Inconel specimens with small surface cracks produced


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in low cycle fatigue bending to classify penetrant systems. The brightness of the in-dications produced after processing a set of specimens with a particular penetrant system is measured using a photometer. A procedure for producing and evaluating the penetrant qualification specimens was reported on by Moore and Larson at the 1997 ASNT Fall Conference. Most commercially available penetrant materials are listed in the Qualified Products List of MIL-I-25135 according to their type, method and sensitivity level. Visible dye and dual-purpose penetrants are not classified into sensitivity levels as fluorescent penetrants are. The sensitivity of a visible dye pene-trant is regarded as level 1 and largely dependent on obtaining good contrast be-tween the indication and the background.

7.25.12 Penetrant Application and Dwell Time

The penetrant material can be applied in a number of different ways which include spraying, brushing, or immersing the parts in a penetrant bath. The method of pene-trant application has little effect on the inspection sensitivity but an electrostatic spraying method is reported to produce slightly better results than other methods. Once the part is covered in penetrant it must be allowed to dwell so the penetrant has time to enter any defect present.

Figure 7-81: Drain-dwell.

There are basically two dwell mode options, immersion-dwell (keeping the part im-mersed in the penetrant during the dwell period) or drain-dwell (letting the part drain during the dwell period). Prior to a study by Sherwin, the immersion-dwell mode was generally considered to be more sensitive but recognized to be less eco-nomical because more penetrant was washed away and emulsifiers were contami-nated more rapidly. The reasoning for thinking this method was more sensitive was that the penetrant was more migratory and more likely to fill flaws when kept com-


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pletely fluid and not allowed to loose volatile constituents by evaporation. However, Sherwin showed that if the specimens are allowed to drain-dwell, the sensitivity is higher because the evaporation increases the dyestuff concentration of the penetrant on the specimen. As pointed-out earlier, sensitivity increases as the dyestuff concen-tration increases. Sherwin also cautions that the samples being inspected should be placed outside the penetrant tank wall so that vapors from the tank do not accumu-late and dilute the dyestuff concentration of the penetrant on the specimen.

Figure 7-82: Immersion-dwell.

Penetrant Dwell Time Penetrant dwell time is the total time that the penetrant is in contact with the part surface. The dwell time is important because it allows the penetrant the time neces-sary to be drawn or to seep into a defect. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The time required to fill a flaw depends on a number of variables which include the following:

• The surface tension of the penetrant.

• The contact angle of the penetrant.

• The dynamic shear viscosity of the penetrant, which can vary with the di-ameter of the capillary. The viscosity of a penetrant in microcapillary flaws is higher than its viscosity in bulk, which slows the infiltration of the tight flaws.

• The atmospheric pressure at the flaw opening.

• The capillary pressure at the flaw opening.

• The pressure of the gas trapped in the flaw by the penetrant.

• The radius of the flaw or the distance between the flaw walls.


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• The density or specific gravity of the penetrant.

• Microstructural properties of the penetrant.

The ideal dwell time is often determined by experimentation and is often very spe-cific to a particular application. For example, AMS 2647A requires that the dwell time for all aircraft and engine be at least 20 minutes while the ASTM E1209 only requires a 5 minute dwell time for parts made of titanium and other heat resistant alloys. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry.

7.25.13 Penetrant Removal Process

The penetrant removal procedure must effectively remove the penetrant from the surface of the part without removing an appreciable amount of entrapped penetrant from the defect. If the removal process extracts penetrant from the flaw, the flaw indication will be reduced by a proportional amount. If the penetrant is not effec-tively removed from the part surface, the contrast between the indication and the background will be reduced. As discussed in Contrast Sensitivity Section, as the con-trast increases so does visibility of the indication.

Figure 7-83: Penetrant removed by a waterspray.


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Removal Method Penetrant systems are classified into four methods of excess penetrant removal. These include the following:

1. Method A: Water-Washable

2. Method B: Post Emulsifiable, Lipophilic

3. Method C: Solvent Removable

4. Method D: Post Emulsifiable, Hydrophilic

Method C, Solvent Removable, is used primarily for inspecting small localized areas as this method requires hand wiping the surface with a cloth moistened with the sol-vent remover, and this process is too labor intensive for most production situations. Of the three production penetrant inspection methods, Method A, Water-Washable, is the most economical to apply. Water-washable or self-emulsifiable penetrants con-tain an emulsifier as an integral part of the formulation. The excess penetrant may be removed from the object surface, with a simple water rinse. These materials have the property of forming relatively viscous gels upon contact with water, which results in the formation of gel-like plugs in surface openings. While they are completely soluble in water, given enough contact time, the plugs offer a brief period of protec-tion against rapid wash removal. Thus, water-washable penetrant systems provide ease of use and a high level of sensitivity.

When removal of the penetrant from the defect due to over-washing of the part is a concern, a post-emulsifiable penetrant system can be used. Post-emulsifiable pene-trants require a separate emulsifier to break the penetrant down and make it water washable. The emulsifier is usually applied by dipping the object. Hydrophilic emulsi-fiers may also be sprayed on the object but spraying is not recommended for lipo-philic emulsifiers because it can result in non-uniform emulsification if not properly applied. Brushing the emulsifier on to the part is not recommended because the bris-tles of the brush may force emulsifier into discontinuities causing the entrapped penetrant to be removed. The emulsifier is allowed sufficient time to react with the penetrant on the surface of the part but not given time to make its way into defects to react with the trapped penetrant. The penetrant that has reacted with the emulsi-fier is easily cleaned away. Controlling the reaction time is of essential importance when using a post-emulsifiable system. If the emulsification time is too short, an excessive amount of penetrant will be left on the surface leading to high background levels. If the emulsification time is too long, the emulsifier will react with the pene-trant entrapped in discontinuities making it possible to deplete the amount needed to form an indication.


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Figure 7-84: Part is being moved in the emulsifier.

The hydrophilic post emulsifiable method (Method D) is more sensitive than the lipo-philic post emulsifiable method (Method B). Since these methods are generally only used when very high sensitivity is needed, Method D is almost always used making Method B virtually obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time. While emulsifica-tion time should be controlled as closely as possible, a variation of one minute or more in the contact time will have little effect on flaw detectability when a hydro-philic emulsifier is used, but a variation of as little as 15 to 30 seconds can have a significant effect when a lipophilic system is used. Using an emulsifier involves add-ing a couple of steps to the penetrant process and ,therefore, slightly increases the cost of an inspection. When using an emulsifier, the penetrant process includes the following steps (extra steps in bold): 1. pre-clean part, 2. apply penetrant and allow to dwell, 3. pre-rinse to remove first layer of penetrant, 4. apply hydrophilic emulsifier,and allow contact for specified time, 5. rinse to remove excess pene-trant, 6. dry part, 7. apply developer and allow part to develop, and 8. inspect. (see some of the steps below).

Penetrant

Figure 7-85: To the left: Part with penetrant. To the right: Part with penetrant

and emulsifier.


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Emulgeret vandafvaskelig penetrant

Vandafskylning

Figure 7-86: To the left: The emulsifier has emulsified the pene-trant on the

parts surface, but not the penetrant in the discontinuitie. To the

right: Excess penetrant and emulsifier on the parts surface are

being removed with water. Rinse Method and Time for Water-Washable Penetrants The method used to rinsing the excess penetrant from the object surface and the time of the rinse should be controlled so as to prevent over-washing. It is generally recommended that a coarse spray rinse or an air agitated, immersion wash tank be used. When a spray is being used, it should be directed at a 45° angle to the part surface so as to not force water directly into any discontinuities that may be present. The spray or immersion time should be kept to a minimum through frequent inspec-tions of the remaining background level.

Hand Wiping of Solvent Removable Penetrants When a solvent removable penetrant is used, care must also be taken to carefully remove the penetrant from the part surface while removing as little as possible from the flaw. The first step in this cleaning procedure is to dry wipe the surface of the part in one direction using a white lint free cotton rag. One dry pass in one direction is all that should be used to remove as much penetrant as possible. Next, the surface should be wiped with one pass in one direction with a cleaner moistened rag. One dry pass followed by one damp pass is all that is recommended. Additional wiping may sometimes be necessary but keep in mind that with every additional wipe, some of the entrapped

Emulsified water-washable penetrant

Water-Washing


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penetrant will be removed and inspection sensitivity will be reduced.

To study the effects of the wiping process, Japanese researchers manufactured a test specimen out of acrylic plates that allowed them to view the movement of the penetrant in a narrow cavity. The sample consisted of two pieces of acrylic with two thin sheets of vinyl clamped between as spaces. The plates were clamped in the corners and all but one of the edges sealed. The unsealed edge acted as the flaw. The clear-ance between the plates varied from 15 microns (0.059 inch) at the clamping points to 30 microns (0.118 inch) at the midpoint between the clamps. The distance between the clamping points is be-lieved to be 30 mm (1.18 inch).

Although the size of the flaw represented by this specimen is large, an interesting observation was made. They found that when the surface of the specimen was wiped with a dry cloth, penetrant was blotted and removed from the flaw at the corner ar-eas were the clearance between the plate was least. When the penetrant at the side areas was removed, penetrant moved horizontally from the center area to the ends of the simulated crack where capillary forces are stronger. Therefore, across the crack length, the penetrant surface has a parabola-like shape where the liquid is at the surface in the corners but depressed in the center. This shows that each time the cleaning cloth touches the edge of a crack, penetrant is lost from the defect. This also explains why the bleedout of an indication is often largest at the corners of cracks.

7.25.14 Use and Selection of a Developer

The use of developer is almost always recommended. One study reported that the output from a fluorescent penetrant could be multiplied by up to seven times when a suitable powder developer was used. Another study showed that the use of devel-oper can have a dramatic effect on the probability of detection (POD) of an inspec-tion. When a Haynes Alloy 188, flat panel specimen with a low-cycle fatigue crack was inspected without a developer, a 90 % POD was never reached with crack lengths as long as 19 mm (0.75 inch). The operator detected only 86 of 284 cracks and had 70 false-calls. When a developer was used, a 90 % POD was reached at 2 mm (0.077 inch), with the inspector identifying 277 of 311 cracks with no false-calls. However some authors have reported that in special situations the use of a devel-oper may actually reduce sensitivity. These situations primarily occur when large, well defined defects are being inspected on a surface that contains many nonrelevant indications that cause excessive bleedout.


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Figure 7-87: Nonaqueous Wet Solvent Developer.

Type of Developer Used and Method of Application Nonaqueous developers are generally recognized as the most sensitive when prop-erly applied. There is less agreement on the performance of dry and aqueous wet developers but the aqueous developers are usually considered more sensitive. Aque-ous wet developers form a finer matrix of particles that is more in contact with the part surface. However, if the thickness of the coating becomes too great, defects can be masked. Also aqueous wet developers can cause leaching and blurring of indica-tions when used with water washable penetrants. The relative sensitivities of devel-opers and application techniques as ranked in Volume II of the Nondestructive Test-ing Handbook are shown in the table below. There is general industry agreement with this table, but some industry experts feel that water suspendible developers are more sensitive than water-soluble developers. Sensitivity ranking of developers per the Nondestructive Testing Handbook.


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Ranking 1 2 3 4 5 6 7 8 9 10

Developer Form Nonaqueous Wet Solvent

Plastic Film water-soluble

Water Suspendible water-soluble

Water Suspendible Dry Dry Dry Dry

Method of Application Spray Spray Spray Spray

Immersion Immersion

Dust Cloud (Electrostatic) Fluidized Bed

Dust Cloud (Air Agitation) Immersion (Dip)

Table 7-3: Sensitivity Ranking (highest to lowest) Developer Form Application

Technique.

Table 7-4 lists the main advantages and disadvantages of the various developer

types.


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Developer Advantages Disadvantages

Dry

Indications tend to remain brighter and more distinct over time

Easily to apply

Does not form contrast background so cannot be used with visible systems

Difficult to assure entire part surface has been coated

Soluble

Ease of coating entire part

White coating for good con-trast can be produced which work well for both visible and fluorescent systems

Coating is translucent and provides poor contrast (not recommended for visual systems)

Indications for water wash-able systems are dim and blurred

Suspendible

Ease of coating entire part

Indications are bright and sharp


Indications weaken and become diffused after time

Nonaqueous

Very portable

Easy to apply to readily ac-cessible surfaces


Indications show-up rapidly and are well defined

Provides highest sensitivity

Difficult to apply evenly to all surfaces

More difficult to clean part after inspection

Table 7-4: List of main advantages and disadvantages of the various developer

types.


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7.25.15 Quality Control Temperature The temperature of the penetrant materials and the part being in-spected can have an effect on the results. Temperatures from 27 to 49oC (80 to 120oF) are reported in the literature to produce optimal results. Many specifications allow testing in the range of 4 to 52oC (40 to 125oF). A tip to remember is that surfaces that can be touched for an extended period of time without burning the skin are generally below 52oC (125oF).

Since the surface tension of most materials decrease as the temperature increases, raising the temperature of the penetrant will increase the wetting of the surface and the capillary forces. Of course, the converse is also true and lowering the tempera-ture will have a negative effect on the flow characteristics. Raising the temperature will also raise the speed of evaporation of penetrants, which can have a positive or negative effect on sensitivity. The impact will be positive if the evaporation serves to increase the dye concentration of the penetrant trapped in a flaw up to the concen-tration quenching point and not beyond. Higher temperatures and more rapid evapo-ration will have a negative effect if the dye concentration is caused to exceed the concentration quenching point or the flow characteristics are changed to the point where the penetrant does not readily flow.

The method of processing a hot part was once commonly employed. Parts were ei-ther heated or processed hot off the production line. In its days, this served to in-crease inspection sensitivity by increasing the viscosity of the penetrant. However, the penetrant materials used today have 1/2 to 1/3 the viscosity of the penetrants on the market in the 1960's and 1970's. Heating the part prior to inspection is no longer necessary and no longer recommended.

Penetrant The quality of a penetrant inspection is highly dependent on the quality of the pene-trant materials used. Only products meeting the requirements of an industry specifi-cation, such as AMS 2644, should be used. The performance of a penetrant can be affected by contamination and aging. Contamination by another liquid will change the surface tension and contact angle of the solution, and virtually all organic dyes deteriorate over time resulting in a loss of color or fluorescent response. Therefore, regular checks must be performed to insure that the material performance has not degraded.

When the penetrant is first received from the manufacturer, a sample of the fresh solution should be collected and stored as a standard for future comparison. The standard specimen should be stored in an opaque glass or metal, sealed container. Penetrants that are in-use should be compared regularly against the standard speci-men to detect changes in color, odor and consistency. When using fluorescent pene-trants, a brightness comparison per the requirements of ASTM E 1417 is also often required. This check involves placing a drop of the standard and the in-use pene-


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trants on a piece of Whatman #4 filter paper and making a side by side comparison of the brightness of the two spots under UV light.

Additionally, the water content of water washable penetrants must be checked regu-larly. When water contaminates oil-based penetrants, the surface tension and con-tact angle of the mixture will increase since water has a higher surface tension than most oil-based penetrants In self-emulsifiable penetrants, water contamination can produce a gel break or emulsion inversion when the water concentration becomes high enough. The formation of the gel is an important feature during the washing processes but must be avoided until the stage in the process. Data indicates that the water contamination must be significant (greater than 10%) for gel formation to occur. Most specification limit water contamination to around 5% to be conservative. Non-water-based, water washable penetrants are checked using the procedure specified in ASTM D95 or ASTM E 1417. Water-based, water washable penetrants are checked with a refractometer. The rejection criteria is different for different pene-trants so the requirements of the qualifying specification or the manufacturer's in-structions must be consulted.

Application of the Penetrant The application of the penetrant is the step of the process that requires the least amount of control. As long as the surface being inspected receives a generous coat-ing of penetrant, it really doesn't matter how the penetrant is applied. Generally, the application method is an economic or convenience decision.

It is important that the part be thoroughly cleaned and dried. Any contaminates or moisture on the surface of the part or within a flaw can prevent the penetrant mate-rial from entering the defect. The part should also be cool to the touch. The recom-mended range of temperature is 4 to 52oC (39 to 125 F). Wash Temperature and Pressure The wash temperature and pressure and time are three parameters that are typically controlled in penetrant inspection process specification. A coarse spray or an immer-sion wash tank with air agitation is often used. When the spray method is used, the water pressure is usually limited to 276 kN/m2 (40 psi). The temperature range of the water is usually specified as a wide range (ex. 10 to 38C (50 to 100 F) in AMS 2647A.) A low-pressure, coarse water spray will force less water into flaws to dilute and/or remove trapped penetrant and weaken the indication. The temperature will have an effect on the surface tension of the water and warmer water will have more wetting action than cold water. Warmer water temperatures may also make emulsi-fiers and detergent more effective. The wash time should only be as long as neces-sary to decrease the background to an acceptable level. Frequent visual checks of the part should be made to determine when the part has be adequately rinsed.


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Drying Process The temperature used to dry parts after the application of aqueous wet developer or prior to the application of a dry powder or a nonaqueous wet developer, must be controlled to prevent "cooking" of the penetrant in the defect. High drying tempera-ture can affect penetrants in a couple of ways. First, some penetrants can fade at high temperatures due to dye vaporization or sublimation. Second, high tempera-tures can cause the penetrant to dry in the the flaw preventing it from migrating to the surface to produce an indication. To prevent harming the penetrant material, drying temperature should be kept to under 71 degree centigrade. The drying should be limited to the minimum length of time necessary to thoroughly dry the compo-nent being inspected.

Developer The function of the developer is very important in a penetrant inspection. It must draw out of the discontinuity a sufficient amount of penetrant to form an indication, and it must spread the penetrant out on the surface to produce a visible indication. In a fluorescent penetrant inspection, the amount of penetrant brought to the sur-face must exceed the dye's thin film threshold of fluorescence of the indication will not fluoresce. Additionally, the developer makes fluorescent indications appear brighter than indications produced with the same amount of dye but without the de-veloper.

In order to accomplish these functions, a developer must adhere to the part surface and result in a uniform, highly porous layer with many paths for the penetrant to be moved due to capillary action. Some developers are applied wet and other dry, but the desired end result is always a uniform, highly porous, surface layer. Since the quality control requirements for each of the developer types is slightly different, they will be covered individually. Dry Powder Developer A dry powder developer should be checked daily to ensure that it is fluffy and not caked. It should be similar to fresh powdered sugar and not granulated like powered soup. It should also be relatively free from specks of fluorescent penetrant material from previous inspection. This check is performed by spreading a sample of the de-veloper out and examining it under UV light. If there are ten or more fluorescent specks in an 10 cm diameter area, the batch should be discarded.

Apply a light coat of the developer by immersing the test component or dusting the surface. After the development time, excessive powder can be removed by gently blowing on the surface with air not exceeding 35 kPa or 5 psi.

Wet Soluble/Suspendible Developer Wet soluble developer must be completely dissolved in the water and wet suspendi-ble developer must be thoroughly mixed prior to application. The concentration of powder in the carrier solution must be controlled in these developers. The concentra-


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tion should be checked at least weekly using a hydrometer to make sure it meets the manufacturer's specification. To check for contamination, the solution should be ex-amined weekly using both white light and UV light. If a scum is present or the solu-tion fluoresces, it should be replaced. Some specification require that a clean alumi-num panel be dipped in the developer, dried, and examined for indications of con-tamination by fluorescent penetrant materials.

These developers are applied immediately after the final wash. A uniform coating should be applied by spraying, flowing or immersion of the component. They should never be applied with a brush. Care should be taken to avoid a heavy accumulation of the developer solution in crevices and recesses. Prolonged contact of the compo-nent with the developer solution should be avoided in order to minimize dilution or removal of the penetrant from discontinuities. Solvent Suspendible Solvent suspendible developers are typically supplied in an sealed aerosol spray can. Since the developer solution is in a sealed vessel, direct check of the solution are not possible. However, the way that the developer is dispensed must be monitored. The spray developer should produce a fine, even coating on the surface of the part. Make sure the can is well shaken and apply a thin coating to a test article. If the spray produces spatters or other an uneven coating the can should be discarded.

When applying a solvent suspendible developer, it is up to the inspector to control the thickness of the coating. When using a visible penetrant system, the developer coating must be thick enough to provide a white contrasting background but not heavy enough to mask indications. When using a fluorescent penetrant system, a very light coating should be used. The developer should be applied under white light condition and should appear evenly transparent. Development Time Part should be allowed to develop for a minimum of 10 minutes and no more than 2 hours before inspecting.

Lighting After a component has been properly processed, it is ready for inspection. While automated vision inspection systems are sometimes used, the focus here will be on inspection performed visually by a human inspector as this is the dominate method. Proper lighting is of great importance when visually inspecting a surface for a pene-trant indication. Obviously, the lighting requirements are different for an inspection conducted using a visible dye penetrant than they are for an inspection conducted using a fluorescent dye penetrant. The lighting requirements for each of these tech-niques, as well as how light measurements are made, is discussed below.


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Figure 7-88: UV-lamp.

Lighting for Visible Dye Penetrant Inspections When using a visible penetrant, the intensity of the white light is of principal impor-tance. Inspections can be conducted using natural lighting or artificial lighting. When using natural lighting, it is important to keep in mind that daylight varies from hour to hour so inspector must stay constantly aware on the lighting conditions and make adjustment when needed. To improve uniformity in lighting from one inspection to the next, the use of artificial lighting is recommended. Artificial lighting should be white whenever possible and white flood or halogen lamps are most commonly used. The light intensity is required to be 100 foot-candles at the surface being inspected. It is advisable to choose a white light wattage that will provide sufficient light, but avoid excessive reflected light that could distract from the inspection.

Lighting for Fluorescent Penetrant Inspections Then a fluorescent penetrant is being employed, the ultraviolet illumination and the visible light inside the inspection booth is important. Penetrant dyes are excited by the UV of 365-nm wavelength and emit visible light somewhere in the green-yellow range between 520 and 580 nm. The source of ultraviolet light (UV) is often a mer-cury arc lamp with a filter. The lamps emit many wavelengths and a filter is used to remove all but the UV and a small amount of visible light between 320 and 410 nm. Visible light of wavelengths above 410 nm interferes with contrast, and UV emissions below 320 nm include some hazardous wavelengths.

Standards and procedures require verification of lens condition and light intensity. Black lights should never be used with a cracked filter as output of white light and harmful black light will be increased. The cleanliness of the filter should also be checked as a coating of solvent carrier, oils, or other foreign materials can reduce the intensity by up to as much as 50%. The filter should be checked visually and cleaned as necessary before warm-up of the light.


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Since fluorescent brightness is linear with respect to ultraviolet excitation, a change in the intensity of the light (from age or damage) and a change in the distance of the light source from the surface being inspected will have a direct impact on the inspec-tion. For UV lights used in component evaluations, the normally accepted intensity is 1000 microwatts per square centimeter when measured at 15 inches from the filter face (requirements can vary from 800 to 1200 µW/cm2). The required check should be performed when a new bulb is installed, at startup of the inspection cycle, if a change in intensity is noticed, or every eight hours of continuous use. Regularly checking the intensity of UV lights is very important because bulbs loose intensity over time. In fact, a bulb that is near the end of its operating life will often have an intensity of only 25 percent of its original output. A maximum intensity of 5000 µw/cm2 is recommended at the inspection area. The reason is that the fluorescent part of the penetrant is sensitive to UV-light in fact, the UV-light can make the flourescens fade away. Consequencely, a UV-lamp should never be placed directly above the tank containing fluorescent penetrant liquid. Black light intensity will also be affected by voltage variations. A bulb that produces acceptable intensity at 120 volts will produce significantly less at 110 volts. For this reason it is important to provide constant voltage to the light. Also, most UV light must be warmed up prior to use and should be on for at least 15 minutes before beginning an inspection. When performing a fluorescent penetrant inspection, it is important to keep white light to a minimum as it will significantly reduce the inspectors ability to detect fluo-rescent indications. Light levels of less than 2 fc are required by most procedures with some procedures requiring less than 0.5 fc at the inspection surface. Procedures require a check and documentation of ambient white light in the inspection area. When checking black light intensity at 15 inches a reading of the white light pro-duced by the black light may be required to verify white light is being removed by the filter. Light Measurement Light intensity measurements are made using a radiometer. A radiometer is an in-strument that translate light energy into an electrical current. Light striking a silicon photodiode detector causes a charge to build up between internal layers. When an external circuit is connected to the cell, an electrical current is produced. This current is linear with respect to incident light. Some radiometers have the ability to measure both black and white light, while others require a separate sensor for each meas-urement. Whichever type is used, the sensing area should be clean and free of any materials that could reduce or obstruct light reaching the sensor. Radiometers are relatively unstable instruments and readings often change considerable over time. Therefore, they should be calibrated at least every six months. Ultraviolet light measurements should be taken using a fixture to maintain a minimum distance of 15 inches from the filter face to the sensor. The sensor should be centered in the light field to obtain and record the highest reading. UV spot lights are often focused, so intensity readings will vary considerable over a small area. White lights are seldom


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focused and depending on the wattage, will often produce in excess of the 100 fc at 15 inches. Many specifications do not require the white light intensity check to be conducted at a specific distance.

7.25.16 System Performance Check System performance checks involve processing a test speci-men with known defects to determine if the process will reveal discontinuities of the size required. The specimen must be processed following the same procedure used to process production parts. A system performance check is typically required daily, at the reactivation of a system after maintenance or repairs, or any time the system is suspected of being out of control. As with penetrant inspections in general, results are directly dependent on the skill of the operator and, therefore, each operator should process a panel.

The ideal specimen is a production item that has natural defects of the minimum acceptable size. Some specification delineate the type and size of the defects that must be present in the specimen and detected. Surface finish is will affect washability so the check specimen should have the same surface finish as the production parts being processed. If penetrant systems with different sensitivity levels are being used, there should be a separate specimen for each system.

There are some universal test specimens that can be used if a standard part is not available. The most commonly used test specimen is the TAM or PSM panel.

These panel are usually made of stainless steel that has been chrome plated on one half and surfaced finished on the other half to produced the desired roughness. The chrome plated section is impacted from the back side to produce a starburst set of cracks in the chrome. There are five impacted areas to produce range of crack sizes. Each panel has a characteristic “signature” and variances in that signature are indi-cations of process variance. Panel patterns as well as brightness are indicators of process consistency or variance, see picture above.

Care of system performance check specimens is critical. Specimens should be han-dled carefully to avoid damage. They should be cleaned thoroughly between uses and storage in a solvent is generally recommended. Before processing a specimen, it should be inspected under UV light to make sure that it is clean and not already pro-ducing an indication.


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7.25.17 Nature of the Defect

The nature of the defect can have a large affect on sensitivity of a dye penetrant inspection. Sensitivity is defined as the smallest defect that can be detected with a high degree of reliability. Typically, the crack length at the sample surface is used to define size of the defect. A survey of any probability-of-detection curve for penetrant inspection will quickly lead one to the conclusion that crack length has a definite af-fect on sensitivity. However, the crack length alone does not determine whether a flaw will be seen or go undetected. The volume of the defect is likely to be the more important feature. The flaw must be of sufficient volume so that enough penetrant will bleed back out to a size that is detectable by the eye or that will satisfy the di-mensional thresholds of fluorescence.

Figure 7-89: Example of fluorescent penetrant inspection probability of detec-

tion (POD) curve from the Nondestructive Evaluation (NDE) Capabilities Data Book.

Above is an example of fluorescent penetrant inspection probability of detection (POD) curve from the Nondestructive Evaluation (NDE) Capabilities Data Book. Please note that this curve is specific to one set of inspection conditions and should not be interpreted to apply to other inspection situations.


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In general, penetrant inspections are more effective at finding

• small round defects than small linear defects. Small round defects are generally easier to detect for several reasons. First, they are typically volu-metric defects that can trap significant amounts of penetrant. Second, round defects fill with penetrant faster than linear defects. One research effort found that elliptical flaw with length to width ratio of 100, will take the pene-trant nearly 10 times longer to fill than a cylindrical flaw with the same vol-ume.

• deeper flaws than shallow flaws. Deeper flaws will trap more penetrant than shallow flaws, and they are less prone to over washing.

• flaws with a narrow opening at the surface than wide open flaws. Flaws with narrow surface openings are less prone to over washing.

• flaws on smooth surfaces than on rough surfaces. The surface rough-ness of the part primarily affects the removability of a penetrant. Rough sur-faces tend to trap more penetrant in the various tool marks, scratches, and

pits that make up the surface. Removing the penetrant from the surface of the part is more difficult and a higher level of background fluores-cence or over washing may occur.

• flaws with rough fracture surfaces than smooth fracture surfaces. The surface roughness that the fracture faces is a factor in the speed at which a penetrant enters a defect. In general, the penetrant spreads faster over a surface as the surface roughness increases. It should be noted that a particular penetrant may spread slower than others on a smooth surface but faster than the rest on a rougher surface.

• flaws under tensile or no loading than flaws under compression load-ing. In a 1987 study at the University College London, the effect of crack closure on detectability was evaluated. Researchers used a four-point bend fixture to place tension and compression loads on specimens that were fabri-cated to contain fatigue cracks. All cracks were detected with no load and with tensile loads placed on the parts. However, as compressive loads were placed on the parts, the crack length steadily decreased as load increased until a load was reached when the crack was no longer detectable.

7.25.18 Health & Safety Precautions in Dye Penetrant Inspection When proper health and safety precautions are followed, dye penetrant inspection operations can be completed without harm to inspection personnel. However, there are a number of health and safety related issues that must be addressed. Since each inspection operation will have its own unique set of health and safety concerns that must be addressed, only a few of the most common concerns will be discussed here.


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Chemical Safety Whenever chemicals must be handled, certain precautions must be taken as directed by the material safety data sheets (MSDS) for the chemicals. Before working with a chemical of any kind, it is highly recommended that the MSDS be reviewed so that proper chemical safety and hygiene practices can be followed. Some of the penetrant materials are flammable and, therefore, should be used and stored in small quanti-ties. They should only be used in a well ventilated area and ignition sources avoided. Eye protection should always be worn to prevent contact of the chemicals with the eyes. Many of the chemicals used contain detergents and solvents that can dermati-tis. Gloves and other protective clothing should be worn to limit contact with the chemicals.

Ultraviolet Light Safety Ultraviolet (UV) light or "black light" as it is sometimes called, has wavelengths rang-ing from 180 to 400 nanometers. These wavelengths place UV light in the invisible part of the electromagnetic spectrum between visible light and X-rays. The most familiar source of UV radiation is the the sun and is necessary in small doses for cer-tain chemical processes to occur in the body. However, too much exposure can be harmful to the skin and eyes. Excessive UV light exposure can cause painful sun-burn, accelerate wrinkling and increase the risk of skin cancer. UV light can cause eye inflammation, cataracts, and retinal damage.

Because of their close proximity, laboratory devices, like UV lamps, deliver UV light at a much higher intensity than the sun and, therefore, can cause injury much more quickly. The greatest threat with UV light exposure is that the individual is generally unaware that the damage is occurring. There is usually no pain associated with the injury until several hours after the exposure. Skin and eye damage occurs at wave-lengths around 320 nm and shorter which is well below the 365 nm wavelength, where penetrants are designed to fluoresce. Therefore, UV lamps sold for use in LPI application are almost always filtered to remove the harmful UV wavelengths. The lamps produce radiation at the harmful wavelengths so it is essential that they be used with the proper filter in place and in good condition.


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7.25.19 References and Resources Cartz, Louis, Nondestructive Testing, ASM Intl, Metals Park, OH, 1995, ISBN: 0871705176

Introduction to Capillary Testing Theory, Borovikov, A.S. (Ed.), Minsk, Nauka i Tekhnika Publishing, 1988

Liquid Penetrant Testing, Nondestructive Testing Handbook, Volume 2, Tracy, Noel (Tech. Ed.), Moore, Patrick (Ed.) American Society for Nondestructive Testing, Co-lumbus, OH, 1999, ISBN 1-57117-028-6

Larson, B.F., Study of the Factors Affecting the Sensitivity of Liquid Penetrant In-spections: Review of Literature Published from 1970 to 1998, FAA Technical Report Number DOT/FAA/AR-01/95, Office of Aviation Research, Washington, DC, Jan 2002 (pdf 1.0 meg)

Flaherty, J. J., History of Penetrants: The First 20 Years, 1941-61, Materials Evalua-tion, Vol. 44, No. 12, November 1986, pp. 1371-1374, 1376, 1378, 1380, 1382

Boisvert, B.W., Hardy, G., Dorgan, J.F., and Selner, R.H., The Fluorescent Penetrant Hydrophilic Remover Process, Materials Evaluation, February 1983, pp. 134-137.

Sherwin, A. G., Overremoval Propensities of the Prewash Hydrophilic Emulsifier Fluo-rescent Penetrant Process, Materials Evaluation, March 1993, pp. 294-299.

Robinson, Sam J., Here Today, Gone Tomorrow! Replacing Methyl Chloroform in the Penetrant Process, Materials Evaluation, Vol. 50, No. 8, August 1992, pp. 936-946.

Rummel, W., Cautions on the Use of Commercial Aqueous Precleaners for Penetrant Inspection, Materials Evaluation, Vol. 16, No. 5, August 1998, pp. 950-952.

Glazkov, Y.A., Some Technological Mistakes in the Application of Capillary Inspection to Repairs of Gas Turbin Engines, translation from Defektoskopiya - The Soviet Jour-nal of Nondestructive Testing, Vol. 26, No. 3, New York, NY Plenum/Consultants Bu-reau, January 1990, pp. 361-367.

Glazkov, Yu . A., Bruevich, E.P., and Samokhin, N.L, Special Features of Application of Aqueous Solutions of Commercial Detergents in Capillary Flaw Inspection, Defek-toskopiya - The Soviet Journal of Nondestructive Testing, Vol. 19, No. 8, August 1982, pp. 83-87.

De Graaf, E. and De Rijk, P., Comparison Between Reliability, Sensitivity, and Accu-racy of Nondestructive Inspection Methods, 13th Symposium on Nondestructive Evaluation Proceedings, San Antonio, TX, published by NTIAC, Southwest Research Institute, San Antonio, TX, April 1981, pp. 311-322.


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Thomas, W.E., An Analytic Approach to Penetrant Performance, 1963 Lester Honor Lecture, Nondestructive Testing, Vol. 21, No. 6, Nov.-Dec. 1963, pp. 354-368.

Senda, T., Maeda, N., Kato, M., Ebata, M., Ooka, K., and Miyoshi, S., Factors In-volved in Formation of Penetrant Testing Indications, NDE in the Nuclear Industry: Proceedings of the 6th International Conference, Zurich, Switzerland, November - December 1984, pp. 807-810.

Brittain, P. I., The Amplifying Action of Developer Powders, QUALTEST 3 Conference, Cincinnati OH, Oct 1984.

Rummel, W. D., Probability of Detection as a Quantitative Measure of Nondestructive Testing End-To-End Process Capabilities, Materials Evaluation, January 1998, pp. 35.

Nondestructive Testing Handbook, Vol. 2, Liquid Penetrant Tests, Robert McMaster, et al., American Society for Nondestructive Testing, 1982, pp. 283-319.

Rummel, W.D. and Matzkanin, G. A., Nondestructive Evaluation (NDE) Capabilities Data Book, Published by the Nondestructive Testing Information Analysis Center (NTIAC), NTIAC #DB-95-02, May 1996.

Alburger, J.R., Dimensional Transition Effects in Visible Color and Fluorescent Dye Liquids, Proceedings, 23rd Annual Conference, Instrument Society of America, Vol. 23, Part I, Paper No. 564.

Deutsch, S. A, Preliminary Study of the Fluid Mechanics of Liquid Penetrant Testing, Journal of Research of the National Bureau of Standards, Vol. 84, No. 4, July-August 1979, pp. 287-291.

Kauppinen, P. and Sillanpaa, J., Reliability of Surface Inspection Methods, Proceed-ings of the 12th World Conference on Nondestructive Testing, Amsterdam, Nether-lands, Vol. 2, Elsevier Science Publishing, Amsterdam, 1989, pp. 1723-1728.

Vaerman, J. F., Fluorescent Penetrant Inspection Process, Automatic Method for Sen-sitivity Quantification, Proceedings of 11th World Conference on Nondestructive Test-ing, Volume III, Las Vegas, NV, November 1985, pp. 1920-1927.

Thomas, W.E., An Analytic Approach to Penetrant Performance, 1963 Lester Honor Lecture, Nondestructive Testing, Vol. 21, No. 6, Nov.-Dec. 1963, pp. 354-368.

Clark, R., Dover, W.D., and Bond, L.J., The Effect of Crack Closure on the Reliability of NDT Predictions of Crack Size, NDT International, Vol. 20, No. 5, Guildford, United Kingdom, Butterworth Scientific Limited, October 1987, pp. 269-275.


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7.26 Magnetic Particle Flow Test This method can be used for steel structures.

7.26.1 Introduction to Magnetic Particle Inspection (MPI) Magnetic particle inspection is a non-destructive testing method used for defect de-tection. MPI is fast and relatively easy to apply and part surface preparation is not as critical as it is for some other NDT methods. These characteristics makes MPI one of the most widely utilized non-destructive testing methods. MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components. The only requirement from an inspection standpoint is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective. The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries uses magnetic particle inspection for de-termining a component's fitness-for-use. Some examples of industries that uses magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore struc-tures and underwater pipelines.

Figure 7-90: To the left: Underwater Inspection. To the right: Inspection of

Castings.


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7.26.2 Basic Principles In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two non-destructive testing methods: magnetic flux leakage testing and vis-ual testing. Consider a bar magnet. It has a magnetic field in and around the magnet. Any place that a mag-netic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole. When a bar magnet is broken in the centre of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the North Pole and renters at the South Pole. The magnetic field spreads out when it encounter the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus, it is called a flux leakage field. If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

Figure 7-91: Illustration of cluster of magnetic particles at a crack.


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The first step in a magnetic particle inspection is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, ei-ther in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus form-ing a visible indication that the inspector can detect.

7.26.3 History of Magnetic Particle Inspection Magnetism is the ability of matter to attract other matter to it. The ancient Greeks were the first to discover this phenomenon in a mineral they named magnetite. Later on Bergmann, Becquerel, and Faraday discovered that all matter including liquids and gasses were affected by magnetism, but only a few responded to a noticeable extent.

The earliest known use of magnetism to inspect an object took place as early as 1868. Cannon barrels were checked for defects by magnetizing the barrel then slid-ing a magnetic compass along the barrel's length. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass. This was a form of non-destructive testing but the term was not commonly used until some time after World War I.

In the early 1920’s, William Hoke realized that magnetic particles (colour metal shavings) could be used with magnetism as a means of locating defects. Hoke discovered that a surface or sub-surface flaw in a magnetized material caused the magnetic field to distort and extend beyond the part. This discovery was brought to his attention in the machine shop. He noticed that the metallic grindings from hard steel parts, which were be-ing held by a magnetic chuck while being ground, formed patterns on the face of the parts which corresponded to the cracks in the surface. Apply-ing a fine ferromagnetic powder to the parts caused a build up of powder over flaws and formed a visible indication. The image shows a 1928 Electro-Magnetic Steel Testing Device (MPI) made by the Equipment and Engineering Com-pany Ltd. (ECO) of Strand, England.

In the early 1930’s, magnetic particle inspection (MPI) was quickly replacing the oil-and-whiting method (an early form of the liquid penetrant inspection) as the method of choice by the railroad to inspect steam engine boilers, wheels, axles, and the tracks. Today, the MPI inspection method is used extensively to check for flaws in a large variety of manufactured materials and components. MPI is used to check mate-rials such as steel bar stock for seams and other flaws prior to investing machining


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time during the manufacturing of a component. Critical automotive components are inspected for flaws after fabrication to ensure that defective parts are not placed into service. MPI is used to inspect some highly loaded components that have been in-service for a period of time. For example, many components of high performance race cars are inspected whenever the engine, drive train and other systems are overhauled. MPI is also used to evaluate the integrity of structural welds on bridges, storage tanks, and other safety critical structures.

7.26.4 Magnetism Magnets are very common items in the workplace and household. Uses of magnets range from holding pictures on the refrigerator to causing torque in electric motors. Most people are familiar with the general properties of magnets but are less familiar with the source of magnetism. The traditional concept of magnetism centres around the magnetic field and what is know as a dipole. The term "magnetic field" simply describes a volume of space where there is a change in energy within that volume. This change in energy can be detected and measured. The location where a magnetic field can be detected exiting or entering a material is called a magnetic pole. Mag-netic poles have never been detected in isolation but always occur in pairs and, thus, the name dipole. Therefore, a dipole is an object that has a magnetic pole on one end and a second equal but opposite magnetic pole on the other. A bar magnet can be considered a dipole with a north pole at one end and South Pole at the other. A magnetic field can be measured leaving the dipole at the North Pole and returning the magnet at the South Pole. If a magnet is cut in two, two magnets or dipoles are created out of one. This sectioning and creation of dipoles can continue to the atomic level. Therefore, the source of magnetism lays in the ba-sic building block of all matter...the atom.

The Source of Magnetism All matter is composed of atoms, and atoms are composed of protons, neutrons and electrons. The protons and neutrons are located in the atom's nucleus and the electrons are in con-stant motion around the nucleus. Electrons carry a negative electrical charge and produce a magnetic field as they move through space. A magnetic field is produced whenever an electrical charge is in motion. The strength of this field is called the magnetic moment.

This may be hard to visualize on a subatomic scale but con-sider electric current flowing through a conductor. When the electrons (electric current) are flowing through the conductor, a magnetic field forms around the conductor. The magnetic field can be detected using a compass. The magnetic field will place a force on the compass needle, which is another example of a dipole.


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Since all matter is comprised of atoms, all materials are affected in some way by a magnetic field. However, not all materials react the same way. This will be explored more in the next section.

7.26.5 Magnetic Materials When a material is placed within a magnetic field, the magnetic forces of the mate-rial's electrons will be affected. This effect is known as Faraday's Law of Magnetic Induction. However, materials can react quite differently to the presence of an ex-ternal magnetic field. This reaction is dependent on a number of factors such as the atomic and molecular structure of the material, and the net magnetic field associated with the atoms. The magnetic moments associated with atoms have three origins. These are the electron orbital motion, the change in orbital motion caused by an external magnetic field, and the spin of the electrons. In most atoms, electrons occur in pairs. Each electron in a pair spins in the opposite direction. So when electrons are paired together, their opposite spins cause there magnetic fields to cancel each other. Therefore, no net magnetic field exists. Alternately, ma-terials with some unpaired electrons will have a net magnetic field and will react more to an external field. Most materials can be classified as ferromagnetic, diamagnetic or paramagnetic.

Diamagnetic metals have a very weak and negative suscepti-bility to magnetic fields. Diamagnetic materials are slightly re-pelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Dia-magnetic materials are solids with all paired electron and, therefore, no permanent net magnetic moment per atom. Diamagnetic properties arise from the realignment of the electron orbits under the influence of an external magnetic field. Most ele-ments in the periodic table, including copper, silver, and gold, are diamagnetic.

Paramagnetic metals have a small and positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic properties are due to the presence of some unpaired electrons and from the realign-ment of the electron orbits caused by the external magnetic field. Paramagnetic ma-terials include magnesium, molybdenum, lithium, and tantalum.

Ferromagnetic materials have a large and positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. Ferro-magnetic materials have some unpaired electrons so their atoms have a net mag-netic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atoms moments (1012 to 1015) are aligned parallel so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnified state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong


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magnetic field within the part. Iron, nickel, and cobalt are examples of ferromagnetic materials. Components with these materials are commonly inspected using the mag-netic particle method.

7.26.6 Magnetic Domains Ferromagnetic materials get their magnetic properties not only because their atoms carry a magnetic moment but also because the material is made up of small regions known as magnetic domains. In each domain, all of the atomic dipoles are coupled together in a preferential direction. This alignment develops as the material develops its crystalline structure during solidification from the molten state. Magnetic domains can be detected using Magnetic Force Microscopy (MFM) and images of the domains like the one shown below can be constructed.

Magnetic Force Micros-copy (MFM) image showing the magnetic domains in a piece of heat treated carbon steel.

Figure 7-92: Magnetic Force Microscopy (MFM) image showing the magnetic

domains in a piece of heat treated carbon steel.

During solidification a trillion or more atom moments are aligned parallel so that the magnetic force within the domain is strong in one direction. Ferromagnetic materials are said to be characterized by "spontaneous magnetization" since they obtain satu-ration magnetization in each of the domains without an external magnetic field being applied. Even though the domains are magnetically saturated, the bulk material may not show any signs of magnetism because the domains develop themselves are ran-domly oriented relative to each other. Ferromagnetic materials become magnetized when the magnetic domains within the material are aligned. This can be done by placing the material in a strong external magnetic field or by passing electrical cur-rent through the material. Some or all of the domains can become aligned. The more domains that are aligned, the stronger the magnetic field in the material. When all of

the domains are aligned, the material is said to be magnetically saturated. When a

material is magnetically saturated, no additional amount of external magnetization force will cause an increase in its internal level of magnetization. An example of these domains is shown in Figure 7-93.


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Unmagnetized Material Magnetized Material

Figure 7-93: Illustrations of unmagnetized respectively magnetized materials.

7.26.7 Magnetic Field Characteristics

Magnetic Field In and Around a Bar Magnet As discussed previously a magnetic field is a change in energy within a volume of space. The magnetic field surrounding a bar magnet can be seen in the magneto-graph below. A magnetograph can be created by placing a piece of paper over a magnet and sprinkling the paper with iron filings. The particles align themselves with the lines of magnetic force produced by the magnet. The magnetic lines of force show where the magnetic field exits the material at one pole and reenters the mate-rial at another pole along the length of the magnet. It should be noted that the mag-netic lines of force exist in three-dimensions but are only seen in two dimensions in the image.

It can be seen in the magnetograph that there are poles all along the length of the magnet but that the poles are concentrated at the ends of the magnet. The area where the exit poles are concentrated is called the magnet's North Pole and the area where the entrance poles are concentrated is called the magnet's South Pole.


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Magnetic Fields in and around Horseshoe and Ring Magnets Magnets come in a variety of shapes and one of the more common is the horseshoe (U) magnet. The horseshoe magnet has north and south poles just like a bar magnet but the magnet is curved so the poles lie in the same plane. The magnetic lines of force flow from pole to pole just like in the bar magnet. However, since the poles are located closer together and a more direct path exists for the lines of flux to travel between the poles, the magnetic field is concentrated between the poles.

If a bar magnet was placed across the end of a horseshoe magnet or

if a magnet was formed in the shape of a ring, the lines of magnetic force would not even need to enter the air. The value of such a magnet where the magnetic field is completely contained with the material probably has limited use. However, it is important to understand that the magnetic field can flow in loop within a material when the concept of circular magnetism is later covered.

General Properties of Magnetic Lines of Force Magnetic lines of force have a number of important properties, which include:

• They seek the path of least resistance between opposite magnetic poles. In a single bar magnet as shown to the right, they attempt to form closed loop from pole to pole.

• They never cross one another.

• They all have the same strength.

• Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability.

• Their density decreases with increasing distance from the poles.

• They are considered to have direction as if flow-ing, though no actual movement occurs. They flow from the South Pole to the North Pole within the material and North Pole to South Pole in air.


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7.26.8 Electromagnetic Fields Magnets are not the only source of magnetic fields. In 1820, Hans Christian Oersted discovered that an elec-tric current flowing through a wire caused a nearby compass to deflect. This indicated that the current in the wire was generating a magnetic field.

Oersted studied the nature of the magnetic field around the long straight wire. He found that the mag-netic field existed in circular form around the wire and that the intensity of the field was directly proportional to the amount of current carried by the wire. He also found that the strength of the field was strongest close to the wire and diminished with distance from the con-ductor until it could no longer be detected.

In most conductors, the magnetic field exists only as long as the current is flowing (i.e. an electrical charge is in motion). However, in ferromagnetic materials the electric current will cause some or all of the magnetic domains to align and a residual magnetic field will re-main.

Oersted also noticed that the direction of the magnetic field was dependent on the direction of the electrical current in the wire. A three-dimensional representation of the magnetic field is shown below.

There is a simple rule for remembering the direction of the magnetic field around a conductor. It is called the right-hand rule. If a person grasps a conductor in ones right hand with the thumb pointing in the direction of the current, the fingers will circle the conductor in the direction of the magnetic field.


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A word of caution about the right-hand rule For the right-hand rule to work, one important thing that must be remembered about the direction of current flow. Standard convention has current flowing from the positive terminal to the negative terminal. This convention is credited to Benjamin Franklin who theorized that electric current was due to a positive charge moving from the positive terminal to the negative terminal. However, it was later discovered that it is the movement of the negatively charged electron that is responsible for electrical current. Rather than changing several centuries of theory and equations, Franklin's convention is still used today.

7.26.9 Magnetic Field Produced by a Coil When a current carrying conductor is formed into a loop or several loops to form a coil, a magnetic field develops that flows through the centre of the loop or coil along the longitudinal axis and circles back around the outside of the loop or coil. The magnetic field circling each loop of wire combines with the fields from the other loops to produce a concentrated field down the centre of the coil. A loosely wound coil is illustrated below to show the interaction of the magnetic field. The magnetic field is essentially uniform down the length of the coil when it is wound tighter.


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The strength of a coil's magnetic field increases not only with increasing current but also with each loop that is added to the coil. A long straight coil of wire is called a solenoid and can be used to generate a nearly uniform magnetic field similar to that of a bar magnet. The concentrated magnetic field inside a coil is very useful in mag-netizing ferromagnetic materials for inspection using the magnetic particle testing method. Please be aware that the field outside the coil is weak and is not suitable for magnetize ferromagnetic materials.

7.26.10 Quantifying Magnetic Properties (Magnetic Field Strength, Flux Density, Total Flux and Magnetiza-tion)

Until now, only the qualitative features of the magnetic field have been discussed. However, it is necessary to be able to measure and express quantitatively the vari-ous characteristics of magnetism. Unfortunately, a number of unit conventions are in use as shown below. SI units will be used in this material. The advantage of using SI units is that they are traceable back to an agreed set of four base units - meter, kilogram, second, and Ampere.

Quantity SI Units

(Sommerfeld) SI Units

(Kennelly) CGS Units (Gaussian)

Field H A/m A/m oersteds

Flux Density (Magnetic Induction)

B tesla tesla gauss

Flux φ weber weber maxwell

Magnetization M A/m - erg.Oe-1.cm-3

The units for magnetic field strength H are ampere/meter. A magnetic field strength of 1 ampere/meter is produced at the center of a single circular conductor of diame-ter 1 meter carrying a steady current of 1 ampere.

The number of magnetic lines of force cutting through a plane of a given area at a right angle is known as the magnetic flux density B. The flux density or magnetic induction has the tesla as its unit. One tesla is equal to 1 Newton/ (A/m). From these units it can be seen that the flux density is a measure of the force applied to a particle by the magnetic field. The Gauss is CGS unit for flux density and is commonly used by US industry. One gauss represents one line of flux passing through one square centimeter of air oriented 90 degrees to flux flow.

The total number of lines of magnetic force in a material is called magnetic flux. The strength of the flux is determined by the number of magnetic domains that are aligned within a material. The total flux is simply the flux density applied over an area. Flux carries the unit of a weber, which is simply a tesla-square meter.


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The magnetization is a measure of the extent to which an object is magnetized. It is a measure of the magnetic dipole moment per unit volume of the object. Magnetiza-tion carries the same units as a magnetic field; amperes/meter.

Conversion between CGS and SI magnetic units.

7.26.11 The Hysteresis Loop and Magnetic Properties

A great deal of information can be learned about the magnetic properties of a mate-rial by studying its hysteresis loop. A hysteresis loop shows the relationship between the induced magnetic flux density B and the magnetizing force H. It is often referred to as the B-H loop. An example hysteresis loop is shown below.


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The loop is generated by measuring the magnetic flux B of a ferromagnetic material while the magnetizing force H is changed. A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed line as H is increased. As the line demonstrates, the greater the amount of current applied (H+), the stronger the magnetic field in the component (B+). At point "a" almost all of the magnetic domains are aligned and an additional increase in the magnetizing force will produce very little increase in magnetic flux. The mate-rial has reached the point of magnetic saturation. When H is reduced back down to zero, the curve will move from point "a" to point "b." At this point, it can be seen that some magnetic flux remains in the material even though the magnetizing force is zero. This is referred to as the point of retentivity on the graph and indicates the remanence or level of residual magnetism in the material. (Some of the magnetic domains remain aligned but some have lost there alignment.) As the magnetizing force is reversed, the curve moves to point "c", where the flux has been reduced to zero. This is called the point of coercivity on the curve. (The reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero.) The force required to remove the residual magnetism from the material, is called the coercive force or coercivity of the material.

As the magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction (point "d"). Re-ducing H to zero brings the curve to point "e." It will have a level of residual mag-netism equal to that achieved in the other direction. Increasing H back in the posi-tive direction will return B to zero. Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The curve will take a different path from point "f" back the saturation point where it with complete the loop.

From the hysteresis loop, a number of primary magnetic properties of a material can be determined.

1. Retentivity - A measure of the residual flux density corresponding to the saturation induction of a magnetic material. In other words, it is a material's ability to retain a certain amount of residual magnetic field when the mag-netizing force is removed after achieving saturation. (The value of B at point B on the hysteresis curve.)

2. Residual Magnetism or Residual Flux - the magnetic flux density that remains in a material when the magnetizing force is zero. Note that residual magnetism and retentivity are the same when the material has been mag-netized to the saturation point. However, the level of residual magnetism may be lower than the retentivity value when the magnetizing force did not reach the saturation level.

3. Coercive Force - The amount of reverse magnetic field which must be ap-plied to a magnetic material to make the magnetic flux return to zero. (The value of H at point C on the hysteresis curve.)


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4. Permeability - A property of a material that describes the ease with which a magnetic flux is established in the component.

5. Reluctance - Is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit.

7.26.12 Permeability

As previously mentioned, permeability is a material property that describes the ease with which a magnetic flux is established in the component. It is the ratio of the flux density to the magnetizing force and, therefore, represented by the following equa-tion:

μ = Β/Η

It is clear that this equation describes the slope of the curve at any point on the hysteresis loop. The permeability value given in papers and ref-erence materials is usually the maximum per-meability or the maximum relative permeability. The maximum permeability is the point where the slope of the B/H curve for unmagnetized material is the greatest. This point is often taken as the point where a straight line from the origin is tangent to the B/H curve. The relative permeability is arrived at by taking the ratio of the material's permeability to the permeability in free space (air).

μ(relative) = μ(material) / μ(air)

where: μ(air) = 4π x 10^-7 Hm^-1


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The shape of the hysteresis loop tells a great deal about the material being magnetized. The hys-teresis curves of two different materials are shown in the graph.

Relative to the other material, the materials with the wide hysteresis loop has:

• Lower Permeability

• Higher Retentivity

• Higher Coercivity

• Higher Reluctance

• Higher Residual Magnetism

The material with the narrower loop has:

• Higher Permeability

• Lower Retentivity

• Lower Coercivity

• Lower Reluctance

• Lower Residual Magnetism.

In magnetic particle testing the level of residual magnetism is important. Residual magnetic fields are affected by the permeability, which can be related to the carbon content and alloying of the material. A component with high carbon content will have low permeability and will retain more magnetic flux than a material with low carbon content.

7.26.13 Magnetic Field Orientation and Flaw Detectability

To properly inspect a component for cracks or other defects, it is important to under-stand that orientation between the magnetic lines of force and the flaw is very im-portant. There are two general types of magnetic fields that can be established within a component.


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A longitudinal magnetic field has magnetic lines of force that run parallel to the long axis of the part. Lon-gitudinal magnetization of a component can be accom-plished using the longitudinal field set up by a coil or solenoid. It can also be accomplished using permanent or electromagnets.

A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part. A circular magnetic field is induced in an article by either passing current through the component or by passing current through a conductor surrounded by the component.

The type of magnetic field established is determined by the method used to magnet-ize the specimen. Being able to magnetize the part in two directions is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect. This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part.

As can be seen in the image below, if the magnetic field is parallel to the defect, the field will see little disruption and no flux leakage field will be produced.

An orientation of 45 to 90 degrees between the magnetic field and the defect is nec-essary to form an indication. Since defects may occur in various and unknown direc-


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tions, each part is normally magnetized in two directions at right angles to each other. If the component below is considered, it is known that passing current through the part from end to end will establish a circular magnetic field that will be 90 degrees to the direction of the current. Therefore, defects that have a significant dimension in the direction of the current (longitudinal defects) should be detectable. Alternately, transverse-type defects will not be detectable with circular magnetiza-tion.

7.26.14 Magnetization of Ferromagnetic Materials There are a variety of methods that can be used to establish a magnetic field in a component for evaluation using magnetic particle inspection. It is common to classify the magnetizing methods as either direct or indirect.

Magnetization Using Direct Induction (Direct Magnetization)

With direct magnetization, current is passed directly through the component. Recall that whenever current flows a magnetic field is produced. Using the right-hand rule, which was introduced earlier, it is known that the magnetic lines of flux form normal to the direction of the current and form a circular field in and around the conductor. When using the direct magnetization method, care must be taken to ensure that good electrical contact is established and maintained between the test equipment and the test component. Improper contact can result in arcing that may damage the component. It is also possible to overheat components in areas of high resistance such as the contact points and in areas of small cross-sectional area.

There are several ways that direct magnetization is commonly accomplished. One way involves clamping the component between two electrical contacts in a special


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piece of equipment. Current is passed through the component and a circular mag-netic field is established in and around the component. When the magnetizing cur-rent is stopped, a residual magnetic field will remain within the component. The strength of the induced magnetic field is proportional to the amount of current passed through the component.

A second technique involves using clamps or prods, which are attached or placed in contact with the component. Elec-trical current flows through the component from contact to contact. The current sets up a circular magnetic field around the path of the current.

Magnetization Using Indirect Induction (Indirect Magnetization) Indirect magnetization is accomplished by using a strong external magnetic field to establish a magnetic field within the component. As with direct magnetization, there are several ways that indirect magnetization can be accomplished.

The use of permanent magnets is a low cost method of establishing a magnetic field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component. Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromag-nets only exhibit a magnetic flux when electric current is flowing around the soft iron core. When the magnet is placed on the component, a mag-netic field is established between the north and south poles of the magnet. Another way of indirectly inducting a magnetic field in a material is by using the magnetic field of a current carrying conductor. A circular magnetic field can be es-tablished in cylindrical components by using a central conductor. Typically, one or more cylindrical components are hung from a solid copper bar running through the inside diameter. Current is passed through the copper bar and the resulting circular magnetic field establishes a magnetic field within the test components.


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The use of coils and solenoids is a third method of indirect magnetization. When the length of a component is several times larger than its diame-ter, a longitudinal magnetic field can be estab-lished in the component. The component is placed longitudinally in the concentrated magnetic field that fills the centre of a coil or solenoid. This magnetization technique is often referred to as a "coil shot."

7.26.15 Magnetizing Current As seen in the previous pages, electric current is often used to establish the mag-netic field in components during magnetic particle inspection. Alternating current and direct current are the two basic types of current commonly used. Current from single phase 110 volts, to three phase 440 volts are used when generating an electric field in a component. Current flow is often modified to provide the appropriate field within the part. The type of current used can have an effect on the inspection results so the types of currents commonly used will be briefly reviewed.

Direct Current Direct current (DC) flows continuously in one direction at a constant voltage. A bat-tery is the most common source of direct current. As previously mentioned, current is said to flow from the positive to the negative terminal when in actuality the elec-trons flow in the opposite direction. DC is very desirable when performing magnetic particle inspection in search of subsurface defects because DC generates a magnetic field that penetrates deeper into the material. In ferromagnetic materials, the mag-netic field produced by DC generally penetrates the entire cross-section of the com-ponent; whereas, the field produced using alternating current is concentrated in a thin layer at the surface of the component.

Alternating Current Alternating current (AC) reverses in direction at a rate of 50 or 60 cycles per second. In the United States, 60 cycles current are the commercial norm but 50 cycles cur-rent are common in many countries. Since AC is readily available in most facilities, it is convenient to make use of it for magnetic particle inspection. However, when AC is used to induce a magnetic field in ferromagnetic materials the magnetic field will be limited to narrow region at the surface of the component. This phenomenon is known as "skin effect" and it occurs because induction is not a spontaneous reaction and the rapidly reversing current does not allow the domains down in the material time to align. Therefore, it is recommended that AC be used only when the inspection is limited to surface defects.


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Rectified Alternating Current Clearly, the skin effect limits the use of AC since many inspection applications call for the detection of subsurface defects. However, the convenient access to AC, drive its use beyond surface flaw inspections. Luckily, AC can be converted to current that is very much like DC through the process of rectification. With the use of rectifiers, the reversing AC can be converted to a one-directional current. The three commonly used types of rectified current are described below.

Half Wave Rectified Alternating Current (HWAC) When single phase alternating current is passed through a rectifier, current is al-lowed to flow in only one direction. The reverse half of each cycle is blocked out so that a one directional, pulsating current is produced. The current rises from zero to a maximum and then returns to zero. No current flows during the time when the re-verse cycle is blocked out. The HWAC repeats at same rate as the unrectified current (50 or 60 hertz typical). Since half of the current is blocked out, the amperage is half of the unaltered AC. This type of current is often referred to as half wave DC or pulsating DC. The pulsa-tion of the HWAC helps magnetic particle indications form by vibrating the particles and giving them added mobility. This added mobility is especially important when


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using dry particles. The pulsation is reported to significantly improve inspection sen-sitivity. HWAC is most often used to power electromagnetic yokes. Full Wave Rectified Alternating Current (FWAC) (Single Phase) Full wave rectification inverts the negative current to positive current rather than blocking it out. This produces a pulsating DC with no interval between the pulses. Filtering is usually performed to soften the sharp polarity switching in the rectified current. While particle mobility is not as good as half-wave AC due to the reduction in pulsation, the depth of the subsurface magnetic field is improved.

Three Phase Full Wave Rectified Alternating Current Three phase current is often used to power industrial equipment because it has more favourable power transmission and line loading characteristics. This type of electrical current is also highly desirable for magnetic particle testing because when it is recti-fied and filtered, the resulting current very closely resembles direct current. Station-ary magnetic particle equipment wired with three phase AC will usually have the abil-ity to magnetize with AC or DC (three phase full wave rectified), providing the in-spector with the advantages of each current form.

7.26.16 Longitudinal Magnetic Fields, Distribution and Intensity When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be established in the component. The component is often placed longitudinally in the concentrated magnetic field that fills the centre of a coil or solenoid. This magnetiza-tion technique is often referred to as a "coil shot."

The magnetic field travels through the component from end to end with some flux loss along its length as shown in the image to the right. Keep in mind that the mag-netic lines of flux occur in three dimensions and are only shown in 2D in the image. The magnetic lines of flux are much denser inside the ferromagnetic material than in air because ferromagnetic materials have much higher permeability than air. When the concentrated flux within the material comes to the air at the end of the compo-nent, it must spread out since the air can not support as many lines of flux per unit volume. To keep from cross-ing as they spread out, some of the magnetic lines of flux are forced out the side of the component.

When a component is magnetized along its complete length, the flux loss is small along its length. Therefore, when a component is uniform in cross section and magnetic permeability, the flux density will be relatively uniform throughout the component. Flaws that run normal to the magnetic lines of flux will disturb the flux lines and often cause a leakage field at the surface of the component.


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When a component with considerable length is magnetized using a solenoid, it is possible to magnetize only a portion of the component. Only the material within the solenoid and about the same width on each side of the solenoid will be strongly magnetized. At some distance from the solenoid, the magnetic lines of force will abandon their longitudinal direction, leave the part at a pole on one side of the sole-noid and return to the part at an opposite pole on the other side of the solenoid. This occurs because the magnetizing force diminishes with increasing distance from the solenoid, and, therefore, the magnetizing force may only be strong enough to align the magnetic domains within and very near the solenoid. The unmagnetized portion of the component will not support as much magnetic flux as the magnetized portion and some of the flux will be forced out of the part as illustrated in the image below. Therefore, a long component must be magnetized and inspected at several locations along its length for complete inspection coverage.

Solenoid - An electrically energized coil of insulated wire, which produces a magnetic field within the coil.


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7.26.17 Circular Magnetic Fields, Distribution and Intensity As discussed previously, when current is passed through a solid conductor, a mag-netic field forms in and around the conductor. The following statements can be made about the distribution and intensity of the magnetic field.

• The field strength varies from zero at the center of the component to a maxi-mum at the surface.

• The field strength at the surface of the conductor decreases as the radius of the conductor increases when the current strength is held constant. (However, a lar-ger conductor is capable of carrying more current.)

• The field strength outside the conductor is directly proportional to the current strength. Inside the conductor the field strength is dependent on the current strength, magnetic permeability of the material, and if magnetic, the locations on the B-H curve.

• The field strength outside the conductor decreases with distance from the con-ductor.

In the images below, the magnetic field strength is graphed versus distance from the center of the conductor. It can be seen that in a nonmagnetic conductor carrying DC, the internal field strength rises from zero at the center to a maximum value at the surface of the conductor. The external field strength decrease with distance from the surface of the conductor. When the conductor is a magnetic material, the field strength within the conductor is much greater than it was in the nonmagnetic con-ductor. This is due to the permeability of the magnetic material. The external field is exactly the same for the two materials provided the current level and conductor ra-dius are the same.


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The magnetic field distribution in and around a solid conductor of a nonmag-netic material carrying direct current.

The magnetic field distribution in and around a solid conductor of a magnetic material carrying direct current.

When the conductor is carrying alternating current, the internal magnetic field strength rises from zero at the center to a maximum at the surface. However, the field is concentrated in a thin layer near the surface of the conductor. This is known as the "skin effect." The skin effect is evi-dent in the field strength versus distance graph for a magnet conductor shown to the right. The external field decreases with increasing distance from the surface as it does with DC. It should be remembered that with AC the field is constantly varying in strength and direction.

The magnetic field distribution in and around a solid conductor of a magnetic material carrying alternating current.


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In a hollow circular conductor there is no magnetic field in the void area. The mag-netic field is zero at the inside wall surface and rises until it reaches a maximum at the outside wall surface. As with a solid conductor, when the conductor is a magnetic material, the field strength within the conductor is much greater than it was in the nonmagnetic conductor due to the permeability of the magnetic material. The exter-nal field strength decrease with distance from the surface of the conductor. The ex-ternal field is exactly the same for the two materials provided the current level and conductor radius are the same.

The magnetic field distribution in and around a hollow conductor of a nonmag-netic material carrying direct current.

The magnetic field distribution in and around a hollow conductor of a magnetic material carrying direct current.


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When AC is passed through a hollow circu-lar conductor the skin effect concentrates the magnetic field at the outside diameter of the component.

As can be seen in the field distribution images, the field strength at the inside surface of hollow conductor carrying a circular magnetic field produced by direct magnetization is very low. Therefore, the direct method of magnetization is not rec-ommended when inspecting the inside diameter wall of a hollow component for shallow defects. The field strength in-creases rather rapidly as one moves in from the ID so if the defect has significant depth, it may be detectable.

However, a much better method of magnetizing hollow components for inspection of the ID and OD surfaces is with the use of a central conductor. As can be seen in the field distribution image to the right, when current is passed through a nonmagnetic central conductor (copper bar) the magnetic field produced on the inside diameter surface of a magnetic tube is much greater and the field is still strong enough for defect detection on the OD surface.

The magnetic field distribution in and around a hollow conductor of a magnetic material carrying alternating current.

The magnetic field distribution in and around a nonmagnetic central conductor carrying DC inside a hollow conductor of a magnetic material.


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7.26.18 Demagnetization

After conducting a magnetic particle inspection, it is usually necessary to demagnet-ize the component. Remanent magnetic fields can:

• affect machining by causing cuttings to cling to a component.

• interfere with electronic equipment such as a compass.

• create a condition known as "arc blow" in the welding process. Arc blow may cause the weld arc to wonder or filler metal to be repelled from the weld.

• cause abrasive particle to cling to bearing or faying surfaces and increase wear.

Removal of a field may be accomplished in several ways. This random orientation of the magnetic domains can be achieved most effectively by heating the material above its curie temperature. The Curie temperature for low carbon steel is 770 de-grees C or 1390 degrees F. When steel is heated above its curie temperature, it will become austenitic and loose its magnetic properties. When it is cooled back down it will go through a reverse transformation and will contain no residual magnetic field. The material should also be placed with it long axis in an east-west orientation to avoid any influence of the Earth's magnetic field.

It is often inconvenient to heat a material above it Curie temperature to demagnetize it so another method that returns the material to a nearly unmagnetized state is commonly used. Subject-ing the component to a reversing and decreasing mag-netic field will return the dipoles to a nearly randomly oriented throughout the material. This can be accom-plished by pulling a component out and away from a coil with AC passing through it. The same can also be ac-complished using an electromagnetic yoke with AC se-lected. Also, many stationary magnetic particle inspec-tion units come with a demagnetization feature that slowly reduces the AC in a coil in which the component is placed.

A field meter is often used to verify that the residual flux has been removed from a component. Industry standards usually require that the magnetic flux be re-duced to less than 3 gauss after completing a magnetic particle inspection.


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7.26.19 Measuring Magnetic Fields When performing a magnetic particle inspection, it is very important to be able to determine the direction and intensity of the magnetic field. As discussed previously, the direction of the magnetic field should be between 45 and 90 degrees to the long-est dimension of the flaw for best detectability. The field intensity must be high enough to cause an indication to form, but not too high or nonrelevant indications may form that could mask relevant indications. To cause an indication to form, the field strength in the object must produce a flux leakage field that is strong enough to hold the magnetic particles in place over a discontinuity. Flux measurement devices can provide important information about the field strength. Since it is impractical to measure the actual field strength within the material, all the devices measure the magnetic field that is outside of the material. There are a num-ber of different devices that can be used to detect and measure an external magnetic field. The two devices commonly used in magnetic particle inspection are the field indicator and the Hall Effect meter, which is also often called a gauss meter. Pie gages and shims are devices that are often used to provide an indication of the field direction and strength but do not actually yield a quantitative measure. They will be discussed in a later section.

Field Indicators Field indicators are small mechanical devices that utilize a soft iron vane that will be deflected by a magnetic field. The X-ray image be-low shows the inside working of a field meter looking in from the side. The vane is attached to a needle that rotates and moves the pointer for the scale. Field indicators can be adjusted and calibrated so that quantitative information can be obtained. However, the measurement range of field indicators is usually small due to the mechanics of the device. The one shown to the right has a range from plus twenty gauss to minus twenty gauss. This limited range makes them best suited for measuring the residual magnetic field after demagnetiza-tion.


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Hall-Effect (Gauss/Tesla) Meter A Hall-effect meter is an electronic device that provides a digital readout of the magnetic field strength in gauss or tesla units. The meters use a very small conductive or semiconductor ele-ment at the tip of the probe. Electric current is passed through the conductor. In a magnetic field, the magnetic field exerts a force on the moving electrons which tends to push them to one side of the conductor. A build-up of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The presence of this measurable transverse voltage is called the Hall-effect after Edwin H. Hall who discovered it in 1879.

The voltage generated Vh can be related to the external mag-netic field by the following equation.

Vh = I B Rh / b Where: Vh is the voltage generated. I is the applied direct current. B is the component of the magnetic field that is at a right an-gle to the direct current in the Hall element. Rh is the Hall Coefficient of the Hall element. b is the thickness of the Hall element.

Probes are available with either tangential (transverse) or axial sensing elements. Probes can be purchased in a wide variety of sizes and configura-tions and with different measurement ranges. The probe is placed in the magnetic field such that the magnetic lines of force intersect the major dimensions of the sensing element at a right angle. Placement and orientation of the probe is very im-portant and will be discussed in a later section.


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7.26.20 Portable Magnetizing Equipment

To properly inspect a part for cracks or other defects, it is important to become fa-miliar with the different types of magnetic fields and the equipment used to generate them. As discussed previously, one of the primary requirements for detection of a defect in a ferromagnetic material is that the magnetic field induced in the part must intercept the defect at a 45 to 90 degrees angle. Flaws that are normal (90 degrees) to the magnetic field will produce the strongest indications because they disrupt more of the magnet flux.

Therefore, for proper inspection of a component, it is important to be able to estab-lish a magnetic field in at least two directions. A variety of equipment exists to es-tablish the magnetic field for MPI. One way to classify equipment is based on its portability. Some equipment is designed to be portable so that inspections can be made in the field and some is designed to be stationary for ease of inspection in the laboratory or manufacturing facility. Port-able equipment will be discussed first.

Permanent magnets Permanent magnets are sometimes used for magnetic parti-cle inspection as the source of magnetism. The two primary types of permanent magnets are bar magnets and horseshoe (yoke) magnets. These industrial magnets are usually very strong and may require significant strength to remove them from a piece of metal. Some permanent magnets require over 50 pounds of force to remove them from the surface. Because it is difficult to remove the magnets from the com-ponent being inspected, and sometimes difficult and danger-ous to place the magnets, their use is not particularly popu-lar. However, permanent magnets are sometimes used by a diver for inspection in an underwater environment or other areas, such as in an explosive environment, where electromagnets cannot be used. Permanent magnets can also be made small enough to fit into tight areas where electromagnets might not fit.


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Electromagnets Today, most of the equipment used to create the magnetic field used in MPI are based on electromagnetism. It uses an elec-trical current to produce the magnetic field. An electromagnetic yoke is a very common piece of equipment that is used to establish a magnetic field. It is basically made by wrapping an electrical coil around a piece of soft ferromagnetic steel. A switch is included in the electrical circuit so that the current and, therefore, also the magnetic field can be turn on and off. They can be powered with alternating current from a wall socket or by direct current from a battery pack. This type of magnet generates a very strong magnetic field in a local area where the poles of magnet touch the part to be in-spected. Some yokes can lift weights in excess of 40 pounds.

Figure 7-94: To the left: Portable yoke with battery pack. To the right: Portable

magnetic particle kit.


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Prods Prods are handheld electrodes that are pressed against the surface of the component being inspected to make contact for passing electrical current through the metal. The current passing between the prods creates a circular magnetic field around the prods that is can be used in magnetic particle inspection. Prods are typically made from copper and have an insulated handle to help protect the operator. One of the prods has a trigger switch so that the current can be quickly and easily turned on and off. Sometimes the two prods are connected by an insulator as shown in the image to facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld inspections.

If proper contact is not maintained between the prods and the component surface, electrical arcing can occur and cause damage to the component. For this reason, the uses of prods are not allowed when inspecting aerospace and other critical compo-nents. To help to prevent arcing, the prod tips should be inspected frequently to en-sure that they are not oxidized, covered with scale or other contaminant, or dam-aged.

The figure below shows two prods used to create a current through a conducting part. The resultant magnetic field roughly depicted gives an estimation of the pat-terns expected with magnetic particle on an unflawed surface.

Portable Coils and Conductive Cables Coils and conductive cables are used to establish a longitudinal magnetic field within a component. When a preformed coil is used, the component is placed against the inside surface on the coil. Coils typically have three or five turns of a copper cable within the molded frame. A foot switch is often used to energize the coil. Conductive cables are wrapped around the component. The cable used is typically 00 extra flexi-


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ble or 0000 extra flexible. The number of wraps is determined by the magnetizing force needed and, of course, the length of the cable. Normally the wraps are kept as close together as possible. When using a coil or cable wrapped into a coil, amperage is usually expressed in ampere-turns. Ampere-turns are the amperage shown on the amp meter times the number of turns in the coil.

Figure 7-95: To the left: Portable coil. To the right: Conductive Cable.

Portable Power Supplies Portable power supplies are used to provide the necessary electricity to the prods, coils or cables. Power supplies are commercially available in a variety of sizes. Small power supplies generally provide up to 1,500 A of half wave direct current or alter-nating current when used with a 4.5 meter 0000 cable. They are small and light enough to be carried and operate on either 120 V or 240 V electrical services. When more power is necessary, mobile power supplies can be used. These units come with wheel so that they can be rolled where needed. These units also operate on 120 V or 240 V electrical services and can provide up to 6,000 A of AC or half-wave DC when 9 meters or less of 0000 cable is used.


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7.26.21 Stationary Magnetizing Equipment Stationary magnetic particle inspection equipment is designed for use in labo-ratory or production environment. The most common stationary system is the wet horizontal (bench) unit. Wet hori-zontal units are designed to allow for batch inspections of a variety of com-ponents. The units have head and tail stocks, similar to a lathe but with elec-trical contact that the part can be clamped between for the production of a circular magnetic field using direct magnetization. The tail stock can be moved and locked into place to accommodate parts of various lengths. To assist the operator in clamping the parts, the contact on the headstock can be moved pneu-matically via a foot switch. Most units also have a movable coil that can be moved into place so the indirect magnetization can be used to produce a longitudinal magnetic field. Most coils have five turns and can be obtained in a variety of sizes. The wet magnetic particle solu-tion is collected and held in a tank. A pump and hose system is used to apply the particle solution to the components being inspected. Either the visible or fluorescent particles can be used. Some of the systems offers a variety of options in electrical current used for magnetizing the component. The operator has the option to use AC, half wave DC, or full wave DC. In some units, a demagnetization feature is built in, which uses the coil and decaying AC. To inspect a part using a head-shot, the part is clamped between two electrical contact pads. The magnetic solu-tion, called a bath, is then flowed over the surface of the part. The bath is then interrupted and a magnetizing cur-rent is applied to the part for a short duration, typically 0.5 to 1.5 seconds. (Precautions should be taken to prevent burning or overheating of the part.) A circular field flowing around the circumference of the part is created. Leakage fields from defects then attract the particles forming indi-cations.


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When the coil is used to establish a longitudinal magnetic field within the part, the part is placed on the inside surface of the coil. Just as done with a head shot, the bath is then flowed over the surface of the part. A magnet-izing current is applied to the part for a short duration, typically 0.5 to 1.5 seconds, just after coverage with the bath is interrupted. (Precautions should be taken to pre-vent burning or overheating of the part.) Leakage fields from defects attract the particles forming visible indica-tions.

The wet horizontal unit can also be used to establish a cir-cular magnetic field using a central conductor. This type of a setup is used to inspect parts that are hollow such as gears, tubes, and other ring-shaped objects. A central conductor is an electrically conductive bar that is usually made of copper or aluminium. The bar is inserted through the centre of the hollow part and the bar is then clamped between the contact pads. When current is passed through the central conductor, a circular magnetic field flows around the bar and enters into the part or parts being in-spected.

7.26.22 Multidirectional Magnetizing Equipment Multidirectional units allow the component to be mag-netized in two directions, longitudinally and circumfer-entially, in rapid succession. Therefore, inspections are conducted without the need for a second shot. In mul-tidirectional units, the two fields are balanced so that the field strengths are equal in both directions. These quickly changing balanced fields produce a multidirec-tional field in the component providing detection of defects lying in more than one direction. Just as in conventional wet-horizontal systems, the electrical current used in multidirectional magnetiza-tion may be alternating, half-wave direct, or full-wave. It is also possible to use a combination of currents de-pending on the test applications. Multidirectional mag-


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netization can be used for a large number of production applications, and high vol-ume inspections. To determine adequate field strength and balance of the rapidly changing fields, it requires a little more effort when multidirectional equipment is used. It is desirable to develop the technique using a component with known defects oriented in at least two directions, or a manufactured defect standard. Quantitative Quality Indicators (QQI) are also often used to verify the strength and direction of magnetic fields. 7.26.23 Lights

Magnetic particle inspection can be performed using particles that are highly visible under white lighting conditions or particles that are highly visible under ultraviolet lighting conditions. When an inspection is being performed using the visible color contrast particles, no special lighting is re-quired as long as the area of inspection is well lit. A light intensity of at least 1000 lux (100 fc) is recommended when visible particles are used, but a variety of light sources can be used.

When fluorescent particles are used, special ul-traviolet light must be used. Fluorescence is de-fined as the property of emitting radiation as a result of and during exposure to radiation. Parti-cles used in fluorescent magnetic particle inspec-tions are coated with a material that produces light in the visible spectrum when exposed to the near-ultraviolet light. This "particle glow" provides high contrast indi-cations on the component anywhere particles collect. Particles that fluoresce yellow-green are most common because this color matches the peak sensitivity of the hu-man eye under dark conditions. However, particles that fluoresce red, blue, yellow, and green colors are available.

Ultraviolet Light Ultraviolet light or "black light" is light in the 1,000 to 4,000 Angstroms (100 to 400 nm) wavelength range in the electromagnetic spectrum. It is a very energetic form of light that is invisible to the human eye. Wavelengths above 4,000 Angstroms fall into the visible light spectrum and are seen as the color violet. UV is separated ac-cording to wavelength into three classes: A, B, and C. The shorter the wavelength, the more energy that is carried in the light and the more dangerous it is to the hu-man cells.


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Class UV-A UV-B UV-C

Wavelength Range 3,200–4,000 Angstroms 2,800–3,200 Angstroms 2,800–1,000 Angstroms

The desired wavelength range for use in nondestructive testing is between 3,500 and 3,800 Angstroms with a peak wavelength at about 3,650 A. This wavelength range is used because it is in the UV-A range, which is the safest to work with. UV-B will do an effective job of causing substances to fluoresce, however, it should not be used because harmful effects such as skin burns, and eye damage can occur. This wave-length of radiation is found in the arc created during the welding process. UV-C (1,000 to 2,800) is even more dangerous to living cells and is used to kill bacteria in industrial and medical settings.

The desired wavelength range for use in NDT is obtained by filtering the ultraviolet light generated by the light bulb. The output of a UV bulb spans a wide range of wavelengths. The short wave lengths of 3,120 A to 3,340 A are produced in low lev-els. A peak wavelength of 3650 A is produced at a very high intensity. Wavelengths in the visible violet range (4050 A to 4350 A), green-yellow (5460 A), yellow (6220 A) and orange (6770 A) are also usually produced. The filter allows only radiation in the range of 3200 to 4000 angstroms and a little visible dark purple to pass.

Basic Ultraviolet Lights UV bulbs come in a variety on shapes and sizes. The more common types are the low pressure tube, high pressure spot, and the high pressure flood types. The tubu-lar black light is similar in construction to the tubular fluorescent lights used for of-fice or home illumination. These lights use a low pressure mercury vapor arc. Tube lengths of 6 to 48 inches are common. The low pressure bulbs are most often used to provide general illumination to large areas rather than for illumination of compo-nents to be inspected. These bulbs generate a relatively large amount of white light that is a concern as inspection specifications require less than two foot candles of white light at the inspection surface.

Flood lights are also used to illuminate the inspection area as they provide even illu-mination over a large area. Intensity levels for flood lamps are relatively low because the energy is spread over a large area. They generally do not generate the required UV light intensity at the given distance that specifications require.


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Spot lights on the other hand provide concentrated energy that can be directed to the area of inspec-tion. A spot light will generate a six inch diameter circle of high intensity light when held fifteen inches from the inspection surface. 100 watt mercury vapor lights are most commonly used, but higher watt-age's are available.

In the high pressure mercury vapor spot or flood lamps, UV light is generated by a quartz tube inside the bulb. This tube contains two electrodes that es-tablish an arc. The distance between electrodes is such that a starting electrode must be used. A re-sister limits the current to the starting electrode that establishes the initial arc that vaporizes the mercury in the tube. Once this low level arc is established and the mercury is vaporized the arc between the main electrodes is established. It takes approximately five minutes to "warm up" and establish the arc between the main electrodes. This is why specifications require a "warm up time" before using the high pressure mercury vapor lights. Flood and spot black lights pro-duce large amounts of heat and should be handled with caution to prevent burns. This condition has been eliminated by newer designs that include cooling fans. The arc in the bulb can be upset when exposed to an external magnetic field, such as that generated by a coil. Care should be taken not to bring the lamp close to strong magnetic fields, but if the arc is upset and extinguished, it must be allowed to cool before it can be safely restarted.

High Intensity Ultraviolet Lights The 400 watt metal halide bulbs or "super lights" can be found in some facilities. This super bright light will provide adequate lighting over an area of up to ten times of that covered by the 100 watt bulb. Due to their high in-tensity, excessive light reflecting from the surface of a component is a concern. Moving the light a greater dis-tance from the inspection area will generally reduce this glare. Another type of high intensity light available is the micro discharge light. This particular light produces up to ten times the amount of UV light conventional lights pro-duce. Readings of up to 60,000 uW/cm2 at 15 inches can be achieved.


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7.26.24 Magnetic Field Indicators Determining whether a magnetic field is of adequate strength and in the proper di-rection is critical when performing magnetic particle testing. As discussed previously, knowing the direction of the field is important because the field should be as close to perpendicular to the defect as possible and no more than 45 degrees from normal. Being able to evaluate the field direction and strength is especially important when inspecting with a multidirectional machine, because when the fields are not balanced properly a vector field will be produced that may not detect some defects.

There is actually no easy to apply method that permits an exact measurement of field intensity at a given point within a material. In order to measure the field strength it is necessary to intercept the flux lines. This is impossible without cutting into the material and cutting the material would immediately change the field within the part. However, cutting a small slot or hole into the material and measuring the leakage field that crosses the air gap with a Gauss meter is probably the best way to get an estimate of the actual field strength within a part. Nevertheless, there are a number of tools and methods available that are used to determine the presence and direction of a field surrounding the component.

Gauss Meter or Hall Effect Gage A Gauss meter with a Hall Effect probe is commonly used to measure the tangential field strength on the surface of the part. As discussed in some detail on the "Measur-ing Magnetic Fields" page, the Hall Effect is the transverse electric field created in a conductor when placed in a magnetic field. Gauss meters, also called Tesla meters, are used to measure the strength of a field tangential to the surface of the magnet-ized test object. The meters measure the intensity of the field in the air adjacent to the component when a magnetic field is applied.

The advantages of Hall Effect devices are; they provide a quantitative measure of the strength of magnetizing force tangential to the surface of a test piece, they can be used for measurement of residual magnetic fields, and they can be used repeti-tively. Their main disadvantages are that they must be periodically calibrated, and they cannot be used to establish the balance of fields in multidirectional applications.

Quantitative Quality Indicator (QQI) The Quantitative Quality Indicators (QQI) or Artificial Flaw Standard are often the preferred method of assuring proper field direction and adequate field strength. The use of QQI is also the only practical way of ensuring balanced field intensity and di-rection in multiple-direction magnetization equipment. QQI are often used in con-junction with a Gauss meter to establish the inspection procedure for a particular component. They are used with the wet method only and, as other flux sharing de-vices; they can only be used when continuous magnetization is used.

The QQI is a thin strip of either 0.002 or 0.004 inch thick AISI 1005 steel. A photo etch process is used to inscribe a specific pattern, such as concentric circles or a plus sign. QQI are nominally 3/4 inch square, but miniature shims are also available. QQI


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must be in intimate contact with the part being evaluated. This is accomplished by placing the shim on a part etched side down, and taping or gluing it to the surface. The component is then magnetized and particles applied. When the field strength is adequate, the particles will adhere over the engraved pattern and provide informa-tion about the field direction. When a multidirectional technique is used, a balance of the fields is noted when all areas of the QQI produce indications. Some of the advantages of QQI are: they can be quantified and related to other pa-rameters; they can accommodate virtually any configuration with suitable selection; and they can be reused with careful application and removal practices. Some of the disadvantages are: the application process is somewhat slow, the parts must be clean and dry; shims cannot be used as a residual magnetism indicator as they are a flux sharing device; they can be easily damaged with improper handling and will cor-rode if not cleaned and properly stored.

Figure 7-96: Left is a photo of a typical QQI shim. The photo on the right shows

the indication produced by the QQI when it is applied to the surface a part and a

magnetic field is established that runs across the shim from right to left.

Pie Gage The pie gage is a disk of highly permeable material divided into four, six, or eight sections by nonferromagnetic material. The division serve as artificial defects that radiate out in different directions from the centre. Diameter of the gage is ¾ to 1 inch. The divisions between the low carbon steel pie sections are to be no greater than 1/32 inch. The sections are furnace brazed and copper plated. The gage is placed on the test piece copper side up, and the test piece is magnetized. After par-ticles are applied, and excess removed, the indications provide the inspector the ori-entation of the magnetic field.


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The principal application is on flat surfaces such as weldments or steel castings where dry powder is used with a yoke or prods. The pie gage is not recommended for precision parts with complex shapes, for wet-method applications, or for proving field magnitude. The gage should be demagnetized between readings. Several of the main advantages of the pie gage are: it is easy to use and it can be used indefinitely without deterioration. The pie gage has several disadvantages, which include: it retains some residual magnetism so indications will prevail after removal of the source of magnetization, it can only be used in relatively flat areas, and it cannot be reliably used for determination of balanced fields in multidirectional magnetization.

Slotted Strips Slotted strips, also known as Burmah-Castrol Strips, are pieces of highly permeable ferromagnetic material with slots of different widths. They are placed on the test object as it is inspected. The indications produced on the strips give the inspector a general idea of the field strength in a particular area.

Advantages of these strips are: they are relatively easily applied to the component; they can be used successfully with either the wet or dry method when using the continuous magnetization; they are repeatable as long as orientation to the magnetic field is maintained and they can be used repetitively. Disadvantages include: they cannot be bent to complex configuration; and they are not suitable for multidirectional field applications since they indicate defects in only one direction.


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7.26.25 Magnetic Particles As mentioned previously, the particles that are used for magnetic particle inspection are a key ingredient as they form the indications that alert the inspector to defects. Particles start out as tiny milled (a machining process) pieces of iron or iron oxide. A pigment (somewhat like paint) is bonded to their surfaces to give the particles col-our. The metal used for the particles has high magnetic permeability and low reten-tivity. High magnetic permeability is important because it makes the particles attract easily to small magnetic leakage fields from discontinuities, such as flaws. Low re-tentivity is important because the particles themselves never become strongly mag-netized so they do not stick to each other or the surface of the part. Particles are available in a dry mix or a wet solution.

Dry Magnetic Particles Dry magnetic particles can typically be purchased in red, black, grey, yellow and several other colours so that a high level of contrast between the particles and the part being inspected can be achieved. The size of the magnetic particles is also very important. Dry magnetic particle products are produced to include a range of particle sizes. The fine particles are around 50 µm (0.002 inch) in size are about three times smaller in diameter and more than 20 times lighter than the coarse particles (150 µm or 0.006 inch), which make them more sensitive to the leakage fields from very small discontinuities. However, dry testing particles cannot be made exclusively of the fine particles. Coarser particles are needed to bridge large discontinuities and to reduce the powder's dusty nature. Additionally, small particles easily adhere to surface contamination, such as remanent dirt or moisture, and get trapped in surface roughness features producing a high level of background. It should also be recognized that finer particles will be more easily blown away by the wind and, therefore, windy conditions can reduce the sensitivity of an inspection. Also, reclaiming the dry particles is not recommended because the small particles are less likely to be recaptured and the "once used" mix will result in less sensitive inspections.


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The particle shape is also important. Long, slender particles tend to align themselves along the lines of magnetic force. However, research has shown that if dry powder con-sists only of long, slender particles, the ap-plication process would be less than desir-able. Elongated particles come from the dispenser in clumps and lack the ability to flow freely and form the desired "cloud" of particles floating on the component. There-fore, globular particles are added that are shorter. The mix of globular and elongated particles results in a dry powder that flows well and maintains good sensitivity. Most dry particle mixes have particle with L/D ratios between one and two. Wet Magnetic Particles Magnetic particles are also supplied in a wet suspension such as water or oil. The wet magnetic particle testing method is generally more sensitive than the dry because the suspension provides the particles with more mobility and makes it possible for smaller particles to be used since dust and adherence to surface contamination is reduced or eliminated. The wet method also makes it easy to apply the particles uni-formly to a relatively large area. Wet method magnetic particles products differ from dry powder products in a num-ber of ways. One way is that both visible and fluorescent particles are available. Most nonfluorescent particles are ferromagnetic iron oxides, which are either black or brown in colour. Fluorescent particles are coated with pigments that fluoresce when exposed to ultraviolet light. Particles that fluoresce green-yellow are most common to take advantage of the peak colour sensitivity of the eye but other fluorescent col-ours are also available. (For more information on the colour sensitivity of the eye, see the penetrant inspection material.)

The particles used with the wet method are smaller in size than those used in the dry method for the reasons mentioned above. The particles are typically 10 µm (0.0004 inch) and smaller and the synthetic iron oxides have particle diameters around 0.1 µm (0.000004 inch). This very small size is a result of the process used to form the particles and is not particularly desirable, as the particles are almost too fine to set-


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tle out of suspension. However, due to their slight residual magnetism, the oxide particles are present mostly in clusters that settle out of suspension much faster than the individual particles. This makes it possible to see and measure the concen-tration of the particles for process control purposes. Wet particles are also a mix of long slender and globular particles. The carrier solutions can be water- or oil-based. Water-based carriers form quicker indications, are generally less expensive, present little or no fire hazard, give off no petrochemical fumes, and are easier to clean from the part. Water-based solutions are usually formulated with a corrosion inhibitor to offer some corrosion protection. However, oil-based carrier solutions offer superior corrosion and hydrogen embrit-tlement protection to those materials that are prone to attack by these mechanisms.

7.26.26 Suspension Liquids Suspension liquids used in the wet magnetic particle inspection method can be either a well refined light petroleum distillate or water containing additives. Petroleum-based liquids are the most desirable carriers because they provided good wetting of the surface of metallic parts. However, water-based carriers are used more because of low cost, low fire hazard, and the ability to form indications quicker than solvent-based carriers. Wa-ter-based carriers must contain wetting agents to disrupt surface films of oil that may exist on the part and to aid in the dispersion of magnetic particles in the carrier. The wetting agents create foaming as the solution is moved about, so anti-foaming agents must be added. Also, since water promotes corrosion in ferrous materials, corrosion inhibitors are usually added as well. Petroleum based carriers are primarily used in sys-tems where maintaining the proper particle concen-tration is a concern. The petroleum based carriers require less maintenance because they evaporate at a slower rate than the water-based carriers. Therefore, petroleum based carriers might be a better choice for a system that only gets occasional use and adjusting the carrier volume with each use is undesirable. Modern solvent carri-ers are specifically designed with properties that have flash points above 200 de-grees F and keep nocuous vapours low. Petroleum carriers are required to meet cer-tain specifications such as AMS 2641.


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7.26.27 Testing Practices Dry Particle Inspection In this magnetic particle testing technique, dry particles are dusted onto the surface of the test object as the item is magnetized. Dry particle inspection is well suited for the inspections conducted on rough surfaces. When an electro-magnetic yoke is used, the AC or half wave DC current creates a pulsating magnetic field that provides mobility to the powder. The primary applications for dry powders are ungrounded welds and rough as-cast surfaces.

Dry particle inspection is also used to detect shallow subsurface cracks. Dry particles with half wave DC is the best approach when inspecting for lack-of-root penetration in welds of thin ma-terials. Half wave DC with prods and dry particles is commonly used when inspecting large castings for hot tears and cracks.

Steps in performing an inspection using dry particles Prepare the part surface - the surface should be relatively clean but this is not as critical as it is with liquid penetrant in-spection. The surface must be free of grease, oil or other moisture that could keep particles from moving freely. A thin layer of paint, rust or scale will reduce test sen-sitivity but can sometimes be left in place with adequate results. Specifications often allow up to 0.003 inch (0.076 mm) of a nonconductive coating (such as paint) and 0.001 inch max (0.025 mm) of a ferromagnetic coating (such as nickel) to be left on the surface. Any loose dirt, paint, rust or scale must be removed. Apply the magnetizing force - Use permanent magnets, an electromagnetic yoke, prods, a coil or other means to establish the necessary magnetic flux. Dust on the dry magnetic particles - Dust on a light layer of magnetic particles. Gently blow off the excess powder - With the magnetizing force still applied, remove the excess powder from the surface with a few gently puffs of dry air. The force of the air needs to be strong enough to remove the excess particle but not strong enough to dislodge particle held by a magnetic flux leakage field. Terminate the magnetizing force - If the magnetic flux is being generated with an electromagnet or an electromagnetic field, the magnetizing force should be ter-minated. If permanent magnets are being used, they can be left in place. Inspect for indications - Look for areas where the magnetic particles are clus-tered.


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Wet Suspension Inspection Wet suspension magnetic particle inspection, or more commonly wet magnetic parti-cle inspection, involves applying the particles while they are suspended in a liquid carrier. Wet magnetic particle inspection is most commonly performed using a sta-tionary, wet, horizontal inspection unit but sus-pensions are also available in spray cans for use with an electromagnetic yoke. A wet inspection has several advantages over a dry inspection. First, all the surfaces of the component can be quickly and easily covered with a relatively uni-form layer of particles. Second, the liquid carrier provides mobility to the particles for an extended period of time, which allows enough particles to float to small leakage fields to form a visible indi-cation. Therefore, wet inspection is considered best for detecting very small discontinuities on smooth surfaces. On rough surfaces, however, the particles (which are much smaller in wet sus-pensions) can settle in the surface valleys and loose mobility rendering them less effective than dry powders under these condi-tions. Steps in performing an inspection using wet suspensions Prepare the part surface - Just as is required with dry particle inspections, the surface should be relatively clean. The surface must be free of grease, oil and other moisture that could prevent the suspension from wetting the surface and preventing the particles from moving freely. A thin layer of paint, rust or scale will reduce test sensitivity, but can sometimes be left in place with adequate results. Specifications often allow up to 0.003 inch (0.076 mm) of a nonconductive coating (such as paint) and 0.001 inch max (0.025 mm) of a ferromagnetic coating (such as nickel) to be left on the surface. Any loose dirt, paint, rust or scale must be removed. Apply the suspension - The suspension is gently sprayed or flowed over the sur-face of the part. Usually, the stream of suspension is diverted from the part just be-fore the magnetizing field is applied. Apply the magnetizing force - The magnetizing force should be applied immedi-ately after applying the suspension of magnetic particles. When using a wet horizon-tal inspection unit, the current is applied in two or three short busts (1/2 second) which helps to improve particle mobility. Inspect for indications - Look for areas where the magnetic particles are clus-tered. Surface discontinuities will produce a sharp indication. The indications from subsurface flaws will be less defined and loose definition as depth increases.


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7.26.28 Inspection using Magnetic Rubber The magnetic rubber technique was developed for detecting very fine cracks and is capable of revealing finer cracks than other magnetic techniques. Additionally, the technique can be use to examine difficult to reach areas, such as the threads on the inside diameter of holes, where the moulded plugs can be removed and examined under ideal conditions and magnification if desired. The trade-off, of course, is that inspection times are much longer. The technique uses a liquid (uncured) rubber containing suspended magnetic parti-cles. The rubber compound is applied to the area to be inspected on a magnetized component. Inspections can be performed using either an applied magnetic field, which is maintained while the rubber sets (active field), or the residual field from magnetization of the component prior to pouring the compound. A dam of modelling clay is often used to contain the compound in the region of interest. The magnetic particles migrate to the leakage field caused by a discontinuity. As the rubber cures, discontinuity indications remain in place on the rubber. The rubber is allowed to completely set, which takes from 10 to 30 minutes. The rubber cast is removed from the part. The rubber conforms to the surface contours and provides a reverse replica of the surface. The rubber cast is examined for evi-dence of discontinuities, which appear as dark lines on the surface of the moulding. The moulding can be retained as a permanent record of the inspection. Magnetic rubber methods require similar magnetizing systems used for dry method magnetic particle tests. The system may include yokes, prods, clamps, coils or cen-tral conductors. Alternating, direct current, or permanent magnets may be used to draw the particles to the leakage fields. The direct current yoke is the most common magnetization source for magnetic rubber inspection.

7.26.29 Continuous and Residual Magnetization Techniques In magnetic particle inspection, the magnetic particles can either be applied to the component while the magnetizing force is applied, or after it has been stopped. Continuous magnetization de-scribes the technique where the magnetizing force is ap-plied and maintained while the magnetic particles are dusted or flowed onto the surface of the component. In a wet horizontal testing unit the application of the particles is stopped just before the magnetizing force is applied but since particles are still flowing over and covering the surface, this is considered continuous magnetization. Residual magnetization, on the other hand, describes the technique where the magnetizing force is applied to magnetize the component and then stopped before ap-plying the magnetic particles. Only the residual field of the magnetized component is used to attract magnetic particles and produce an indication.


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The continuous technique is generally chosen when maximum sensitivity is required because it has two distinct advantages over the residual technique. First, the mag-netic flux will be highest when current is flowing and, therefore, leakage fields will also be strongest. Field strength in a component depends primarily on two vari-ables...the applied magnetic field strength and the permeability of the test object. Viewing the upper right portion of the hysteresis loop below, it is evident that the magnetic flux will be strongest when the magnetizing force is applied. If the magnet-izing force is strong enough, the flux density will reach the point of saturation. When the magnetizing force is removed, the flux density will drop to the retentivity point. The two grey traces show the path the flux density would follow if the magnetizing force was applied and removed at levels below that required to reach saturation. It can be seen that the flux density is always highest while the magnetizing current is applied. This is independent of the permeability of a material.

However, the permeability of the material is very important. High permeability mate-rials do not retain a strong magnetic field so flux leakage fields will be extremely weak or nonexistent when the magnetizing force is removed. Therefore, materials with high magnetic permeability are not suited for inspection using the residual tech-nique. When the residual technique is used to inspect materials with low permeabil-ity, care should be taken to ensure that the residual field is of the necessary strength to produce an indication. Defects should be relatively large and surface breaking to have a high probability of detection using the residual method.


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The second advantage of the continuous technique is that when current is used to generate the magnetizing force, it can provide added particle mobility. Alternating or pulsed direct current will cause the particles to vibrate and move slightly on the sur-face of the part. This movement allows the particles to travel to leakage sites. More particles mean brighter indications compared to those formed using the residual technique. One disadvantage of the continuous method is that heating of the component occurs when using direct magnetization. For example, when prods are used, they may cre-ate areas of localized heating when the continuous technique is used. This may be acceptable on components that will be further processed removing this condition but machined or in-service components may be adversely affected by this condition. While generally not recommended, the residual technique does have its uses. It is commonly used in automated inspection systems to inspect materials with high re-tentivity. To speed throughput, automated systems often magnetize the parts and then submerge them in an agitated magnetic particle bath or pass them through a spray station. Closely controlled automated systems provided good results using the residual magnetism technique.

7.26.30 Field Direction and Intensity

Field Direction As discussed previously, determining the direction of the field is important when conducting a magnetic particle inspection because the defect must produce a signifi-cant disturbance in the magnetic field to produce an indication. It is difficult to detect discontinuities that intersect the magnetic field at an angle less than 45 degrees. When the orientation of a defect is not well established, components should be mag-netized in a minimum of two directions at approximately right angles to each other. Depending on the geometry of the component, this may require longitudinal mag-netization in two or more directions, multiple longitudinal and circular magnetization or circular magnetization in multiple directions. Determining strength and direction of the fields is especially critical when inspecting with a multidirectional machine. If the fields are not balanced a vector field will be produced that may not detect some de-fects.

Depending on the application, pie gages, QQI, and a gauss meter can be used to check the field direction. The PIE gage is generally only used with dry powder inspec-tions. QQI shims can be used in a variety of applications but are the only method recommended for use in establishing balanced fields when using multidirectional equipment.

Field Strength The applied magnetic field must have sufficient strength to produce a satisfactory indication, but not so strong that it produces nonrelevant indications or limits particle mobility. If the magnetizing current is excessively high when performing a wet fluo-rescent particle inspection, particles can be attracted to the surface of the part and not allowed to migrate to the flux leakage fields of defects. When performing a dry


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particle inspection, an excessive longitudinal magnetic field will cause furring. Furring is when magnetic particles build up at the magnetic poles of a part. When the field strength is excessive, the magnetic field is forced out of the part before reaching the end of the component and the poles along its length attract particle and cause high background levels. Adequate field strength may be determined by:

• performing an inspection on a standard specimen that is similar to the test component and has known or artificial defects of the same type, size, and lo-cation as those expected in the test component. QQI shims can sometimes be used as the artificial defects.

• using a gauss meter with a Hall Effect probe to measure to the peak values of the tangent field at the surface of the part in the region of interest. Most specifications call for a field strength of 30 to 60 gauss at the surface when the magnetizing force is applied.

• looking for light furring at the ends pipe and bar when performing dry parti-cle inspections of pipe, bar and other uncomplicated shapes.

Formula for calculating current levels should only be used to estimate current re-quirements. The magnetic field strength resulting from calculations should be as-sessed for adequacy using one of the two method discussed above. Likewise, pub-lished current level information should also be used only as a guide unless the values have been established for the specific component and target defects of the inspec-tion at hand.

Using a PIE Gage A PIE gage is placed copper side up and held in contact with the component as the magnetic field and particles are applied. Indications of the leakage fields provide a visual representa-tion of defect direction within the component. PIE gages work well on flat surfaces, but if the surface is concave or convex inaccurate readings may occur. The PIE gage is a flux sharing device and requires good contact to provide accurate read-ings.

Using Quantitative Quality Indicator (QQI) Shims Quantitative Quality Indicator (QQI) flaw shims are used to establish proper field direction and to ensure adequate field strength during technique development. The QQI flaw shim is the most efficient means of determining balance and effectiveness of fields. The QQI are also flux sharing devices and must be properly attach so as not allow particle to be trapped under the artificial flaw. Application using super glue is the preferred way of attaching the artificial flaw, but does not allow for reuse of the shims. Shims can also be attached with tape applied to just the edge of the shim. It is recommended that the tape be impervious to oil, not be fluorescent, and be 1/4 to 1/2 inch in width.


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The QQI must be applied to locations on the component where the flux density may very. One example would be the center area of a yoke or Y shaped component. Of-ten the flux density will be near 0 in this area. If two legs of a Y are in contact with the pad in circular magnetization it must be determined if current is flowing evenly through each leg. A QQI on each leg would be appropriate under such conditions.

QQI's can be used to establish system threshold values for a defect of a given size. By attaching a QQI shim with three circles (40%, 30% and 20% of shim thickness) to the threshold values for a specific area of the component can be established. Be-gin by applying current at low amperage and slowly increasing it until the largest flaw is obtained. The flux density sound is verified and recorded using a Hall effects probe. The current is then increased until the second circle is identified and the flux density is again recorded. As the current is raised more, the third ring is identified and the current values are recorded.

Hall Effects Gauss Meter There is several types of Hall effects probes that can be used to measure the magnetic field strength. Transverse probes are the type most commonly used to evaluate the field strength in magnetic particle testing. Transverse probes have the Hall Effect element mounted in a thin, flat stem and they are used to make measurements be-tween two magnetic poles. Axial probes have the sensing element mounted such that the magnetic flux in the direc-tion of the long axis of the probe is measured. To make a measurement with a transverse probe, the probe is positioned such that the flat surface of the Hall Effect element is transverse to the magnetic lines of flux. The Hall Effect voltage is a function of the angle at which the magnetic lines of flux pass through the sensing element. The greatest Hall Effect voltage occurs when the lines of flux pass perpendicularly through the sensing ele-ment. If not perpendicular, the output voltage is related to the cosine of the differ-ence between 90 degrees and the actual angle. The peak field strength should be measured when the magnetizing force is applied. The field strength should be measured in all areas of the component to be inspected.


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7.26.31 Particle Concentration and Condition Particle Concentration The concentration of particles in the suspension is a very important parameter in the inspection process and must be closely controlled. The particle concentration is checked af-ter the suspension is prepared and continued regularly as part of the quality system checks. ASTM E-1444-01 requires concentration checks to be performed every eight hours or every shift change. The standard process used to perform the check requires agitating the carrier for a minimum of thirty minutes to en-sure even particle distribution. A sample is then taken in a pear-shaped 100 ml centrifuge tube having a stem gradu-ated to 1.0 ml in 0.05 ml increments for fluorescent parti-cles, and graduated to 1.5 ml. in 0.1 ml increments for visi-ble particles. The sample is then demagnetized so that the particles do not clump together while settling. The sample must then remain undisturbed for a minimum of 60 minutes for a petroleum-based carrier or 30 minutes for a water-based carrier, unless shorter times have been documented to produce results similar to the longer settling times. The volume of settled particles is then read. Acceptable ranges are 0.1 to 0.4 ml for fluorescent particles and 1.2 to 2.4 ml for visible particles. If the particle concentration is out of the acceptable range, particles or the carrier must be added to bring the solution back in compliance with the requirement. Particle loss is often attributed to "drag out". Drag out occurs because the solvent easily runs off components and is recaptured in the holding tank. Particles, on the other hand, tend to adhere to components, or be trapped in geometric features of the component. These particles will be "drug out" or lost to the system, and will eventually need to be replaced.

Particle Condition After the particles has settled, they should be examined for brightness and agglomeration. Fluorescent particles should be evaluated under ultraviolet light and visible particles under white light. The brightness of the particles should be evaluated weekly by comparing the particles in the test solution to those in an unused reference solution that was saved when the solution was first prepared. The brightness of the two solutions should be relatively the same. Additionally, the particles should appear loose and not lumped to-gether. If the brightness or the agglomeration of the particles is noticeably different from the reference solution, the bath should be replaced.


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7.26.32 Lighting Magnetic particle inspection predominately relies on visual inspection to detect any indications that are formed. Therefore, lighting is a very important element of the inspection process. Obviously, the lighting requirements are different for an inspec-tion conducted using visible particles than they are for an inspection conducted using fluorescent particles. The lighting requirements for each of these techniques, as well as how light measurements are made, are discussed below. Light Requirements When Using Visible Particles Magnetic particle inspections conducted using visible particles can be conducted us-ing natural lighting or artificial lighting. When using natural lighting, it is important to keep in mind that daylight varies from hour to hour. Inspector must stay constantly aware of the lighting conditions and make adjustments when needed. To improve uniformity in lighting from one inspection to the next, the use of artificial lighting is recommended. Artificial lighting should be white whenever possible and white flood or halogen lamps are most commonly used. The light intensity is required to be 100 foot-candles at the surface being inspected. It is advisable to choose a white light wattage that will provide sufficient light, but avoid excessive reflected light that could distract from the inspection.

Light Requirements When Using Fluorescent Particles

Ultraviolet Lighting When performing a magnetic particle inspection using fluorescent particles, the condition of the ultraviolet light and the ambient white light must be monitored. Stan-dards and procedures require verification of lens condition and light intensity. Black lights should never be used with a cracked filter as output of white light and harmful black light will be increased. The cleanliness of the filter should also be checked as a coating of solvent carrier, oils, or other foreign materials can reduce the intensity by up to as much as 50%. The filter should be checked visually and cleaned as necessary before warm-up of the light.

For UV lights used in component evaluations, the nor-mally accepted intensity is 1000 microwatts per square centimetre when measured at 15 inches from the filter face (requirements can vary from 800 to 1200). The required check should be performed when a new bulb is in-stalled, at start-up of the inspection cycle, if a change in intensity is noticed, or every eight hours if in continuous use. Regularly checking the intensity of UV lights is very important because bulbs loose intensity over time. In fact, a bulb that is near the end of its operating life will often have an intensity of only 25 percent of its original output. Black light intensity will also be affected by voltage variations, so it is important to provide constant voltage to the light. A bulb that produces acceptable intensity at 120 volts will produce significantly less at 110 volts.


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Ambient White Lighting When performing a fluorescent magnetic particle inspection, it is important to keep white light to a minimum as it will significantly reduce the inspector’s ability to detect fluorescent indications. Light levels of less than 2 fc are required by most procedures with some procedures requiring less than 0.5 fc at the inspection surface. When checking black light intensity at 15 inches a reading of the white light produced by the black light may be required to verify white light is being removed by the filter.

White Light for Indication Confirmation While white light is held to a minimum in fluorescent inspections, procedures may require that indications be evaluated under white light. The white light requirements here are the same as when performing an inspection with visible particles. The minimum light intensity at the surface being inspected must be 100 foot-candles.

Light Measurement Light intensity measurements are made using a radiometer. A radiometer is an instrument that translates light energy into an electrical current. Light striking a silicon photodiode detector causes a charge to build up between internal layers. When an external circuit is connected to the cell, an electrical current is produced. This current is linear with respect to incident light. Some radiometers have the ability to measure both black and white light, while others require a separate sensor for each measurement. Whichever type used, the sensing area should be clean and free of any materials that could reduce or obstruct light reaching the sensor. Radiometers are relatively unstable instruments and readings often change considerably over time. Therefore, they must be calibrated regularly. They should be calibrated at least every six months. A unit should be checked to make sure its calibration is current before taking any light readings.

Ultraviolet light measurements should be taken using a fixture to maintain a minimum distance of 15 inches from the filter face to the sensor. The sensor should be centred in the light field to obtain and record the high-est reading. UV spot lights are often focused so inten-sity readings will vary considerable over a small area. White lights are seldom focused and depending on the wattage, will often produce in excess of the 100 fc at 15 inches. Many specifications do not require the white light intensity check to be conducted at a specific dis-tance.


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7.26.33 Eye Consideration Eye Adaptation Just as lighting is an important consideration in the inspection process, so is the eyes response to light. Scientists have recently discovered that a special, tiny group of cells at the back of the eye help tell the brain how much light there is, causing the pupil to get bigger or smaller. The change in pupil diameter is not instantaneous and, therefore, eyes must be given time to adapt to changing lighting conditions. When performing a fluorescent mag-netic particle inspection, the eye must be given time to adapt to the darkness of the inspection booth before beginning to look for indications. Dark adaptation time of at least one minute is required by most procedures. Some studies recommend adaptation time of five min-utes if entering an inspection area from direct sunlight. Technicians should be aware of, and adhere to, the adaptation time procedures re-quirements as it is quite easy to overlook and begin inspection before the eyes have adjusted to the darkened conditions.

Eyeball Fluorescence When ultraviolet light enters the human eye, the fluid that fills the eye fluoresces. This condition is called eyeball fluorescence, and while it is considered harmless, it is annoying and interferes with vision while it exists. When working around ultraviolet lights, one should be careful not to look directly into lights and to hold spot lights to avoid reflection. UV light will be reflected from surfaces just as white light will and, therefore it is advisable to consider placement of lights to avoid this condition. Spe-cial filtered glasses may be worn by the inspector to remove all UV light from reach-ing the eyes but allowing yellow-green light from fluorescent indications to pass. Technicians should never wear darkened or photo chromatic glasses as these glasses also filter or block light from fluorescent indications.

7.26.34 Examples of Visible Dry Indications

One of the advantages that a magnetic particle inspection has over some of the other nondestructive evaluation methods is that flaw indications generally resemble the actual flaw. This is not the case with NDT methods such as ultrasonic and eddy current inspection, where an electronic signal must be interpreted. When magnetic particle inspection is used, cracks on the surface of the part appear as sharp lines that follow the path of the crack. Flaws that exist below the surface of the part are less defined and more difficult to detect. Below are some examples of magnetic par-ticle indications produced using dry particles.


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Figure 7-97: Indication of a crack in a saw blade.

Figure 7-98: Indication of cracks in a weldment.

Figure 7-99: Indication of cracks originating at a fastener hole.


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Figure 7-100: Before and after inspection pictures of cracks emanating from a

hole.

Figure 7-101: Indication of cracks running between attachment holes in a hinge.


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7.26.35 Examples of Fluorescent Wet Indications The indications produced using the wet magnetic particles are sharper than dry par-ticle indications formed on similar defects. When fluorescent particles are used, the visibility of the indications is greatly improved because the eye is drawn to the "glowing" regions in the dark setting. Below are a few examples of fluorescent wet magnetic particle indications.

Figure 7-102: Magnetic particle wet fluorescent indication of cracks in a drive

shaft.

Figure 7-103: Magnetic particle wet fluorescent indication of a crack in a bear-

ing.


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Figure 7-104: Magnetic particle wet fluorescent indication of a crack in the crane

hook.

Figure 7-105: Magnetic particle wet fluorescent indication of a crack at a sharp

radius.


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Figure 7-106: Magnetic particle wet fluorescent indication of a crack in casting.

Figure 7-107: Magnetic particle wet fluorescent indication of cracks at a fastener

hole.


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e

d

7.27 Strain gauging

In principle this method can be used on steel, concrete and masonry structures. It is

most commonly used on steel structures. The strain gauge is the fundamental sens-

ing element in many types of sensors such as pressure sensors, load cells, torque

sensors etc. The strain gauge may also be used to monitor the strain at a given loca-

tion on a structure.

Most strain gauges are foil types. When the foil is subjected to strain the resistance of the foil changes in a defined way. When the strain gauge is connected into a Wheatstone bridge the change of the resistance and thereby the change of strain in the strain gauge may be registered.

7.27.1 Measurement Principle In principle, a strain gauge is an electrical-resistance wire bonded to a backing sheet of foil. It is usually cemented onto the object to be measured. When the object is loaded, the gauge follows the deformation of the material, and its resistance changes in proportion to the deformation. The extremely small changes in the resistance in the gauge are recorded, and then used to determine the strain in the material.

Figure 7-108: Different types of Strain Gauges (TML Gauge).

7.27.2 Gauge Construction The fundamental construction of the Strain Gauge:

a) Supporting foil b) Resistor wire c) Connections d) Glue joint e) Supporting plate

a

b

c


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7.27.3 Applications Strain gauge measurements shed light on deformation, stresses and loads in just about any structure. Strain gauges are often used in conjunction with live load testing of bridges. The strain gauges are placed at locations where the live load causes large strains and at hot-spots where the live-load may lead to fatigue fracture. These locations are de-termined on the basis of a Finite Element analysis of the considered structure. A number of different types of sensors are available. A sensor may consist of a single strain gauge measuring the strain in only one direction, two strain gauges measuring the strain in two directions (usually at an angle of 90o) or three strain gauges. Using a sensor consisting of three strain gauges allow the principal strains to be deter-mined (in the case of plane strain). Properly mounting a strain gauge is critical to its performance in ensuring that the applied strain of a material is accurately transferred through the adhesive and back-ing material to the foil itself. The mounting of strain gauges should therefore always be performed by trained professionals. Strain gauge measurements are usually only used for structures with a complicated geometry and/or structures where some parameters influencing the stress distribu-tion are unknown or subject to considerable uncertainty. The method is not typical for bridge inspection and is primarily used to monitor bridges having high loads or high deterioration levels. The gauge length required depends on the material of the structure being tested and the strain gradient induced. Strain gauges are readily available with gauge lengths ranging from less than 1 mm, for high strain gradients in homogeneous materials, to 100 mm for small strain gradients in concrete. For direct attachments to masonry structures the latter gauges are appropriate. The results of the measurements may be used to calibrate the Finite Element model used to evaluate the stress distribution in the structure. This assures a more reliable evaluation of the load-carrying capacity and/or fatigue life of the structure. Examples: • Machines - also rotating parts • Bridges • Offshore structures • Cranes • Pressure vessels • Concrete structures • Prototypes of all kinds.


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Figure 7-109: Squeezing Machine.

Figure 7-110: MR-Train shaft.


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Figure 7-111: Rail on Great Belt Bridge (Denmark).


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Measurement results from the bridge. The above curves show how the wind influ-ences on the rail. The straight downward going line between the peaks shows how a moderate wind (as it turns) influences on the rail. The peaks in the red circle show a blast of wind. The measurements are done in very short intervals, about 200 read-ings per/sec.

Figure 7-112: Telescope-crane.

Figure 7-113: Pressure Vessel.

7.27.4 Structural design Characteristic of the development of structural designs is that, at some point in the design phase, computations have to be made to determine the strength of the vari-


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ous sections of the structure. Often, advanced software will be used in this con-nection employing e.g. the Finite Element Method (FEM). Only if all the assumptions - such as size of load, load frequency, and support conditions - are completely cor-rect will such computations give a true picture of structural deformation and stress. Once the structure has been erected, strain gauges placed at critical points in the structure can be used to monitor very closely whether the assumptions made were satisfactory or whether the computational model needs to be adjusted. Strain gauge measurements provide a solid basis for making changes to give the structure the desired lifetime, and so that future structures can be built as efficiently and eco-nomically as possible. Below an example: Measurements are done on a shaft of a ferry. The curves shows vibration and torsion as it turns.

7.27.5 Fitness for purpose Computational methods in modem fracture mechanics can be used to evaluate the fitness for purpose of defective structures, e.g. structures with welding defects. These methods require a precise determination of the type, size and location of such


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defects. This can be done using non-destructive test methods: ultrasound, eddy cur-rent, and others. It is also necessary to know precisely the stresses in the defective areas. Strain gauge measurements are the most reliable non-destructive test method in this situa-tion, and it should be used where there is uncertainty about the stresses. Strain gauging results can be used to assess: • Safety • Remaining lifetime • Mode of vibration • Repair and reinforcement options.

Figure 7-114: Safety (doors of a guard less train).

7.27.6 Testing and documentation Strain Gauge measurements can be performed as: • Static testing, such as is used in the certification of pressure vessels • Dynamic testing - recording variations in load over time • Wireless signal transfer - measuring rotary machine parts using telemetric equipment • Testing with a friction gauge - a quick and economical test of stress variations • Residual stress testing, e.g. checking results of stress-relief annealing • Bolt tension testing - measurement of bolt preload.


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Figure 7-115: Engineers on the job where a pipe is tested under an explosion

test.


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7.27.7 Mechanical Strain Gauge There are many types of mechanical strain gauges, but the most common is the Demec gauge.

The Demec gauge is essentially a bar with a small conical point at one end and a spring-loaded pivoted lever at the other. On one side of the pivot is another small conical point and a dial gauge records the lever’s position on the other side of the pivot. Small targets with central conical recesses are stuck to the structure, with their distance apart being equal to the Demec gauge length in the middle of its measuring range.

The relationship between the dial gauge reading and strain depends on the gauge length and lever arm ratio and is provided with the Demec gauge.

Demec gauges are available with gauge lengths from 50 to 2000 mm, but 200 mm is the most common and well suited for masonry structures.

The accuracy of the Demec gauge of 200 mm is ± 6 x 10-6. This is, however only theoretical as the accuracy also depends on the skills of the operator. To compensate for measuring mistakes made by the operator it is advantageous to use the same operator.


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7.28 Electromagnetic Testing (ET) or Eddy Current Testing Electrical currents are generated in a conductive material by an induced alternating magnetic field. The electrical currents are called eddy currents because they flow in circles at and just below the surface of the material. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the materials conductive and permeability properties, can be detected with the proper equipment.

Equipment and testing of part of aerospace industry

Principle


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7.29 Radiography (RT) Radiography involves the use of penetrating gamma or X-radiation to examine parts and products for imperfections. An X-ray generator or radioactive isotope is used as a source of radiation. Radiation is directed through a part and onto film or other im-aging media. The resulting shadowgraph shows the dimensional features of the part. Possible imperfections are indicated as density changes on the film in the same manner as medical X-ray shows broken bones.

The principle

X-ray generator

Isotope

X-ray on bridgedeck


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7.30 Sonic Methods For relatively homogeneous materials ultrasonic methods are well-established proce-dures but the high resolution at shallow depths of penetration of high frequency causes the ultrasonic methods to be inappropriate for masonry. Instead sonic meth-ods have been developed.

The principle of operation of the sonic method is that one face of the structure is hit with a hammer and the impact is recorded by an adjacent accelerometer. Another accelerometer on the opposite face of the structure records the arrival of the trans-mitted compression wave, and the time between transmission and reception is calcu-lated (time domain).

If this procedure is repeated over a regular grid, variations in the transmission time of the compression wave over the structure can be graphed.

The transmission time of the compression wave depends on the density of material and the presence of voids through which the wave will not travel. If a void is pre-sent, the wave will travel around it and thereby lengthening the transmission time. If the void is large or near either the transmit-side or the receive-side accelerometer, a signal may not be received at all.

A study of the variation in transmission times over a structure will indicate changes in density or the presence and extent of voids.

The sonic methods have been used with success on structures where both sides of the structures are accessible. However internal construction features such as changes in wall thickness, internal arches and changes in fill material from concrete to rubble or earth are hard to interpret.


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7.31 Accelerometers An accelerometer produces a continuous electrical record of acceleration in a given direction. It is a small device which is easy to use and calibrate and therefore it is often used for investigating masonry structures.

Accelerometers have been used to make comparative measurements of effects of different loading events or the effect of a loading event on different structures. This usage may be able to identify that one loading event causes twice the acceleration of another, but as the acceleration cannot be related to the strength of the structure, not much will have been learned.

Accelerometers can be used to obtain displacement by double integration of the ac-celeration signal. Thereby displacements due to transient loading can be measured in locations which are otherwise not easily accessible.

The displacements are not very accurate at low frequencies depending on the char-acteristics of the particular accelerometer. Furthermore the structures fail because of excessive stress or strain, not excessive acceleration, which is very much a second-order effect.


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8. Economic analysis

8.1 General When a bridge deteriorates very often rehabilitation work is necessary. It may not obvious which repair methods to use, and at what time to carry out the works. In other words, the optimum repair strategy is not obvious.

Many factors may influence the choice of repair strategy, such as:

• The urgency of repair. • The repair cost estimate. • The available funds. • The traffic hazards caused by damage to the bridges. • The inconvenience to the public in case of closure of the bridge (no trains pass-

ing the line, …). • The inconvenience during the repair works. • Other repair works on the same railway line.

An economic analysis is carried out as part of the special inspection in order to select the repair strategy which is economically optimum (most profitable) for the bridge owner or for the society as a whole. It has to be noted, that this analysis is not in-cluded in the extended principal inspection.

The economic analysis takes into account only those factors, which can be measured in the 'unit': money. The analysis is carried out in order to determine which strategy is the optimum for the bridge, given the premises at the time of decision.

When a strategy is selected for the bridge (which includes activities over a period of 20 to 30 years), it does not mean that the decision-makers are stuck with this strat-egy for the next 20 or 30 years. Circumstances may change, and another strategy may become more profitable. The development of traffic volume, the interest rate, the inflation may change, and the development of damage to the structure may not be as expected.

If the decision makers suspect that a chosen strategy is no longer the optimum, a new special inspection has to be carried out, including an economic analysis, in order to determine which strategy is the optimum given the new premises.

Obviously, if the chosen strategy implies a replacement or an exhaustive repair of a bridge, there is not much room for a later change of strategy.


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8.2 Present Value Method Economic analyses are normally carried out by the 'Present Value Method'.

The basic idea is that all amounts connected to a repair strategy are 'discounted' to the same year in order to compare costs that occur at different times.

This section describes in more detail how to carry out economic analyses for bridge repair strategies.

The analysis is carried out by performing the following steps:

• Identify the relevant repair strategies.

• Determine the size and distribution of costs (repair, maintenance) connected to each strategy.

• Calculate the present value of each strategy.

• Choose the strategy with the lowest present value as the economical optimum strategy.

In connection with a repair work it is often necessary to choose between various strategies. Shall one choose an expensive repair with a long service life or a cheaper repair with a short service life ? Another problem is the time at which the repair should be carried out. Should it be done as soon as possible, can it be deferred, or can it wait until the structure is replaced ? An economic calculation method that can help in such decisions - the present value method - will be described in the following.

In the present value calculations, the costs for repairs, operation and maintenance may be calculated year for year within a chosen time-horizon; the timing of each cost is based on the service life of each repair. The annual amounts are then dis-counted back to the initial year using a given discount rate. In this way the present value of each years expenditure is obtained.

By summing the present values, a value for the strategy in question is obtained that can be compared with the corresponding value for other strategies. The strategy for which the cumulative present value is lowest is the economic optimum for the struc-ture considered in isolation.

The cumulative present value makes it possible to compare strategies in which the costs are spread over varying periods, as all costs are converted to the initial year. The further in the future a cost falls due, the lower is the present value of the cost. This effect is proportional to the discount rate adopted.

To put it simply, the present value is the amount that must be deposited in the bank today to cover a cost that will fall due at the time the repair is carried out.

The present value is calculated by:


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( )nn rII

+=

11

where: In is the present value of a cost I in year n

I is the cost in year n calculated based on the chosen price level (normally the current price)

n is the number of years until the costs falls due

r is the discount rate decided by the management authority

The present value calculation is thus carried out in fixed prices (those of the initial year) with a chosen price level and a chosen discount rate.

By fixed prices is understood the initial years prices. Inflation and development of wages, taxes, etc. should not be incorporated in the calculations.

For an economic evaluation of alternative solutions is the most important parame-ters:

• Repair

• Maintenance strategies equal content

• The service life of the structural components

• The time frame for the calculations

• Time for repair and maintenance

• Residual value

• Discount rate

8.2.1 Repair Strategies Special Inspection: In order to cover the relevant range of strategies, 2 to 4 (in special cases fewer or more) fundamentally different strategies are investigated as part of the special in-spection. The strategies normally fall within the following groups:

• Total replacement of the whole bridge or the bridge components in question.

• Thorough (major) repair of the relevant bridge components.

• Interim repair, and after some time a thorough repair/replacement.

• Doing nothing.

Example:


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A slab/girder reinforced concrete bridge with a thin slab suffers from overload. Struc-tural cracks and spalling of the lower concrete cover have developed in a few deck panels. If nothing is done, one or more panels are expected to fail within 5 years. Relevant strategies may be:

A: Replacement of the slab, or possibly of the whole superstructure.

B: Strengthening of the slab by pouring a new reinforced top layer on top of the existing slab, with anchors into the existing concrete. Injection of coarse cracks and local replacement of spalled concrete.

C: Replacement of spalled concrete. After approximately 5 years the damage is expected to have reoccurred to the same extent. At that time the repair is re-peated. After additionally 5 years the damage is expected to be so serious that the slab has to be replaced.

D: Doing nothing. After 5 years the slab is expected to fail, and it is replaced or repaired.

Strategy A and D may seem very close to each other. But in reality, they are not:

In A, a replacement of the slab is planned in advance, and it is possible to make the replacement with very little disturbance to the railway line.

In D, we let the slab fail. When it has failed, the bridge must be closed to traffic whi-le investigations and rehabilitation design is carried out. Thus, the bridge may be closed for several months, causing very high inconvenience for the users of the rail-way. In reality, the bridge must be closed well in advance of any possible failure of the bridge or bridge component. It is not acceptable to allow traffic on the bridge when there is a known risk of failure. Therefore strategy D may require the bridge to be closed well in advance of any possible failure which – taken uncertainties into account may be 5-10 years.

Extended Principal Inspection: Comprised in the extended principal inspection are general considerations regarding future maintenance activities. This includes a description of the need for major reha-bilitation jobs and further inspections.

8.2.2 Service Life The service life of the bridge components in question is estimated for each mainte-nance strategy. Service life estimations is based on experience of the different main-tenance methods used in the maintenance strategies.

Estimation of the service life should be based on considerations where ordinary pre-ventive and corrective maintenance is carried out on the component.


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Safety considerations can reduce the service life relative to that estimated on the basis of the selected maintenance strategy due to outdated components.

Determination of the optimal repair time is associated with the evaluation of the de-velopment of damage and their influence on durability and safety of the components. One has to evaluate how fast the component is deteriorating and when the function requirements are no longer fulfilled, i.e. the end of its service life. This evaluation is often complex because it includes an evaluation of the continuing deterioration and the damage time-dependent development.

When the possible damage development is evaluated it is important to describe the most possible development. Additional “safety factors” applied when determinating the damage development on the “safe side” should not be used when the economic optimal time for repair should be found. It can be wrong (and costly) to repair too early as it is too repair to late. However when there is a risk to the saefy of the bridge and railway users, the design codes safety limit should be used.

8.2.3 Time Frame The time frame is laid down on the basis of the service life of the main repair work necessary to carry out. The same time frame should always be used for the different maintenance strategies to ensure that they are economic comparable.

The time frame should be chosen so long that cost that become due after the time frame has only little or no influence on the accumulated net present value.

Normally is chosen a time frame of 25 years but it may be longer if the discount rate is low.

8.2.4 Time of Repair Repair time for the different strategies are based on experience. By postponement of a repair work normally the damage extent is increased and will consequently result in an increase in repair costs later.

The repair time is therefore based on economic optimal service life of the different components. In that way a minimum present value is reached for each repair strat-egy.

By stipulation of the repair time for each part of the maintenance strategy it should be taken in consideration the general costs such as traffic management and by that collect the different repair works in different time phases.

To help choosing the optimal time for execution of a repair service life models and present value calculations may be used. General, where the optimal time for execu-tion is found, the present value of the maintenance strategy will increase if the works is carried out in advance or postponed considering the optimal repair time.


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This means that the economic optimal time for repair may differ depending on the discount rate used in the present value calculations.

Due to budgetary limitations it may be necessary to postpone the works. This means that the present value of the maintenance strategy increases due to increases in the extent of repair work (increases in damage etc.). The costs due to increase in repair work should overdue the economic advantage by postponement.

8.2.5 Residual Value As consequence of using the same time frame for the different maintenance strate-gies it will often be a residual value of a repair work which service life is not reached within the time frame. This residual value should be incorporated in the strategies.

The present accumulated value should contain the residual value with opposite sign.

8.2.6 Discount Rate The discount rate should be determined by the management authority and be based on the societies possibility for return of investments.

The societies return of investments is depended on the interest rate and the inflation in the economy. The discount rate is normally the interest rate minus the inflation.

If the discount rate is high, the pay pack time of investments should be low (i.e. “throw away and buy new”). Opposite if the discount rate is low it will pay back to invest in components that will have long service life, which means durable compo-nents and carry out proper maintenance.

The above considerations points in the direction of using different discount rates which may be exemplified by investment in computes (i.e. installations) where the discount rate normally had to be high compared to concrete structures where it had to be low (concrete structures is expected to last for a long period).

However to keep things simple normally one discount rate is used covering both in-stallations and structural components.

8.2.7 Sensitivity Analysis The sensitivity analysis should prove the changes in present value due to changes in the different factors that are involved in the different maintenance strategies.

A sensitivity analyse should show the increase in the costs due to postponement of the works.

Only the parameters that have a significant effect on the different strategies need to be investigated. Parameters that have the same effect will increase or decrease the present value of all the strategies by the same factor.


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Be aware that it often pays to postpone a strategy if the damage costs develops with a lower rate than the used discount rate.

8.2.8 Optimum solution – special inspection Within each strategy stated in the special inspection report an analysis is carried out in order to determine in detail which works to carry out and at what times. This is called finding the optimum 'solution' within the individual strategy.

The relevant repair methods must be considered incl. the extent of the repair. (In the above example, should replacement of the slab include repair of girders?) The repair works must be described in so much detail that the cost estimate can be suffi-ciently accurate.

When determining when to carry out repair works, the development of damage and the discount rate are the dominating factors. If you postpone a repair, normally the extent of damage and thus the repair cost will increase. However, if the annual in-crease in repair cost is less than the discount rate, it could be profitable to postpone the repair. When performing an economic analysis the bridge owner has to provide you with the value of the discount rate to take into account.

If you continue postponing a repair work, the extent of damage (and thus the cost) will in most cases increase slowly and linearly until a point where the cost rises dra-matically. This is because at some point the problem can not be solved by the pro-posed repair method, and a more extensive and expensive method has to be used. (E.g. if damage to the superstructure is allowed to develop, it may at some point be necessary to use interim supports when carrying out the repair). It is very often profitable to postpone repair works until just before this kind of 'jumps' in the repair costs.

The present value method is used to determine which solution is optimum within each strategy. (The solution with the lowest present worth is the optimum).

When comparing strategies, it is important that all strategies cover the same com-ponents of the bridge. If for example strategy 1 comprises repair of the superstruc-ture while strategy 2 is a replacement of the whole bridge, both strategies must in-clude all costs regarding repair and maintenance of the whole bridge. Otherwise the strategies are not comparable.


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9. Reporting of Extended Principal Inspection

9.1 General In order to facilitate comparison of extended principal inspection reports made by different people, and in order not to forget important aspects of the inspection, the reporting is made using a standardised table of contents.

The report of an extended principal inspection contains a text section, and appendi-ces with the detailed registrations made on site. This section gives a short summary of the content of the extended principal inspection report.

9.2 Text Section In the following the chapters that the extended principal inspection report must in-clude are described:

9.2.1 Cover Page The cover page of the extended principal inspection report must comprise the follow-ing information:

• Identification of the bridge owner (e.g. Central Railway)

• Identification of the bridge (Bridge-ID according to the bridge management sys-tem – if a management system is used - and bridge name).

• 'Extended Principal Inspection of ... (the components in question)', e.g. 'Ex-tended Principal Inspection of bearings and girders'.

• Date of the extended principal inspection.

• Name of the company performing the extended principal inspection.

9.2.2 Front Page The front page of the extended principal inspection report must comprise the follow-ing information:



• 'Extended Principal Inspection of ... (the components in question)', e.g. 'Ex-tended Principal Inspection of bearings and girders'.

• Date of the extended principal inspection.

• Identification of the extended principal inspection engineer(s) (name and com-pany).


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9.2.3 Summary The summary must contain all relevant information from the other chapters in a short form. This chapter must include a comprehensive overview of the registrations and conclusions on the damage to the bridge. It must comprise description of the extent of registrations, conclusions on cause and extent of damage, and recommen-dation for rehabilitations and further activities. However, the summary should not be more than 1-2 pages in length.

9.2.4 Motivation of the extended principal inspection This chapter describes why and by whom the inspection is initiated. It tells which bridge components are the objects of NDT-inspections and which visible damage has been registered.

9.2.5 Background documents This chapter lists the background material that has been available for the inspection, such as:

• Inventory report and previous principal inspection report.

• Reports from previous extended principal inspections or special inspections on

the same bridge or from similar bridges with similar damage.

• 'As built' drawings.

• Materials specifications for steel, concrete, masonry etc.

• Structural and hydraulics calculations if relevant.

9.2.6 Registrations This chapter describes the registrations from the inspection. For each of the test methods used, the extent and location is described, and a summary of the results is given. The detailed record of all registrations is enclosed in the appendices.

9.2.7 Evaluation of registrations In this chapter the inspection engineer describes the probable deterioration mecha-nisms and causes of damage based on the registrations. The chapter must also in-clude an estimate of the actual damage of the bridge components investigated.

The damage mechanism should be described in detail. This means that in cases of corrosion, 'saline soil' is not sufficient as explanation. You must also explain where the water comes from, how the chlorides have reached the reinforcement, etc.


9-3/311 9-3

For the bridge components of NDT-measurements the extent of damage based on the NDT-measurements is described. Thus, this section includes a summary of the interpretations of the test results from all the NDT-methods used in the inspection and the registrations from the visual inspection.

The chapter also includes the condition rating of the components inspected. A brief motivation for each component rating is given based on the previous sections. E.g. bridge deck, piers etc..

Based on the registrations from this inspected a condition rating of each bridge com-ponent has been made. The condition rating is a number of 1 to 6 and is based on the following guidelines:

1: A condition which warrants rebuilding / rehabilitation immediately. 2: A condition which requires rebuilding / rehabilitation on a programmed basis. 3: A condition which requires major / special repairs. 4: A condition which requires routine maintenance. 5: A sound condition. 6: Not applicable. 0: Not inspected.

9.2.8 General considerations regarding future maintenance activities This chapter describes the inspection engineer's recommendation of future activities. The need for major rehabilitation jobs and further inspections is included in this chapter. The description does not include budgets for the activities. If there is any doubt of the carrying capacity of the bridge recommendation of calcu-lations must be included in this chapter.

Based on the condition rating, the results from the NDT-investigations and the dam-age type, extent and cause on the selected bridge components a recommendation of an economic analyse must be made in order to select the optimal / best maintenance strategy for the bridge.

9.3 Appendices The extended principal inspection report comprises those relevant of the following appendices.

9.3.1 A: Background Material This appendix includes copies of the inventory report of the bridge, the previous principal inspection report (if any), previous extended principal inspection reports and special inspection reports regarding the bridge components chosen for NDT-investigations.


9-4/311 9-4

9.3.2 B: Selected Drawings This appendix includes selected drawings of the bridge itself and the bridge compo-nents of which the NDT-investigations are carried out. The selected drawings may be very useful to describe details of the design of the bridge.

9.3.3 C: Visual Inspection This appendix includes the registrations from the visual inspection of all the bridge components included in the inspection. General orientation of the bridge and the bridge components under investigation, numbering of components and damage pat-tern are most conveniently shown on sketches.

An overview sketch of the whole structure is often suited to register the extent of damage (which columns have spalling of cover, which girders have shear and flex-ural cracks, etc.).

The appendix should include photo pages for the photographs taken during the in-spection. The photographs should always include the following:

• Overview photographs showing the approach and surface, and the elevation of the bridge.

• Photographs showing the general design of the bridge components under inves-tigation.

• Photographs describing the damage to the structure. Overview as well as close-up photographs.

• Photogrpahs showing details of the registrations, e.g. exposed reinforcement, corrosion of steel surfaces, deteriorated stone of masonry structures, etc..

9.3.4 D: NDT-method no. 1 Depending on the complexity of the NDT-method a general description of the princi-ples of the method is described in this appendix. This appendix includes the registrations from one of the NDT-methods used in this extended principal inspection. The appendix should include sketches of the areas of measurements and of the measuring grid if used e.g. for HCP, Impact-Echo, Impulse Response (s’MASH) etc. The appendix should also include relevant photographs re-lated to the NDT-investigation (of break-ups etc.). Always note the dimensions of the bridge component in question. (Diameter of col-umn; width, depth, spacing and length of girders, etc.).

Mapping of HCP (Half-cell Potential measurements) readings is shown on sketches.

Mapping of s’MASH (impulse response measurements) readings is shown on sketches.


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Mapping of Impact-Echo readings is shown on sketches.

9.3.5 E - ?: NDT-method no. 2 - ? This appendix includes registrations from another NDT-method. The content of this appendix is similar to the content mentioned in section 9.3.4.

9.3.6 F.. Other In some cases it is appropriate to include other appendices than the 'standard' ones. These are numbered 'F', 'G'.....


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10. Reporting of Special Inspection

10.1 General As the reporting of an extended principal inspection, reporting a special inspection is made using a standardised table of contents.

The special inspection report contains a text section, and appendices with the de-tailed registrations made on site. This section gives a short summary of the content of the special inspection report.

10.2 Text Section In the following the chapters that the special inspection report must include are de-scribed:

10.2.1 Cover Page The cover page of the special inspection report must comprise the following informa-tion:



• 'Special inspection of ... (the components in question)', e.g. 'Special inspection of bearings and girders'.

• Date of the special inspection.

• Name of the company performing the special inspection.

10.2.2 Front Page The front page of the special inspection report must comprise the following informa-tion:



• 'Special inspection of ... (the components in question)', e.g. 'Special inspection of bearings and girders'.

• Date of the special inspection.

• Identification of the special inspection engineer(s) (name and company).


10-2/311 10-2

10.2.3 Summary The summary must contain all relevant information from the other chapters in a short form. This chapter must include a comprehensive overview of the registrations and conclusions on the damage to the bridge. It must comprise description of the extent of registrations, conclusions on cause and extent of damage, and the pro-posed repair strategy including cost estimate and time schedule. However, the sum-mary should not be more than 2-3 pages in length.

10.2.4 Motivation of the special inspection This chapter describes why and by whom the inspection is initiated. It tells which bridge components are the objects of the inspection and which visible damage has been registered.

10.2.5 Background documents This section lists the background material that has been available for the inspection, such as:

• Inventory report and previous principal inspection report.




• Materials specifications for steel, concrete and masonry.


10.2.6 Registrations This chapter describes the registrations. On the basis of the visual inspection and prior knowledge the structure may be divided into homogeneous areas. A homoge-nous area is defined as an area where the parameters affecting the deterioration – and the deterioration itself – of the structure exhibits only a random variation. For each of the homogeneous areas a damage hypothesis is prepared. These hy-pothesis are described in this section.

For each of the test methods used, the extent and location is described, and a sum-mary of the results is given.

The detailed record of all registrations is enclosed in the appendices.

10.2.7 Evaluation of registrations This section includes an interpretation of the test results from the NDT-investigations. E.g. for chloride-measurements: do the measurements show risk of chloride initiated corrosion of the reinforcement – are the values of the chloride con-tent larger than the critical chloride content in the depth of reinforcement.


10-3/311 10-3

In this chapter the special inspection engineer describes the probable deterioration mechanisms and causes of damage, based on the registrations. The chapter must also include an estimate of the actual damage of the bridge components investi-gated. It should also include a description of the expected development of damage if no action is taken.

The damage mechanism should be described in detail. This means that in cases of corrosion, 'saline soil' is not sufficient as explanation. You must also explain where the water comes from, how the chlorides have reached the reinforcement, etc.

It is also important to explain the differences in damage appearance: Why are some columns damaged while others are undamaged, why is only the centre girder cracked, etc.

10.2.8 Repair strategies This chapter describes the relevant repair strategies for the bridge.

The description of each strategy should comprise:

• A general description of the 'idea' of the strategy, e.g. 'Replacement of the whole bridge', 'Interim repair, followed by major rehabilitation after 10 years'.

• List of all activities with year and cost estimate, e.g.:

Activity Year Cost

Interim repair of bridge deck 2005 5 mio. Rs.

Replacement of bridge deck 2015 45 mio. Rs.

If the contents and extent of the activities are not obvious, they should be detailed, e.g: 'Interim repair of deck comprises repair of honeycombs at 8 locations, and ce-ment mortar injection of approximately 50 meters of cracks'. 'Replacement of deck comprises replacement of the deck slab on the whole bridge, including expansion joints, edge beams, ballast and tracks. The existing girders are re-used'.

Description of possible disturbance to the traffic.

'Present value' of the strategy, calculated following the 'present value method'.

Remember that 'doing nothing' may very well be one of the possible strategies. This strategy must be examined as well. This strategy will have no repair or maintenance costs, but it may imply severe inconvenience for the users of the railway.


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10.2.9 Recommendation of follow-up activities This chapter describes the special inspection engineer's recommendation of future activities.

Normally the recommendation will be to carry out the repair strategy with the lowest present value, as this should be the optimum thing to do.

However, in some cases the recommendation may be to carry out further, more de-tailed investigations, or to monitor the development of damage for some time before making final conclusions on the optimal repair of individual bridge components or the bridge as a whole.

If there is any doubt of the carrying capacity of the bridge recommendation of calcu-lations must be included in this chapter.

10.3 Appendices The special inspection report comprises those relevant of the following appendices.

10.3.1 A: Background Material This appendix includes copies of the inventory of the bridge, the previous principal inspection report, previous extended principal inspection reports and special inspec-tion reports regarding the bridge components of the special inspection.

10.3.2 B: Selected Drawings This appendix includes selected drawings of the bridge itself and of the bridge com-ponents for the special inspection.

10.3.3 C: Visual Inspection This appendix includes the registrations from the visual inspection of the bridge components included in the special inspection. General orientation of the bridge and the bridge components under investigation, numbering of components and damage pattern are most conveniently shown on sketches.


The appendix should include photo pages for the photos taken during the inspection. The photos should always include the following:

• Overview photos showing the approach and surface, and the elevation of the bridge.

• Photos showing the general design of the bridge components under investiga-tion.

• Photos describing the damage to the structure. Overview as well as close-up photos.


10-5/311 10-5

• Photos showing details of the registrations, e.g. exposed reinforcement, corro-sion of steel surfaces, deteriorated stone of masonry structures, etc..

10.3.4 D: NDT-method No. 1 Depending on the complexity of the NDT-method a general description of the princi-ples of the method is described in this appendix. This appendix includes the registrations from one of the NDT-methods used in the special inspection. The appendix should include sketches of the areas of measure-ments and of the measuring grid if used e.g. for HCP, Impact-Echo, Impulse Re-sponse (s’MASH) etc. The appendix should also include relevant photos related to the NDT-investigation (of break-ups etc.). Always note the dimensions of the bridge component in question. (Diameter of col-umn; width, depth, spacing and length of girders, etc.).

Mapping of HCP (Half Cell Potential measurements) readings is shown on sketches.



10.3.5 E - ?: NDT-method No. 2 - ? This appendix includes registrations from another NDT-method. The content of this appendix is similar to the content mentioned in section 10.3.4.

10.3.6 F: Economic analysis This appendix may contain present value calculations of the strategies under investi-gation.

10.3.7 G.. Other In some cases it is appropriate to include other appendices than the 'standard' ones. These are numbered 'H', 'I'.....


11-1/311 11-1

11. References

[1] CEB: Durable Concrete Structures. Design Guide. Second Edition, reprint 1997.

[2] Geiker, M.: Durability of concrete – chloride induced corrosion. Course 59203, Danish Technical University, 2000.

[3] Larsen, E. S.: Service Life Prediction of Cementitious Materials. SBI Report 221, 1992.

[4] Larsen, E. S.: Inspection of Structures – Special Inspection – Compendium for support and Inspiration (in Danish). VEJ-EU, 1997.

[5] Mattsson, E.: Electro chemistry and corrosion (in Swedish). 2end edition, Cor-rosion Institute, Stockholm 1984.

[6] Nielsen, A.; Eeg, R. and Sorensen, H.: Building Materials – Metal (in Danish) Polyteknisk Forlag 1998.

[7] Sowden A.M. (1990). The Maintenance of Brick and Stone Masonry structures. ISBN: 0-419-14930-9. E. & F.N. Spon. First Edition

[8] Federal Lands Highway Program. Surface Nondestructive Test (NDT) Methods http://www.cflhd.gov/agm/engApplications/BridgeSystemSubstructure/211SurfaceNDTMethods.htm

[9] Departments of the Army. Seismic Design Guidelines for Upgrading Existing Buildings, appendix E. http://www.usace.army.mil/inet/usace-docs/armytm/tm5-809-10-2/app-e.pdf

[10] Ron Grieve. Non-destructive Testing of Concrete and Masonry Buildings. The construction Specifier, October 2005

5721063-07_L010-VerA_NDT_manual_App_frontpages.doc

APPENDIX A

Handout of Slides from Classroom Training in NDT-Methods


APPENDIX A1

Introduction to the Class Room Training in NDT and UWI

Appendix A1, Page 1 of 28

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Non Destructive Testing and Underwater Inspections

Concrete, Steel and Masonry Bridges

2SlideNon Destructive Testing and Underwater Inspection - 8 February, 2006

0. Welcome by Central Railway, LTR and Ramboll

1. Presentation of the lecturers by AKN

2. Presentation of the participants by AKN

3. Presentation of the Ramboll Group by AKN

4. Lecture program for NDT and UWI by LTP

5. General introduction to deterioration mechanisms by LTP

6. General introduction to systematic operation and maintenance by LTP

7. Special inspection by LTP

8. Structural assessment – Case by LTP

Agenda


LTP

Cross-Out

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1. Presentation of the lecturers


Asger Knudsen:Head of department of “Bridge Management and Materials Technology”15 years of experience with bridge inspections, NDT-testing, condition assessment and bridge management systems

Lene Torrnaes Helbo:Project coordinator of the NDT & UWI pilot project.Extended experience with inspections, NDT-testing, condition assessment, petrographic analysis, deterioration evaluation and bridge management systems.

Peter H. Moeller:NDT- and corrosion expert, cathodic protection expert Extended experience with NDT-testing, inspections, condition assessment, deterioration evaluation, rehabilitation

Morten Daroe Tranholm Jensen:NDT-specialistExtended experience with NDT-testing, inspection and condition assessment

Joergen LenlerNDT-expert in Steel investigations – especially ultra sonic testingExtended experience with NDT-testing of steel structures

1. Presentation of the lecturers


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2. Presentation of the participants


Position in Central Railway

Experience within the fields of NDT and UWI

2. Presentation of the participants


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3. Presentation of the Ramboll Group


Ramboll was established in Copenhagen in 1945 as Ramboll & Hannemann, named after its founders: B.J. Ramboll, D.techn.Sc., and I.G. Hannemann, D.techn.Sc

History of the Ramboll Group

In 2003 the company merged with the publicly listed Scandiaconsult AB originally established in Stockholm as Orrje& Co. AB in 1947 by five engineers: Alfred Orrje, BengtWård, Hans Hilborn, BjörnRomson and Lars Berlin.



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The Ramboll Group in brief

The Ramboll Group is a leading Nordic provider of knowledge services with a broad specialisation, operating globally in the main business segments of buildings, infrastructure, environment, energy, oil and gas, IT and management.

Our customers have access to our large network of specialists from 91 offices in the Nordic region and another 50 permanent or project offices around the world.



Our organisation

Ramboll Gruppen A/S

Ramboll Danmark A/S

Ramböll AB (Sweden)

Ramboll Norge A/S

Ramboll Finland OY

Ramboll Informatik A/S

Ramboll Management A/S



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Our Nordic coverage

The Ramboll Group ranks among Top 10 consultancies in Europe and Top 25 globally, a ranking primarily maintained by a strong domestic market position.

With 91 offices covering the Nordic region, being a local partner is a key strategic focus of the Group.



Our global coverage

The Ramboll Group reaches out to the rest of the world with experience from projects in more than 100 countries.

Ramboll offices abroadBelgium (Bruxelles)Germany (Hamburg, Munich) Greenland (Nuuk, Sisimiut)Estonia (Tallinn)India (Chennai, Delhi, Hyderabad)Latvia (Riga)Lithuania (Vilnius)Laos (Vientiane)Qatar (Doha)Romania (Bucharest)Russia (Moscow, St. Petersburg) Thailand (Bangkok)



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The customers’ knowledge bank – fields of specialisation

The specialisation of Ramboll is wide, covering almost any aspect of engineering, IT and management. We have divided our skills into a number of primary fields of specialisation, which can be accessed by customers through any of our 105 permanent offices worldwide.

Constant research and development initiatives support our ability to provide state-of-the-art solutions.

Buildings Infrastructure, transport and traffic




Water and environment Energy, oil and gas

Telecommunications Industry



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Waste Health

International development projects Facilities management




Architecture & landscape architecture Geotechnical and rock engineering

Management Information technology



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Our value keywords

TrustHonesty and integrity, openness and cooperation

QualityQuality and value for the consumer

InnovationDevelopment, improvement, exploitation and sharing knowledge

Commitment:Responsibility, focus, initiative and high motivation

EmpowermentDecentralisation and delegation of authority



Our human resources

2004

54Engineers, %

13Other graduate staff, %

41,5Average age, years

92/8Gender distribution among line managers

70/30Male/Female, %

12Other staff groups, %

21Technicians, %

4.029Number of permanent employees

Employee facts in the Ramboll Group at 31 December 2004



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Bridge Management and Materials TechnologyThe department


The department comprises app. 30 persons.

21 have an engineering degree or other academic degrees of similar level.

We cover all relevant fields of expertise in relation to operation and maintenance of civil works such as:

minor and major bridges

tunnels

ports

We have theoretical as well as practical expertise on materials technology for the relevant construction materials.


Bridge Management and Materials TechnologyThe department

Our competences cover areas like:

Routine inspections

Special inspections including laboratory and on site testing

Risk analysis for assessment of the importance of damage on safety and durability

Rehabilitation projects and supervision

Monitoring

Maintenance management systems

Operation and maintenance contracts

Materials technology (concrete, surfacing, waterproofing, steel, natural building stone, etc.)

Research and development



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India 2005/2006


Bridge Management and Materials TechnologyMaterial technology

Concrete:

A team of experts within the field of cement and concrete technology offers individual services to companies in materials supply, production and use of concrete and repairs on a worldwide basis. Among other things, Ramboll's expertise covers specialized knowledge regarding:

Deterioration mechanisms

Curing design

Mix design optimisation or trouble-shooting

At Ramboll's laboratory, materials can be prepared for different tests.

The laboratory includes a chemical laboratory, a concrete mixinglaboratory as well as optical polarization microscopes, scanning electron microscope and equipment for automatic air void analysis.



Bridge Management and Materials TechnologyMaterial technology

Steel:

Ramboll evaluates deterioration mechanisms in steel structures.

Masonry:

Ramboll evaluates deterioration mechanisms in masonry structures.

Natural building stone:

Ramboll has extensive experience in the selection and evaluation of stone and with the use of stone in buildings.

Roadway surfacing and waterproofing:

Ramboll has state of the art knowledge regarding road surfacing and waterproofing.



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Mobile concrete laboratory for onsite testingMobile concrete laboratory for onsite testing

Purpose with mobile laboratory:

all planned test equip-ment are available

most unplanned test equipment are available

all relevant practical tools are available



Selected references - inspection and maintenance of bridges

• Great Belt connection (Denmark)

• Oeresundsbron (Sweden and Denmark)

• Tete bridge (Mozambique)

• Large bridges & tunnels for the Danish Road Directorate (Denmark)

• Several bridges for the Danish Railways (Denmark)

• Hooghley bridge (India)

• Victoria Falls Bridge (Zambia)

• Steel bridges (Greece)

• Riveted steel bridge (Denmark)

• Soerstraumen bridge – monitoring system (Norway)

• Haicang suspension bridge (China)

• Progreso pier (Mexico)



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Great belt (Denmark)Inspections / repair projects / life time evaluations of concrete structures



Oeresundsbron (Sweden and Denmark)Inspection of steel and concrete structures – bridge and tunnel



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Tete bridge (Mozambique)Inspection of 5 span suspension bridge



Danish Road DirectorateManagement and maintenance of large bridges and tunnels



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Danish Road DirectorateManagement and maintenance of large bridges and tunnels



Danish RailwaysMaintenance and repair of several bridges



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2nd. Hooghley River Bridge (India)Maintenance manual and DANBROweb management system



Victoria Falls BridgeInspection and fatigue analysis/assessment



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Inspection and evaluation of 2 steel bridges (Greece)



Inspection / NDT test of rivet steel bridge(Road and railway - Denmark)



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Soerstraumen Bridge (Norway)Monitoring system



Haicang suspension bridge (China)Maintenance manuals



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Progreso Pier (Mexico)Inspection of a 65-yr. concrete pier with stainless steel


1937: Pier with stainless steel

1965: Pier with carbon steel

4. Non Destructive Testing and Underwater Inspections

Lecture program


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Classroom training

4. Lecture program


Classroom training and field inspections

4. Lecture program


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4. Lecture program



4. Lecture program


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NDT-methods – above water

4 Lecture program

ConcreteImpact Echo equipment

ConcreteSpraying indicators (pH)

ConcreteCover meter

ConcreteCorrosion rate meter

ConcreteHalf cell potential measurements

Concrete, steel and masonryBoroscope

Concrete, steel and masonryCrack detection microscope

Concrete, steel and masonryCrack measuring gauge

Used for structures made of:NDT-method



4 Lecture program

ConcreteEvaluation of concrete cores

Concrete and masonryCoring equipment

ConcreteChloride content

Concrete and masonryGround Penetration Radar

Concrete and masonrySchmidt Hammer

ConcretePull-off/Bond test

ConcreteCAPO test

ConcreteImpulse response equipment



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4 Lecture program

SteelStrain gauging

SteelMagnetic particle testing

SteelDye penetrant

SteelMagnetic thickness gauge

SteelUltrasonic testing

SteelUltrasonic Thickness gauge

Concrete, steel and masonryStructural scan equipment

Concrete, steel and masonryStructural testing system

SteelAcoustic emission monitoring



NDT-methods – under water

4 Lecture program

Concrete and masonryCoring equipment

ConcreteChloride content

Concrete and masonrySchmidt Hammer

ConcreteCover meter

Concrete and steelUltrasonic testing

SteelUltrasonic thickness gauge



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NDT-methods – above and under water

4 Lecture program

Agenda:

1. Theory – Technical Method Description

2. Applications and Limitations

3. Test Planning and Execution of Field Tests

4. Interpretation and Reporting of Results

5. Application Summary

6. References


Field inspectionsNDT-training - Overview

4 Lecture program

Visual, Boroscope, Crack Detection Microscope, Crack Measuring Gauge, Ultrasonic Thickness Gauge, Ultrasonic Testing, Magnetic Thickness Gauge, Dye Penetrant

Sandhurst

No. 1/9

Visual, CAPO, Boroscope, Schmidt Hammer, Spraying indicators, Crack Detection Microscope, Crack Measuring Gauge, Chloride Content

Thane Creek Bridge

No. 25/1

PhotoInspections and NDTBridge


NDT-Course

India 2005/2006



4 Lecture program

Visual, Schmidt Hammer, Boroscope, Crack Detection Microscope, Crack Measuring Gauge

Karjat-Lonavala

No. 107/2

Visual, Impact-Echo, HCP, Cover Meter, Boroscope, Schmidt Hammer, Spraying indicators, Chloride Content

Diva-Panvel

No. 49/2




4 Lecture program

Visual, Schmidt Hammer, Boroscope, Crack Detection Microscope, Crack Measuring Gauge

Neera Bridge

No. 149



NDT-Course

India 2005/2006


Field inspectionsUWI-training - Overview

4 Lecture program

Water soundings and scour. Level I inspection.

Level III inspection including: Schmidt hammer, Coring, Ultra Sonic Testing of Concrete, Cover Meter, Chloride Content.

MumbraCreek Bridge

No. 40/1

Water soundings and scour. Level I inspection.

Thane Creek Bridge

No. 25/1




4 Lecture program

Water soundings and scour. Level I and level II inspections.

Wardha-Nagpur

Kistna Bridge

No. 807/1

Water soundings and scour. Level I and level II inspections.

Wardha-Nagpur

Dham Bridge

No. 768/2



NDT-Course

India 2005/2006



4 Lecture program

Water soundings and scour.

Level I inspection.

Daud –Kurduwadi

BheemaBridge

No. 301/1




APPENDIX A2

General Introduction to Deterioration Mechanism


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5. General introduction to deterioration mechanisms

5.A Concrete Bridges


Selected deterioration mechanisms

Corrosion of reinforcement

Carbonation

Chloride ingress

Alkali Silica Reaction (ASR)

Initial defects (honey combing etc.)

Chemical attack

Acid attack

Sulphate attack

Seawater attack

5.A Deterioration mechanisms in concrete bridges


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Initiation: Chloride ingress, carbonation.

Result: Reduction of cross-section, surface cracks, spalling concrete.

Visual appearance: Wide cracks in a pattern, spalling concrete.

Growth: Chloride: Very fast (pitting). Carbonation: slow.

Typical areas: Splash zones on marine structures, areas with small concrete cover.

NDT-methods: HCP, corrosion rate, break-ups, crack detection, boroscope.

Rehabilitation methods:- Preventive: Reduce moisture and chloride, cathodic protection.

- Corrective: Replacement of reinforcement and concrete, replacement of waterproofing on road carrying bridges.




Carbonation Chlorides Diffusion of oxygen Moisture (H2O)

Deterioration of coating of ferric oxide

Possibility of corrosion



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Corrosion products (black) with a small volume are created in environments with high humidity and limited access of oxygen (lack of oxygen). This is often observed by chloride initiated corrosion.

In environments with plenty of oxygen corrosion products with more volume are created (brown). This is often observed at corrosion caused by carbonation of the concrete cover.

Black

Black

Brown

Brown - yellow




The ingress of chloride in concrete depends on e.g.:

How the ingress is happening:

Diffusion

Capillary suction (ascension)

Water pressure

Migration (electrical field)

And factors as:

Concrete cover (Concrete) quality (w/c-ratio, cement type and –content, pH-value of the cellular liquid, cellular system, appearance of pozzolanes, defects (cracks, inhomogeneities), the air void system).

Thickness of concrete cover.

Chloride impact from the environment (concentration, exposure)



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Alkali silica reaction (ASR)

Initiation: Reactive material, alkaline environment, moisture.

Result: Reduction of concrete strength, surface cracks, spalling concrete, pop-outs.

Visual appearance: Narrow cracks in a pattern, spalling concrete.

Growth: Fast/slow depending of the reactive aggregate. Acceleration: ingress of e.g. sodium-chlorides.

Typical areas: Splash zones on marine structures, etc.

NDT-methods: Concrete cores, Impact-Echo, Impulse Response, Boroscope.

Rehabilitation methods:- Preventive: Reduce moisture/chloride ingress, surface treatment.

- Corrective: Replacement of concrete.




4 components must be present to cause ASR:

Harmless ASR – interior cracking/solution of reactive grain

Damaging ASR – cracking in concrete

ASR

Reactive Silica (e.g. SiO2·H2O)

Alkali (Na+, K+)

Ca(OH)2

Water (H2O)



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To cause damage due to ASR a certain minimum amount of alkali reactive grains has to be contained in the concrete. The value of the minimum amount depends on the type of reactive material.

For ASR to cause expansion and cracking of the concrete the alkali content of the concrete has to exceed a certain value – this value depends on the type of reactive aggregate. It has to be noted that alkalis from outside (e.g. sodium from salt water) also contributes to the alkalis in the reactions.

The relative humidity of the concrete typically has to be > 80% RH.

Alkaline environment in the concrete is necessary for the reactions (pH > 12) – thus no reactions will take place if the concrete is carbonated.

Reactive sand aggregateReactive sand aggregate

Alkali silica reaction in a sand aggregate. Photo: 2.5x3.5 mm.



Initial defects

Initiation: Poor vibration, poor casting.

Result: Honeycombing, spalling.

Visual appearance: Cracking, spalling concrete, stones in the concrete surface.

Growth: Slow.

Typical areas: Areas with poor conditions for casting/vibration e.g. areas with heavy/close reinforcement and relatively low concrete cover.

NDT-methods: Break-ups, Impulse Response, boroscope.

Rehabilitation methods:- Preventive: Good workmanship and QA while casting in critical areas.

- Corrective: Replacement of concrete.



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Initial defects



Chemical attacks

Sulphate and acid attacks – the mechanism:

Sulphates are found in sea water, sewage water etc.

A number of sulphate compounds can attack the concrete.

First the compounds convert Ca(OH)2 to gypsum.

Then the compounds react with the aluminate parts (C3A) of the cement paste and form the chemical compound of ettringite.

The result of the chemical reactions is loss of concrete strength and expansion of the cement paste which causes cracking and spalling in the outer layer of concrete.

Ca(OH)2 + SO42- ⇒ CaSO4 + 2OH2-



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5.B Steel Bridges



Corrosion of steel

Electrochemical corrosion

Chink corrosion

Galvanic corrosion

Atmospheric corrosion

Ageing of steel

5.B Deterioration mechanisms in steel bridges


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Corrosion of steel – Electrochemical Corrosion

The water drop dissolve O2 from the atmosphere.

Where the water layer is thin the oxygen reach the steel very fast and acts as an electron acceptor according to the cathode process.

Inhomogeneities in the steel surface result in a certain area to become anode.

The circuit is working and the steel corrodes anodic in the centre.



Corrosion of steel – Chink Corrosion

Chink corrosion is a case of electrochemical corrosion.

Two plates overlap each other, and there is water in the overlaps (in the chink).

The free water surface absorbs oxygen and the metal surfaces in the marginal zones become cathodes.

In the chink the oxygen have difficulties to penetrate and the metal surfaces become anodes and is corroded. This corrosion is not visible from the outside and it is therefore dangerous.



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Corrosion of steel – Galvanic Corrosion

Galvanic corrosion is initiated when electrical contact occurs between two metals in a moist environment.

The more anodic metal releases ions and is corroded.

The smaller ratio between anode and cathode the more severe corrosion.

Prevention of galvanic corrosion:

Remove the moisture.

Disable electrical contact by:greasinginserts of plastic coating



Corrosion of steel – Atmospheric Corrosion

Atmospheric corrosion is corrosion on un-protected steel surfaces exposed to the atmosphere.

If the air humidity > 65 % relative humidity adsorption of connected water film occurs and thereby electrochemical corrosion occurs.

Different factors influence the risk and velocity of the corrosion.

Temperature: Increase of 10 oC → double corrosion velocity.

Air pollution: In an industrial environment the creation of soot is high. The soot contains sulphur and carbon. The sulphur acid acts as electrolyte and a strong corrosion cell is created between carbon and steel. In marine environment the large amount of salts in the air may cause stronger corrosion than in the inner of the country.



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Ageing of steel

Impact on steel at very low temperatures may result in fracture without any large deformation as seen at normal temperatures.

The brittle fracture form may also be seen on very old steels at normal temperatures: brittle fracture due to aging of steel.



Ageing




5.C Masonry Bridges


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Chemical/biological attack

Water and waterborne acids

Sulphates

Pollution

Erosion

Particles in flowing water and wind

frost attack

salt crystallization

plant root action

5.C Deterioration mechanisms in masonry bridges

Stress-related effects

movement of foundation

movement/consolidation/washout

of infill

vibration

overloading

moisture movement

thermal movement


Chemical/biological attack (Water)

Initiation: Water ingress.

Result: Loose sandy or friable mortar, loss of mortar.

Visual appearance: Raked joints, loss of bricks, stains due to precipitation of dissolved material.

Growth: Usually slow

Typical areas: Piers: Under water and in the splash zone. Abutments or other structures partly covered by water/soil.

NDT-methods: Cores, crack detection, boroscope.

Rehabilitation methods:- Preventive: Channel water away, or use strong impermeable mortars

- Corrective: Mechanical repointing using a waterproof or polymer-modified mortar.



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Chemical/biological attack (Water)

Absolute pure water will have no direct chemical effect but some of the constituents of mortar are slightly soluble and will leach away slowly.

Rainwater contains dissolved carbon dioxide forming a very mild acid which dissolves calcium carbonate by production of soluble bicarbonate.

Lime mortars will eventually be destroyed by percolating rainwater because calcium carbonate is their main binding agent.



Chemical/biological attack (Sulphate)

Initiation: Reaction between sulphate ions in water solution and the tricalciumaluminate (C3A) phase in mortars

Result: Net expansion that causes both local disruption of the mortar bed and stresses in the brickwork as a result of the expansion.

Visual appearance: Small horizontal cracks are sometimes visible in the centre of each bed joint. Rendered masonry may exhibit a network of cracks.

Typical areas: Will only occur in wet or saturated conditions and where there is a source of water-soluble sulphate compound.

NDT-methods: Cores, crack detection.

Rehabilitation methods:

- Preventive: Keep the masonry dry, exclude sulphates, use mortars that are not affected by sulphates.

- Corrective: Correction of faults that are causing unintended wetting. In serious cases it might be necessary to demolish and rebuild.



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Chemical/biological attack (Sulphate)

A sulphate attack will only occur in a wet or saturated environment where there is a source of water-soluble sulphate compound.

It will never take place in dry or slight damp masonry.

Sulphates may be present in groundwater and can affect masonry below the waterproofing membrane - and affect masonry in contact with the ground such as retaining walls, bridges and tunnels



Erosion (frost)

Initiation: Expansion of water freezing in the pore system of materials.

Result: Spalling of material, softening and erosion of the mortar (indistinguishable from chemical erosion)

Visual appearance: Spalling, loss of mortar

Typical areas: Water-saturated or near-saturated conditions in porous material.

NDT-methods: Cores, sonic methods.


- Preventive: Eliminate saturation of construction.

- Corrective: Mechanical repointing with a mortar containing a waterproofing or polymer additive.



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Erosion (Salt crystallization)

Initiation: Expansion of hydrated salts in the pore structure

Result: Spalling of material, softening and erosion of the mortar

Visual appearance: The spalling, softening and erosion of mortar will usually be associated with salt crystals.

Typical areas: Normally occurs in warm conditions where there is a rapid drying of water causing the salts to crystallize.

NDT-methods: Cores.


- Preventive: Using appropriate materials and detailing



Erosion (Abrasion and Stress related effects)

Abrasion:

Abrasion by particles in wind and water often acts in combination with other processes.

The appearance will normally be a loss of surface and change of colour and texture

Stress-related effects:

Step cracking or splitting of the mortar beds is common where movement or tension/shear forces occur.

The most common source is invasion by plant roots which then split the porous mortar as they grow



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5.D Underwater structures


Selected deterioration mechanisms - UWI

Basically the deterioration mechanisms of underwater structures include the same mechanisms as for structures above water – however the degree of deterioration may differ from the ones above water.

Deterioration due to chloride ingress will typically only be actual to a depth of app.0.5 m from the minimum water line.

NOTE: Severe corrosion may occur in this zone and the corrosion products will be less voluminous.

5.D Deterioration mechanisms of underwater structures


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Scour

Erosive action of running water carrying away material.

Aggradation (long term changing of the conditions):

Deposition of material – elevation the riverbed.

Reduction of the waterway the surface of water will rise.

Unintended horizontal forces to the piers/abutments during flood.

Failure of piers / abutments failure of bridge.

Degradation (long term changing of the conditions):

Lowering the river bed caused by erosion.

Undermining of foundation failure of piers / abutments failure of bridge.




Scour

General scour:

Erosion / removal of material from the whole width of the waterway.

Takes place over a short period of time (due to increased water speed).

Due to obstructions of the waterway upstreams or downstream.

Local scour:

Erosion / removal of material from part of the waterway.

Occurs where obstructions (piers etc.) changes the flow of water creating accelerations and vortex.

Depends on the shape of the obstruction (pier).




APPENDIX A3

General Introduction to Systematic Operation and Maintenance


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6. General Introduction to Systematic Operation and Maintenance


Systematic Operation and Maintenance

Systematic maintenance and operation requires:

Overview of assets.

Overview of documentation.

Overview of condition.

Activity management.

- Planning.

- Budgeting.

- Optimization of resources.

- Follow-up on execution and economy.

Several approaches could be used – the next slide shows one approach.

6 General introduction to operation and maintenance


NDT-Course

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Systematic Operation and Maintenance Chain of activities


No

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bilit

atio

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ed

No

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bilit

atio

n ne

ed

Afte

r 1-6

yea

rs

Principal Inspection Report

Special Inspection Report, Strategies

Optimization Projects to Execute

Rehabilitation Design Tender Documents

Execution 'As Built' Documentation

Activities Output

Routine Inspection

Routine Maintenance

Updating of Principal Inspection

Extended PrincipalInspection

Economic Evaluation

Report, General Considerationsof Rehabilitations

Rehabilitation needs

Rehabilitation needs


Systematic Operation and MaintenanceOptimisation

The challenge:

The funds are often not sufficient for maintaining all structures in perfect condition.

Basic demands for safety and capacity must be fulfilled.



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Systematic Operation and MaintenanceOptimisation

PurposeFinding a set of long-term rehabilitation strategies (one for each structure) that meet the available funds and has the lowest total cost for society.

PrincipleProjects required for reasons of safety etc. will be carried out.Projects with high cost increase over time will be carried out and projects with low cost increase will be postponed.

ResultBudget for selected maintenance/repair work for each structure for e.g. a 5-year period.Penalty (the cost of not having sufficient means to carry out all work at the optimum time).



Inspection types and frequencies



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Inspection Types

Routine Inspection

Principal Inspection

Extended Principal Inspection

Special Inspection

Monitoring (online or ad-hoc)



Routine Inspection

PurposeIdentify and register suddenly occurred damage and thereby ensuring safetyRegister the need for and supervise the execution of routine maintenance and cleaning

MethodSuperficial visual inspection

FrequencyNormal routine inspection usually calendar based (day/week) Extended routine inspection usually calendar based (1⁄2 year – 1 year)

ReportingStandard forms / check listsLists of standard worksRequisitions of maintenance works

PersonnelUsually limited needs for education (can be carried out by maintenance crew with some training)



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PurposeDetect deterioration before it gets seriousRegister the need for rehabilitation worksRegister the need for special inspectionsKeep track of the condition of the structure

MethodDetailed visual inspection of all visible parts at close range registering:

Condition ratingDescription of damageNeed for rehabilitation, including year and cost estimateNeed for special inspectionPhotosYear for next principal inspection




Condition rating of the bridge components:

0: No damage. As new.1: Insignificant damage. No action needed.2: Minor damage. Repair when convenient.3: Damage. Repair soon (or: evaluate more closely the need for repair).4: Severe damage. Repair is urgent.5: Extreme damage. Action must be taken immediately.

FrequencyUsually every 1-6 years – depending on the condition of the structure



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ReportingInspection report with registrations and recommendationsOverview of condition rating for the structureDescription of the condition of the structural elements (damages) List of structural elements to be repairedRunning forecasts of budget needs 5-10 years aheadList of structural elements that require special inspectionThe time to the next principal inspection (for the various structural elements)

PersonnelGood knowledge of damage types, causes and consequences, a certain knowledge of material technology and structural behaviour of bridges, good knowledge of maintenance and repair methods (usually carried out by engineers)




PurposeDetect deterioration before it gets seriousRegister the need for general rehabilitation worksKeep track of the condition of the structureDetermine knowledge about the type and extent ofdamage of selected bridge components

FrequencyIrregular intervals – is performed instead of a principle inspection if there is doubt about damage type and extent of some bridge components



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Method

Detailed visual inspection of all visible parts at close range registering:Condition rating (defined as in principal inspection)Description of damageGeneral considerations of the need for rehabilitationsNeed for special inspectionPhotosYear for next principal inspection

Non Destructive Testing of selected bridge components




ReportingInspection report with registrations andrecommendationsOverview of condition rating for the structureDescription of the condition of the structural elements (damages) General considerations of need for rehabilitationThe time to the next principal inspection (for the different structural components)

PersonnelGood knowledge of damage types, causes and consequences, a certain knowledge of material technology and structural behaviour of bridges, good knowledge of maintenance and repair methods, experts with experience in on site investigations, laboratory analysis, structural analysis etc. (usually carried out by experienced engineers)



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Special Inspection

Purpose – overall

To obtain detailed knowledge about the type and extent of damage resulting in the below benefits for the owner of the structure:

Optimal use of budgetsExtended service lifetime of structuresAvoidance of unforeseen costs



Special Inspection

Purpose – specific

Determine cause and extent of damage.

Assess probable future development of damage.

Set up and analyze alternative rehabilitation strategies (usually 1-3 strategies).

Make cost estimates.

Assess the technical and economical consequences of a (e.g. 5-year) postponement of each strategy. This part analyse may be left out.

Establish basis for rehabilitation design.



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Special Inspection

On site NDT-investigations:

Half Cell Potential

Galvapulse (corrosion rate)

s’MASH (Impulse Response)

Impact-Echo

Ground Penetration Radar

Boroscope

Break-ups to reinforcement

Covermeter

Capo test

Etc.

Laboratory analysis:

Chloride content

Moisture content

Carbonation depth

Concrete quality (microstructure, w/c ratio, air void distribution etc.)

Alkali Silica Reactivity

Etc.

Test methods – Concrete Structures

Detailed investigation comprising combinations of:



Special Inspection


Ultrasonic testing gauge

Ultrasonic testing

Magnetic particle testing

Dye penetrant

Magnetic thickness gauge

Strain gauge

Boroscope

Etc.


Fracture toughness

Chemical composition

Pressure testing

Etc.

Test methods – Steel Structures




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Special Inspection


Schmidt hammer

Ground penetration radar

Coring

Etc.


Evaluation of masonry cores

Etc.

Test methods – Masonry structures




Special Inspection

Method:

Identify relevant strategies, typically

Thorough repair now that solves the problem permanently.

Interim repair now. Thorough repair later.

Do nothing now. When the structure is no longer safe, replace it.

For each strategy:

Estimate all direct over a long period (typically 50 years)

Calculate the net present value (NPV) of the costs

The optimum strategy is the one with the lowest net present value



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Special InspectionSpecial Inspection

FrequencySpecial Inspections are carried out at irregular intervals when there is doubt about:

Damage mechanism/interaction

Damage cause

Damage type and extent

Damage development/growth



Special Inspection

Reporting

Inspection report on a ”scientific level”Description of the structure and the problemPresentation of the test methods usedThe registrations (measurements/test results)Evaluation of the registrations (What is wrong? What willhappen if nothing is done? What should be done ?)Recommendations for remedial actions

PersonnelExperts with experience in on site investigations, laboratory analysis, structural analysis etc. (usually carried out byexperienced engineers with significant knowledge on therequired topics)



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MonitoringMonitoring


Monitoring of the condition and behavior of bridges is an integrated part of bridge management.

Benefits:

Monitoring critical parts of the structure gives detailed information of the actual condition.

Reduced direct costs by postponing and tailoring the need for rehabilitation.

Reduced traffic interference and traffic regulations.




Economy:

The monitoring approach is often more economic than the traditional approach in the field of rehabilitations.

Early-warning of damages or safety risks.

The basis for making decisions and prioritization regarding maintenance and repair activities is improved

Fewer unexpected major costs and cost reductions as repairs may be postponed by several years and/or tailored.


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In-situ monitoring:

Frequent inspection with traditional methods (HCP etc.), providing a picture of the development of the properties that are registered.

On-line-monitoring

Continuous (or very frequent) measuring of specific properties, performed by sensors placed in the structure.

Possible to monitor areas where access is difficult or impossible.

Traffic interference is reduced.

The data can be collected directly from a data logger on site or be downloaded using Internet or telephone line




Management solution for monitoringSMARTmonitoring – an integrated module in the SMARTmanagementsystem.


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Under Water Inspections for Indian Railways are conducted according to:

FHWA-DP-80-01, Underwater Inspection of BridgesUS Department of Transportation, Federal Highway AdministrationNovember 1989

Three levels of inspection are considered:

Under Water Inspection (UWI)

Level I: Purely visual inspection, corresponding to Principal Inspection, supplemented by water depth soundings

Level II: Level I supplemented by cleaning selected areas for closer inspection, corresponding to Extended Principal Inspection

Level III: Detailed investigation of specific elements, using Non-Destructive Testing, corresponding to Special Inspection



UWI - Level I:

Visual, tactile inspection using large sweeping motions of the hands where visibility is limited.

Major damage or deterioration due to over-stress or severe deterioration (spalling) or corrosion should be detected.

The continuity of the full length of all members should be confirmed.

Undermining or exposure of normally buried elements should be detected.

The inspection should be conducted over the total exterior surface of each underwater structural element.



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UWI - Level II:

Detailed inspection requiring parts of the structure to be cleaned of marine growth.

The cleaning of piers and abutments are performed in areas of app. 0.30 m x 0.30 m in 3 different levels on each face of the element.

The thoroughness of cleaning should be governed by what is necessary to identify and register the condition of the underlying material.

Damaged areas should be measured and the extent and severity of the damage should be documented.




UWI - Level III:

Highly detailed inspection of a critical structures, structural components, or members where extensive repair or possible replacement is contemplated.

Hidden or interior damage must be detected.

Loss of cross sectional area must be detected.

The material homogeneity must be evaluated.

The level III inspection includes extensive cleaning, detailed measurements and selected Non Destructive Tests.





APPENDIX A4

Special Inspection


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7. Special Inspection


Agenda

A. Inspection Program

B. Planning

C. Execution

D. Assessment and Reporting

E. Report template

7 Special Inspection


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A. Inspection program

An inspection program should describe:

1. Purpose

2. Result

3. Extent

4. Background information

5. Basic assumptions

6. Inspection activities to be carried out




1. Purpose

– To maintain the functionality

– To fulfill the requirements regarding safety and aesthetic appearance

– To obtain knowledge of the condition of the structure

– To determine a repair strategy

– To determine the need for future inspections



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2. Result (inspection report)

– Level of detail (consider the need for future follow up on damage development)

– Who will be using the report (are certain parts to be handed over to e.g. contractors)

– Format (electronic, paper, management system)

3. Extent

– What structural parts are to de included ?

– Assessment of all or selected structural parts

– Number of repair strategies




4. Background information

– Drawings

– Previously inspection reports or investigations

– Access facilities

– Information on wires, cables etc.

– Requirements regarding special educations (e.g. with respect to working on the railway and on highways)

– Restrictions regarding the time for carrying out the inspections (e.g. night work)



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5. Basic assumptions

– Requirements regarding the time for carrying out maintenance and repair (e.g. coordination with other repair works or inspections on the same railway line)

– Requirements regarding the aesthetic appearance of the structure

– Accuracy in economic assessments




6. Inspection activities

– On site activities (measurements, sampling, visual inspection etc.)

– Laboratory investigations

– Structural assessment /Static evaluations



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B. Planning

1. Review of the background information

2. Introductory visual inspection

3. Set up hypotheses

4. Select inspection activities

5. Clear up all practical matters



B. Planning

1. Review of the background information– Identification of critical areas using drawings, previous inspections

and investigations, any static evaluations or structural assessments, etc.

2. Introductory visual inspection– Identification/verification of relevant damage mechanisms and

selection of areas for detailed inspection (in situ and/or laboratory analyses)



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B. Planning

3. Set up hypotheses – Set up hypotheses regarding damage mechanism and extent (define

homogenous areas) - Remember to consider what to do if your hypothesis is not confirmed

4. Select inspection activities– Taylor the activities to verify the hypotheses by selecting the right

combination of in-situ investigations and laboratory analyses –including selecting the sufficient number of measurements/samples



B. Planning

4. Select inspection activities (continued)– Usually the structure in divided into homogenous areas, i.e. areas

which are expected to have the same damage extent

– A superficial investigation is usually carried out on a large area and detailed investigation is carried out on selected small local areas (typically both the most critical areas and areas that are expected to be in good condition)

Large area with a small level of detail

Small area with a high level of detail



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B. Planning

4. Select inspection activities (continued)– Visual non-deteriorated areas of locally deteriorated structural

components must be included into the inspection in order to verify that the deterioration is as assumed in the hypothesis.



B. Planning

We have finished our investigation and can conclude that the building is in perfect condition !



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B. Planning

5. Clear up all practical matters

– Assess facilities (traffic restrictions, Sky lifts, Sky Climbers, boats etc.)

– Assistance from contractors (e.g. core drilling, break-ups etc.)

– Necessary tools/equipment (e.g. for NDT-measurements, break-ups, core drilling equipment etc.)

– Assess the duration of the inspection and the personnel required

– Consider the climatic conditions (wind, waves etc.)



C. Execution

Follow the planning if the conditions are as assumed –otherwise adjust the inspection activities to theconditions !

The process is iterative, so be aware of theconsequences of changes on the budget and durationof the inspection !



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C. Execution

Set up hypothesis on damage cause and extentSet up hypothesis on damage cause and extentSet up hypothesis on damage cause and extent

Select the necessary inspection activities for verification of the hypothesis

Select the necessary inspection activities for verification of tSelect the necessary inspection activities for verification of the he hypothesishypothesis

Carry out the inspection and evaluate the resultsCarry out the inspection and evaluate the resultsCarry out the inspection and evaluate the results

Is the hypothesis verified ?Is the hypothesis verified ?Is the hypothesis verified ?

Prepare inspection report with recommended repair strategyPrepare inspection report with recommended repair strategyPrepare inspection report with recommended repair strategy

YES

NO



D. Assessment and reporting

Table of contents

• Motivation of the special inspection

• Back ground documents

• Registrations (description of the condition)

• Evaluation of registrations (damage mechanism, cause, extent, location and development)

• Repair strategies

• Recommendation of follow up activities



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1. Background documents

– Inventory report – previous inspection reports

– Description of the structure (administrative data, structural layout, materials etc.)

– History (construction, repair, changes etc.)

– Other matters (traffic, etc.)




2. Registrations

– Introductory visual inspection

– Inspection extent and methods

– Homogeneous areas and damage hypothesis

– Summary of registrations (divided into structural parts)

- Visual registrations (damage type and extent)

- Results from on site and laboratory investigations



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3. Evaluation of registrations (divided into structural parts)

– Damage mechanism

– Damage cause and extent

– Damage location

– The effect of the damages on the structural part considered and on other structural parts

– Damage development in the future




4. Repair strategies

Short description of the selected repair strategies

A. Limited / temporary repairing

B. Thorough repair of bridge component

C. Replacement of bridge component

Detailed description of each strategy (technical description, time schedule for repair, consequences for the traffic)

Economical and technical evaluation of the different strategies

5. Recommendation of follow up activities

Usually the repair strategy with the lowest net present value

If any doubt of the load carrying capacity – structural calculations must be carried out



NDT-Course

India 2005/2006


E. Report Template

Report templates for extended principal inspection and for special inspection has been prepared for this pilot project.




APPENDIX A5

Structural Assessment - Case


NDT-Course

India 2005/2006

8. Structural Assessment - Case

A Combined Road and Railway Bridge


A. Introduction

B. Activity program

C. Phase 1 – Fatigue analysis

D. Phase 1 - Inspections

E. Phase 2 – Strengthening

F. Phase 3 – Inspection program for detection of fatigue cracks

Outline8 Structural Assessment - Case


NDT-Course

India 2005/2006


Riveted steel bridge from 1937

Combined railway and road

Total length app. 185m (5 spans of 31.5m and 1 bascule span of 28.4m)

The superstructure includes 2 main girders and cross beams per 5.25m as well as two railway girders

A. Introduction8 Structural Assessment - Case


A. Introduction

During a routine inspection cracks were observed in the cross beams at the joints between the main girders and the cross beams (14 positions)

8 Structural Assessment - Case


NDT-Course

India 2005/2006


A. Introduction

The end of the cracks were identified by magnetic particle testing

Holes were drilled in the steel plates to stop the development of cracks



B. Activity program

1. Structural fatigue analysisIdentification of critical jointsFatigue analysis of joints identified as being criticalProgram of further inspections

2. Strengthening projectIdentification of the cause of the damages (observed cracks)Pilot projectFull scale strengthening project

3. Inspection program with respect to fatigue cracksInspections in the period before strengtheningInspections after strengthening

Based on the observations of cracks the following program was setup:



NDT-Course

India 2005/2006


C. Phase 1 - Fatigue analysis

Based on the drawings critical joints were identified.

Fatigue analysis were carried out for the critical joints. The calculations were based on a Finite-Element model using shell elements in the program LUSAS.



C. Phase 1 - Fatigue analysis

The calculated stresses in critical elements were calibrated with results from strain gauge measurements. The plot shows the calculated stresses versus the measured stresses at a specific points of the structure as a function of time.



NDT-Course

India 2005/2006


Based on the rate of utilization the joints were divided into three groups:

Critical areas/joints u ≥ 1.00

Potential critical areas/joints 0.80 ≤ u ≤ 1.00

Not critical areas/joints u < 0.80

C. Phase 1 - Fatigue analysis8 Structural Assessment - Case


Examples of critical areas / joints



NDT-Course

India 2005/2006


Examples of critical areas / joints



An inspection program was prepared comprising:

Superficial visual inspection of the entire superstructure

Detailed visual inspection of all potential critical and critical joints (u > 0.80)

NDT-inspections of selected part of critical joints

The inspection program was dynamic (if cracks were detected visually or with NDT then the areas of NDT-inspections were expanded)

D. Phase 1 - Inspections8 Structural Assessment - Case


NDT-Course

India 2005/2006


Visual inspection

No additional cracks observed at main or cross girder

Gaps and corrosion between L-profile and railway girder in the joint to cross beams were observed

Corrosion damages were observed



NDT-inspections:

Ultrasonic tests of rivets


Rivet

PlatePlate

Grinding of surface to a plane section

Probe



NDT-Course

India 2005/2006



Few cracks were found in rivets

Yes

Yes

Yes

No

Calibration

1

4(3)

Number of defects

Signals in different levels on the two sides / probably linear misalignment

D

Heavy signal below the rivet head / probably crack

C

Low indication of crack below the rivet head / probably background noise

B

No significant signal / no signalA

IllustrationDescriptionType



Calibration of ultrasonic tests on rivets



NDT-Course

India 2005/2006



NDT-inspections – steel plates

Ultrasonic tests of platematerial (cracks and irregularities on and behind the surface can be identified)

Magnetic particle tests ofplate material (surface related cracks can be identified)




NDT-inspections – steel plates

No surface related cracks were identified

Few material irregularities below the surface were identified in the plate material



NDT-Course

India 2005/2006


E. Phase 2 - Strengthening



Maintenance program before execution of the strengthening project:

Regular monitoring (app. every two weeks during 5 months) of cracks observed for developing.

Maintenance program after the strengthening project:

Monitoring of selected critical areas visually and by NDT-tests every four years

F. Phase 3 – Inspection program for detection of fatigue cracks8 Structural Assessment - Case


NDT-Course

India 2005/2006


Inspection program before execution of the strengthening project:

Regular monitoring was carried out. The monitoring included visual inspection and NDT-inspection (dye penetrant test or magnetic particle testing).



Inspection program after the strengthening project:

Monitoring of selected critical areas visually and by NDT-tests every four years.

The areas / joints are selected based on the calculated utilization rate and an evaluation of the consequences for the structure:

Most severe consequences: Derailing of trains and collapse of bridge.

Severe consequences: Derailing of trains but no collapse of bridge.

Less severe consequences: No derailing of trains, no collapse of bridge but local failure of neighboring bridge elements.

Insignificant consequences: No interference with neighboring bridge elements, no collapse of bridge and no derailing of trains.



NDT-Course

India 2005/2006



Utilization rate

Consequences

Critical Potentialcritical

Not critical

Most severe Severe Insignificant

Most severe

Severe

Insignificant

Most severe

Severe Insignificant




Detail 10: Inspection year 2007, hereafter every four years.


Detail 17a: Inspection year 2007, hereafter every four years.

Detail 20b: Inspection year 2007, hereafter every four years.






APPENDIX A6

Crack Measuring Gauge and Crack Detection Microscope


NDT-Course

India 2005/2006

Crack Measuring GaugeCrack Detection Microscope

NDT – Concrete, Steel and Masonry

2SlideCrack Measuring GaugeCrack Detection Microscope - 21 February, 2006

Crack measuring gauge and Crack detection microscope -Measuring Concept

Visual measurement of crack widths

Light weight and portable

Typical Applications

Measurements of cracks induced by load or deterioration

Concrete, masonry or steel structures

Identification of crack width and development in crack width

Introduction


NDT-Course

India 2005/2006


Introduction

Benefits:

Low costs

Great accuracy

Fast measuring

Easy to use

Light weight and portable


Agenda






6. References


NDT-Course

India 2005/2006




Measuring Principle – The Instrument

Crack Measuring Gauge

Comparison of visible crack with predefined accurate scale

Crack Detection Microscope

Optical magnification

Build in light and measuring scale



NDT-Course

India 2005/2006


Measuring Principle – What is Measured?

The crack width opening at the surface is measured

It is a visual comparison between a predefined scale and the appearance of the crack at the surface

Often several points along a crack are measured in order to gain more accuracy



Measurements

For a general determination of cracks in a structure a crack measuring gauge is used

Several cracks are measured and often a hand sketch is made

Often the cracks are marked with chalk

If the exact width of a crack is wanted a crack detection microscope is used

The crack is measured in several points



NDT-Course

India 2005/2006


Accuracy

In general an accuracy of 0,01 mm is possible

Cracks smaller than 0,05 mm are hard to see for an un aided eye –a crack detection microscope is needed

The complex nature of crack propagation causes the crack width to deviate substantially along the crack mouth opening

The accuracy is in general limited by the number of measurements



Factors of General Influence

Broken edges near the surface will often make a crack seem larger than it is – this effect becomes more and more dominant in time

Temperature and load has great influence on the crack width


Diva-Panvel Bridge


NDT-Course

India 2005/2006




Common Applications - Damage

Cracks in concrete caused by:OverloadingShrinkage and temperatureASR, Frost, corrosion

Cracks in masonry caused by:OverloadingSettling of the foundationLoss of strength (aging)

Cracks in steel caused by:OverloadingFatigueHydrogen brittlenessCorrosion (loss of capacity)



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India 2005/2006


Common Applications - Structural Elements

Bridge decks, edge beams and wing walls

Concrete in general – crack free concrete has yet to be invented!!!

Masonry arches and walls

All steel members subjected to tension and shear



Theoretical Possible Applications/Limitations

Obviously any cracked structure can be measured if it is visible

Measurements must be performed “hands-on” – hence internal or other inaccessible areas cannot be measured



NDT-Course

India 2005/2006


Input to Rehabilitation Strategies

There are almost infinite application possibilities of the crackmeasuring gauge and the crack detection microscope regarding input to rehabilitation strategies

The main output for any survey including crack measurements will in general be to estimate:

The cracks influence on the structural integrity

Crack propagation / development

Need of rehabilitation

Feasible rehabilitation techniques





NDT-Course

India 2005/2006


Test Planning

1. Initial Visual Survey

Identify the general crack pattern

Find areas suitable for making measurements

2. Forecast of Results – Creating a Hypothesis

Cause of cracking

Expected size and deviation of crack widths


Diva-Panvel Bridge


Test Planning

3. Selection of Test Areas

Choose representative areas for coarse measurements

Choose the critical areas for accurate measurements

4. Estimating the Appropriate Test Quantity

Often it will be optimal to make a larger set of coarse measurements combined with a smaller set of accurate measurements


Diva-Panvel Bridge


NDT-Course

India 2005/2006


Test Planning

5. Practical Preparations

Create sketch sheets for visual registrations

Make a time plan

Create a list of the planned investigation

6. To Bring (tools)

A normal hammer

Camera

Chalk for marking

Measuring tape and folding rule for measuring crack lengths


Diva-Panvel Bridge


Execution of Field Tests

7. Conduct Measurements

Make a sketch (table) with the cracks to be measured

Conduct measurements

A relative precise indication of the measuring point is needed for successive measurements

8. Calibrate Measurements

Take out concrete/masonry cores in selected cracks to calibrate the measurements for different crack widths



NDT-Course

India 2005/2006



9. Evaluate Measurements and Calibration

Consider whether additional measurements are necessary based on current results

10.Registration

Make a thorough visual registration



Summary – Planning and Execution

Planning






6. To Bring (Tools)

Execution




10.Registration



NDT-Course

India 2005/2006




Calibration and Reliability

The reliability of the measurements is mainly dependent on:

The correct areas and number of measurements has been chosen

The inspector has been thorough doing the measurements

A calibration by drilling cores or making break ups may identifycorrelations between:

Crack width and crack depth

Crack width and corrosion extent of rebars

Crack width and concrete condition/quality (Petrographical analysis)



NDT-Course

India 2005/2006


Reporting of Results

Report:

Description of measuring strategy and equipment

Results - including variations

Cause of cracking (if known)

Developments in different positions




Appendix:

Sketch of the general crack pattern

Field sketches and all results

Photo documentation



NDT-Course

India 2005/2006




5. Application Summary – Concrete Bridges

Damage

X(x)XXXXX(x)XXX(Freeze-thaw)

(x)XXXXXX(x)XXXASR

XXXXXXXXXXXInitial defects

XXXXXXXXXXStructural problems

(x)XXXXXChloride penetration

(x)(x)XXXXXXCarbonation

XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Cra

ck d

ete

ction

NDT-Method


NDT-Course

India 2005/2006



Always bring your Crack Measuring Gauge !!!

It is useful anywhere - anytime



APPENDIX A7

Boroscope


NDT-Course

India 2005/2006

Boroscope

NDT – Concrete, Steel and Masonry

2SlideBoroscope - 24 February, 2006

Boroscope - Measuring Concept

A tube packed with optical fibers - a viewing lens and light in one end and an eyepiece for viewing

Light is provided via a custom made light source through a fiber optic cable

The tip is inserted into a hole or otherwise inaccessible areas

Images or video-clips are recorded


Investigation of post tensioned cables in concrete structuresIdentification of corrosion

Evaluation of grouting and cable ducts

Evaluation of concrete condition

Investigation of the inside of closed box girdersDetection of corrosion

Introduction


NDT-Course

India 2005/2006


Introduction

Benefits:

“Hidden” areas can be inspected

Fast inspection

Inspection costs of build in items e.g. cables is reduced

Inspection can be recorded

The inside of a structure can be inspected with a minimum of damage


Agenda







NDT-Course

India 2005/2006


Boroscope



A Boroscope (or Endoscope) consist of:

A light source

A flexible fibre optic cable

An optical viewing unit where the tip is used both for illumination and “seeing”

A great number of different specialized configurations are available

Power source can be both battery or main power



NDT-Course

India 2005/2006



Most Boroscopes has optional connection of video or photo gear

View by video screen should not replace traditional view – The “resolution” of the human eye is far better than any digital chips!!

Newer equipment has build in video and photo features – but some are lacking the option of “normal view”




The Boroscope is used for visual registration

It can also be used for measuring the position of inaccessible items or damages

Thickness of masonry



NDT-Course

India 2005/2006


Measurements

The essential thing to consider when using the Boroscope is to select the appropriate position and number of inspection points

Shooting of video and photos should be done after a thorough registration



Accuracy

The accuracy is mainly limited by the thoroughness of the inspector

Also physical restrictions such as range and amount of light limits the accuracy of the equipment


Nira Bridge


NDT-Course

India 2005/2006


Boroscope


Common Applications - Damages

Poor injection of grout in cable ducts for post tensioned cables

Corrosion on post tensioned cables

Detection of deteriorated or lacking mortar in masonry bridges

Measurement of wall / arc thickness

Corrosion on the inside of closed steel profiles (Tubes and box profiles)



NDT-Course

India 2005/2006



Post tensioned concrete members

Beams

Decks

Girders

Cantilever wings

Box girders – concrete and steel

Joints (inaccessible parts)

Face, soffit and base of arch bridges


Diva-Panvel Bridge



Any structure where inspection through small holes is relevant

The feasibility of the Boroscope is limited by:

Geometry, depths to 50 cm

Lack of light – the distance from the tip to a surface should be less than 2 meters

Accessibility for the inspector



NDT-Course

India 2005/2006


Case 1: Skovdiget

Twin bridges from 1966 constructed as post tensioned concrete box-girder bridges.

220 m long with 12 spans which carries a 4 - lane highway over two roads, a parking area and a train line

Problems:Cable ducts insufficiently injected with grouting

Corrosion on cables and rebar

Massive problems with water and salt causing extensive precipitation and ASR cracks



Case 1: Skovdiget

1965-67 ConstructionDue to unfortunate design and poor workmanship the bridge began to deteriorate shortly after construction

1978 Major rehabilitation of Eastern BridgeThe cost of rehabilitation was 3.5 mill US$ (1978) - almost identical to the cost of a new Western Bridge !During the rehabilitation a Boroscope was used for the first time in Denmark for inspection of cables

1978-98 Inspection and test-loading of Western Bridge

Inspection plan for the Western Bridge -including inspections 4 times a year. Load test in 1984, 88 and 93.Several inspections of cables and the inside of box girders has been made in resent years



NDT-Course

India 2005/2006


Case 1: Skovdiget


Good injection Poor injection


Case 1: Skovdiget



NDT-Course

India 2005/2006


Case 1: Skovdiget



Case 1: Skovdiget



NDT-Course

India 2005/2006


Case 2: Steel box girder bridge

Small box girder bridge

Extensive corrosion on external girder faces – especially near the joints

Inspection included an investigation of the inside of the girder




Extensive corrosion on external girder faces – especially near the joints

Inspection included an investigation of the inside of the girder

It turned out that there was extensive corrosion inside the girder

The cause of corrosion was high humidity inside the girder because it was not sealed properly



NDT-Course

India 2005/2006








NDT-Course

India 2005/2006



Evaluation of otherwise inaccessible areas

Reduction of uncertainties for the overall condition rating



Boroscope


NDT-Course

India 2005/2006


Test Planning

1. Initial Visual SurveyIdentify the general condition

Locate potential critical areas

Find areas suitable for inspection


Evaluate possible correlation between external and internal conditionExpected damage mechanisms – corrosion, delamination, washing out of mortar



Test Planning


Aim to investigate both good and bad areas

Select areas where extrapolation of the condition is possible


Make few thorough investigations rather than many coarse



NDT-Course

India 2005/2006


Test Planning



Make a time plan


6. To Bring (tools)

Chalk for marking

Measuring tape and folding rule for measuring depths





Make a sketch (table) with indication of each investigation

Conduct investigation


Drilling of cores or break ups can be used for calibration



NDT-Course

India 2005/2006




Consider whether additional measurements are necessary based on current results

10.Registration

Make a thorough registration




Planning






6. To Bring (Tools)

Execution




10.Registration



NDT-Course

India 2005/2006





The reliability of the measurements is mainly dependent on:

The number of “black spot” – inaccessible areas

The inspectors ability of telling the difference between real flaws/ damages and e.g. harmless discoloration

A calibration by drilling cores or making break ups may give theinspector essential hints at what to look for in order to detectdamages

Calibrations will serve as further documentation and minimize uncertainties



NDT-Course

India 2005/2006


Damage Identification

Successive comparison of photos and videos

Compare internal and external visual registrations and if any, other NDT measurements

Make an overview of all registrations – this will often give a good idea of the deterioration pattern




Report:

Background for making the investigation

Extend and position of the investigation

Summary of the results

Result evaluation/evaluation of hypothesis – note if it some areas was inaccessible

Description of needed rehabilitation



NDT-Course

India 2005/2006



Appendix:

Sketch of all investigations and a “rating” for each investigation


Photo documentation



Boroscope


NDT-Course

India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Bo

rosco

pe

Crack d

etection

NDT-Method



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Bo

rosco

pe

Crack d

etection

NDT-Method



APPENDIX A8

Half Cell Potential Measurements


NDT-Course

India 2005/2006

Half-cell potential

NDT - Concrete

2SlideHalf-cell potential - 21 February, 2006

Overview of recent corrosion activity

Introduction


NDT-Course

India 2005/2006


Introduction

Typical application:Pitting corrosionUniform corrosion


Agenda






6. References


NDT-Course

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Method



Instrument:Half cell (Copper rod in a container filled with Copper sulphate and having a porous plug)

Voltmeter

Reinforcement contact


Measuremet:Voltage between reinforcement and electrode


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Potential measurement, theory

Fe Fe++ + 2e-

A steel rod placed in water will start to dissolve. The process will be reduced by the increasing electrical difference, and eventually stop.




Fe Fe++ + 2e- Zn Zn++ + 2e-

A less noble metal like zinc will also dissolve in Water, but the process will continue much longer than for the steel rod, and much stronger electrical differences will be created.



NDT-Course

India 2005/2006



v -+

If the 2 metal rods are connected an electrical voltage can be measured. This is the difference in potential between the 2 metals.



Measurements


Reinforcement in concrete Porous plug Half cell (Zn-cell)

Metal in stable solution will have a stable potential

Concrete environment surrounding the reinforcements affects the potential


NDT-Course

India 2005/2006


Corrosion potentials, EKP-measurement

Potential: quality of passive layer on the reinforcement surface

Au Ag Cu Pb Fe Zn Al

+ (Noble) Less noble -

Reinforcement in good concrete(passivation)

Reinforcement in poor concrete

Potential

Potential criterion: quality of passive layer on the reinforcement surface


Simple interpretation of results

Potential evaluation (ASTM C876)

• If the potential is more positive then -200 mV the risk of corrosion is less than 10%.

• If the potential is more negative then -350 mV the risk of corrosion is more than 90%.

Conclusion:Poor correlation



NDT-Course

India 2005/2006


Half-cell potential, theoryRefined interpretation of results

12

Oxygen access affects critical potential level, and oxygen access must be evaluated. This is done from concrete resistance.

Conclusion:

Resistance must be evaluated


Corrosion evaluation, resistance

Corrosion form is mainly controlled by concrete resistance: - Very high resistance prevents corrosion- Low resistance increases risk of macro cell corrosion- Low resistance reduces potentials in general

Conclusion: Resistance must be evaluated

Corrosion involves current through concrete Long vs. Short distance through concrete



NDT-Course

India 2005/2006



Principe: Potential gradient from fixed current

Pros: Independent of contact resistance

Cons: Errors from reinforcement, time consuming




Practice: AC-resistance between reference cell and reinforcement, parallel to potential measurement.

Pros: Fast and direct connected to potential mapping

Cons: Errors from contact resistance

Conclusion:

Potential and resistance is measured simultaneously.



NDT-Course

India 2005/2006


Aim of measurements

Measurements of potential and resistance is used for:- Determination of actual condition- Determination of future condition development- Estimation of corrosion cause- Estimation of corrosion problems (structural

damage, spalling)

These results are used for:- Evaluation of recent and future need for

repair/corrosion prevention- Estimation of cost and methods for repair/corrosion

prevention.



Accuracy


Accuracy:- Location of corroding area approx. 10-20 cm- Potentials, approx. 20 mV from measurement, much more from seasonal

changes.- Resistance, approx. 50%

Note:On areas with very sharp gradients, small changes in the location of the measuring point can have high influence on the results.


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The potentials are greatly influenced by:

- Moisture content

- Concrete resistance

- Concrete cover

- Chloride content

The potentials are to a much smaller degree influenced by carbonation.



Method


NDT-Course

India 2005/2006



The results are used for evaluation of corrosion from:

Deep carbonation

General chloride ingress

Local chloride ingress (cracks, pile base)

Water leaks through cracks




The method is usually used for evaluation of corrosion problems in:

Bridges: decks, columns, facades

Tunnels: Inside surface

Parking decks

Housing: facades, balconies



NDT-Course

India 2005/2006



Apart from the usual applications the method can to some degree be used for evaluation of corrosion problems in:

- Masonry and stone constructions

- Underwater constructions

- Constructions covered with soil

Further more the method can be used for evaluation of cathodic protection in constructions surrounded by air, water and soil.

The method can be used for evaluation of problems from stray current.




The method has several limitations:

- Reinforcement should have electrical continuity

- The surface should be free of electrical isolating surface treatment

- Potentials will only come from the reinforcement close to the electrode

- Corroding areas with small concrete cover are often not detected

- Corroding areas without contact to the concrete will not be detected.



NDT-Course

India 2005/2006


Case 1: Soderledstunnel, Stockholm, Sweden

1.5 km concrete tunnel

Spalling due to carbonation and small concrete cover

Corrosion due to chloride ingress and moderate concrete cover





Carbonation

Chloride

No corrosion


NDT-Course

India 2005/2006


Case 2, Shell Parking house, Denmark

- Optimal location for breake ups- Evaluation of overall area of damage



Case 3, access balcony, Copenhagen, Denmark

Evaluation of the effect of stopping water leakage

Before 1 year after 3 years after



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Case 4, Great Belt Bridge, Denmark


Inspection of cathodic protection from water anodes


Case 4, Great Belt Bridge, Denmark


Protection criteria: Potential lower than -850 mVAll underwater areas protected.


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Expected input to maintenance strategies

• Were is the corrosion

• How much corrosion there is

• Size of the areas with corrosion

• What type of damage (structural damage, spalling)

• Is a repair necessary

• What type of repair

• Time before the repair must be done

• Area of the repair



Half cell potential


NDT-Course

India 2005/2006


Test Planning


- General environmental impact- General concrete quality- Corrosion signs- Deviant environmental impact- Deviant concrete quality

2. Forecast of Results –Creating a Hypothesis



Test Planning


- How critical are the damage (safety, economical)

- Magnitude of variations


- Sharp gradients Close spacing- Low resistance Close spacing



NDT-Course

India 2005/2006


Test Planning

Practical Preparations

- Mark measuring grid- Make 2 reinforcement contact

points- Calibration brake-ups- Access of water and electrode

To Bring (tools)



Test Planning


To Bring (tools)

• Measurement unit• Measurement device• Cables• Contact to reinforcement• Jack hammer• Repairer equipment and

material• Generator• Cover meter (metal detector)


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- Visual registration of corrosion signs- 2 reinforcement contact points- Measure the resistance between the 2

points- Mark measuring grid with sufficient

accuracy - Calibrate instrument- Wetting the measure points- Conduct measurement- Control results during execution- Dynamic adjustment of measure-point

spacing (gradients, resistance)




5. Calibrate Measurements:

- Brake-ups- Chlorides- Carbonation- Concrete cover

Select calibration points at:

- Most corroding areas (serious damage?)

- Typical areas (general condition)- Not corroding areas (control of

Hypothesis



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Potential measurement, calibration

Remember:

Check the apparently undamaged areas






Direct

Coluredplot (potential)

Calculated corrosion risk (potential and gradient)


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India 2005/2006


Interpretation of results

Potentials

Potential gradients

Resistance

Calibrating brake-ups

Visual damange

Experience Supplementary measurementschlorides, carbonation

Experience Interpretation

1 2 3 4 5




Planning





Execution




8. Interpretation



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India 2005/2006


Method



Potentials have no direct connection to corrosion rate

Potentials have no direct connection to corrosion history

Numerous factors influence the potentials and the influence can only be estimated.

Calibration by direct inspection is vital and experience is necessary



NDT-Course

India 2005/2006




Sharp gradients, low resistance: chloride initiated corrosion, risk of cross-section reduction

Low gradients, high resistance: carbonation initiated corrosion, risk of spalling

General:Damage identification is difficult and supplementary measurements are usually necessary.



Report:

General conclusions

Eventually illustrative plots



NDT-Course

India 2005/2006



Appendix:

- Measured potentials and resistance- Measuring grid- Calibrating brake-ups- Relevant metrological data



Method


NDT-Course

India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



APPENDIX A9

Corrosion Rate Meter


NDT-Course

India 2005/2006

Corrosion rate

NDT - Concrete

2SlideCorrosion rate - 24 February, 2006

Overview of corrosion rate

Introduction


NDT-Course

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Introduction, typical application

Future development of damage due to - Pitting corrosion- Uniform corrosion


Agenda






6. References


NDT-Course

India 2005/2006


Corrosion rate



Instrument:Half cell instrument

+ AmmeterContact ring


Measurement:Change in voltage between reinforcement and electrode, when a fixed current is applied


NDT-Course

India 2005/2006


Corrosion rate, polarisation resistance

Corrosion rate (current) vs. Potential

0,01 0,1 1.0Log corrosion current

Poten

tial

Applied current causes changes in potentials




Principe: A fixed current is forced into the reinforcement and the change in potential is measured:

- Small changes = high corrosion current- Big changes = low corrosion current

Corrosion current vs. Potential

0,01 0,1 1.0Log corrosion current

Poten

tial

Slow corroding area, passivated

Fast corroding area, not passivated

Conclusion:

Corrosion current can be estimated from the measured potential changes.



NDT-Course

India 2005/2006



Corrosion density = corrosion current / measured corroding area

Corrosion rate = 11,6 x corrosion density


The integrated corrosion rate determines corrosion, and thereby the cross–section reduction responsible for strength reduction. The acceptable strength reduction is calculated from structural analysis

The integrated corrosion rate determines the rust formation, andthe concrete conditions determines the volume of the rust products corrosion, and thereby the cross–section reduction responsible for strength reduction. The acceptable strength reduction is calculated from structural analysis


From corrosion current to time till damage


Measuring of corrosion current

Determination of active corroding area

Calculation of corrosion density

Determination of recent Determination of recentcross section rust formation

Calculation of corrosion rate (µm/year)

Calculation of time till spalling

Calculation of acceptable Evaluation of volume of new rust cross section reduction rust products

Calculation of time till Evaluation of acceptable structural damage rust formation


NDT-Course

India 2005/2006


General Principe of measuring

70 mm

Reinforcement contact

Confined measuring area

Guard ring creating a uniform current density within the measuring area

Central potential measuring

Ring for current distribution



Main measuring methods

Linear polarization method:Polarization current in both directions and longer polarization duration

Theoretical more correct but much more laborious and time consuming (5 minutes/measuring).

Galva-pulse method:Polarization pulse in one directions (5-30 sec. duration).

Theoretical not very correct but less laborious and much faster (½ minute/measuring).

Rambøll’s conclusion:

The linear polarization method has not proven significantly more accurate, and the fast pulse method allows a much more detailed measuring grid, which overall gives a far more precise evaluation of the corrosion condition.

Half cell method (for comparison):5 sec/measuring and not laborious.



NDT-Course

India 2005/2006


Measuring Principe, additional measuring

Simultaneously with corrosion rate, the usual parameters (reinforcement potential and resistance) from the half-cell method is measured.



What is measured

The instrument measures a calculated value of the corrosion current. From this value the rate of cross section reduction and the formation of spalling and delaminaition must be estimated



NDT-Course

India 2005/2006


Aim of measurements, in general

The measurements (including the measurements of potential and resistance) is used for:- Determination of actual condition- Determination of future condition development- Estimation of corrosion cause- Estimation of corrosion problems (structural damage, spalling)

These results are used for:- Evaluation of recent and future need for repair/corrosion

prevention- Estimation of cost and methods for repair/corrosion prevention.



What damage is measured

The main damage which can be evaluated from this method is:- Cross section reduction - Development of spalling



NDT-Course

India 2005/2006


Accuracy


Accuracy in determination of corrosion rate:- Better than a factor of 10, if uniform corrosion - Calculation of pitting corrosion is more insecureLocation of corroding area approx. 10-20 cm

Note:On areas with very sharp gradients, small changes in the location of the measuring point can have high influence on the results.

0 40 80 120 160 200 240 280 320 360 400 440 480 520 560 600 640 6800

20

40

60

80

100

cm

cm

0-5 5-10 10-15 15-20

Corrosionrate at the inside casting joint

µm/year



Determination of corrosion rate are greatly influenced by:

- Seasonal changes - Reinforcement layout- Corrosion distribution


0

10

20

30

40

50

60

21 Aug 21 Dec 21 Apr 21 Aug 21 DecAve

rage

cor

rosi

on ra

te (m

icro

A/cm

2)

-10

-5

0

5

10

15

20

25

Tem

pera

ture

(Cel

cius

)Corrosion

Temperature


NDT-Course

India 2005/2006


Method



The results are usually used for evaluation of corrosion from:

Carbonation

General chloride ingress

Local chloride ingress (cracks, pile base)




NDT-Course

India 2005/2006



The method is usually used for evaluation of corrosion problems in:



Parking decks





The GP method works in wet structures, where interpretation of the potentials measurements is difficult

Fast and reliable information of the corrosion activity in dry and semi-dry structures is obtained by a combined use of the GP method and the HCP method

From multiple GP measurements taken over a period of time, an average rate of cross section loss can be estimated.

Monitoring by continuous measurement is possible



NDT-Course

India 2005/2006




- Reinforcement should have electrical continuity

- The surface should be free of electrical isolating surface treatment

- Potentials will only come from the reinforcement close to the electrode

- Corroding areas without contact to the concrete will not be detected.









NDT-Course

India 2005/2006




Carbonation

Chloride

No corrosion




100%

0 2 5 10

Corrosion rate (µm/year)

Percentage of brake-ups with visual signs of corrosion


NDT-Course

India 2005/2006


Case 2: Skovdiget bridge, Copenhagen, Denmark

0,6 km concrete bridge

Severe corrosion at column base due to chloride ingress poor concrete quality



Case 2, corrosion rate, september

2000 2001

2002

0 30 60 90 120 150 180 210 240 270 300 330 3600

33

66

100

133

166

200

2004



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India 2005/2006



• Were is the corrosion

• How much corrosion there is

• How fast is corrosion propagating

• Size of the areas with corrosion

• What type of damage (structural damage, spalling)







Half cell potential


NDT-Course

India 2005/2006


Test Planning


- General environmental impact- General concrete quality- Corrosion signs- Areas with deviant environmental

impact- Areas with deviant concrete quality




Test Planning


- How critical are the damage (safety, economical)

- Magnitude of variations- Will other measurements tell us

what wee need


- Sharp gradients Close spacing- Low resistance Close spacing



NDT-Course

India 2005/2006


Test Planning


- Mark measuring grid- Locate reinforcement- Make 2 reinforcement contact points- Calibration brake-ups- Access of water and electrode



Test Planning


To Bring (tools)

• Measurement unit• Measurement device• Cables• Contact to reinforcement• Jack hammer• Repair equipment and

material• Generator• Cover meter (metal detector)


NDT-Course

India 2005/2006




- Visual registration of corrosion signs- 2 reinforcement contact points- Measure the resistance between the 2

points- Mark measuring grid with sufficient

accuracy - Calibrate instrument- Wetting the measure points- Conduct measurement- Control results during execution- Dynamic adjustment of measure-point

spacing (gradients, resistance)




5. Calibrate Measurements:

- Brake-ups- Chlorides- Carbonation- Concrete cover




hypothesis



NDT-Course

India 2005/2006



Calibration from brake-ups:- Reinforcement area within guard

ring (size, number, crossing)- Active corroding area- Corrosion depth- Carbonation- Concrete cover




hypothesis



Potential measurement, calibration

Remember:

Check the apparently undamaged areas



NDT-Course

India 2005/2006





Direct with evaluation presented with colors

Colored plot, gives overview



7. Interpretation


The following criteria are recommended by FORCE for evaluation of GP-measurements on concrete structures:

Corrosion current density Evaluation Time till damageAbove 15 mA/cm2 High < 10 years

Between 5 & 15 mA/cm2 Moderate 10-25 years

Between 1 & 5 mA/cm2 Low 25-50 years

Below 1 mA/cm2 Negligible > 50 years

These values are valid when measured with the GalvaPulse instrument as corrosion current with correct rebar area and uniform corrosion distribution. Time till damage varies heavily with several factors individual to each structure and must therefore be evaluated individually.


NDT-Course

India 2005/2006


Interpretation of results

Potentials

Potential gradients

Resistance

Calibrating brake-ups:(active/passive,corrosion depth and -uniformity,reinforcement area)

Visual damage

Experience

Supplementary measurements (chlorides, carbonation)

Evaluation of corrosion development(seasonal, concrete environment)

Evaluation of damage development(spalling, structural damage)

1 2 3 4 5


Corrosion current



Planning





Execution




8. Interpretation



NDT-Course

India 2005/2006


Method



Measured corrosion rate are heavily influenced by seasonal changes

Measured corrosion rate have no direct connection to corrosion history

Numerous factors influence the potentials and the influence can only be estimated.

Calibration by direct inspection isvital and experience is necessary



NDT-Course

India 2005/2006



Measuring of corrosion current

Determination of active corroding area

Calculation of corrosion density

Determination of recent Determination of recentcross section rust formation

Calculation of corrosion rate (µm/year)

Calculation of time till spalling

Calculation of acceptable Evaluation of volume of new rust cross section reduction rust products

Calculation of time till Evaluation of acceptable structural damage rust formation





Local high corrosion rate, sharp gradients: chloride initiated corrosion, risk of cross-section reduction

Uniform, moderate corrosion rate: carbonation initiated corrosion, risk of spalling

General:Damage identification is difficult and supplementary measurements are usually necessary.


NDT-Course

India 2005/2006



Report:

General conclusions

Eventually illustrative plots




Appendix:

- Measured corrosion rates- Measured potentials and

resistance- Measuring grid- Calibrating brake-ups- Relevant metrological data



NDT-Course

India 2005/2006


Method



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



APPENDIX A10

Covermeter


NDT-Course

India 2005/2006

Cover meter

NDT - Concrete

2SlideCovermeter - 24 February, 2006

Cover meter - Measuring Concept

The equipment consist of sensor and a recording instrument

Advanced metal-detector

Measurement of concrete cover, and rebar size

Fast (very fast) overview

Introduction


NDT-Course

India 2005/2006


Introduction

Advanced metal detector:

Fast screening of a large areas

Evaluation of ???

Estimation of extent of repair

Estimation of where repair is needed

Locating of vital reinforcement

Validation of drawings


Agenda







NDT-Course

India 2005/2006


NDT - Concrete




Principe (electrical Conductivity):

AC-current (pulse) is run through the coils in the sensor head, and the resulting current is measured.


NDT-Course

India 2005/2006




Rebar size:Signal strength from 2 positions is measured and compared.


Accuracy

Cover meter investigations are reliable and easy to reproduce

The deeper a rebar is located, the harder it is to detect

Typical Accuracy: + 1 mm (cover 10-30 mm)+ 2 mm (cover 30-65 mm)+ 5% (cover > 65 mm)

Typical max. cover depth: 130 mm (Ø 8 mm), 180 mm (Ø 32 mm)



NDT-Course

India 2005/2006




Rebar orientation Rebar spacing


NDT - Concrete


NDT-Course

India 2005/2006



Corrosion from chloride ingress Spalling caused by carbonation




Concrete Bridges

Facade, soffit, top side of bridge deck, columns

Housing facades

Cylindrical Structures

Silos, Tanks, Chimneys



NDT-Course

India 2005/2006



Cover meter can be used on all non-metallic materials (concrete, masonry, wood, stone etc.)

Special metal detectors can locate steel at depth of ½-1 meter

With proper equipment, underwater measurements are possible

Very rough surfaces will reduce accuracy

Go / no-go factorsExpected concrete coverExpected rebar spacingExpected other metallic objects



Case 1: Soderledstunnel

1.5 km concrete tunnel in Stockholm, Sweden





NDT-Course

India 2005/2006


Case 1: Soderledstunnel

Very small concrete cover Deep carbonatisation



Case 2: Bernstorffstunnel, facade

Problem: Spalling due to carbonation of concrete



NDT-Course

India 2005/2006


Case 2: Bernstorffstunnel, facade

Non-uniform concrete quality causes variations in carbonation depth

Break-up no. 1: Carbonation approx 10 mm Break-up no. 2: Carbonation approx. 25 mm



Case 2: The Sorterende Bridge

A 300 m long concrete bridge in Denmark

Deep chloride ingress in concrete piers during 25 years lifetime. Concrete cover is important when evaluating the time to corrosion initiation.



NDT-Course

India 2005/2006


Case 2: The Sorterende Bridge

Determination of chloride content at the reinforcement level


S2.2-Ø , Kote -0,5 m

0,235 0,121 0,061 0,0210,3360,000

0,100

0,200

0,300

0,400

0,500

0-10 10-20 20-30 30-50 50-70D ybde ba g ove rfla de [mm]Concrete cover

Rein

force

men

t level



Fast screening of large areas

Identifying good and damaged areas

Estimating the extent of needed repair

Locating of areas to be repaired



NDT-Course

India 2005/2006


NDT - Concrete


Test Planning


Focus on visible damage and signs of small concrete cover

Accessibility



NDT-Course

India 2005/2006


Test Planning


2 Selection of Test Areas

Risk of damages (reinforcement drawings)

Identification of critical areas

Identification of critical elements

Include intact and damaged areas in each test-grid

3 Estimating the Appropriate Test Quantity

Rebar size, etc.

Dynamic test planning

4 Rebar size and direction (drawings)



5. Conduct MeasurementsMake a superficial visual survey in order to confirm the feasibility of the planned tests

Mark up test grid

6. Calibrate MeasurementsCreate Excel-graphs and view the results

Make a swift visual registration / survey of the test grid, use also a normal hammer

Mark up where cores should be drilled or break-ups be made for on-site calibration



NDT-Course

India 2005/2006



5 Control rebar size

Position of 2 rebars from outer rebar net is located

Position of 1 rebar from outer rebar net is marked for approx. 2 m

Cover of rebars in the inner rebar net is measured between outer rebars

6 Calibrate instrument




7 Conduct Measurements

Position of 2 rebars from outer rebar net is located

Position of 1 rebar from outer rebar net is marked for approx. 2 m

Cover of rebars in the inner rebar net is measured between outer rebars



NDT-Course

India 2005/2006




Verify concrete cover

Examine rebar size

Does the results match with the hypothesis?!

Decide whether additional steps must be taken (e.g. extra cores or break-ups)

8. Registration

Make a thorough visual registration, geometry, brake ups etc.




Planning




Execution

4. Control rebar size

Calibrate Instrument

Conduct Measurements

Evaluate Measurements and Calibration

Registration



NDT-Course

India 2005/2006


NDT - Concrete



It must be evaluated how reliable / accurate the measurements are:

How good is the correlationbetween measurements and calibration?

Are the measured areas representative for the whole element / structure?


Error:

2 rebars


NDT-Course

India 2005/2006




Height

Distance from joint Cover (mm)

The areas which are found to be damaged are pointed out

Spalling



Report:

The conclusions of the measurements should be summarized

Overall condition, damage type and extend

Possible repair methods

Are further measurements needed? (describe benefits)

Avoid inserting plots of the measurements

The technical presentation of the measurements should be constricted to the appendix



NDT-Course

India 2005/2006



Appendix:

Registration of position and geometryIs used as a tool for interpretation

Gives the reader an overview of exactly where there measurements has been made

If successive measurements are expected thorough registrations are necessary for comparison of results


Height

Distance from joint Cover (mm)



Appendix:

Registration of position and geometry

Is used as a tool for interpretation





NDT-Course

India 2005/2006



Appendix:

Measurements and Calibration

These are equally important and should be presented accordingly

Focus on rehabilitation strategy

Good and bad areas

Degree of damage and repair methods

Description of relevant uncertainties



NDT - Concrete


NDT-Course

India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial & co

rrosio

n

rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Co

ver m

ete

r

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial & co

rrosio

n

rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Co

ver m

ete

r

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



APPENDIX A11

Spraying Indicators


NDT-Course

India 2005/2006

Spraying indicator

NDT – Concrete, masonry, steel

2SlideSpraying indicators (pH) - 24 February, 2006

Determination of surface pH

Introduction


NDT-Course

India 2005/2006



Risk of corrosion, primarilycorrosion caused by carbonation


Agenda






6. References


NDT-Course

India 2005/2006


Spraying indicator, pH


Theory, carbonation in concrete


pH

12

9

Concrete cover

carbonated un-carbonated

Carbonation causes a sharp drop in pH from more than 12 to less than 9.

In moist conditions (RH >60%) areas with pH<9 will corrode

Carbonation depth increases with time and the rate of carbonation can be predicted

carbonation depth


NDT-Course

India 2005/2006


Measuring Principle

Instrument:98% Alcohol with 1% Phenolphthalein

+ Manually spray-can


Measurement:Registration of colored areas

In concrete the carbonation depth is measured.


What is measured

If the current pH value of the surface is above or bellow the shift-value of the indicator in use.

The measurement is preformed on every point of the sprayed surface. Even small points with deviating pH are detected.

For the Phenolphthalein the shift-value is approx. pH 9.



NDT-Course

India 2005/2006


Aim of measurements

Evaluation of current corrosion risk:In concrete-, mortar-, soil environment the risk of corrosion is small if pH is above approx. 9


Evaluation of aggressive liquids:Risk of degradation of concrete, mortar and natural stone in contact with running water depends highly on the pH-value of the water.

Evaluation of future corrosion riskIn concrete the time dependent inward process of pH-reduction from carbonation can be predicted and the future risk of corrosion can be evaluated



The main damage which can be evaluated from this method is:- Cross-section reduction

from uniform corrosion

- Development of spalling



NDT-Course

India 2005/2006


Accuracy


A spraying indicator will show if the pH of the surface is over or under the shift pH value of the indicator

The usual indicator (phenopthalein) has a shift value of approx 9. Other indicators

Other indicators exist. Shift values of approx 2, 4, 9, 12 can be found. Some of the indicators can be combined and thereby have multiple color changes, like the rainbow-indicator

The measured value is only correct on very freshly exposed surfaces



The use of spraying indicators is influenced by:

- Exposure time of the surface - Pollution from dust and water- Pulverized cement grains- Aggregates and non-cement materials- Corrosion



NDT-Course

India 2005/2006


Spraying indicators pH



The results are used for evaluationof surfaced based corrosion from:

General carbonation

Carbonation along cracks and local defects (holes, pour)




NDT-Course

India 2005/2006



The method is usually used for evaluation of carbonation problems in:



Parking decks





Deciding if rust stains are caused by active corrosion or just light corrosion from casting

Estimation of chloride ingress

Improving accuracy of chloride profiles.

Estimation of corrosion risk in masonry structures.

Estimation of strength of lime based mortars

Corrosion conditions in fast corroding areas (very low pH)

Spraying indicators for chloride level exist



NDT-Course

India 2005/2006




- Testing immediately after exposing the surface to be tested

- Deeper carbonation at areas with very small concrete cover (areas with high risk of corrosion caused by carbonation).

- Dust from freshly broken or water running from un-carbonated concrete will have clear influence of the surface colour.


Carbonation front

Surface

Reinforce-ment








NDT-Course

India 2005/2006




Results from more than 100 brake ups:- Carbonation near the ground: 5 mm

- Carbonation 0.5 – 1,5 m above ground 5-10 mm

- Carbonation at more than 1,5 m above ground 10-15 mm

- Carbonation at areas with poor concrete quality: 25 mm

- Carbonation at 0,5-1,0 mm cracks: 45 mm



Combined with measurement of concrete coverspraying indicators are expected to give informationon corrosion caused by carbonation:• Size and number of corroding areas

• Future formation of new corroding areas

• Risk of spalling

• Risk of structural damage

• Can further damage be prevented







NDT-Course

India 2005/2006




Test Planning

1. Visual inspection

- General environmental impact- General concrete quality- Corrosion signs- Areas with deviant environmental

impact- Areas with deviant concrete quality




NDT-Course

India 2005/2006


Test Planning

3. Selection of Test Areas- How critical are the damage (safety,

economical)- Magnitude of variations- Will other measurements tell us

what wee needSpraying indicators are generally use onbrake-ups and on cores taken for otherinvestigations, and the test areas areusually governed by these

investigations


Uniform concrete quality - open spacingUniform environment - open spacing



Test Planning


- Immediately testing on brake-ups- Close wrapping of cores for later

testing



NDT-Course

India 2005/2006


Test Planning


To Bring (tools)

• Spraying indicator• Jack hammer• Air pump• Ruler for measuring of carbonation depth• Repair equipment and material• Generator• Cover meter (metal detector)




- Visual registration areas to be inspected- Make a fresh broken surface (do not use

the hammer direct on the surface of the reinforcement)

- Blow dust away from the surface- Make photos before the use of the

spraying indicator - Spray the indicator at the surface- Wait for 5 minutes- Measure the carbonation depth with care- Inspect the indicator at the reinforcement

surface



NDT-Course

India 2005/2006



Planning





Execution




Method


NDT-Course

India 2005/2006



Generally, spraying indicators workvery well if the tests are executed correctly, and no calibration is needed.

At concrete with fine cracks and at very old concrete (70-100 years) tests from micro analyses will locally show a deeper carbonation. However, usually the difference issmall.





Identification of damage risk due to carbonation

Identification of critical crack size

Identification of damage due to poor compacting


NDT-Course

India 2005/2006



Report:

General conclusions

Eventually calculated average and coefficient of variation of carbonation depth




Appendix:- Measured values of carbonation

depth- Statistics


Placing and result of carbonation measurement

Carb. depth >cover

Carb. depth = cover

Carb. depth > cover


NDT-Course

India 2005/2006


Spraying indicator (pH, concrete)



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



APPENDIX A12

Impact-Echo


NDT-Course

India 2005/2006

Impact Echo

NDT - Concrete

2SlideImpact Echo - 21 February, 2006

Impact Echo - Measuring Concept

The equipment consist of a Transducer (Receiver), a steel ball (Impactor) and a laptop

Principle: “hit and measure”

On-site measurements and analysis


Delamination and deterioration of concrete

Bridge decks, beams and piers

Introduction


NDT-Course

India 2005/2006


Introduction

Benefits:


Qualitative measurements


Estimation of what kind of repair is needed


Agenda






6. References


NDT-Course

India 2005/2006


Impact Echo



The Impact Echo equipment consist of:

Displacement transducers for measuring surface movements

Arrangement with different sizes of steel balls (“impactors”) for making the impact

A laptop with custom made signal amplifier and software

The transducer is linked to the laptop with Impact-Echo software for data acquisition, processing and storage.



NDT-Course

India 2005/2006



A short-duration stress pulse is introduced into the test element by mechanical impact

Three types of stress waves are generated: P-wave (spherical wavefront waves) S-wave (spherical wavefront waves) R-wave (surface wave)

The P-wave will, when reaching a material with another acoustic impedance be reflected and return to the surface.

A sensitive displacement transducer picks up the successive arrival of the P-wave to the surface.




A. Mechanical impulse on the surface

B. Measuring of the surface movement

C. Frequency analysis of surface movement

D. Evaluation



NDT-Course

India 2005/2006


Measurements

The aim of an Impact-Echo investigation is in general to make a fast screening of a large area and locate position and depth of flaws and damages

The measurements are performed within a predefined grid

Results are stored, analyzed and presented on the laptop.



Measurements

The red cursor/line indicate the peak frequency from which a depth is calculated

The blue cursor indicate a predefined depth (used for fast overview of location of the flaw / defect)



NDT-Course

India 2005/2006


Accuracy

Two reflections

One reflection

No reflection – the distance between surface and flaw isto small



Accuracy

The deeper a damage is located, the larger it must be to be detected

The surface of the tested media must be fairly smooth in order to avoid distortion of the waves

The instrument is very accurate and factors such as how the steel ball hits the surface or flaws in the surface makes the uncertainties of the instrument insignificant

0.5 m

Concrete Bridge Deck

Detectable flaws

Undetectable flaw



NDT-Course

India 2005/2006


Accuracy

The limits of depth and size of flaws which can be detected are given by the wavelength of the impact wave

The wavelength is primarily governed by the size of the steel ball used for the impact

A relation between flaw size/depth and size of the steel ball has been established:



Accuracy

It is always necessary to make an on-site calibrationof the measurements

The calibration should validate the used wave speed and interpretation of the signal

Hence the accuracy of an investigation is found and documented by the calibration

Test experience increases the accuracy



NDT-Course

India 2005/2006



Heterogeneous materials can cause strongly distorted signals

Humidity will often alter the wave speed in the material



Impact Echo


NDT-Course

India 2005/2006



Delaminations

Caused by ASR, Corrosion etc.

Voids

Often seen beneath reinforcement with too little spacing

Honeycombs and Poor Consolidation of concrete

E.g. near reinforcement

Geometry




Concrete Bridges

Deck, girder, pier etc.

Pavement on Bridges

Floor Slabs and Walls


Silos, Tanks, Chimney



NDT-Course

India 2005/2006



Any part of a structure which has a sizeable flaw of defect parallel to the surface

Very rough surfaces cannot be tested – the surface must be smooth enough for the steel ball to make reproducible impacts

Go / no-go factors

Surface roughness

Expected flaw size

Expected flaw depth



Case 1: Road surface/Water proofing

Concrete bridge: Special inspection of pavement and water proofing.

Large area - 18.000 m2

Much traffic - 50.000 cars pr.day.

Time restriction – Traffic restriction only allowed between hours 9 am and 3 pm



NDT-Course

India 2005/2006



Determinations of the velocity of propagation in the road surface.

Velocity found to be ~ 3400 m/s




Assessments of:- delaminations- internal voids- crushed layers- slip

from the Impact-Echo signal

Concrete cores to calibrate the measurements



NDT-Course

India 2005/2006


Case 2: Delaminations in concrete

Concrete bridge from 1917.

Railway bridge.

Any delaminations will cause problems with stability in the middle section of the arch




The top part of the intrados of the arc was selected as test area

0,5 m between points in the length

5 concrete cores to calibrate the measurements in good and bad areas



NDT-Course

India 2005/2006



A 9 mm steel ball was used – this enabled detection of flaws in depths between 70 and 600 mm

The arc thickness in the top is 400 mm and in the bottom 1000 mm

In the top the measurements showed a thickness of 400 mm

The wave speed is set to 3500 m/s




In the lower sections measurements showed a delamination in a depth of 150 mm

This was validated by a concrete core

Note that smaller cracks and delamination create “noise”



NDT-Course

India 2005/2006



The measurement shows delaminating in 1 - 1,5 m from north and south arch face.

In the middle 8 m the measurements showed no delaminations





Qualitative investigation of flaws / geometry





NDT-Course

India 2005/2006


Impact Echo


Test Planning

1. Initial Visual SurveyFocus on visible damages and

Practical hindrances

Accessibility


Thorough investigation of background material



Expected damagestype

size and depth



NDT-Course

India 2005/2006


Test Planning

3. Selection of Test AreasInclude non-damaged and damaged areas in each test-grid

Choose representative grids

Consider possible uncertainties/errors from edges or surface conditions


Expected extend of flaws

Expected variationDynamic test planning



Test Planning

5. Practical PreparationsCreate sketch sheets for visual registrations

Make a time plan


6. “To Bring” (tools)A normal hammer

Camera

Chalk for marking the grids

Measuring tape and folding rule

Equipment for core drilling



NDT-Course

India 2005/2006




Mark up the test grid


8. Calibrate MeasurementsView all the results and find signals indicating damage


Mark up where cores should be drilled for on-site calibration





Examine cores and core holes

Evaluate actual vs. expected condition of the coresDo the results match with the hypothesis?!

Decide whether additional steps must be taken (e.g. extra cores)

10.RegistrationMake a thorough visual registration, geometry, cores etc.



NDT-Course

India 2005/2006



Planning






6. “To Bring” (tools)

Execution




10.Registration



Impact Echo


NDT-Course

India 2005/2006




Establish correlation between measurements and calibration

Wave speed, depths etc.

Identify signals from geometry and flaws

Estimate how representative the measurements are for the whole element / structure. This is essential as we are dealing with a “point test”





If possible the damaged areas are subdivided by the type of repair which is found to be necessary, e.g.:

1. Shallow removal and repair of concrete cover (cheap)

2. Removal and repair of concrete to a depth behind the reinforcement (expensive)



NDT-Course

India 2005/2006



Report:




Result evaluation/evaluation of hypothesis

Estimate of the reliability of the investigation





Appendix:

We are dealing with a complex method – an introduction to the method should therefore always be made



Gives the reader an overview of exactly where the measurements has been made




NDT-Course

India 2005/2006



Appendix:



Often it is a good idea to make a separate appendix with registration of concrete cores


Good and bad areas




Application Summary

Impact Echo


NDT-Course

India 2005/2006


Application Summary – Concrete bridges

Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Imp

act E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Imp

act E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method


NDT-Course

India 2005/2006


References



APPENDIX A13

Impulse Response (s’MASH)


NDT-Course

India 2005/2006

Impulse Response

NDT - Concrete

2SlideImpulse Response - 21 February, 2006

Impulse Response - Measuring Concept

The equipment consist of an instrumented hammer, a geophone and a laptop

Principle: “Hit and measure”

On-site measurements and analysis


Delamination and deterioration of concrete

Bridge decks, beams and piers

Introduction


NDT-Course

India 2005/2006


Introduction

Benefits:


Identification of good and bad areas


Estimation of what kind of repair is needed


Agenda






6. References


NDT-Course

India 2005/2006


NDT - Concrete


A low-strain impact with an instrumented rubber tipped hammer sends stress waves through the tested element.

The element acts in bending mode and a velocity transducer (geophone), placed adjacent to the impact point, receives this response. Response to the impact is logged in the time domain

Both the hammer and the velocity transducer are linked to a portable field computer with s´MASH software for data acquisition, processing and storage.




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India 2005/2006



The time trace of the hammer force and the velocity transducer are processed into frequencies using the Fast Fourier Transform (FFT) algorithm.

Dividing the resultant velocity spectrum by the force spectrum then derives the “mobility” and ”stiffness”of the structural element tested.

Areas which react differently in bending mode can be identified. The difference may be due to voids, honeycombing, deteriorated concrete, etc. Q N P

Frequency

VF

o

o

Mobility

o

oVF m

fm

Rigid Base

SoftBase

f f



Measurements

The aim of an Impulse Response investigation is in general to make a fast screening of a large area and locate damages

The measurements are performed within a predefined grid

Results are exported to an Excel-file where the graphs of mobility vs. frequency are subjected to a standard analysis which is presented in five surface plots

1234567891011121314S1

S2

S3

S4

S5

Column

Row

Average Mobility 0-10 10-20 20-30



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Measurements

Average Mobility

A parameter found by calculating the average value of the mobility for frequencies between 100 and 800 Hz (red line)

Stiffness

Dynamic stiffness in MN/mm derived from the mobility slope between 0-50 Hz

Is in fact the inverse slope of the initial part of the mobility curve

Average Mobility

Stiffness



Measurements

Mobility SlopeParameter defined as the slope of the mobility curve within the range of 100 to 800 Hz

Voids IndexParameter found by dividing the peak mobility with the average mobility

Mobility x SlopeAverage Mobility multiplied with the Mobility Slope



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Correlation between calculated parameters and actual damages

Measurements

Honeycomb in concrete

Void in concrete



Measurements

Average MobilityAverage mobility from 100 to 800 Hz

StiffnessInverse slope from 0 to 50 Hz

Mobility SlopeSlope from 100 to 800 Hz

Voids IndexPeak divided by average mobility

Mobility x Slope



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India 2005/2006


Accuracy

Impulse Response investigations are reliable and easy to reproduce

The deeper a damage is located, the harder it is to detect

Flaws smaller than 0.25 m times 0.25 m cannot be found

The instrument is very accurate and factors such as how the hammer hits the surface or flaws in the surface makes the uncertainties of the instrument insignificant

The greatest source of uncertainty is the blow with the hammer

0.5 m

Concrete Bridge Deck

Detectable flaws

Undetectable flaw



Accuracy

It is always necessary to make an on-site calibration of the measurements

The calibration establishes the connection between the measured relative differences and the actual variation of the condition

Hence the precision of an investigation is found and documented by the calibration

Test experience increases the precision considerably

1234567891011121314S1

S2

S3

S4

S5

Column

Row


Core 1 Core 2Core 3



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India 2005/2006



Stiffness, geometry and support conditions

Example:

Systematic variation of stiffness

Caused by support conditions and geometry

s1 s2 s3 s4 s5

Wing Part of box

girder



NDT - Concrete


NDT-Course

India 2005/2006



Delaminations Caused by ASR, Corrosion etc.

VoidsOften seen beneath reinforcement with too little spacing

Honeycombs and Poor Consolidation of concrete

E.g. near reinforcement

Damages due to OverloadingCould be from accidents such as a derailed train or a ship collision




Concrete Bridges

Deck, girder , etc.

Pavement on Bridges


Pile Integrity



Cladding on Buildings



NDT-Course

India 2005/2006



Concrete Bridges

Deck, girder , etc.

Pavement on Bridges


Pile Integrity







Concrete Bridges

Deck, girder , etc.

Pavement on Bridges


Pile Integrity






NDT-Course

India 2005/2006



Concrete Bridges

Deck, girder , etc.

Pavement on Bridges


Pile Integrity







Any part of a structure where a correlation between damage and response to an impact is thinkable can be tested

A sound way of estimating such a correlation is by looking at changes in stiffness caused by the damages

Very rough surfaces cannot be tested – the surface must be smooth enough for the hammer to make reproducible impacts

Go / no-go factorsExpected flaw sizeExpected flaw depth

Expected dynamical performance



NDT-Course

India 2005/2006


Case 1: Aalborg

5 Span Prestressed Concrete Bridge

“Butterfly” Cross Section

Prestressed in the Longitudinal Direction

Severe Deterioration due to ASR

Set to Demolition in September 2005 because the Underpass is to be widened



Case 1: Aalborg





Set to Demolition in September 2005 because the Underpass is to be widened



NDT-Course

India 2005/2006


Case 1: Aalborg

Photo taken on a rainy day

Water is coming through the bridge deck

Coarse cracks are also present where the concrete is wet on the under side



Case 1: Aalborg



NDT-Course

India 2005/2006


Case 1: Aalborg

Visible damages due to ASR:

Extensive cracking

White precipitation and stalactites

Water is coming through the deck

Damages are limited to the wing only



Case 1: Aalborg



NDT-Course

India 2005/2006


Case 1: Aalborg



Case 1: Aalborg

ResultsMeasurements conducted every 4 m along the entire bridge

8 measurements across

Average Mobility inserted on sketch



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India 2005/2006


Case 1: Aalborg

Delaminated Concrete Intact Concrete Local Damage



Case 2: The Great Belt Link

East Bridge, a 6,790 m long suspension bridge

West bridge, a 6,611 m long combined rail and road bridge

An 8,000 m long immersed rail tunnel



NDT-Course

India 2005/2006



Ship collision on the West Bridge

Upper picture: Ship prior to collision

Lower picture: Ship after the collision

The front crane is broken




The front crane of the ship collided with the southern cantilever wing

The impact was in an upward direction as indicated

The damage was visible both on soffit and top side



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NDT-Course

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Investigations were conducted both on the under and the top side of the cantilever wing

Impulse Response was used to identify the extend of the damage

Cores were drilled for calibration

1234567891011121314S1

S2

S3

S4

S5

Column

Row


Core 1 Core 2Core 3




1234567891011121314S1

S2

S3

S4

S5

Column

RowCore 1 Core 2Core 3



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Impulse Response and visual inspections was used to create a damage assessment

The assessment was used to make a calculation of the capacity

A repair project was made and has been executed









NDT-Course

India 2005/2006


NDT - Concrete


Test Planning


Focus on visible damages and


Accessibility





Expected damages



NDT-Course

India 2005/2006


Test Planning


Include intact and damaged areas in each test-grid

Choose representative grids

Consider possible uncertainties/errors from geometry etc.


Flaw size

Dynamic test planning



Test Planning


Make a time plan



Camera






NDT-Course

India 2005/2006


Test Planning


Make a time plan



Camera








Mark up test grid

Measure

8. Calibrate MeasurementsCreate Excel-plots and view the results


Mark up where cores should be drilled for on-site calibration



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India 2005/2006




Examine cores and core holes

Evaluate actual vs. expected condition of the coresDoes the results match with the hypothesis?!






Planning







Execution




10.Registration



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India 2005/2006


NDT - Concrete





Selection of parameters to be used for interpretation: Average Mobility, Voids Index etc.

Find out whether the degree of damage is detectable

Estimate how representative the measurements are for the whole element / structure


“Error”: Not deteriorated concrete but two construction joints


NDT-Course

India 2005/2006




If possible the damaged areas are subdivided by the type of repair which is found to be necessary, e.g.:

1. Shallow removal and repair of concrete cover (cheap)

2. Removal and repair of concrete to a depth behind the reinforcement (expensive)


Damaged Areas



Report:









NDT-Course

India 2005/2006



Appendix:









Appendix:








NDT-Course

India 2005/2006



Appendix:



Often it is a good idea to make a separate appendix with registration of concrete cores


Good and bad areas




Application Summary

NDT - Concrete


NDT-Course

India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Imp

ulse

resp

on

se

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Imp

ulse

resp

on

se

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method


NDT-Course

India 2005/2006


References

Davis, A.G. : ”The non-destructive impulse response test in North America: 1985-2001”, NDT & E International 36 (2003), 185-193, Elsevier Science Ltd.

Ottosen, N.S, Ristinmaa, M & Davis, A.G, : ”Theoretical interpretation of impulse response test of embedded concrete structures”, Div. of Solid Mechanics, Lund University, Lund, Sweden (to be published in ASCE).



APPENDIX A14

CAPO-test


NDT-Course

India 2005/2006

CAPO-test

NDT - Concrete

2SlideCAPO-test - 8 February, 2006

Cut And Pull-Out-test -Measuring Concept

The equipment consists of:

- a diamond drill unit

- a diamond recess router

- an expansion unit

- a hydraulic pull machine

- CAPO-inserts.

Introduction


NDT-Course

India 2005/2006


Cut And Pull-Out-test - Measuring ConceptPrinciple:

A special designed expansion bolt is placed in the structure and pulled out.

A certain fracture geometry (cone) is achieved.

The pull out force is correlated to the compression strength of the concrete.

On-site measurements / results.

Typical ApplicationsCompression strength of fresh concrete.Compression strength of “old” concrete.Bridge decks, beams, piers, tunnel walls etc.

Introduction


Introduction

Benefits:

Fast and relative precise estimation of the compression strength (on site results).

Early development of concrete strength can be followed.

Less repair work compared to drilling out concrete cores for laboratory testing.

Compression strength (condition / durability) in the surface layer.

Possible method for heavy reinforced structures.


NDT-Course

India 2005/2006


Agenda






6. References


CAPO-test


NDT-Course

India 2005/2006


Measuring Principle

The compression strength is correlated to the force necessary to pull out an expansion bolt from the concrete, if the fracture shape is a cone with a specific angle.

During the pull-out process the following fracture process is developed:

1. Tensile cracking occurs starting from the pull-out insert head.

2. A band of micro cracks develops between the pull-out insert head and the counter pressure forming a cone.

3. Internal rupture occurs forming a tensile/shear crack from the edge of the counter pressure to the edge of the pull-out insert head.


Pull-outinsert head



The pull-out force needed to pull out the expansion bolt is measured.

At the laboratory calibration of the equipment a table is generated to transform the readings of the pull-out force to 150 mm x 300 mm standard cylinder compressive strength.

Based on the pull-out force and the laboratory calibration a compression strength is calculated.



NDT-Course

India 2005/2006


Measurements

The aim of the CAPO-test is to estimate the concrete compression strength.

The test is performed in intact areas of the structure in between the reinforcement.

The test is performed in the concrete surface – the depth of the hole drilled for the expansion bolt is approximately 45 mm.

Final results are given as a compressive strength in MPa.



Precision

The variation of the test results are typically within 10-20%.

The variation of the test results are influenced by the variations in the concrete homogeneity and condition.

As a rule of thumb it is recommended that 3 CAPO test determinations are carried out per segment (homogeneous areas). If the purpose is to determine the characteristic compression strength of the concrete at least 5 test must be performed in each area.



NDT-Course

India 2005/2006


Precision

The test method is very sensitive to a careful performance.

Calibration of the test results can be made by drilling out few cores for laboratory testing.

Determination of characteristic strength from CAPO-test can be expected to be within app. 20% of the actual characteristic strength (found from strength testing on an "infinite" number of cores)“.




The geometry of the pull-out proportions has to be correct. Only failure type ”x” is acceptable.

Incorrect testing causes the failure types “Y” and “Z”, and the test result should be rejected.



NDT-Course

India 2005/2006



The test surface has to be plane to secure the right counter pressure and perpendicular to the centreline of the expansion unit.

The inserts shall be placed so that all reinforcement is out side the expected conic failure surface by at least one bar diameter or the maximum aggregate size, whichever is the greatest.




The minimum thickness of the concrete tested should be at least 100 mm.

The centres of test positions should be at least 200 mm.

The pull-out force has to be supplied at a constant rate –following the test instructions.



NDT-Course

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The instrument has to be calibrated (in an authorised laboratory).

To reach the peak load no oil leaks must occur from the pull machine.

The expansion unit has to be fully expanded to make sure toobtain a plane pressure surface.

The test measures the strength in a very small area. The presence of coarse aggregates or minor deficiencies in the concrete at the test location may affect the measured strength.




Milling of the recess with sharp edges (by keeping the miller at a right angle to the surface all the time).

Assembling and tightening the various parts of the expansion bolt and jack in the right sequence.

Fastening the insert without rotating it.

Honeycombing, cracks or other damages in the concrete will influence the result.

Content of moisture in concrete.



NDT-Course

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CAPO-test



Initial defects.

Structural problems.



NDT-Course

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Girder

Pier

Bridge deck

Tunnel walls and ceilings

Beams




All concrete structures.

Masonry structures.



NDT-Course

India 2005/2006


Case – Concrete Tunnel

Purpose:

Estimation of the concrete compression strength for structural calculations.

Structure:

App. 290 m long tunnel divided into 19 segments of app. 15 m.

Test plan:5 segments were chosen for CAPO tests. In each of the 5 segments 3 tests were performed in the Eastern respectively in the Western tube.

A total of 30 CAPO-tests were carried out.

5 concrete cores were tested in the laboratory (compression strength and density).




Test plan:



NDT-Course

India 2005/2006



Test results:




Calibration results:

Transformation of measured strength to cylinder strength (Danish Code of calculation):

Cylinder strength: 49.1 MPa x 1.25 = 61.3 MPa



NDT-Course

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Comparing CAPO-test result with laboratory test:

CAPO-test: 59 MPa

Laboratory test: 61 MPa

Result from tests: 60 MPa

Characteristic strength based on the Danish code of calculation:

39 MPa




Fast estimation of the level of compression strength.

Does the structure need strengthening?

Fast estimation of the homogeneity of the material.



NDT-Course

India 2005/2006


CAPO-test


Test Planning


Focus on visible damages


Accessibility


Investigation of background material: expected level of strength - what is the strength assumed to be?

What level of strength is needed in the future?



NDT-Course

India 2005/2006


Test Planning

3. Selection of Test AreasInclude tests in all homogeneous areas (eg. if different concretes has been used). The tests must be carried out in intact areas.No cracks.No honeycombing.No signs of leaking.Plane surface.Tests are carried out in the middle of the reinforcement mesh – distance from reinforcement to failure surface > 15 mm.



Test Planning


Level of compression strength:

At least 3 tests in each area.

Characteristic strength:

At least 5 tests in each area.



NDT-Course

India 2005/2006


Test Planning



Make a time schedule


Prepare registration sheets for the results



Test Planning

To Bring (tools)

CAPO-equipment

Cover meter

Hammer (laminations)

Camera

Chalk for marking the reinforcement

Folding rule

Water and water pump

Equipment for repairing the holes



NDT-Course

India 2005/2006



5. Conduct Measurements:

A. Locate the reinforcement (cover meter).

B. Select test areas as described in the test planning.

C. Mark and name the test locations.

D. Coring the center hole.

E. Make sure that the hole is deep enough (app. 65 mm) and that allparts of the core is removed.

F. Rinse the hole with water.

G. If the surface is not plan – plane the surface using a diamond surface planning unit.





H. Routing the recess carefully – eg. start routing for each 45o of the hole and then do the routing for the rest of the hole.

I. Rinse the hole with water.

J. Fell the recess with a finger to verify that there are no brokenedges.

K. Add a CAPO-insert to the expansion unit.

L. Greasing of the expansion unit.



NDT-Course

India 2005/2006




M.Mounting of the expansion bolt in the hole.

N. Tighten the expansion bolt.

O. Attach the counter pressure and the coupling.





P. Mounting of the hydraulic pull machine (the handle needs to be at a fully extended position).

Q. Turn the handle of the pull machine to make sure there is no gap between the counter pressure and the pull machine.

R. Turn on the display at the pull machine and start pulling at a steady velocity (app. one rotation every 2 sec.).



NDT-Course

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S. Keep on turning till fracture.

T. Note the peak load.

U. Make registrations of the hole, cone and CAPO-insert. The CAPO-insert has to be fully expanded (plane).

Even if you are a an experienced inspector – USE THE MANUAL.




6. Calibrate MeasurementsThe tests may be calibrated with laboratory compression tests of few concrete cores.

7. Evaluate MeasurementsExamine cones and holes.

Examine CAPO-insert.

Does the results match with the hypothesis?

8. RegistrationMake a thorough visual registration.

Take photos of the hole, the cone and the CAPO-insert.



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Planning





Execution



May be performed.

7. Evaluate Measurements Registration



CAPO-test


NDT-Course

India 2005/2006



It must be evaluated how reliable / accurate the tests are:

Establish correlation between test results and calibration (if carried out)

If there is a significant variation of the test results, which cannot be explained by “failure” of the test -3 new tests have to be made.




Report:


Extend and positions of the investigation

Summary of the results – mean value


Is the level of strength as demanded?

If the characteristic compression strength is needed use the national standards to transform the measured strength to a characteristicstrength.


Does the test areas represent the structure?

Are there many visual damages, laminations etc.?



NDT-Course

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Appendix:

Sketch of registration of position and geometry



Table showing the results from the test

Table showing the results from the calibration (if any) – laboratory test.




Appendix:



NDT-Course

India 2005/2006


CAPO-test



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CA

PO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method


NDT-Course

India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CA

PO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method


6. References

Germann Petersen, C.: “LOK-test and CAPO-test pullout testing, twenty years experience”, Presented at the Non-Destructive Testing in Civil Engineering conference in Liverpool, 1997.

Germann Petersen, C. & Poulsen, E.: “Pull-out testing by LOK-test and CAPO-test with particular reference to the in-place concrete of the Great Belt Link”, DanskBetoninstitut A/S, 1993.

Germann Instruments A/S, “CAPO-test manual for CAPO-test equipment with Electronic Microprocessor Gauge Hydraulic Pullmachine”, 1996.

Construction Materials Managenemt: “In-situ compressive strength testing of precast concrete tunnel lining segments using CAPO test”, 1990.

Germann Petersen, C.: “CAPO-test”, Nordisk Betong, 1980.

CEN: “Testing Concrete – Determination of pull-out force”, European standard, pr EN-ISO 8046, 1994.



APPENDIX A15

Pull-Off / Bond Test


NDT-Course

India 2005/2006

Pull-off / Bond-test

NDT - Concrete

2SlidePull-off / BOND-test - 14 February, 2006

Bond test - Measuring Concept

Equipment for planning the

surface.

Equipment for cleaning the

surface and gluing on the disk.

Equipment for producing a

partial core.

Hydraulic pull machine.

Introduction


NDT-Course

India 2005/2006


Bond test - Measuring Concept

Principle:

A disk is glued on a prepared testing surface.

A partial core is cut around the disk.

The disk is pulled off.

The pull off force is correlated to the adhesion strength or thetensile strength of the concrete or the overlay.

On-site measurements / results.

Introduction


Typical Applications:

Adhesion strength between two layers.

Tensile strength of concrete.

Bridge decks, tunnel walls etc.

Benefits:

Fast evaluation of the adhesion strength (on site results).

Fast evaluation of the concrete tensile strength.

Is typically used in rehabilitation projects.

Introduction


NDT-Course

India 2005/2006


Agenda







BOND-test


NDT-Course

India 2005/2006


Measuring Principle

The adhesion strength or the tensile strength is correlated to the force necessary to pull off a disk glued to the surface.


Pull-outinsert head

2

4dFf

π=


Measuring Principle

The force needed to pull off a disk glued to the overlay surface is measured.

Depending on the type of fracture one of the following parameters is measured:

The concrete tensile strength.

The adhesion strength.

The overlay tensile strength.



NDT-Course

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Measurements

Typically the aim of the Bond test is to evaluate the adhesion between two layers (eg. the adhesion between an repair of concrete and the “old” concrete).

If performing a sufficient number of tests the bond test can also be used to estimate the characteristic value of the concrete tensile strength.

The test is performed in the concrete surface – the depth of the core drilled has the approximate depth of 25 mm into the substrate layer (concrete).

Final results are given as a strength in MPa.



Precision

The variation of the test results are typically within 10-20%.

Usually the exact value of the strength is not needed from the test. Only information of the adhesion between the two layers are typically needed as “acceptable” or “non-acceptable”.

The acceptance criteria could be a demand of a certain level of the mean value and “no values below a certain value”.



NDT-Course

India 2005/2006



The test results are influenced be:

The strength of the glue. One must wait for the glue to harden before carrying out the pull off.

Variations in the overlay and concrete homogeneity and condition.

The cleaning of the casting joint before casting of the overlay.

The size of the disk to be glued on the surface of the overlay.

Cleaning of the testing surface before gluing the disk to the surface.



BOND-test


NDT-Course

India 2005/2006


Common Applications

Damage:

Initial defects – bad adhesion between two layers of material.

Structural elements:

Girder

Pier

Bridge deck

Tunnel walls and ceilings

Beams





All structures with an overlay of a material on top of a concrete layer:

Adhesion between waterproofing and concrete deck.

Adhesion between tiles (eg. granite) and concrete floor.

Etc.



NDT-Course

India 2005/2006


Input to Rehabilitation

The results from the bond test can be used in two phases of the rehabilitation:

Before repair:

Is the strength of the concrete surface good enough for carrying the repair?

After repair (or during the repair as a quality assurance):

Is the adhesion between the two layers acceptable?



BOND-test


NDT-Course

India 2005/2006


Test Planning


Focus on visible damages of the overlay


Accessibility


What is the acceptable level of strength?

Identify critical areas as for instance areas where the cleaning before casting the overlay might have been poor.



Test Planning


Include tests in both expected “bad” and “good” areas when testing the concrete tensile strength.

No signs of visual damage must be observed in the overlay - this might lead to fracture in the overlay instead of testing the adhesion.



NDT-Course

India 2005/2006


Test Planning


The quantity depends on the size of the test area.

The absolute minimum of tests should be 3 within every homogeneous area.



Test Planning



Prepare registration sheets for the results

To Bring (tools)

Bond test equipment

Camera

Chalk for marking the test areas

Folding rule

Water and water pump

Equipment for repairing the holes.



NDT-Course

India 2005/2006




A.Surface planning: The surface is planned with a diamond planning wheel.

B.Cleaning the testing surface. The dry surface is steel brushed and any dust/powder is blown away.

C.A clean disk is glued to the clean surface (using a strong glue eg. 10 MPa).

D.The glue is hardening (2-5 min. in normal temperatures).





E. Partial coring: A partial core is cut perpendicular to the surface with a coring bit that has the disk diameter as the inner diameter.

F. Pull-off: The disk is pulled off using a hydraulic pull machine with a counter pressure placed centrally on the planned surface.

G. Note the peak load.



NDT-Course

India 2005/2006



6. Evaluate Measurements

Examine the core to determine the type of fracture and homogeneity.

7. Registration

Make a visual registration.

Take photos of the hole and the core.




Planning





Execution



7. Registration



NDT-Course

India 2005/2006


BOND-test


Interpretation

a) Failure in substrate: The adhesion strength is higher than the strength of the substrate. The tensile strength of the concrete is measured.

b) Failure in adhesion layer: The adhesion strength is lower than the strength of the substrate. The adhesion strength is measured.

c) Failure in overlay: The adhesion strength is higher than the strength of the overlay. The tensile strength of the overlay is measured.



NDT-Course

India 2005/2006



Typically the result needed is an acceptance or non-acceptance of the adhesion of the two layers.

If performed prior to a repair – an evaluation of the tensile strength of the concrete is needed to evaluate if the concrete is able to carry to repair.




Appendix:

Sketch of registration of position

Gives the reader an overview of exactly where there measurements has been made.

Photos of the holes and the cores.

Measurements

Table showing the results from the test including the pull force, the strength, the type of fracture and comments to the fracture.



NDT-Course

India 2005/2006


BOND-test



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bo

nd

-test/

Pu

ll-off

CA

PO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method


NDT-Course

India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/M

icro a

naly

ses

Co

res

Bre

ak u

p

Gro

un

d p

en

etra

tion

rad

ar

Imp

ulse

resp

on

se

Imp

act E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bo

nd

-test/

Pu

ll-off

CA

PO

-test

Sch

mid

t ham

mer

Bo

rosco

pe

Cra

ck d

ete

ction

NDT-Method



APPENDIX A16

Schmidt Hammer


NDT-Course

India 2005/2006

Surface Hardness Schmidt hammer

NDT - Concrete

2SlideSchmidt hammer - 24 February, 2006

Schmidt hammer - Measuring Concept

One instrument the Schmidt Hammer

Principle: “Push and measurer”

On-site measurements


Concrete hardness

Concrete strength

Inhomogeneities

Introduction


NDT-Course

India 2005/2006


Introduction

Benefits:

Fast screening of a large area

Easy to use

On-site estimate of concrete strength


Agenda






6. References


NDT-Course

India 2005/2006


Schmidt hammer



The components:

Outer body

Plunger

Hammer mass

The main spring

Indicator



NDT-Course

India 2005/2006



The hammer mass hits the plunder and the rebound is measured.



Measurements

The Schmidt hammer measure the stiffness of the concrete.

There is no theoretical relationship between the stiffness and the concrete strength

Empirical correlations between the concrete strength and the rebound number



NDT-Course

India 2005/2006


Accuracy

In laboratory the accuracy is ± 15 %

In the field the accuracy is ± 25%

The accuracy can deviate even more if great care when selecting test points is not taken




Smoothness of test surface

Size, shape, and rigidity of the specimens

Surface and internal moisture conditions of the concrete

Type of coarse aggregate

Type of cement

Type of mould

Carbonation of the concrete surface



NDT-Course

India 2005/2006


Schmidt hammer



Carbonation

Frost

ASR

Casting defects

Location of use of different concrete types (strength)

Deteriorated mortar



NDT-Course

India 2005/2006



Piers

Bridge decks

Wing walls

Beams

Girders

Mortar between masonry bricks


Thane Creek Bridge



Any structural element where the surface stiffness can be used for evaluating the condition

If the Schmidt hammer is used for detecting differences only - its usefulness increases significantly

One measurement is never accurate! - But:

Even few measurements may indicate the order of magnitude of the strength ( 5 vs. 50 MPa)



NDT-Course

India 2005/2006


Case: Englandsvej

Bridge under Airport runway at Copenhagen Airport

Larger loading due to larger plains

The carrying capacity could be fulfilled if a certain concrete strength could be documented

Before making Capo test and drilling of cores the Schmidt hammer was used to evaluate the order of magnitude of the concrete strength




Estimating the concrete strength


Locating heterogeneous areas



NDT-Course

India 2005/2006


Schmidt hammer


Test Planning

1. Initial Visual SurveyIdentify the general condition

Locate potential critical areas

Find surfaces suitable for inspection


Estimation of concrete strength and the correlated value expected for the Schmidt hammer

Evaluate if measuring it is sufficient to find the order of magnitude of the strength


Diva-Panvel Bridge


NDT-Course

India 2005/2006


Test Planning


Select test areas from the hypothesis

The exact location of each measuring point must be found on site


If more accurate measurements are wanted the quantity must be decided from the dispersion of the result on site

If less accuracy is sufficient 3-7 measurements within each assumed homogeneous areas will do in most cases



Test Planning



Make a time plan


6. To Bring (tools)

A normal hammer

Chalk for marking


3. 3. Test Planning and Execution of Field Tests

Diva-Panvel Bridge


NDT-Course

India 2005/2006



7. Conduct MeasurementsMake a sketch (table) with indication of each or groups test points


Evaluate dispersion of results

Make additional measurements

8. Calibrate MeasurementsCalibration can be made by drilling cores or making Capo-tests

It is always a good idea to use a normal hammer as “a second opinion”


Nira Bridge



9. Evaluate Measurements and CalibrationFor more accurate measurements it may be necessary to make a statistical analysis on site to confirm that the wanted reliability of the measurements has been achievedCalibration can be made by Capo test and concrete cores

10.RegistrationMake a thorough visual registrationA photo of the surface of all measuring points to use for successive evaluation



NDT-Course

India 2005/2006



Planning






6. To Bring (Tools)

Execution




10.Registration



Schmidt hammer


NDT-Course

India 2005/2006



It is essential to decide the level of reliability before conducting the measurements

Statistics should always be used to evaluate the results

For less accurate measurements calculating the mean value and the standard deviation is mostly sufficient

For more accurate measurements to use for e.g. carrying capacity calculations the used design code will decide which parameters should be calculated

Calibration with a capotest or concrete core and successive analysis of the core




Make an overview of all registrations – this will often give a good idea of the deterioration pattern

A direct damage identification is not possible – but in combination with:

Analysis of a core

Spraying indicators

Chloride measurements

etc.

Schmidt hammer measurements is able of locating damaged areas



NDT-Course

India 2005/2006



Report:

Background for making the investigation – including the wanted reliability / accuracy



Result evaluation/evaluation of hypothesis – note if it some areas was inaccessible




Appendix:

Sketch of all investigations and a result of each measurement orgroup of measurements


Photo documentation



NDT-Course

India 2005/2006


Schmidt hammer



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method


NDT-Course

India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



APPENDIX A17

Ground Penetration Radar


NDT-Course

India 2005/2006

Ground Penetrating Radar (GPR)

NDT - Concrete

2SlideGPR- 24 February, 2006

Ground Penetrating Radar - Measuring ConceptAn antenna is dragged over the surface of the measuring area

The antenna emits electromagnetic waves and receives the reflectionsThe reflections from various depths are visible real-time during the investigation

Typical ApplicationsGeotechnical surveysInspection of bridgesInspection of roadsInspection of concrete structures in general

Introduction


NDT-Course

India 2005/2006


Introduction

Benefits:

Fast, Inexpensive surveys

Real-time data processing

Large comprehensive data sets

Flexible setup options to suit any type of investigation


Introduction


NDT-Course

India 2005/2006


Agenda






6. References


Method


NDT-Course

India 2005/2006



A GPR equipment consist of

An antenna for emitting and receiving radar waves

A computer device which processes and stores reflected waves

Complex equipment - available in many configurations

The frequency of the antenna decides the penetration depth and resolution




An electromagnetic pulse is emitted into an object

The waves will be reflected if they encounter:

A new material (e.g. steel, air)

Changes in the moisture content

The amount of reflected energy is dependent on the material parameter called the

Dielectric Permittivityoften just Permittivity



NDT-Course

India 2005/2006


Measurements

Measurements are most often made in parallel lines

Measurements are not continuous but a number of pulses send out in even intervals along the line

Real-time plot show length and “time” which is proportional to the depth



Accuracy

The accuracy or resolution of a GPR survey is mainly dependent on which antenna is used

High frequency antennas at 1-2 GHz makes it possible to detect e.g. reinforcement – but only at shallow depth (less than 1 m)

Lower frequency antennas 0.1 to 1 GHz can penetrate several meters into the structure

Reinforcement will distort “deep”measurements



NDT-Course

India 2005/2006


Accuracy

The quality of the radar image, in terms of detail in depth, is dependent upon:

The length of the pulse

The characteristics of the ground or material under investigation

An indication of the appropriate equipment characteristics is shown below (Source: The European GPR Association)



Method


NDT-Course

India 2005/2006



Increased moisture

Chloride

Air in concrete

Geometrical deviations




Bridge decks

Bridge piers

Wing wall

Pavement

Geophysical surveys

Reinforced structures – localize rebar and prestressed cables



NDT-Course

India 2005/2006



Any flaw/damage or change in material or geometry which represent a sufficiently large change in electromagnetic properties

E.g. delamination are very hard to locate (Unless they are very large)

But the conditions which cause delamination can be detected



Case 1: Aalborg





Concrete very wet

Set to Demolition in September 2005 because the Underpass was widened



NDT-Course

India 2005/2006


Case 1: Aalborg

Investigation with GPR from the top side

Longitudinal and transverse measurements

Focus on locating areas where the concrete has a high moisture content

Locating post tensioned cable ducts



Case 1: Aalborg



NDT-Course

India 2005/2006


Case 1: Aalborg



Case 1: Aalborg


Deteriorated Area

Post tensioned cables

PVC-ducts

Empty space in box girder


NDT-Course

India 2005/2006




Verification of “As Build” drawings (geometry)

Identification of areas in need of repair

Development of damages

Achieved by successive measurement at intervals of e.g. two or three years



Method


NDT-Course

India 2005/2006


Test Planning

1. Initial Visual SurveyVisible damages


Accessibility



Identification of critical areas and elements

Expected damagestype

size

depth



Test Planning


Consider making a 100 % survey

Make multi directional surveys in “interesting” areas


Use two different setups when possible

Include “extra” surveys in the initial time plan



NDT-Course

India 2005/2006


Test Planning



Make a time plan


Choose which antennas to bring

6. To Bring (tools)

Camera


Marking pins





Mark up test grid or setup marking pins


8. Calibrate MeasurementsIf possible conduct visual calibration (e.g. measure known geometry)

Mark up where cores or breaks pus should be made for on site calibration



NDT-Course

India 2005/2006




Examine results and calibration

Evaluate actual vs. expected condition of the coresDoes the results match with the hypothesis?!






Planning





Execution




8. Registration



NDT-Course

India 2005/2006


Method





Find out whether the degree of damage is detectable

Estimate how representative the measurements are for the whole element / structure



NDT-Course

India 2005/2006



A lot of software for processing the raw data has been developed in recent years

Specialized software for finding different kin of damage / build in items are available

Analysis of data is one of the key issues for damage identification




Report:









NDT-Course

India 2005/2006



Appendix:

An introduction to the method should therefore always be made







Method


NDT-Course

India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

un

d p

en

etra

tion

rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

un

d p

en

etra

tion

rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method


NDT-Course

India 2005/2006


6. References

Geophysical Survey Systems Inc.



APPENDIX A18

Chloride Content


NDT-Course

India 2005/2006

Chloride content

NDT - Concrete

2SlideChloride content- 24 February, 2006

Chloride content - Measuring Concept (the RCT principle)

Holes are drilled in a concrete structure

The drilling dust from different depths is collected

The dust and thereby the chlorides are dissolved in a solution

The Chloride content can be found by measuring the potential of the solution


Concrete piers in salt water

Submerged concrete structures

Bridges subjected to de-icing salts

Introduction


NDT-Course

India 2005/2006


Introduction

Benefits:

Estimate the risk of for initiation of corrosion

Prediction of initiation of corrosion via Fich’s II law

Estimate the risk of accelerated ASR damage

With RCT on-site measurements are possible


Agenda







NDT-Course

India 2005/2006


Chloride content of hardened concrete


Measuring Principle – Dust Collection

The collection of dust can be done with a normal drill and some plastic bags

Customized equipment reduces errors and uncertainties

Standard rules for collecting dust has been made – also to reduce uncertainties

Dust is collected from at least three holes within a square of 15x15 cm



NDT-Course

India 2005/2006



The collection of dust can be done with a normal drill and some plastic bags

Customized equipment reduces errors and uncertainties

Standard rules for collecting dust has been made – also to reduce uncertainties

Dust is collected from at least three holes within a square of 15x15 cm




The chloride content varies the most near the surface

Selection of drilling intervals for dust collection should reflect this

The Danish standard is as shown on the graph

These intervals are sufficient for making ingress analysis using Fich’s II law



NDT-Course

India 2005/2006


Measuring Principle – What is measured?

Estmation of chloride content and chloride profiles.

Total Cl- ionsWater soluble Cl- ions – dangerous to corrosion

Chloride content is determined on a powder of concrete.

Crushed samples – single measurements and large scale profilesGrinding (cores)– detailed profilesDrilling – sampling in the field

Two methods of determinationTitration method – Precise estimationsRCT (Rapid Chloride Test) – Quick estimations



Total chloride content by the RCT method

Extraction of chlorides:

1.5 g of fine powder is poured into an RCT ampoule

Chloride phases is dissolved in the ampoule:

Quick measurements: Shake the ampoule in 5 min

More precise measurements: Shake the ampoules and let the dissolution take place over 12-24 hours.



NDT-Course

India 2005/2006



Preparations of the RCT electrode:

Electrode is filled with an electrode wetting agent

Air entrapped within the wetting agent is carefully avoided

Connect the electrode to the to the electrometer

Electrode is calibrated by four calibration liquids of known Cl-

concentration.


0,005% Cl- : ca. 100 mV0,020% Cl- : ca. 72 mV0,050% Cl- : ca. 49 mV0,500% Cl- : ca. –5 mV



Measurements:

The tip of the electrode is lowered into the RCT ampoule

Record the mV reading when the value becomes stable (stir the electrode a couple of times)

The reading of mV are transformed to Cl-

concentration by a logarithmic scale.



NDT-Course

India 2005/2006


Total chloride content by the titration method

The Principle:12 g of fine powder is dispersed in water

Chloride bearing phases is dissolved by concentrated nitric acid and water.

The solution is filtered

The solution is treated with silver nitrate in excess, which cause the Cl- ions to precipitate as silver chloride

The silver nitrate in excess is back titrated with an ammonium thiocyenatesolution


The amount of precipitated silver chloride is proportional to the chloride content of the concrete and is calculated by

Cl- = 3,545(V1N1-V2N2)/m

V1: total amount of silvernitrate

V2: titrated ammonium cyenate solution

m: Weight of powder sample

N1, N2: Normalisation factors


Accuracy

Practical limitations of the accuracy:

Thoroughness collecting dust samples

The number of holes which has been drilled

The natural variation in chloride content

Current measuring method are sufficiently accurate to predict the risk of chloride initiated corrosion within at least 5 – 10 years



NDT-Course

India 2005/2006


Accuracy

The RCT method

Based on linear interpolationbetween 4 predetermined concentrations

The accuracy at very low or very high concentrations is less – but is also less relevant !!!

An on- site measurement is less accurate than a measurement after 24 hours



Accuracy

The titration method is slightly more accurate than the RCT – but RCT is faster, less costly and require less drilling dust

Total Cl- ions is measured but …

Only Water soluble Cl- ions are dangerous in terms of corrosion (can be estimated by RCTW)

The theory of chloride ingress and chloride initiated corrosion is still debated



NDT-Course

India 2005/2006



Max aggregate size – larger aggregates decreases accuracy

Fe-Ions in the dust sample – these ions will ruin the measurements

Cracks in the surface

Carbonation – pushes the chloride ions away, presence of chloride rules out carbonation and presence of carbonation rules out chloride



Chloride content


NDT-Course

India 2005/2006



Chloride initiated corrosion on reinforcement in concrete

Alkali Aggregate Reactions in concrete




Bridge:

Decks

Piers

Columns

Marine structures

Harbours

Houses

Wind mill foundations



NDT-Course

India 2005/2006



Concrete structure subjected to salt

Including submerged areas – Chloride content can be meassuredfrom cores

Measurements are only possible if dust ca be collected or if a core can be drilled



Case: Kalvebod

Two sets of twin bridges

Build in 1978 – 1982

Post tensioned concrete box girder bridge

Very good concrete quality



NDT-Course

India 2005/2006


Case: Kalvebod

Part of the great coastal bridge project conducted from 1996 to 2000 including over 20 coastal bridges

An investigation was conducted in 1996

The successive investigation was conducted in 2005:

Chloride content

Half Cell potential, resistance –

Corrosion rate measurementsDrilling of core above and sub surface



Case: Kalvebod

Test plan

Chloride content


Corrosion rate measurements

Drilling of core above and sub surface

Carbonation

Concrete cover



NDT-Course

India 2005/2006


Case: Kalvebod

Test planChloride content


Corrosion rate measurements

Drilling of core above and sub surface

Carbonation

Concrete cover

The variation with the height was investigated

Measurements with half cell potential was conducted at identical levels



Case: Kalvebod

Results:

Chloride at low levels

Little chloride near rebars (40 mm)



NDT-Course

India 2005/2006


Case: Kalvebod

Level 0 m



Case: Kalvebod

Level 0,3 m



NDT-Course

India 2005/2006


Case: Kalvebod

Development:1996 - 2005



Case: Kalvebod

Forecast of chloride ingress



NDT-Course

India 2005/2006



Locating areas with critical chloride content in the depth of the reinforcement – hence the areas where a removal of concrete is necessary can be identified

Estimating the time of which chloride initiated corrosion on thereinforcement in the concrete will start – Precautions can be initiattet before the reinforcement starts to corrode



Method


NDT-Course

India 2005/2006


Test Planning


Locate accessible areas

Overview of damages



Expected variation of chloride exposure / content

Expected concrete cover



Test Planning


Areas without significant damage

Use a covermeter to avoid reinforcement


Expected variations

Aim of the investigation –estimation of repair need or estimation of initiation of corrosion

The wanted reliability



NDT-Course

India 2005/2006


Test Planning


Create sketches to register the position of the measurements

Investigate drawings of the reinforcement

6. To Bring (tools)

Drilling equipment

Plastic bags for dust collection

“Dust blower” for cleaning holes





Drill and collect dust

Field RCT

RCT after 24 hours for verification

8. Calibration

Break ups to detect corrosion on the reinforcement

The electrode is calibrated prior to making the RCT measurements



NDT-Course

India 2005/2006




Amount of dust which has been collected

Is the measured profile as expected (Fe – pollution)

10.Registration

Make a thorough visual registration




Planning







Execution

7. Calibration



10.Registration



NDT-Course

India 2005/2006


Method



The reliability the RCT measurements itself is seldom calibrated

Finding the critical chloride content by break ups is only possible in rare cases

In general values for the critical chloride content is between 0.05 % and 0.1 % weight compared to dry concrete



NDT-Course

India 2005/2006



Corrosion is found in the break ups: The critical chloride content in the depth of the reinforcement has been reached

The depth of carbonation may be found inderectly




Report:

Background for making the measurements – also including a result summary of earlier measurements if any are available

Result summary

Description of variations and development



NDT-Course

India 2005/2006



Appendix:

All results

Sketches and photos of positions where measurements has been conducted

Comparison of measurements



Method


NDT-Course

India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



APPENDIX A19

Coring Equipment


NDT-Course

India 2005/2006

Core Drilling

Concrete and Masonry

2SlideCore Drilling - 14 February, 2006

Core Drilling – Concept

(Non) destructive testing.

Damaging small area of the structure.


Bridge decks

Piers

Beams

Girders

Introduction


NDT-Course

India 2005/2006


Core Drilling – Equipment

Usually a portable electric core drill machine is used.

The size of the core varies depending on the actual conditions. For laboratory evaluation cores with a diameter of app. 100 mm and at least 250 mm in length are preferable.

Introduction


Introduction

Benefits:

On-site evaluation

A piece of the actual structure

Relatively mobile

Possibility of laboratory evaluation and thus large information of the concrete.


NDT-Course

India 2005/2006


Agenda






Core Drilling


NDT-Course

India 2005/2006


Common Applications

Calibration of NDT-investigations: HCP, Impact Echo and Impulse Response.

Verification of structural drawings: Rebar, concrete cover, filling of masonry structures etc.

First step in a macro/micro analysis.

First step in laboratory testing of the concrete compression strength.




Cracks

Delaminations

Alkali Aggregate Reactivity

Carbonation

Freeze/Thaw


Thickness of masonry

Filling behind masonry



NDT-Course

India 2005/2006



Deck

Pier/Column

Beam

Foundation

Arches

Abutment




A core can be drilled where ever it is possible to anchor the equipment.


All masonry structures.

Pavement.

Concrete or masonry can be so deteriorated that anchoring is impossible.



NDT-Course

India 2005/2006


Case: Kalvebod Bridge

Twin concrete prestressed bridge

Build in 1980

Six piers in salt water




NDT-measurements

Half Cell Potential

Corrosion Velocity

Cores drilled above and beneath the waterline



NDT-Course

India 2005/2006








NDT-Course

India 2005/2006






The drilling of cores itself seldom gives direct input to strategies.

The core gives valuable information for calibration of other NonDestructive Testing that has been carried out prior to the core drilling.

Drilling out cores gives the basis of fast screening tests if it is followed up by laboratory analysis. E.g. fast screening of potential risk of alkali aggregate reactivity, freeze-thaw damage etc. This can usually be done by drilling 1-2 concrete cores from a bridge component.



NDT-Course

India 2005/2006


Core Drilling


Test Planning

1. Initial Visual Survey and/or measurements with NDT-equipment such as Impact-Echo or Impulse Response.


Delaminated Concrete Intact Concrete Local Damage


NDT-Course

India 2005/2006


Test Planning

2. Forecast of results

If the cores are drilled out as a calibration of other NDT-measurements an on-site comparison of actual condition of the core and the expectedcondition of the core is made.

The expected extent of laboratory analysis is estimated – this may influence the number of cores needed. And it also influence the size of the cores needed.



Test Planning


The right place to take out the cores depends on

the structure geometry

the condition of the concrete

The information needed from the core.

Cores of “bad” and “good” areas must be represented. If cracking occurs in the bridge component place at least one core on top of the cracking.

Unless the condition of the reinforcement is needed you should avoid drilling out cores in the positions of reinforcement. If reinforcement is to be included in the core make sure to include the whole rebar to avoid problems when carrying out the drilling.



NDT-Course

India 2005/2006


Test Planning


Never drill out cores at the location of prestressed cables.

The majority of the cores should be taken in areas where the results from the previous measurements are inconclusive.

Keep in mind that the cores should represent all types of areas in the structure, which influence the repair strategy.



Test Planning


For calibration of NDT-measurements usually 2-4 cores in each area is enough.

For a fast screening for e.g. the concrete composition, the risk of AAR, the risk of freeze-thaw damage etc. usually only 1-2 cores are needed in each homogeneous area.

The number of cores needed is also influenced of the need for laboratory evaluation. Enough material for the needed tests must be present.



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India 2005/2006


Test Planning

Practical PreparationsFor prestressed concrete make sure there are no cables in the core location. Use the as-built drawings and cover meter.

To Bring (tools)As-built drawings.

Coring equipment.

Cover meter.

Common hand tools.

Folding ruler.

Saran wrap, tape and plastic bag (if the moisture content is to be determined).

Chalk or a pen to write on the core before it is baged.



Execution of Field Work

5. Conduct the core drillingLocate the reinforcement including prestressedcables using cover meter and as-built drawings.

Mark the location of the core.

Mount the coring equipment.

Be sure that the equipment cannot move when drilling out the core.

Mark the length of the core need on the coring equipment (add 1-2 cm’s to the length wanted).

Do the drilling.

Carefully break off the core.When breaking off the core you should be careful not to do any damage to the core. If you have trouble getting out the core it has to be noted as you may have caused some defect / cracking to the core.



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India 2005/2006



6. Registration

Before drilling out the core –take a photo of the core location and note if there are any damage (cracking etc.)

On the core the direction to the surface is marked.

If the core is broken make sure to mark every piece (if possible).

Take photos of the core and the hole in the structure.




6. Registration

Right after taking out the core – look for pop-outs (and note if any pop-outs are registered).

Note if you had problems getting out the cores – if you could have damaged the core during this procedure.

Note the depth of the laminations in the hole of the core if any.



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India 2005/2006



Planning


2. Forecast of Results



Execution

5. Conduct the core drilling

6. Registration



Core Drilling


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India 2005/2006



Report:

No written report is usually needed separately for the core drilling procedure – only your registrations from the field is needed. Usually the evaluation and the reporting is carried out as part of reporting other non destructive measurements or as part of the laboratory evaluation.



Core Drilling


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India 2005/2006



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Co

res

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



Damage


(x)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Co

res

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method



APPENDIX A20

Evaluation of Concrete Cores


NDT-Course

India 2005/2006

Evaluation of concrete cores

NDT - Concrete

2SlideEvaluation of concrete cores - 14 February, 2006

Evaluation of concrete cores - ConceptMacro analysis on cores and plane sectionsCarbonation depth measurementsCrack detection on impregnated plane sectionsMicro analysis on thin sectionsAir void analysis on plane sectionsChloride content determinationMoisture analysisResidual reactivity (ASR – Alkali Silica Reactivity)SEM-analysis (SEM – Scanning Electron Microscopy)

Typical ApplicationsAll types of concrete structures: Evaluation of concrete qualityDamaged / deteriorated concrete: Evaluation of damage cause and further development of damageCalibration of other NDT-methods such as Impact-Echo and Impulse Response

Introduction


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Introduction

Benefits:

Obtaining an overall view of the concrete quality

Investigation of damage of the concrete

Necessity of repair based on the conclusions from the two above mentioned investigations

Results are input to deterioration models

Tool in a fast screening of structures (e.g. risk of AAR or not)


Agenda






6. References


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India 2005/2006




Evaluation


A. TechniqueB. What is evaluated?C. Macro analysis on cores and plane sections - overviewD. Impregnated Plane SectionsE. Air void analysis on plane sections F. Chloride ContentG. Micro Analysis - overviewH. Optical Determination of Compression StrengthI. Delayed Ettringite Formation (DEF)J. Scanning Electron MicroscopyK. Moisture Content and Moisture ProfileL. Residual Reactivity (ASR)


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A. Evaluation Principle – Technique


Vacuum-impregnation of full core with fluorescent epoxy resin

Cracks, voids and porous paste connected to the core surface will be filled with fluorescent epoxy resin




Fluorescence impregnated plane section

Cracks, voids and porous paste near the cut surface will be filled with fluorescent epoxy resin

Core from bridge deck, core length 170 mm


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Fluorescence impregnated thin section

The thin section is a 20-micron thick slice of concrete, which has been impregnated with a fluorescent epoxy resin. The thin section is typically 35mm x 45mm in size.

The semi-transparency of the concrete slice allows the examination of the concrete by transmitted light microscopy.


B. Evaluation Principle – What is Evaluated?

Concrete qualityAggregates

Cracks

Carbonation

W/c-ratio

Homogeneity

Damage causesAAR

Carbonation

Freeze-thaw

Moisture content

DEF



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C. Evaluation – Macro analysis on cores, overview

Evaluation and determination of:

- aggregate type

- aggregate content

- cracks (coarse and fine)

- encapsulated air voids

- carbonation

- casting defects, segregation

- condition of joints

- condition of reinforcement

- signs of attack


Phenolphthalein pH-indicatorred colour: not carbonated


D. Evaluation – Impregnated plane sections


Crack detection on impregnated plane sections, under UV-light

Evaluation and determination of

- extent and distribution of cracks(fine, coarse)(crack width > 0,01 mm)

- possible causes (ASR, freeze-thaw etc.)


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Crack detection on impregnated plane sections:

Core from bridge deck

Plane section through impregnated core

Fluorescence impregnated plane section under UV-light




Fluorescence impregnated plane sections:

Varying capillary porosity in small samples for thin section preparation


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E. Evaluation – Air void analysis on plane sections


Evaluation and determination of the air void structure(ASTM C 457, linear traverse):

- air void content A (vol. %)- specific surface α, mm-1- spacing factor, L, mm


F. Evaluation – Chloride content



- Chloride content

- Chloride profiles


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India 2005/2006


G. Evaluation – Micro analysis, overview


Evaluation and determination of - concrete composition- cement type and content- aggregate type and mineralogy- w/c-ratio- air void content and void structure- defects (cracks and inhomogeneities)- aggressive environment (e.g. acid)- moisture conditions and effects- signs of deterioration (e.g. AAR)- strength level- initial defects (casting, curing etc.)


H. Evaluation – Optical determination of compressive strength


Point counting of thin section –paste, air voids and w/c-ratio

Application of Ferets formula for calculation of strength level.

Density of cement paste, T

Co

mp

ress

ive s

tren

gth

, f c

, M

N/

m2

Cr = Vol% cement

V = Vol% water

L = Vol% air


NDT-Course

India 2005/2006


I. Evaluation – Delayed Ettringite Formation (DEF)


Typical crack pattern for a concrete suffering from DEF

Empty “gabs”


J. Evaluation – Scanning Electron Microscopy (SEM)



- phases in the cement or concrete

- depositions (compositions)

- chemical composition - profiles

Ca

MgSi

BS-image


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India 2005/2006


J. Evaluation – Scanning Electron Microscopy (SEM)


Combination of Scanning Electron Microscopy (SEM) and Optical Microscopy:

Ettringite deposits in airvoid

Ordinary Crossed

Back Scatter


K. Evaluation – Moisture Content and Moisture Profiles



moisture content and profiles:

- water content (U %)

- degree of capillary saturation (Scap)

- degree of pressure saturation (Spressure)

- relative humidity (RH %)

0.0 2.0 4.0 6.0 8.0

0-20

20-40

40-60

60-80

80-100

100-120

120-140

140-160

160-180

180-200

200-220

220-230

Kerne 2


Dep

th fro

m s

urf

ace

[mm

]


NDT-Course

India 2005/2006


K. Evaluation – Moisture Content and Moisture Profiles


Moisture content and moisture profiles:0.0 2.0 4.0 6.0 8.0

0-20

20-40

40-60

60-80

80-100

100-120

120-140

140-160

160-180

180-200

200-220

220-230

Kerne 2


Dep

th fro

m s

urf

ace

[mm

]

70

65

70

5

Semi dry

8060RH %

8060Spressure %

959060Scap %

64U %

Very wetWetDryConditionMoisture

NOTE: The values of U depend on the w/c-ratio. The larger the w/c-ratio the larger value of U before the concrete condition is wet. The values in the table are according to a w/c ratio of 0,50.

Scap < 90%: No risk of freeze-thaw damage90% < Scap < 95%: Small risk of freeze-thaw damage

Scap > 95%: Large risk of freeze-thaw damage


L. Evaluation – Residual Activity (ASR)


Evaluation and determination of the potential risk of development of damage due to Alkali

Aggregate Reactions (AAR) and estimation of the residual potential for further reactions

under the following conditions:

- unlimited access for moisture

- unlimited access for moisture and sodium chloride

Limit:

1 ‰ expansion

Exp

an

sio

n [

0/

00]

Storing at 50oC in sodium chloride solution and at 100% relative humidity

Small risk of future harmful cracking due to AAR


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India 2005/2006


Evaluation


A. TechniqueB. What is evaluated?C. Macro analysis on cores and plane sections - overviewD. Impregnated Plane SectionsE. Air void analysis on plane sections F. Chloride ContentG. Micro Analysis - overviewH. Optical Determination of Compression StrengthI. Delayed Ettringite Formation (DEF)J. Scanning Electron MicroscopyK. Moisture Content and Moisture ProfileL. Residual Reactivity (AAR)


Precision

In general the precision of evaluation of concrete cores depends on the experience of the investigator –the evaluation is assumed to be carried out by an experienced investigator.

It is very important for the general precision of the evaluation that the cores as well as the thin sections etc. represents the structure – test planning!

Aggregate, carbonation, homogeneity & cement typeThese parameters can be determined with a high precision within the test sample.

CracksHigh precision within the test sample - it is assumed that the cores are handled correct before entering the laboratory.



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Precision

ASRReactive sand aggregate: Good precision (e.g. one sample for testing the residual reactivity is usually enough).

Reactive stone aggregate: Poor precision (e.g. three samples for testing the residual reactivity is usually required). Note that the representation of the stone aggregate in the test sample is poorer that the representation of the sand aggregate.

Residual reactivity test: A European research project, PARTNER, has shown that the residual reactivity test will expose whether the concrete is reactive or not. E.g. the project has shown that chlorides from outside the concrete will accelerate all kinds of alkali silica reactions.

Air void analysisThe precision is within ± 5% of the test result.

The influence of the sample preparations is essential.

The position of the test sample can be essential – especially for in situ concrete columns etc. where the air content can likely differ from top to bottom of the column.



Precision

Moisture The precision of the test results is approximately ± 10%.

The influence of sealing and storage of the cores is essential.

Optical strength analysisThe precision of the test results of the samples is app. ± 10%. E.g. the analysis may tell whether the strength is 25MPa or 30MPa, etc.

The precision of the results is larger with lower values of the w/c-ratio. For w/c-ratios above 0,70 determination of w/c is more uncertain. Initial cracks will influence the result of the optical strength analysis. The analysis are to be carried out on intact concrete for the best result.



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Handling the cores before they enter the laboratory:

Note if the core was stuck and forced out of the structure (not initial damages).

Be sure to seal the core if moisture analysis are to be made.

Preparation of the test samples e.g. for thin sections, plane sections for automatically rapid air analysis.





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LaminationsCaused by ASR, freeze-thaw, initial defects, etc.

Cracks in generalCaused by ASR, freeze-thaw, initial defects, etc.

Carbonation

Structural problemsEstimation of concrete strength.

Chloride penetrationThe density of the concrete influence the velocity of chloride ingress.




Concrete Bridges

Deck, girder, columns, abutments, etc.

Railway sleeper.

Concrete Floor Slabs and Walls.

Cylindrical Concrete Structures

Silos, Tanks, Chimneys.

Underground Parking Structures.



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In general the evaluation can be used on all types of concrete structures.

Limitations – evaluation is not necessary:

Structures where the cause of damage is obvious e.g. corroded reinforcement due to carbonation of a very small concrete cover.

Structures where the budget of rehabilitation is less than the costs of an evaluation.



Case - Concrete cores from bridge deck

Bridge from 1935

Cores from bridge deck


Concrete:

Ordinary Portland cementW/C-ratio 0.4-0.5Granite and flint in coarse aggregateSand with alkali-silica reactive, porous flint


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Soffit of bridge deck:

Right: Map cracking and transversal cracks with dry white deposits.

Below: Fine parallel longitudinal cracks with dry white deposits.



Core No. 16:

Protective layer at top (left)

Waterproofing

Structural concrete with cracks



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Plane section of structural concrete

Cracks are mainly surface parallel





Thin section:

Massive ettringite formation in cracks show long time moisture exposure of the concrete, indicating that the waterproofing is not protecting against water.

Size: 0,4 mm x 0,6 mm.

Arrow = Crack widthNeedles in crack: Ettringite


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Thin section:Porous flint particle with high number of cracks in right part of photo.Micro cracks and fine cracks in the paste, left part of photo.

W/c-ratio: ~0.40

Size: 2,7 mm x 4,2 mm.



Petrographic evaluation:

Main cause of surface parallel cracking is water saturation of the concrete and freeze-thaw damage.

A secondary cause is alkali silica reaction from reactive aggregate.

The full depth of the core is affected (> 170 mm).

The concrete has not been protected from water for a long time, e.g. the waterproofing is not effective.

If protected from water the concrete is expected to be of an overall high quality as regards to strength, with low w/c-ratio.

If not protected from water and possible de-icing chemicals (sodium chloride) freeze-thaw damage as well as alkali silica reactions will deteriorate the concrete due to 1) lack of an entrained air void system and 2) a critical content of reactive particles in the fine and coarse aggregate.



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Petrographic evaluation – input to repair strategy in inspection report:

Cause of damage is primary freeze-thaw and secondary ASR.

The full depth of the core is affected (> 170 mm).

The waterproofing has not been intact for some years in the position of the core.

If a new waterproofing is not established more damage will occur due to freeze-thaw and ASR.

Conclusion of the inspection (utilizing results from the petrographic analyse and Impulse-Response measurements):

A new waterproofing is needed in 5 years from the inspection time.

At the time of repair approximately 10% of the road area will need concrete repairs to a depth of 40-70 mm from the surface. Locally concrete repairs to a depth of 175-200 mm is needed.




Identifying the cause of damage.

Evaluation of the concrete quality and identifying parameters that influence the future development of damages.

Input to evaluation of the optimal time of rehabilitation.



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Test Planning


Gathering knowledge of the structure – photos of the structure before and after drilling out the core.

Macroscopic analysis of all cores from the same structure.


Do the cores represent the same quality of concrete?

Expected cause of damage (ASR, etc.).

Expected signs of damage (cracks, etc.).



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Test Planning

3. Selection of Sample Areas

If the cores include cracks – make a fluorescence impregnated plane section of a representative core. The crack pattern can give information regarding the cause and extent (depth from surface) of damage.

Make test samples for the relevant tests such as carbonation, residual reactivity (ASR), rapid air (air content).

Select area(s) of thin sections if needed (concrete composition), ASR, etc. Selection of the area(s) should be based on evaluation of the plane section. The damaged area should be represented.


Cut for plane section


Test Planning

4. Estimating the Appropriate Sample Quantity

Depending on the hypothesis the amount of test samples is determined. E.g. in case of ASR caused by reactive stone aggregate: 3 samples for residual reactivity test – in case of ASR caused by reactive sand aggregate: 1 sample for residual reactivity test.

Depending on the homogeneity the amount of test samples is chosen.


Thin section

Note:

The selection of the area for thin sections is very important

Sand aggregate with possible ASR


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Execution of Evaluation

5. Conduct Evaluation

The macro analysis is carried out.

Areas of test samples (plane sections, etc.) are marked on the cores or on sketches.

The samples are prepared by a laboratory technician.

The samples are evaluated by an experienced engineer. All observations are registered.



Execution of Evaluation

6. Evaluate the observations

The observations are evaluated with regard to the cause of damage and future development of damage.

Does the observations match with the hypothesis?!

Decide whether additional steps must be taken (e.g. extra thin sections, laboratory tests etc.)



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Planning



3. Selection of Sample Areas

4. Estimating the Appropriate Sample Quantity

Execution

5. Conduct Evaluation

6. Evaluate the observations





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It must be evaluated how reliable / accurate the evaluations are:

How good is the correlation between evaluation and registrations of the structure?

Was it possible to detect the cause of damage?

Are the evaluated cores representative for the whole element / structure?





Based on the cause of damage identified by the evaluation of the concrete cores the necessity of rehabilitation is estimated.

The evaluations of the concrete cores are often used as a supplementary investigation or calibration to other NDT-methods. In these cases the input from the evaluation is compared with the results from the NDT-method.


Carbonated concrete at the surface


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Report – overall conclusions:

The conclusions of the evaluation should be summarized answering the questions made by the inspector delivering the core(s) to the laboratory

Overall condition of the concrete, damage type and depth

Possible repair methods

Are further tests needed? (describe benefits)



Report – detailed input to inspection report:

The extent of the analysis should be summarised – often more cores are evaluated in different levels (typically a macro analyse is preformed on all cores. Based on the results from this specific cores are chosen for further analysis).

The conclusions of the evaluations should be summarized for every structural element or concrete type (e.g. bridge deck, column, etc.)

Damage type and depthMoisture content (if measured) Potential risk of ASRPossible repair methodsAre further investigations needed? (describe benefits)

Avoid inserting photos from the micro analysisThe technical presentation of the evaluation should be constricted to the appendix



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Interpretation and Reporting of Results



Appendix:

We are dealing with a complex method – an introduction to the laboratory analysis performed should therefore always be made.

Detailed description of observations from the different analysis.

Evaluation of the observations.

Summary of the concrete quality and condition.

Evaluation of the risk of future development of damages.



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Appendix – example of observations from a micro analyse.

In the micro-description qualitative evaluations are made in a scale from 0-3:

Degree Content0 None / little1 Some2 Many3 A lot




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India 2005/2006




(X)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air v

oid

)

AS

R re

activ

ity

Macro

/M

icro a

naly

ses

Co

res

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mitt h

amm

er

Boro

scope

Crack d

etection

NDT-Method

Damage




(X)XXXXXX(x)XXXASR





XXXXXXCorrosion

(Air v

oid

)

AS

R re

activ

ity

Macro

/M

icro a

naly

ses

Co

res

Brea

k up

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Crack d

etection

NDT-Method

Damage


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India 2005/2006


6. References

Thaulow, A. et. al. : ”Estimation of the compressive strength of concrete samples by means of fluorescence microscopy”, Nordisk Betong, 1982.

Mullick, A.K.: “Alkali-silica reaction – Indian Experience”, The Alkali-Silica Reaction in Concrete, Edited by R.N. Swamy, 1992.

Visvesvaraya, H.C. et. al.: “Analysis of Distress Due to Alkali-Aggregate Reaction in Gallery Structures of a Concrete Dam”, Concrete Alkali-Aggregate Reactions, Proceedings of the 7th International Conference 1986, Ottawa, Canada.

Mullick, A.K., et. al.: ”Evaluation of Quartzite and Granite Aggregates Containing Strained Quartz”, Concrete Alkali-Aggregate Reactions, Proceedings of the 7th International Conference 1986, Ottawa, Canada.



APPENDIX A21

Acoustic Emission Monitoring


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Acoustic Emission

NDT – Steel

2SlideAcoustic Emission - 24 February, 2006

Introduction

General purpose: Detection of fatigue problems

Listening for high frequency sounds which indicate the bridge under stress


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Detection of crack growthin steel bridges

Most methods can detectcracks but this methodwill only detect active cracks


Agenda






6. References


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Acoustic emission


Theory, general Principe


Crack growth causes stress redistribution which is associated with the release of elastic waves

These waves can be detected by specially designed sensors


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Measuring Principle

Instruments:- Transducer- Preamplifier- Filter- Amplifier- Signal analysis- Storage

Main problem:Noise


Transducer (from 3x3mm, 0.2g to 30x30mm, 50g)


Measuring Principle, main problem: noise

Signal analyses:- Only relevant frequencies

are measured (resonant transducers)

- Only relevant amplitudes are measured

- Only signals from relevant locations are measured

Combined with straingauge measurements onlyrelevant time periods canmeasured



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What is measured

Measuring instrument:



Aim of measurements

Crack characterization:- Active cracks from structural

stress- Passive cracks

Minimizing repair costs:- No repair of passive cracks- Early and small repairs of

active cracks- Monitoring for postponing

repair



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The main damage which can be evaluated from this method is:- Crack growth from fatigue and excessive loading



Accuracy


Acoustic emission can detect:- Very early stages of crack growth- At what time periods the cracks grow- Relative speed of crack growth- If cracks are passive

Combined with strain gauges acoustic emission can detect what loads causes crack growth

Acoustic emission cannot measure:Absolute speed of crack growth


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The main problem of acoustic emissionis noise. Noise comes from:

- Crack face rubbing

- Crushing of dirt and corrosion products in the crack

- Bolt/rivet fretting

- Electrical noise

At joints with bolts or rivets the use ofthe method be limited because of noisefrom the bolts/rivets





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The main damage which can be evaluated from this method is crackgrowth from fatigue and excessive loading




The method is usually used at area with high risk of fatigue relateddamage. Typically in high stressed joints at bridge girders



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Common Applications – simple Crack-repairs

By drilling a hole in front of the crack tip local stressconcentration will be reduced.

At a later state this might notbe enough to stop the damage

Acoustic emission can detectdamage at a very early stageand thereby improve the possiblity of stopping furtherdamage by simple repairs

Acoustic emission can inspect if the repairs have been successful



Common Applications – assisting visual inspection

Much crack-growth is not seen on the surface, acoustic emissionwill show the entire crack growth


Crack-tip is foundCrack tip is not found


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Common Applications – Postponing repair

- Some cracks will grow very slowly and repair can be postponed

- Slow growth exhibits very little acoustic activity

- Monitoring acoustic activity can show when a more active stageis reached

- Monitoring results can be transmitted anywhere on-line

- Visual inspection will not show the full crack growth


Initiationof crack

Initiation of micro cracks

Crack growth



General: The well defined loading on railway bridges improves the use of acoustic emission measurements

Testing of loading capacity (combined with structural analysis, strain gauge- and deflection measurements)

Monitoring of corrosion in joints

Monitoring of structural damage in concrete structures

Monitoring of reinforcement corrosion in concrete structures

Limitations:

Place for application must be well defined

Noise must be controlled



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Combined with measurement of strain gauge and traffic loadacoustic emission are expected to give information on:• Areas which must be repaired

• Is repair of cracks and other fatigue related symptoms necessary

• Is preventive precautions relevant on areas with high risk of fatigue

• Can restrictions on load capacity prevent further damage

• Can repair be postponed

• Effectiveness of repair

Evaluation of fatigue related problems must be combined with structural

analysis





NDT-Course

India 2005/2006


Test Planning

1. Structural analysis

- General level of fatigue risk- General high-risk areas- High risk elements

2. Visual inspection - Signs of fatigue related

symptoms- Structural flaws- Corrosion

3. Supplementary crack detection

- Dye penetrate etc.


XY

Z


Test Planning


Analysis of acoustic emission requires- Long term monitoring- Use of expensive equipment. To optimize the value of the measurements selection of the areas testedshould be selected on the basis of detailed structural analysis


Acoustic emission is a relative method and supplementary test areas on undamaged areas will improve the reliability of the measurements


NDT-Course

India 2005/2006


Test Planning

- Grinding of the surface

- Contact couplant to transducer

- Magnetic hold-downs for holding the transducer

- Wiring

- Shelter for amplifier and registration unit

- Power, eventually battery driven

Strain gauge measurements

Registration of loading


4. Establishing the measuring points




- Length of the monitoring period (1 year)- Collecting data- Test of influence from noise- Calibration of signal strength- Calibration from test loading



NDT-Course

India 2005/2006



Planning




4. Establishing of the measuring points

Execution




Method


NDT-Course

India 2005/2006



Calibration:- Calibration of transducer from manufacture- Electrical pulse generator to calibrate amplifier-gain- Test of noise influence on periods without traffic- Test of signal locating- Laboratory testing of samples

The acoustic emission method is in its infancy and the reliability can not be estimated precisely. The method should therefore be combined with other methods (structural analysis, dye penetrant, strain gauge, deflection)





Identification of damage risk due to fatigue-induced cracks

Identification of damage risk due to overloading-induced cracks


NDT-Course

India 2005/2006



Report:

General conclusions

Eventually main statistics andillustrative cumulative plots




Appendix:- Measured values, including

time of registration- Equipment used- Calibration - Cumulative plots- Placing of measure points- Correlation with other

measurements



NDT-Course

India 2005/2006



Damage

X

X

X

X

X

X

X

Crack d

etection

X(x)XXXXX(x)XX(X)(Freeze-thaw)

(x)XXXXXX(x)XX(X)ASR

XXXXXXXXXXInitial defects

XXXXXXXXX(X)Structural problems

(x)XXXXChloride penetration

(x)(x)XXXXXCarbonation

XXXXX(X)Corrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Aco

ustic E

missio

n

NDT-Method



APPENDIX A22

Structural Testing System


NDT-Course

India 2005/2006

Structural Testing System

NDT - Concrete

2SlideStructural Test System - 24 February, 2006

Structural Test System - Measuring Concept

Various methods and instruments for measuring

Structural Test System - Typical Applications

Inspection of bridges

Evaluation of bridge strength and capacity

Evaluation of dynamic behavior

Long-term monitoring

Introduction


NDT-Course

India 2005/2006


Introduction

Benefits:

Online monitoring replaces numerous NDT inspections

Input to structural assessment of a deteriorated or insufficient bridge


Agenda






6. References


NDT-Course

India 2005/2006


Method



Various instrumentations Accelometres

Strain gauges

Wind speed and direction

Vehicle control sensors

Temperature transducer

Displacement transducer

Deflection/tiltmeter

High precision differential GPS

Moisture probes / Corrosion probes / Audio and video



NDT-Course

India 2005/2006


Monitoring Principle: Setup


DATA-LOGGER

STRONG MOTION SENSOR

WEAK MOTION SENSOR

Weather station

Settlement sensors

WORKSTATION

Operation CenterMaintenance

Engineer

Asset Manager

Bridge Owner

CorrosionSensors

IP

IP

DeformationSensors

Load Sensors

Temperature Sensors


Measurements: Setup Monitoring System

Identification of needs and problemsFor existing structures the needs and problems are usually identified on the basis of a detailed inspection of the structure using NDT-methods.

Clarify objective and outline layoutFirstly, it is necessary to clarify how monitoring will assist in handling the needs and problems which have been identified. Once this has been done the designer of the system must choose what to measure, where the measurements should be performed and what kind of instrumentation should be used.



NDT-Course

India 2005/2006



Design of systemThe design of the system consists of selecting the proper sensors de-pending on the required accuracy, sampling frequency etc. Further, the designer must select the system for data acquisition, cabling, communication, user interface and operation.

Installation, commissioning and hand-overInstallation of the system should be performed by a hired professional with experience in the installation of structural monitoring systems.





Results from visual inspections Results from on-site and lab. testing Results from monitoring system

1: Has deterioration been initiated ?

2: What is the possible risk and cause of future deterioration ?

3: When will deterioration be initiated ?

4: Decide preventive actions, if any

5: What is the current state of deterioration ?

6: What is the cause of deterioration ?

Critical degree of deterioration

Critical level for initiation

7: When will the critical degree of deterioration be reached ?

8: Decide remedial actions, if any

Models for initiation

Models for damage growth

No Yes


NDT-Course

India 2005/2006


Accuracy

Varies from system to system

A critical parameter is the durability of the system



Method


NDT-Course

India 2005/2006



Corrosion on reinforcement in concrete

Moisture in concrete

Position – settling of the base

Displacement

Chlorides

Carbonation

Fatigue problems in steel structures

Pore pressure




Bridge deck

Piers and columns

Stay cables on cable stay bridges

Main cables and hangers on suspension bridges



NDT-Course

India 2005/2006



The possibilities are numerous! – as are the amount of monitoring and test systems which are available

Often custom designed system are made for larger new structures: Great Belt Link – Mesina Bridge ???

One of the limitations for monitoring of new structures is the service life of the system (probes etc.)

For existing structures a great challenge is to install the probes without making a local distortion



Case 1: Tete Bridge

Suspension bridge with deteriorated cable system

Structural analysis of the bridge determined (required) that the actual cable forces were determined.

Cable forces may be determined using accelerometer measurements on cables being manually set into vibration.Cable force is proportional to eigenfrequencies



NDT-Course

India 2005/2006


Case 1: Tete Bridge

Hangers were instrumented with accelerometers.

This allowed for the determination of cable forces in each individual cable.

Afterwards a repair project of deficient cables was initiated.

The cable shown tothe right is not supporting the bridge !



Case 2: Skovdiget – Corrosion monitoring


Skovdiget Bridges

Twin bridges, constructed 1966.

Post tensioned concrete box-girder bridges.

220 m long, 12 spans

Carries a 4 lane highway


NDT-Course

India 2005/2006




- Access to data through a browser- Instant access to all collected data- Owner, consultants, contractors mayall have access to the system





NDT-Course

India 2005/2006




0 3 0 6 0 9 0 1 2 0 1 5 0 1 8 0 2 1 0 2 4 0 2 7 0 3 0 0 3 3 0 3 6 00

3 3

6 6

1 0 0

1 3 3

1 6 6

2 0 0

0 30 60 90 120 150 180 210 240 270 300 330 3600

33

66

100

133

166

200

0 30 60 90 120 150 180 210 240 270 300 330 3600

33

66

100

133

166

200

W est South East North W e

Degrees from due W est

1999

2000

2001

1

10

100

06-S ep-01

26-S ep-01

16-O ct-01

05-N ov-01

25-N ov-01

15-D ec -01

04 -Jan -02

24-Jan -02

13-Feb-02

05-M ar-02

R es is tance (kO hm )

0

10

20

T em pera tu re (C e lc ius )

A 1/A 2

A 3/A 4

A 5/A 6

T

0.95

1.00

1.05

1.10

19-Apr-01

8-Jun-01

28-Jul-01

16-Sep-01

5-N ov-01

25-D ec-01

13-Feb-02

4-Apr-02

H um idity (V olt)

-10

0

10

20

30

T em perature (C e lcius)

H U M 12 H U M 13 106(C )





NDT-Course

India 2005/2006


Case 3: Skovdiget – Deformation monitoring





Monitoring of deformation of bridge deck (superstructure) in order to monitor development in deterioration. Strain of main girders is measured using light-fibre strain gauges (measure strain over 0,5 m)


NDT-Course

India 2005/2006



Vehicle speed, type, weight and location of bridge is calibrated by 24-hour video-monitoring of bridge.





Estimate of deterioration rate

Load/performance test

Creation of more accurate but less requiring load cases for carrying capacity calculation

Updating of probability based carrying capacity models


NDT-Course

India 2005/2006


Method


Test Planning


Identify feasible solutions


Make analysis of different solution and which benefits they will produce



NDT-Course

India 2005/2006


Test Planning


Insure accessibility


Requirements for the wanted reliability

All data must be gathered and reported

A procedure for data collecting, processing, interpreting and reporting should be made during the planning phase


Vibrating wire gauges

Temperature


Instrumentation


Displacements, vibrations, inclinations and loads

Inclinometers

Load cells

Accelerometers


NDT-Course

India 2005/2006


Foundation

Instrumentation


SettlementPore Pressure


Instrumentation


Foil gauges

Wind speed and direction


NDT-Course

India 2005/2006



Planning







Method


NDT-Course

India 2005/2006



For monitoring systems calibration should be done on regular basis

Other test system are mostly calibrated in advance





Report:







For monitoring an automatic data collection and storage system is recommended



NDT-Course

India 2005/2006



Appendix:

Include all result and data

Registration should be sufficient for making replicate tests




APPENDIX A23

Structural Scan Equipment


NDT-Course

India 2005/2006

Structural Scan Equipment

NDT - Concrete

2SlideStructural Scan Equipment - 24 February, 2006

Structural Scan Equipment - Measuring ConceptX-rays are emitted from an accelerator unit on one side of the component to be tested Film (electronic or plastic) is placed on the other side of the component tested

Flaws, defects, variation of materials can be detected using themethod.

Structural Scan Equipment - Typical ApplicationsInspection of bridgesInspection of roadsReveal flaws inside the concrete, steel or masonry structuresInspection of masonry and concrete structures in general

Introduction


NDT-Course

India 2005/2006


Introduction

Benefits:

Very detailed 2D images showing the “inside” of bridge components (e.g. possible to detect a 20 mm porosity in 1000 mm thick concrete)

Images may be stitched to generate semi 3D images

Real-time analysis (if using “electronic” paper)


Agenda






NDT-Course

India 2005/2006


Method



X-Ray equipment (also called Radiography) consists of a mobile accelerator to emit the X-rays and an image plate to collect and process the signal.

Expensive and complex instrument.

May require shielding in order to avoid radiation danger



NDT-Course

India 2005/2006







HER – Instrument Betatron PXB-7.5Technical data

X-ray source of 2 to 7.5 MeV

Frequencies of more than 30 PHz

Rate of dose is 300 R/hour at 1 m (air)

Powered by 220V / 240V 13A

Weight of accelerator: 111 kg

The depth of measuring in concrete is app. 1.2 m

Typical use of HER

Determination of amount, size and location of reinforcement.

Investigation of pre-stressed cables in cable-ducts

Localization of delaminations and honeycombing


NDT-Course

India 2005/2006



X-rays penetrate the component to be measured

X-rays are attenuated (behavior of waves as they radiate out from a source) dependent on the density and thickness of the object.

The amount of radiation that penetrates the object will determine the brightness and contrast of the image

In concrete rebars appear lighter

In concrete voids and pores will appear darker



Measurements

Measurements are made in singular points

Measurements are typically areas of 30 x 30 cm – will depend on instrument and setup

Measurements may be evaluated real time



NDT-Course

India 2005/2006


Precision

Very detailed images

Small voids in Cable ducts may be located with mm precision

Precision depend on thickness and layout of reinforcement and cables




The type of source used for generating the X-rays

The thickness of the component

Position of source and detector on each side of the component

Layout of reinforcement within the component

The condition of the component

The signal-to-noise ratio



NDT-Course

India 2005/2006


Method



Un-injected cable ducts

Corroded cable strands

Voids

Delaminations



NDT-Course

India 2005/2006



Bridge deck beams

Bridge decks

Columns

Cantilevered sidewalks




Must be able to place source and detector on each side of the component to be measured.

Measurements of members thicker than 1.2 m is not possible



NDT-Course

India 2005/2006


Case: Skovdiget

30 year old bridge suffering for ASR damage, un-injected and corroded cables.

The post-tensioned beams were badly cracked

Beams were investigated using X-rays and later calibrated using break-ups and boroscope investigations



Case: Skovdiget



NDT-Course

India 2005/2006


Case: Skovdiget

Post tensioned beams having severe cracking from ASR damage



Case: Skovdiget



NDT-Course

India 2005/2006


Case: Skovdiget


Area of cable duct with filling (scan across white line)

1 2

1: A slightly higher density could be a thin air containment of a few millimeters

2: A lower density is caused by steel cables


Case: Skovdiget


Area of cable duct without filling

1 & 2: A higher density is caused by lack of filling in the cable-duct.

1 2


NDT-Course

India 2005/2006


Case: Skovdiget

Boroscope investigations

Yellow line indicates location of cable. Black holes indicate location of boroscope holes



Case: Skovdiget

Voiding between concrete and cable ducts

Picture taken longitudinally the cable duct. Voiding between concrete and duct visible.



NDT-Course

India 2005/2006


Case: Skovdiget

Poorly injected cable-ducts




Detailed investigation for location of voids and defects

The case “Skovdiget” showed that the X-rays investigation was able to locate voids and poorly injected cable ducts very precisely.

Locate position of cables

Estimate need for repair



NDT-Course

India 2005/2006


Method


Test Planning


Accessibility is essential as the equipment is rather heavy and security precautions are required



Define optimal usage



NDT-Course

India 2005/2006


Test Planning


Consider limitation in terms of accessibility and penetration depth






Check that measurements are feasible

Insure security for the public


Boroscope

Breakups



NDT-Course

India 2005/2006




Evaluate correlation between interpretation and calibration

8. Registration

Thorough visual registration




Planning





Execution




8. Registration



NDT-Course

India 2005/2006


Method



Great reliability – especially for shallow measurements

Document reliability with calibration


1 2


NDT-Course

India 2005/2006



Report:










Appendix:

Exact registration of positions

All data

Method and investigation description




APPENDIX A24

Introduction to Non-Destructive Testing of Steel Structures


NDT-Course

India 2005/2006

Introduction to NDT

NDT – Steel

2SlideIntroduction to NDT - 14 February, 2006

Introduction

Outline

Introduction to NDT

Overview of nine Common NDT Methods

Selected Applications


NDT-Course

India 2005/2006


Introduction

The use of techniques to determine the integrity of a material, component or structure

or quantitatively measuresome characteristic of an object.

i.e. Inspect or measure without doing harm.

Definition of NDT


Introduction

Methods of NDT

Visual Testing

Dye Penetrant TestingMagnetic Particle Testing

Eddy Current Testing

Ultrasonic Testing

Radiography

Acoustic Emission Testing

Leak Testing

Strain Gauge


NDT-Course

India 2005/2006


Introduction

What are Some Uses of NDT Methods?

Flaw Detection and Evaluation

Leak Detection

Location Determination

Dimensional Measurements

Stress (Strain) and Dynamic Response Measurements

Material Sorting and Chemical Composition Determination


Introduction:

Most basic and common inspection method.

Tools include mirrors, magnifying glasses,bore scopes, and fiberscope,

Visual Inspection

Visual inspection


NDT-Course

India 2005/2006


Introduction:Dye Penetrant Testing

Liquid penetrant is applied

Excess penetrant is removed

Developer is applied

Imperfection is now visible

Dye Penetrant Testing


Introduction:Magnetic Particle Testing

Magnetic field is induced

Visible imperfection

Magnetic Particle Testing


NDT-Course

India 2005/2006


Introduction:




Introduction: Eddy Current Testing

Magnetic FieldFrom Test Coil

Magnetic Field From

Eddy Currents

Eddy Currents

Crack

Electrical currents generated in a conductive material by an induced alternating magnetic field


NDT-Course

India 2005/2006


Introduction:

Eddy current testing is particularly well suited for detecting surface cracks but can also be used to make electrical conductivity and coating thickness measurements.




Introduction:

The radiation can come from an X-ray generator or a radioactive isotope.Radiation is directed through a specimen and onto a film

Top view of developed film

X-ray film

= more exposure= less exposure

The film darkness (density) will vary with the amount of radiation reaching the film through the test object.

Radiography


NDT-Course

India 2005/2006


Introduction:

Radiography equipment

Radiography

Radiography


Introduction:

High frequency sound waves are introduced into a material and they are reflected back from surfaces or imperfections.

f

plate

crack

0 2 4 6 8 10

initial pulse

crackecho

back wallecho

Oscilloscope or LCD screen

Ultrasonic Testing


NDT-Course

India 2005/2006


Introduction:

Ultrasonic Testing

Ultrasonic Testing


Introduction:

Acoustic Emission measures the short bursts of acoustic energy from a stressed material

Acoustic Emission


NDT-Course

India 2005/2006


Introduction: Leak Testing

Leak Testing is used to locate leaks in pipes, pressure vessels etc. You can use listening devices, pressure gauge measurements, bubble test etc.


Introduction: Strain Gauge

A strain gauge is an electrical resistance wire, which measures the resistance changesaccording to the deformation in a material.


NDT-Course

India 2005/2006


Introduction:Inspection of Raw Products

Forgings,

Castings,

Extrusions,

etc.


Introduction:

Cracking

Corrosion

Erosion/Wear

Heat Damage

etc.

Inspection for In-Service Damage


NDT-Course

India 2005/2006


Introduction:

ProbeSignals produced

by various amounts of corrosion thinning.

Periodically, power plants are shutdown for inspection. Inspectors feed eddy current probes into heat exchanger tubes to check for corrosion damage.

Pipe with damage

Power Plant Inspection



Introduction:

Storage Tank InspectionRobotic crawlers use ultrasound to inspect the walls of large above ground tanks for signs of thinning due to corrosion.

Cameras on long articulating arms are used to inspect underground storage tanks for damage.



NDT-Course

India 2005/2006


Introduction:Rail Inspection

Special cars are used to inspect thousands of miles of rail to find cracks that could lead to a derailment.

In other situations manual equipment is used


Introduction:Bridge Inspection

• The US has 578,000 highway bridges.

• Corrosion, cracking and other damage can all affect a bridge’s performance.

• The collapse of the Silver Bridge in 1967 resulted in loss of 47 lives.

• Bridges get a visual inspection about every 2 years.

• Some bridges are fitted with acoustic emission sensors that “listen” for sounds of cracks growing.



APPENDIX A25

Ultrasonic Testing


NDT-Course

India 2005/2006

Ultrasonic

NDT – Steel

2SlideUltrasonic testing - 14 February, 2006

Introduction

Applicable for:Applicable for:

Thickness measurementThickness measurement

Lamination examinationLamination examination

Weld examinationWeld examination

Material defectsMaterial defects

ULTRASOUNDULTRASOUND

and and manymany other other thingsthings


NDT-Course

India 2005/2006

Theory – Technical Method Description

NDT – Steel


Theory

Ultrasound is:

- mechanical vibrations of particles

- over 16.000 - 20.000 oscillations per sec.

2 typical methods are:

Technical Method Description


NDT-Course

India 2005/2006


Theory

Through transmission

Pulse echo



Theory

BW BW



NDT-Course

India 2005/2006


Theory

BW Defect BW Defect Defect



Theory



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India 2005/2006


Theory

Ultrasonic terms

Frequency:

- Number of oscillations per. sec.

- called: Hz - KHz - MHz



Theory

Wavelength:

High frequencies = small wavelength

Low frequencies = big wavelength



NDT-Course

India 2005/2006


Theory

Type of waves:

Longitudinal waves

Transverse waves



Theory

Sound velocity:

A material quality

Steel: 5900 m/sec. - 3230 m/sec.



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India 2005/2006


Theory

Refraction and reflection

- at interfaces with air

- at defects (cracks - lack of sidewall fusion -lack of penetration etc.)

- Air gap bigger than 1/10.000 mm



Theory

4 MHz

0.37 mm in at least 2 directions

Air gap min. 1/10.000 mm



NDT-Course

India 2005/2006


Theory



Theory

a

b

c



NDT-Course

India 2005/2006


Theory



Theory



NDT-Course

India 2005/2006

Snell’s law Reflection and refraction

NDT – Steel

Reflection and refraction



SteelAir

Longitudinal wave

Angle of incidence L

Longitudinal wave

Reflected angle L

Transverse wave

Reflected angle T



NDT-Course

India 2005/2006



Steel

Air

Transverse wave

Angle of incidence T

Transverse wave

Reflected angle T

Longitudinal wave

Reflected angle L




PerspexSteel

L LT

Reflected waves

L

TRefracted waves



NDT-Course

India 2005/2006



L1. BW1. BW

L

L

LL

T

L

Spurious echoes

L2. BW2. BW




Reflection at right anglesReflection at right angles

Interface

Incident waveTransmitted wave

Reflected wave



NDT-Course

India 2005/2006








NDT-Course

India 2005/2006

Probes: Normal probes

NDT – Steel

Normal probes


Probes

Probes

- normal probes

- angle probes

Normal probes


NDT-Course

India 2005/2006


Probes

Normal probes or transducers

Normal probes


Probes

Normal probes

- Sends out longitudinal waves

- Applicable for: Thickness measurement

Lamination examination

Cast defects

Forge defects

Rolling defects

Normal probes


NDT-Course

India 2005/2006


Probes

- ordinary crystals are destroyed at

600 - 700C

- special probes with crystals up to

app. 8000C, usually TR-probes

Normal probes


Probes

Connector

Wires

Backing

Crystal

Sole

Normal probes


NDT-Course

India 2005/2006

Probes: Angle probes

NDT – Steel

Angle probes


Angle probes

Angle probes

- Sends out transverse waves

- Applicable for: Welds

Material examination of pipes

Angle probes


NDT-Course

India 2005/2006


Angle probes

- ordinary crystals are destroyed at

60 - 700C

- special angle probes with crystals

and wedges up to app. 4800C

Angle probes


Angle probes

Angle probe, construction

DampingDamping

PerspexPerspexCrystalCrystal

WiresWires

ConnectorConnector

Angle probes


NDT-Course

India 2005/2006


Angle probes

Angle probes

Angle probes


Welds

Coupling

- To create airtight contact betweenprobe and specimen

- jelly - water/detergent - oil – wallpaper - paste

Angle probes


NDT-Course

India 2005/2006


Welds

tSound path

Angle of refraction

α

Skip distance

Angle probes


Welds

Angle probes


NDT-Course

India 2005/2006


Welds

Defect echoDefect echo

Angle probes


Welds

S

α

d

αcos s = d x α cos s - t 2 = d xx

a

αsin s = a x

Angle probes


NDT-Course

India 2005/2006


Welds

20 mm20 mm 70°

96 mm96 mm

d =7d =7 mmmm

a = 90 mm

Angle probes


Welds

Angle probes


NDT-Course

India 2005/2006


WeldsHalf value method

or

6 dB drop

Angle probes


Welds

0 dB + 12 dB

Angle probes


NDT-Course

India 2005/2006


Welds

Angle probes


Rivets

Angle probes


NDT-Course

India 2005/2006

Equipment & Materials

NDT – Steel


Equipment


NDT-Course

India 2005/2006

Test Planning and Execution of Field Tests

NDT – Steel



Test Planning and Execution

Planning and execution

1. Visual examination



NDT-Course

India 2005/2006





2. Carry out lamination test and thickness measurement in the scanning area

a

b

c







3. Calibrate the range for the first angle probe


Calibration block 1

Calibration block 2


NDT-Course

India 2005/2006







4. Check sensitivity, exit point and actual angle




5. Carry out transfercorrection



NDT-Course

India 2005/2006





6. Set the equipment to the correct gain: Ref. gain + transfercorr. + extra gain





6. Set the equipment to the correct gain: Ref. gain + transfercorr. + extra gain

7. With defect: Register projected distance – depth – length and dB value

Sα

d

a


NDT-Course

India 2005/2006




Carry out transfercorrection

Set the equipment to the correct gain: Ref. gain + transfercorr. + extra gain

With defect: Register projected distance – depth – dB value and length

Repeat the above from opposite side

Interpretation and reporting of results

NDT – Steel


NDT-Course

India 2005/2006


Interpretation and reporting



Report formula



NDT-Course

India 2005/2006


Report formula




APPENDIX A26

Ultrasonic Thickness Gauge


NDT-Course

India 2005/2006

Ultrasonic Thickness measurement

NDT – Steel


NDT – Steel


NDT-Course

India 2005/2006

3SlideUltrasonic thickness measurement - 24 February, 2006

Theory

Thickness measurement

- accuracy app. 3-4/10 mm

- smallest measurable thickness app. 5 mm

- biggest measurable thickness 5-10 m

- surfaces with coating

- other materials than steel



Theory

Thickness measurement on the specimen

100 mm75

t = 15 mm



NDT-Course

India 2005/2006


Theory


Thickness measurement on the specimen, LCD

100 mm

t = 15 mm

Measurement


Theory

Coating

SteelSteel

SteelSteel

Steel

24.5 mmDigital equipment


With coating


NDT-Course

India 2005/2006


Theory

Coating

SteelSteel

SteelSteel

SteelSteel

98 mm4

= 24.5 mm

Analog equipment



Theory

Steelplates with uneven backwall

nom. t = 25 mm

100/125 mm



NDT-Course

India 2005/2006


Theory

TR-probe, construction

Transmittercrystal

Receivercrystal

Acoustic barrier

Soundpath



Theory

Use a TR-probe/small range

nom. t = 25 mm

25 mm



NDT-Course

India 2005/2006


NDT – Steel


Equipment

Equipment and Materials


NDT-Course

India 2005/2006


Equipment



NDT – Steel



NDT-Course

India 2005/2006





Primarely to check the area to

be measured i.e. for paint-dirt-

scale-corrosion etc.

If grinding is necessery

Contact area at least 2 times

the probe diameter






2. Calibrate the gauge with the right probe: Frequency-type-size etc.

Compensate for V-path

Make sure if the equipment can measure through coating if necessary or on elevated temperatures



NDT-Course

India 2005/2006






3. Check calibration frequently and after the last measurement








4. With a lot of readings remember information on the location and point


Where was I??


NDT-Course

India 2005/2006


NDT – Steel


Report formula

Interpretation and Reporting of results



APPENDIX A27

Magnetic Thickness Gauge


NDT-Course

India 2005/2006

Ultrasonic Thickness measurement

NDT – Steel


NDT – Steel


NDT-Course

India 2005/2006


Theory

Thickness measurement

- accuracy app. 3-4/10 mm

- smallest measurable thickness app. 5 mm

- biggest measurable thickness 5-10 m

- surfaces with coating

- other materials than steel



Theory

Thickness measurement on the specimen

100 mm75

t = 15 mm



NDT-Course

India 2005/2006


Theory


Thickness measurement on the specimen, LCD

100 mm

t = 15 mm

Measurement


Theory

Coating

SteelSteel

SteelSteel

Steel

24.5 mmDigital equipment


With coating


NDT-Course

India 2005/2006


Theory

Coating

SteelSteel

SteelSteel

SteelSteel

98 mm4

= 24.5 mm

Analog equipment



Theory

Steelplates with uneven backwall

nom. t = 25 mm

100/125 mm



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Theory

TR-probe, construction

Transmittercrystal

Receivercrystal

Acoustic barrier

Soundpath



Theory

Use a TR-probe/small range

nom. t = 25 mm

25 mm



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Equipment



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Equipment



NDT – Steel



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Primarely to check the area to

be measured i.e. for paint-dirt-

scale-corrosion etc.

If grinding is necessery

Contact area at least 2 times

the probe diameter






2. Calibrate the gauge with the right probe: Frequency-type-size etc.

Compensate for V-path

Make sure if the equipment can measure through coating if necessary or on elevated temperatures



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3. Check calibration frequently and after the last measurement








4. With a lot of readings remember information on the location and point


Where was I??


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NDT – Steel


Report formula

Interpretation and Reporting of results


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Coating thickness measurement

NDT – Steel


NDT – Steel


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3SlideCoating thickness measurement - 14 February, 2006

Theory


Measuring the thickness of coating (film) can be done in more ways:

Magnetic - Eddy Current (ET) - Ultrasonic (UT) -Micrometer or destructive methods

What to use is determined by type of coating -substrate material - thickness of the coating -size and shape of the part etc.


NDT – Steel


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India 2005/2006



Magnetic gauges

Pencil type pull-off

Roll-back dial pull off

Are used on non magnetic coating on ferrous substrates


The thicker the coating the easier it is to pull away the magnet

It has a magnet in one end anda calibrated spring in the other.

By rotating the dial the magnetis pulled from the surface



It is pressed directly onto the surfaceand one coil produces a magnetic field and one detects the changes in magnetic flux.

Electronic magnetic induction gauge

Electromagnetic Induction


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India 2005/2006



Ultrasonic

Uses the pulse-echo technique.

Measures the thickness of coatingson non-metal substrates

The returning signal is convertedinto a high frequency electrical signal


NDT - Steel


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India 2005/2006




The most commonly used method for measuring coating thicknesses is Electromagnetic Induction

On greater steel constructions you often use the 80/20 rule


80/20 rule


Example:

1. 10 m2 areas is selected on a construction (5% of the surface has to be covered)(Each selected area shall be connected)

2. In each area a minimum of 5 fields isselected each of them 50 cm2

3. Make 3 measurements in each fieldCalculate the mean value of these 3 pointsMake 3 measurements in each field and consider them as 1 measurement


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4. Accept criteria:

Only 20% of the total number of singlemeasurements is allowed to be lower than the nominal dry coating thicknessAnd the lowest value from a single measurement shall be at least 80% of the nominal coating thickness




Calibration of DELTASCOPE MP 3:

1. Select calibration foils acc. to thickness in question

2. Select an uncoated substrate specimen representative of the coated specimen

3. Move probe away from any metal and press

4. Place the probe min. 5 times on the uncoated specimen until the value is stable

CAL


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5. Press and the instrument resets to 0

6. Place the lowest thickness foil (STD 1) overthe uncoated specimen and measure min. 5 times.Delete any obvious invalid result.

7. Press or until the displayed value correspondswith the value of the calibration foil

8. Press

ENTER

ENTER




9. Repeat step 6 to 7 for the second and third calibration standard to complete the calibration or end by pressing

10. Check calibration by measuring a calibration foil of known thickness

ENTER



APPENDIX A28

Dye Penetrant Inspection System


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Dye Penetrant Testing

NDT – Steel


NDT – Steel


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3SlideDye Penetrant Testing - 14 February, 2006

Theory

The method is used to detect surface open cracks

It is normally used on non ferromagnetic objects

It comes in 2 versions: Visible dye penetrant and

Fluorescent dye penetrant

Visible dye penetrant and



Theory

Dye Penetrant can be used on:

Metals (aluminium - copper - steel - titanium etc.

Glass

Many ceramic materials


Rubber - plastics


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Theory

Can be applied with a spray can

or


by dipping

or

with a cotton swab


Theory


Is used to find:

Cracks

Overload and impact fractures

Porosity

Defects in welds

Detectable crack width > 0.001 mm


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Theory

The basic principle is the capillary effect

It is the ability of a liquid to climb in small openings by itself

Trees are using this effect to bring water upwards



Theory

There are different ways of removing excess penetrant

Penetrants are classified according to that

Water washable

Solvent removable



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Theory


Contact angle andsurface tension

Droplet

Capillary tube

Liquid

Surface condition (roughness - cleanliness etc.)

Points to deal with:

Precleaning (probably the most important part)

Temperature (27° C - 49° C or 80° to 125° F)


NDT – Steel


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The most common way of carrying out a dye check is by spraying

The system consists of 3 cans:Cleaner Penetrant Developer




In order to check the system, a plate like this can be used

This area is for checking how difficult it is to remove excess penetrant from different surfaces

This area is for checking the sensitivity


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2. Cleaning of the surface. This is very critical. Free of oil, grease, water etc.







3. After cleaning and drying apply penetrant by spraying, brushing or immersing



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3. After cleaning and drying apply penetrant by spraying, brushing or immersing

4. Dwell time. The time the penetrant is in contact with the specimen. See recommendation from the producer




5. Remove excess penetrant

6. Apply developer



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Report formula




APPENDIX A29



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NDT – Steel


NDT – Steel


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3SlideMagnetic Particle Testing - 14 February, 2006

weld

Theory

The basic principle is that

to e.g. ayou apply a magnetic field

weld



Theory

The material to be inspected with magnetic fields must be ferromagnetic

The method can be used both underwater and above



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Theory

If you have a bar magnet you also have a north and south pole

A crack in this magnetwill form a new north and south pole



Theory

If you magnetize a component and add ironparticles the particles will be attracted to the area where a new north and south pole is created



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Theory

Most materials can be classified as either:

Diamagnetic metals like: Gold, silver etc.

Paramagnetic metals like: Magnesium, lithium etc.

Ferromagnetic metals like: Iron, nickel etc.



Theory

Field orientation and flaw detectability

There are 2 general types of fields:

Longitudinal field Circular field

Detectable crack width > 0.001 mm



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Theory

An orientation of 45° to 90° between the field and the defect is necessary

Normally a specimen is magnetized in 2 directions at right angles to each other



Theory

The most common way of creating a magnetic field is the use of a yoke



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Theory

Another way is the use of a coil



NDT – Steel


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Magnetic fields can be created in a lot of different ways

Yoke

Portable Unit

Stationary

etc:




Ultraviolet light Pie gage

Magnetic Indicator strips



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Dry magnetic particles

Wet magnetic particles


Applications and Limitations

NDT - Steel


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Testing Practices

Dry particle inspection

Dry particle inspectionis well suited on rough surfaces, subsurface cracks and on hot specimens

The surface must be free of loose dirt,paint , rust or scale



Testing Practices

Wet suspension Inspection

Wet magnetic particleis well suited for very smalldefects on smooth surfaces

The liquid carrier gives good mobility for the particles



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Process Control

PIE gage shows direction of defects

It does not tell you if the field strength is adequate



Process Control

Hall Effects Gauss meterMeasures the field strength.

The measurement is done with the magnetic lines at right angles to the sensing element



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Process Control

Pear-shaped tube (Sutherland bottle)

This bottle is used to checkthe concentration of the particles

The volume of settledparticles varies depending on the type of particles



Process Control

Lightning is checked ona radiometer



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Planning and Execution


2. Apply magnetizing force

3. Apply suspension

4. Allow particles to flow

5. Inspect for indications

6. Recording of indications



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Interpretation and reporting of Results


Report formula




APPENDIX A30

Strain Gauging


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Strain Gauge

NDT – Steel

2SlideStrain gauge - 24 February, 2006

Introduction

Strain gauge is used for measuringstress and deformation in structures


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Evaluation of structure strength (calibration of calculation model)

Evaluation of loading impact:- Speed restrictions- Locked bearings- Increased loading

Measurement of fatigue risk at high stressed areas


Agenda






6. References


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Strain Gauge


Theory, general Principe


Tension will increase the length and reduce the cross section area

The electrical resistance is proportional to length and inverse proportional to cross section area

Tension will increase the electrical resistance


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Theory


Principle:

A strain gauge is glued onto the object to be measured

The gauge will follow the deformation of the structure

The electrical resistance in the gauge is then measured


Theory


Principle:

Long thin wire glued onto the object to be measured

Typically constantan wire due to small resistance changes with temperature

Typically resistance of 100-1000 Ω is chosen


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Measuring Principle, direct measurement

Measuring problem:

Small variations on a largesignal


Ω

Time


Theory, accuracy


Total stress variations corresponds to deformationchanges less than 0.1%

Total resistance changes: less than 0,1%

Accuracy of resistance measurement: 0.05%

Accuracy required: better than 5% of max. stress

Laboratory conditions + expensive equipment


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Measuring Principle, Outbalancing basic signal

The Wheatstone bridge outbalances the basic resistance of the gage

Direct measurements of the changes in resistance

Simple and precise measurements


+

-

V

Time


Measuring Principle, Temperature compensation

Temperature compensation:

- Strain Gauge material (constantan) with little temperature variations )

- Unstressed strain gauge in the same part of the Wheatstone bridge


+

-


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Temperature compensation:

- Strain Gauge material (constantan) with little temperature variations )

- Unstressed strain gauge in the same part of the Wheatstone bridge




Bending load, - Strain Gauge in tension side

- Strain Gauge in compression side

- Both strain gauges in the same side of the Wheatstone bridge

- Temperature compensation

- Only bending strain is measured

- Signal amplification factor: 2



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What is measured, Measuring Principe

General:

-Only changes in stress are measured

- Linear strain/stress behaviour

-Non-linear stress/loading behaviour can indicate plastic deformations (risk of fatigue)

-Measurement of axial, bending and torsion stress.




Measuring system:

-Measuring strain gauge

-Temperature compensating strain gauge

- Pre amplifier

- Signal filter

- Amplifier

-Data storage



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Short term (continuous) measurement:

- Test loading

- Typical loading

- Short term temperature variations

Long term measurement (monitoring):

- Fatigue development

- Loading conditions (movement of tracks

- Long term temperature variations


Only stress (strain) variations are measured


Testing of residual stress

1 Strain gauge is mounted on the structure

2 Strain gauge resistance is measured

3 The area around the strain gauge is cut free, without introducing new stresses

4 The area with the strain gauge is now without stress

5 Strain gauge resistance is now measured

6 Residual stress level is the difference between the 2 levels



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Measuring Principe, Concrete

Local variations of stiffness makes longer measuring distance necessary

Cracks should be avoided



Measuring Principe, Concrete Reinforcement

Strain Gauges are mounted direct on the reinforcement prior to concrete casting.

Strain gauges can be mounted on existing structures



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Measuring Principe, Masonry

Risk of high difference in stiffness between joint and stone:

Measurement on a single stone

Determination of modulus of elasticity from laboratory measurement or estimation from Smith hammer or ultrasonic velocity


High stressed Local highcontact points stressed joint

Risk of non-uniform stress-distribution stone/joint


Measuring Principe, Concrete/Masonry

Tensioned vibrating wire system:

- A tensioned wire, when plucked, vibrates at a frequency that is proportional to the strain in the wire

- The wire is held in tension between two end flanges.

- Loading of the structure changes the distance between the two flanges and results in a change in the tension of the wire.

- An electromagnet is used to pluck the wire and measure the frequency of vibration.

- Strain is calculated by applying calibration factors to the frequency measurement



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Aim of measurements

Security, high stress from:- Undocumented drawings- Unintended loading- Poor structural design- Poor structural integrity- Poor construction - Deterioration (corrosion)- Poor repair/strengthening

Function:- Increased loading capacity

due to refined modeling ofstructural behavior

- Monitoring for postponingrepair




Reduced loading capacity due to- Poor stiffness of joints- Geometrical deviations in the construction- Geometrical deviations in foundation, abutment - Geometrical deviations in the tracks- Softening from fatigue- Blocked bearings



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Accuracy


Under favorable conditions accuracy can be very high (better than 0,1% of ultimate stress). High precision weights are usually based on strain gauge measurements.

Under usual conditions accuracy of 1-5% of ultimate stress is possible.

Accuracy can be improved by precise test loading

Note:Only stress variations are measured, not the total stress level



The main problem of determination of stress levelfrom strain gauge is the determination of residualstress since, strain gauges only measures stressvariations.

Generally strain gauge measurements are precise,but the measurements are influenced by:

- Slippage in strain gauge bond

- Incorrect placing or direction of gauge

- Temperature

- Electrical noise

- Non-elastic behaviour



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Strain gauge


Common Applications, general

Strain gages can measure how the structure reacts to short time loading



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Common Applications

Applications

Measurements can be done on e.g.

Rotating parts

Bridges

Offshore structures

Pressure vessels

Cranes

Concrete structures



The method is usually used at areas exposed to - Axial load- Bending moment- Torsion- Local high stressed areas



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Common Applications

Increasing weight and speed limits:- More accurate determination of loading capacity- More accurate determination of loading impact

Evaluation of cause and importance of damage:- More precise precautions for preventing further damage- Only repair of serious damage- More precise guidelines for repair/strengthening

General monitoring for:- Postponing of repair - Unusual constructions



Common Applications

Applications

Strain gauges are often used on new structures

During design phase computations are madeto determine the strength of certain sections

After the structure is erected, strain gauges are placed at critical points and monitored


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Common Applications

The Great Belt Bridge in Denmark is constantlymonitored with strain gauges

The results are transferred wireless to a comp.

Applications


Common Applications – Evaluation of cracks

Strain gauges can measure localstress near the crack tip

From simultaneously measurements of relevant factors:

- Train speed

- Train load

- Temperature, Wind speed, etc.

The worst situations can be evaluated and more preciseprecautions can be taken



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Common Applications – Noise reduction in acoustic emission monitoring

Acoustic emission counts are prone to noise.

Relevant emission counts only occurs in high stress-situations

Strain gages can measure periods of high stress

By only counting emission in high stress situations the noise problem is reduced




General: The well defined loading on railway bridges improves the use of strain gauge measurements

Testing of concrete bridges

Testing of masonry bridges

Testing of plastic behaviour

Limitations:

Temperature influence must be controlled

Access to surface is necessary

Place for application must be well defined

Residual stress must be evaluated



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Case 1: The Masnedsund Bridge, Denmark




Total length app. 185m (5 spans of 31.5m and 1 bascule span of 28.4m)


During a routine inspection cracks were observed at 14 locations in the cross beams at the joints between the main girders and the cross beams



Structural Assessment

Fatigue Analysis – Finite Element Model

Identification of the cause of damage (cracks)

Strain Gauge Measurements

Verification/calibration of the model



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Fatigue analysis was carried out using a full 3D Finite Element Model in the program LUSAS.




Identification of the cause of damage (cracks):

Large stresses and

Inappropriate design of detail

There is no connection between the bottom flange of the cross girder and the bottom flange of the main girder

The bottom flange of the cross girder is sharply disrupted before reaching the bottom flange of the main girder


Crack

Joint between main girder and cross girder at the end of a span.


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Purpose of strain gauge measurements:

Calibration of the finite element model

Ensuring that the level of the actual stresses matches the level of the calculated stresses

Verifying the hypothesis of the cause of damage

Execution of strain gauge measurements:

Strain gauges were mounted at the structure at selected locations

A thorough registration of the trains passing the bridge was carried out

Measurements of strains were transformed to stresses for different types of trains passing the bridge




Locations for strain gauge measurements:

Main girders (verify the global load effects)

Cross beams (verify the global load effects)

Railway girders (local load effects on the railway girders)

Cross beams at the end of the spans where cracking has occurred and in the mid of the spans where no cracking has been observed. (By these measurements the hypothesis of the cause of damage could be verified or rejected)

Wind lattice (the effect of the wind lattice on the development of cracks could be analysed)



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The calculated stresses in critical elements were calibrated with results from strain gauge measurements. The plot shows the calculated stresses versus the measured stresses at a specific points of the structure as a function of time.


Str

ess

[MPa]

Measurement CalculationTime [s]



Conclusion:

A good correlation between the strain gauge measurements and the calculations were found.

The Finite Element model was verified.

The hypothesis regarding the cause of damage was verified.

The case showed that cracking is typically not only caused by stresses exceeding the calculated capacity but the combination of large stresses and inappropriate design.



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Case 2: The Forsmo Bridge, Sweden


Riveted steel truss arch bridge from 1912

Single railway line

Total length app. 263m (a main span of 104m length and 50m height, two side spans of 58.5m and one approach span of 42m

As part of a larger program the Swedish National Rail Administration wanted to upgrade the Forsmo Bridge for freight trains with a 25 tonne axle load




Structural Assessment:

A full 3D Finite Element model was established in the LUSAS-program

Strain gauge measurements were carried out to evaluated the interaction between the global and local elements and the stiffness of joints.


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Strain gauge measurements - execution:

Challenges:

24 hours continuous service of the bridge

Limited access to the relevant bridge elements

Typical sections of upper superstructure of cross beams (CB) and longitudinal beams (LB)





Focus on the main issues:

Verification of the global distribution of forces in the primary structure

Distribution of normal forces between the U-elements and the longitudinal beams

Transverse bending in cross beams and distribution of stresses between upper and lower flanges.

Distribution of primary bending moments in longitudinal beams


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A total of 49 strain gauges were mounted at selected locations of the bridge

The results from the strain gauge measurements were plotted against the calculated values

Due to differences between the measured values and the calculated values the finite element model was refined by refining some geometric data

The refinement was carried out several times to improve the model




Strain gauge measurements - results:

By adjusting the model to make the calculated stresses match the measured stresses the modelling of the interaction between the global and local effects improved significantly

By the refining process the stresses in some elements changed by 100% from their initial value

The strain gauge measurements proved to be essential in modelling the interaction between the global and local effects


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Conclusion:

The calculation proved the primary superstructure to have sufficient capacity for a 30 tonne axle load

For the upper superstructure a 25 tonne axle load was possible with a formal fatigue lifetime of 10-30 years if the structure was modified to include extra bearings and modified joints

It was decided to replace the upper superstructure to gain a service lifetime of 100 years for a 30 tonne axle load


Case 2: The Forsmo Bridge, References

Enevoldsen, I., et. al., “Updating finite element models based on site strain measurements for assessments of the Forsmo Bridge”, Structural Engineering International, Nov., 2002.


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The combined measurement of strain gauge and traffic loadis expected to give information on:• Areas which must be repaired/strengthened

• Is repair of cracks and other fatigue related symptoms necessary

• Is preventive precautions relevant on areas with high risk of fatigue

• Can restrictions on speed or load capacity prevent further damage

• Can restrictions on speed or load capacity be lifted

• Can repair be postponed

• Effectiveness of pilot projects for repair/strengthening

Evaluation of fatigue related problems must be combined with structural

analysis



Strain Gauge


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Test Planning


- General level of fatigue risk- General high-risk areas- High risk elements


- Signs of fatigue related symptoms

- Structural flaws- Corrosion

(Spplementary crack detection)- Dye penetrate etc.


XY

Z


Test Planning

3. Selection of Test AreasAnalysis of strain gauge measurements requires- Detailed preparation of inspected areas- Use of expensive equipment. To optimize the value of the measurements selection of the areas testedshould be selected on the basis of detailed structural analysis

4. Determination of stress directionsCorrect alignment of strain gauge direction according to desired stress direction isvital


Strain gauges only measure stress variations. To evaluate the total stress level residual stresses must be evaluated


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Test Planning

- Grinding of the surface

- Precise alignment of strain gauge

- Mounting of strain gauge

- Protection of strain gauge

- Wiring

- Shelter for amplifier and registration unit

- Power, eventually battery driven

Registration of loading and temperature


5. Establishing the measuring points




- Length of the monitoring period (1 h to 1 year)- Collecting data- Test of influence from noise (temperature, noise)- Calibration from test loading



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Planning




4. Calculation of stress direction

5. Establishing of the measuring points

Execution




Strain gauge


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Calibration:- Gauge factor from manufacture- Supplementary measurements on well defined

structural elements- Supplementary measurements adjacent structure

elements to inspect integrity of calculation model- Test of temperature influence on periods without

traffic- Residual stress from structural analysis

Reliability- Generally good reliability, but deterioration in strain

gaugeconnection can cause to low readings

- Long term stability of stress levels is not precise





Identification of stress concentrations due to poor design

Identification of increased stress due to unintended structural behavior:

- Blocked bearings

- Slippage in joints

Identification of increased stress due to increased loading:

- Movement of foundations

- Poor alignment on bridge approaches

- Incorrect track line

- Incorrect track movement (too little, too much)

Identification of increased stress due to corrosion

Identification of increased stress due to overloading-induced cracks


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Report:

General conclusions

Eventually main statistics




Appendix:- Measured values, including

time of registration- Equipment used- Calibration - Cumulative plots- Placing of measure points- Correlation to calculated

stress levels



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Damage

X

X

X

X

X

X

X

Crack d

etection

X(x)XXXXX(x)XX(X)(Freeze-thaw)

(x)XXXXXX(x)XX(X)ASR



(x)XXXXChloride penetration

(x)(x)XXXXXCarbonation

XXXXX(X)Corrosion

(Air vo

id)

ASR

reactivity

Macro

/Micro

analyses

Cores

Break u

p

Gro

und p

enetratio

n rad

ar

Impulse resp

onse

Impact E

cho

Half cell p

oten

tial &

corro

sion rate

Chlo

ride co

nten

ts

Spra

ying in

dica

tors

Cover m

eter

Bond-test/Pu

ll-off

CAPO

-test

Sch

mid

t ham

mer

Boro

scope

Stra

in g

auge

NDT-Method



APPENDIX A31

Introduction to Rehabilitation of Concrete, Steel and Masonry Bridges


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Introduction to rehabilitation

Concrete, steel and masonry bridges

2SlideIntroduction to rehabilitation

Agenda

1. General aspects and considerations

2. Cases

I. Concrete bridge: “Avedore havnevej”

II. Steel riveted bridge: “Masnedsund Bridge”

III. Masonry Bridge: “Moellevej”

3. General introduction to laboratory testing of steel

4. General description of an alternative: Cathodic protection


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1. General aspects and considerations

Concrete, steel and masonry


A. Preventive Actions

B. Corrective Actions

C. Common Repair Strategies

D. Rehabilitations

E. Selecting the Optimal Strategy

Outline


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A. Preventive Actions - Overview


B. Corrective Actions - Overview


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Typically the following repair strategies are evaluated:

Strategy A: – Limited / temporary repair– Continuous minor repairs when damage occur– E.g. including preventive maintenance of non-damaged areas.

Strategy B: – Thorough repair of the bridge component at the time where the entire

component is damaged.– Supplementary preventive maintenance may be carried out with the

purpose of delaying the development in damage.

Strategy C: – Do nothing now. When the structure is no longer safe, replace it.

C. Common Repair Strategies - Overview



Strategy A:

Very small improvement in condition

Often general reduction of condition over the years

Frequent repairs going on


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Strategy B:

Marked improvement in condition

Optimal time of must be considered


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B. Common Repair Strategies - Overview

Strategy C:

Structure can to some degree be adjusted to traffic

Finding the optimal time of is of high importance

Can be combined with strategy A


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Example of strategy C: Skovdiget

Monitoring of condition is critical topostponing of repair

Inspection cost of 1 mill$/year is acceptable



Protected bridge components:

– Local repairs of the “protection” (water proofing, surface treatment, cover etc.) where it is defect and where damage has developed.

– Total replacement of the “protection” when it is defect and when damage has developed in large areas.

– Replacement of the bridge component when it is no longer safe.

D. Rehabilitations – in general


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Chloride contaminated non-protected bridge components (e.g.

piers, columns, retaining walls and edge beams)

Traditional solutions:

– Preventive maintenance before initiating of damage – e.g. surface treatment.

– Repair of concrete cover (is typically carried out when corrosion of the reinforcement is initiated and is developed at the outer layer of reinforcement but not at the main reinforcement).

– Repairs behind the main reinforcement.

– Replacement of bride component.

Other solutions:

– E.g. corrosion: Cathodic protection and chloride extraction.

D. Rehabilitations - Concrete


Bridge components containing reactive aggregates (ASR) or with insufficient amount of air voids

Traditional solutions:

– Preventive maintenance before development of damage e.g. surface treatment (to stop moisture and chloride ingress).

– Local repairs (these might be combined with surface treatment).

– Replacement of the bridge components.

D. Rehabilitations - Concrete


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For each of the strategies all direct costs for rehabilitation is estimated for a period of e.g. 50 years.

The optimal time of rehabilitation is usually right before a jump in the curve of damage development / deterioration.

For each strategy the net present value is calculated – the cheapest strategy is the one with the lowest net present value.

E. Selecting the Optimal Strategy - Overview

Cost of repair

Time

Typical development of the direct cost of repair.

However, traffic needs can have very strong influence on the optimal strategy.

2. Cases

Concrete bridge: “Avedore havnevej”Steel riveted bridge: “Masnedsund Bridge”Masonry Bridge: “Moellevej”


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Case: Avedoere Havnevej bridge

Main rehabilitation, 2005-2006


The Avedoere Havnevej bridgeOutline

1. Description of the bridge structure

2. Condition of the structure

3. Damage mechanisms and causes

4. Rehabilitation project

5. Road layout improvement project

6. Traffic project during rehabilitation works

7. Cables


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Bridge 11-0012

Overview plan

Bridge constructed in 1965Span: app. 60 m, Width app. 40 m, 7 lanes + bicycle tracks and pavementsBridge deck of app. 2400 m2.

Situated in western Copenhagen

Avedøre Havnevej

Avedøre Havnevej

MotorwayM11

MotorwayM11

1. Description of the structure


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Cross sectionConstruction Joint

Edge beam

Elevation


Underpassing road:

Motorway M116 lanes

Morning traffic to central Copenhagen

Afternoon traffic to Mid-Sealand


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Overpassing road:

Avedoere Havnevej7 lanes on the bridge plus bicycle tracks and pavements

Part of road ring O2 around Copenhagen


2. Condition of the structure

Lower side of the bridge deck


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Bridge deck - soffitSpalling concrete


Bridge deck - soffitCracks


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Bridge deck - soffitLocal areas with corrosion depositsPossible leaking


Bridge deck - soffitConstruction joint


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Frontage and edge beams


Upper side of edge beam and railings

Water is not drained away properly


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Poor drainage conditions

Cracks and settlements at bridge endings

Upper side of bridge deck


Abutments


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3. Damage mechanisms and causes

Mechanisms – hypothesis:

The mechanisms involved are probably a combination of frost attacks and chloride penetration

Bridge deck lower side: Progressing chloride induced corrosion

of reinforcement

Frontages of bridge: Progressing chloride induced corrosion of

reinforcement


Damage causes - combined

Environment

Salting

Weather conditions (rain, freezing/thawing)

Traffic

Heavy traffic load

Heavy vehicles; stopping and turning

Construction

Permeable layers (gravel) in pavements and bicycle tracks

Very limited surface sloping in both directions

Design of construction joint

Materials

Concrete from period when highly reactive aggregate materials were used

Conclusion

Leaching membrane water + chloride impact on bridge deck

Leaching in construction joint


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4. Rehabilitation project

Expected extent of repair works:

Full replacement of waterproofing on bridge deck

Replacement of edge beams – railing replaced by crash barriers

Concrete repair works

Replacement of chloride containing concrete in the bridge deck (upper and lower sides)

Replacement of carbonated concrete

Removal of loose concrete in the lower side of bridge deck

Casting of top concrete slab for improvement of surface sloping

Replacement of permeable layers in the pavement – new bituminous surfacing


5. Carrying out and supervision

Bridge deck


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Breaking up the pavement


Removing the concrete cover (chloride-contaminated)


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Removal of honeycombs and other casting defects

Concrete repair works on the bridge deck


Anchoring and reinforcement


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Reinforcement and formwork


Reinforcement and cables


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Casting


Hardening


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Epoxy coating


Waterproofing bituminous membrane


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Testing of the membrane


Membrane – second layer Draining stripe


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Asphalt surfacing

Membrane overlay for next stage


Replacement of the edge beams – mounting of scaffolding and platform


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Breaking up the edge beams


Handling the existing reinforcement


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New reinforcement, different design of the new beam.


Formwork

Connecting reinforcement to the new slab


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Casting

Wires for temperature measurement


Finishing and mounting of crash barriers

New crash barrier


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Concrete repair works on the bridge ending


6. Traffic project during rehabilitation works

Specifications for the temporary traffic project

All roads and motorway approaches and exits must be kept open at all times

Two straight lanes in each direction of Avedoere Havnevej at all times

Left turning lanes on the bridge to be kept open at all times

Bicycle track on both sides of the road

Footpath on the eastern side of the bridge

Extra queuing in the working period must be minimized

Stages of the rehabilitation works must, added up, cover all of the bridge deck

Changing of road layout and the bridge rehabilitation works must be

carried out simultaneously – without any significant interference of the

traffic flow


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Preparation stage


Stage 1 – Repair of west side of the bridge


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Stage 2 – Repair of east side of the bridge


Stage 3 – Repair of the mid section


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Stage 4 – Bicycle tracks and pavements


Finishing stage – traffic islands


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7. Cables

Large amount of cables in the area

Coordination between contractor and cable owners is vital both on an off the bridge


Placement of cables in the concrete slab

Power cable, 50kV withprotection profile

Pipe for power controller cable

Pipe for power cable, 10kV

Empty pipes forfuture use

Pipes forlighting cables Pipes for

signaling cablesPipe for opticaltelecom cables


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Cables


Cables


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Case: The Masnedsund Bridge

Inspection and fatigue assessment/rehabilitation


A. Introduction

B. Activity program

C. Phase 1 – fatigue analysis


E. Phase 2 – Strengthening


The Masnedsund BridgeOutline


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Total length app. 185 m (5 spans of 31.5 m and 1 bascule span of 28.4 m)


A. Introduction


A. Introduction

During a routine inspection cracks were observed in the cross beams at the joints between the main girders and the cross beams (14 positions)


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B. Activity program

1. Structural fatigue analysisIdentification of critical jointsFatigue analysis of joints identified as being criticalProgram of further inspections

2. Strengthening projectIdentification of the cause of the damages (observed cracks)Pilot projectFull scale strengthening project

3. Inspection program with respect to fatigue cracksInspections in the period before strengtheningInspections after strengthening

Based on the observations of cracks the following program was setup:


E. Phase 2 – problem to be repaired


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F. Phase 3 – Inspection program for detection of fatigue cracks after strengthening








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Case: Masonry Bridge - Moellevej

Inspection of cracking and rehabilitation


A. Introduction

B. Special inspection

C. Monitoring

D. Rehabilitation

Masonry Bridge - MoellevejOutline


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Masonry bridge from 1854

Road over bridge

Total length app. 33 m (3 arch spans with a free width of app. 5 m).

Width app. 6 m.

The arches are constructed of masonry bricks and the facades are made of granite

A. Introduction


Observation of coarse cracking parallel to the façade in all three arches approximately 0.3 m from the façade – monitoring of the cracks is suggested.

Beak-ups to the filling material from the top side of the bridge.

B. Special Inspection


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Structural calculations – the capacity of the bridge does not fulfill the demands of the specified class.

Steel testing and calculations of the steel barriers – the steel barriers does not fulfill the demands to function as crash barriers.

Conclusion: Replacement of the crash barriers for safety reasons. This also includes strengthening of the facades.

B. Special Inspection


In the period from the special inspection to the strengthening the cracks were monitored.

Demec pins were placed at three locations.

Initial measurements were carried out.

C. Monitoring


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C. Monitoring


A new reinforced concrete plate was casted on top of the arches to secure the forces horizontal from the crash barrier.

At the same time the concrete plate stiffens the bridge and secures the facades from opening up.

D. Rehabilitation


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D. Rehabilitation


Challenge:

The bridge is a historical monument and thus no changes of the exterior of the bridge must be made!

Creative engineering is needed.

D. Rehabilitation


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D. Rehabilitation


D. Rehabilitation

Hole for anchorage of barrier

Breaking up the bridge deck


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D. Rehabilitation

Job finished!


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4. General description of an alternative

Cathodic protection


Cathodic protectionPurpose and principle

Purpose:

• Reduce or stop ongoing corrosion in the reinforcement

• Prevent corrosion

Principle:

• Corrosion is a electrochemical process leading to dissolvement of the reinforcement

• The corrosion are stopped by sending a larger current in the opposite direction


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Cathodic protectionPurpose and principle


Cathodic protectionBenefits

Traditional main benefits:

• Extent the service lifetime for existing structures

• Reduce directs and indirect costs to repairs

• Increase effectiveness of repairs

• Increase safety for existing structures

• Short period of rehabilitation


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Principe for use and applications

Water:

Piles (steel, concrete)

Sheet piling

Piers

Ground:

Retaining walls

Ground anchors

Tanks

Structure material (concrete, masonry):

Reinforcement

Embedded steel beams

Anchors



Sacrificial anodes:

Low cost

No maintenance

Supplementary character

Not well documented


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Cathodic ProtectionPiles in water, Sheet piling

Water anodes:

Simple, effective and robust

Some protection above water level

Very small cost for installation and maintenance



Anodes embedded in concrete:

Impressed current

High cost for installation and maintenance

High cost reductions from reduced need for repari

Short range


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Corrosion in concrete reinforcement:By early detection from half cell measurement corrosion are stopped by cathodic protection and time consuming brake ups are avoided

Half cell measurement

Repair by brake ups

Repair by cathodic protection

Anodes



Ground Anodes:Impressed currentLong rangeeffectiveModerate cost for installation and maintenance

Ground anode



APPENDIX A32

Introduction to Laboratory Tests of Steel


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3. Introduction to Steel Laboratory Testing

Steel Structures


Steel Laboratory TestingOverview

Laboratory analysis:Fracture toughness


Microanalysis on polished surface

Tensile strength

Corrosion type

Welding

Delaminaition

Vickers Hardness

Intergranular composition


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Steel Laboratory TestingApplication of Laboratory analysis:

Evaluation of parameters with importance to corrosion:


- Stainless steel

- Risk of hydrogen induced cracking

Microanalysis on polished surfaces

- Registration of cracks

- Intergranular composition

Intergranular



Evaluation of parameters with importance to structural design:

Tensile strength:

- Direct testing

- Vickers Hardness

Microanalysis on polished surfaces:

- Delaminaition

- Intergranularcomposition

Fracture toughness



Ageing



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Evaluation of parameters with importance to fatigue:


- Stainless steel

- Risk of hydrogen induced cracking


- Registration of cracks and flaws


- Delaminaition



Evaluation of parameters with importance to welding:



- Registration of integrity




APPENDIX B

Template for Extended Principal Inspection Report

Central Railway

Bridge Id and name Extended Principal Inspection of Selected Bridge Components

March 2006

Rambøll Denmark A/S Bredevej 2 DK-2830 Virum Denmark Phone +45 4598 6000 www.ramboll.dk

Extended Principal Inspection of Selected Bridge Components

March 2006 Ref 5721063-07_L001_Ver2_Report_Template_Ex_princ_2005.doc Version 2 Date 2006-02-24 Prepared by MDTJ / LTP Checked by FNJ Approved by FNJ

Central Railway

Bridge Id and Name

Ref. 5721063-07_L001_Ver2_Report_Template_Ex_princ_2005.doc

Table of contents

1. Summary 1

2. Motivation of the extended principal inspection 1

3. Background Documents 1 3.1 List of Background Material 1

4. Registrations 1 4.1 Registration Overview 2 4.2 Visual Investigation 2 4.3 NDT-Method no. 1 2 4.3.1 Result Summery 3 4.3.2 On-site calibration 3 4.4 NDT-Method no. 2 3 4.4.1 Result Summery 3 4.4.2 On-site calibration 3

5. Evaluation of registrations 3 5.1 Interpretation of the results from NDT-method no. 1 3 5.2 Interpretation of the results from NDT-method no. 2 3 5.3 Cause, extent and location of damage 4 5.3.1 Bridge Component No. 1 4 5.3.2 Bridge Component No. 2 4 5.4 Condition Rating 4

6. General considerations regarding future maintenance activities 4

Appendices A Background Material B Selected Drawings C Visual Inspection D NDT-Method No. 1 E NDT-Method No. 2 etc.

Central Railways Bridge Id and bridge name Extended principal inspection of selected bridge components


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1. Summary

The summary must contain all relevant information from the other chapters in a short form. This chapter must include a comprehensive overview of the registrations and conclusions on the damage to the bridge. It must comprise description of the extent of registrations, conclusions on cause and extent of damage, and recommen-dation for rehabilitations and further activities. However, the summary should not be more than 1-2 pages in length.

2. Motivation of the extended principal inspection

This chapter describes why and by whom the inspection is initiated. It tells which bridge components are the objects of NDT-inspections and which visible damage has been registered.

3. Background Documents

3.1 List of Background Material This section lists the background material that has been available for the inspection, such as:

• Inventory report and previous relevant inspection reports.




• Materials specifications for steel, concrete, masonry etc.


4. Registrations

This chapter describes the registrations from the inspection. For each of the test methods used, the extent and location is described, and a summary of the results is given. The detailed record of all registrations is enclosed in the appendices.



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4.1 Registration Overview This section contains a relevant photo and a table listing the investigation methods which have been used.

Figure 1: Caption text.

Investigation Method

Purpose Quantity

Visual To form a general view of the bridge/test ar-eas. Condition rating of the bridge elements.

All accessible areas.

Impulse Response

Identification of de-lamination etc..

X grids App. Y m2.

Table 1: List of investigations carried out in this extended principal inspection.

4.2 Visual Investigation Describe the general condition and the condition of the investigated bridge compo-nents based on visual assessments.

Choose an informative photo


4.3 NDT-Method no. 1 Give a short introduction stating when, where and why the NDT-method has been used.



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4.3.1 Result Summery

Description of the registrations from the NDT-method. The detailed information re-garding the results are to be enclosed in the appendices.

4.3.2 On-site calibration This section includes a description of the results from the calibration of the NDT-method (if any). For instance for HCP-measurements this section includes a descrip-tion of the registrations from the break-ups made to calibrate the measurements.







5. Evaluation of registrations

In this chapter the inspection engineer describes the probable deterioration mecha-nisms and causes of damage based on the registrations. The chapter must also in-clude an estimate of the actual damage of the bridge components investigated.

5.1 Interpretation of the results from NDT-method no. 1 This section includes an interpretation of the test results from NDT-method no. 1. E.g. for HCP-measurements: do the measurements show areas of corrosion of the reinforcement.

5.2 Interpretation of the results from NDT-method no. 2 This section includes an interpretation of the test results from NDT-method no. 2. E.g. for chloride-measurements: do the measurements show risk of chloride initiated



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corrosion of the reinforcement – are the values of the chloride content larger than the critical chloride content at the depth of reinforcement.

5.3 Cause, extent and location of damage The damage mechanism should be described in detail. This means that in cases of corrosion, 'saline soil' is not sufficient as explanation. It must also be explained where the water comes from, how the chlorides have reached the reinforcement, etc. In the areas of NDT-measurements the extent of damage based on the NDT-measurements is described. Thus, this section includes a summary of the interpreta-tions of the test results from all the NDT-methods used in the inspection and the registrations from the visual inspection.

5.3.1 Bridge Component No. 1

5.3.2 Bridge Component No. 2

5.4 Condition Rating Each inspected element is given a condition rating. A brief motivation for each ele-ment rating is given based on the previous sections. E.g. bridge deck, piers etc..

Based on the registrations from this inspection a condition rating of each bridge component has been made. The condition rating is a number of 1 to 6 and is based on the following guidelines:

1: A condition which warrants rebuilding / rehabilitation immediately. 2: A condition which requires rebuilding / rehabilitation on a programmed basis. 3: A condition which requires major / special repairs. 4: A condition which requires routine maintenance. 5: A sound condition. 6: Not applicable. 0: Not inspected.

6. General considerations regarding future maintenance activities

This chapter describes the inspection engineer's recommendation of future activities. The need for major rehabilitation jobs and further inspections is included in this chapter. The description does not include budgets for the activities. If there is any doubt of the carrying capacity of the bridge recommendation of calcu-lations must be included in this chapter.



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Based on the condition rating, the results from the NDT-investigations and the dam-age type, extent and cause on the selected bridge components a recommendation of an economic analyse must be made in order to select the optimal / best maintenance strategy for the bridge.



APPENDIX A

Background Material



Appendix A page 1

This appendix includes the inventory of the bridge, the previous principal inspection report (if any), previous extended principal inspection reports and special inspection reports regarding the bride components chosen for NDT-investigations.



APPENDIX B

Selected Drawings



Appendix B page 1

This appendix includes selected drawings of the bridge itself and of the bridge com-ponents of which the NDT-investigations are carried out.



APPENDIX C

Visual Inspection



Appendix C page 1

This appendix includes the registrations from the visual inspection of all the bridge components included in the inspection. General orientation of the bridge and the bridge components under investigation, numbering of elements and damage pattern are most conveniently shown on sketches.









Appendix C page 2

Bridge Id and bridge name

Project: Date: Inspector:

Subject:

Insert photo Insert photo

Photo 1: Photo 2:


Photo 3: Photo 4:


Photo 5: Photo 6:



APPENDIX D

NDT-method No. 1



Appendix D page 1

Depending on the complexity of the NDT-method a general description of the princi-ples of the method is described in this appendix. This appendix includes the registrations from one of the NDT-methods used in this extended principal inspection. The appendix should include sketches of the areas of measurements and of the measuring grid if used e.g. for HCP, Impact-Echo, Impulse Response (s’MASH) etc. The appendix should also include relevant photos related to the NDT-investigation (of break-ups etc.). Always note the dimensions of the bridge component in question. (Diameter of col-umn; width, depth, spacing and length of girders, etc.).




The template for this appendix is copied for all the NDT-methods used in this ex-tended principal inspection.



Appendix D page 2



Subject:

Insert photo

Photo 7:

Insert photo

Photo 8:


APPENDIX C

Template for Special Inspection Report

Central Railway

Bridge Id and name Special Inspection of Selected Bridge Components

March 2006

Rambøll Denmark A/S Bredevej 2 DK-2830 Virum Denmark Phone +45 4598 6000 www.ramboll.dk

Special Inspection of Selected Bridge Components

March 2006 Ref 5721063-07_L003_Ver2_SI_Report_Template.doc Version 2 Date 2006-02-24 Prepared by MDTJ / LTP Checked by FNJ Approved by FNJ

Central Railway

Bridge Id and Name

Ref. 5721063-07_L003_Ver2_SI_Report_Template.doc

Table of contents

1. Summary 1

2. Motivation of the special inspection 1

3. Background Documents 1 3.1 List of Background Material 1

4. Registrations 1 4.1 Registration Overview 2 4.2 Visual Investigation 2 4.3 Homogeneous areas and damage hypothesis 3 4.4 NDT-Method no. 1 3 4.4.1 Result Summery 3 4.4.2 On-site calibration 3 4.5 NDT-Method no. 2 3 4.5.1 Result Summery 3 4.5.2 On-site calibration 3

5. Evaluation of registrations 4 5.1 Interpretation of the results from NDT-method no. 1 4 5.2 Interpretation of the results from NDT-method no. 2 4 5.3 Cause, extent and location of damage 4 5.3.1 Bridge component no. 1 4 5.3.2 Bridge component no. 2 4

6. Repair Strategies 4 6.1 Strategy A – Limited/Temporary Repairing 5 6.2 Strategy B – Thorough Repair of Bridge component 5 6.3 Strategy C – Replacement of Bridge component 5

7. Recommendations of follow-up activities 5


Appendices A Background Material B Selected Drawings C Visual Inspection D NDT-Method no. 1 E Economic Analysis

Central Railway Bridge Id and bridge name Special inspection of selected bridge components


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1. Summary

The summary must contain all relevant information from the other chapters in a short form. This chapter must include a comprehensive overview of the registrations and conclusions on the damage to the bridge. It must comprise description of the extent of registrations, conclusions on cause and extent of damage, and the pro-posed repair strategy including cost estimate and time schedule. However, the sum-mary should not be more than 1-2 pages in length.

2. Motivation of the special inspection

This chapter describes why and by whom the inspection is initiated. It tells which bridge components are the objects of the inspection and which visible damage has been registered.

3. Background Documents

3.1 List of Background Material This section lists the background material that has been available for the inspection, such as:

• Inventory report and previous relevant inspection reports.




• Materials specifications for steel, concrete, masonry, etc.


4. Registrations

This chapter describes the registrations from the inspection. For each of the test methods used, the extent and location is described, and a summary of the results is given. The detailed record of all registrations is enclosed in the appendices.



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4.1 Registration Overview This section contains a relevant photo and a table listing the investigation methods which have been used.


Investigation Method

Purpose Quantity

Visual To form a general view of the bridge/test ar-eas. Condition rating of the bridge elements.

All accessible areas.

Impulse Response

Identification of de-lamination etc..

X grids App. Y m2.

Table 1: List of investigations carried out in this special inspection.

4.2 Visual Investigation Describe the general condition and the condition of the investigated bridge compo-nents based on visual assessments.





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4.3 Homogeneous areas and damage hypothesis On the basis of the visual inspection and prior knowledge the structure may be di-vided into homogeneous areas. A homogenous area is defined as an area where the parameters affecting the deterioration – and the deterioration itself – of the structure exhibits only a random variation. For each of the homogeneous areas a damage hypothesis is prepared. These hy-pothesis are described in this section.















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5. Evaluation of registrations

In this chapter the special inspection engineer describes the probable deterioration mechanisms and causes of damage based on the registrations. The chapter must also include an estimate of the actual damage of the bridge components investi-gated. It should also include a description of the expected development of damage if no action is taken. It is noted whether the hypothesis of the cause of damage is con-firmed or not.

5.1 Interpretation of the results from NDT-method no. 1 This section includes an interpretation of the test results from NDT-method no. 1. E.g. for HCP-measurements: do the measurements show areas of corrosion of the reinforcement.

5.2 Interpretation of the results from NDT-method no. 2 This section includes an interpretation of the test results from NDT-method no. 2. E.g. for chloride-measurements: do the measurements show risk of chloride initiated corrosion of the reinforcement – are the values of the chloride content larger than the critical chloride content at the depth of reinforcement.

5.3 Cause, extent and location of damage The damage mechanism should be described in detail. This means that in cases of corrosion, 'saline soil' is not sufficient as explanation. It must also be explained where the water comes from, how the chlorides have reached the reinforcement, etc. It is also important to explain the differences in damage appearance: Why are some columns damaged while others are undamaged, why is only the centre girder cracked, etc.

The extent of damage based on the NDT-measurements is described. Thus, this sec-tion includes a summary of the interpretations of the test results from all the NDT-methods used in the inspection and the registrations from the visual inspection.

5.3.1 Bridge component no. 1

5.3.2 Bridge component no. 2

6. Repair Strategies

This chapter describes the relevant repair strategies for the bridge.

The description of each strategy should comprise:



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• A general description of the 'idea' of the strategy, e.g. 'Replacement of the whole bridge', 'Interim repair, followed by major rehabilitation after 10 years'.

• List of all activities with year and cost estimate, e.g.:

Activity Year Cost

Interim repair of bridge deck 2005 5 mio. Rs.

Replacement of bridge deck 2015 45 mio. Rs.

If the contents and extent of the activities are not obvious, they should be detailed, e.g: 'Interim repair of deck comprises repair of honeycombs at 8 locations, and ce-ment mortar injection of approximately 50 meters of cracks'. 'Replacement of deck comprises replacement of the deck slab on the whole bridge, including expansion joints, edge beams, ballast and tracks. The existing girders are re-used'.

Description of possible disturbance to the traffic.

'Present value analysis' of the strategy, calculated following the 'present value method'.

Remember that 'doing nothing' may very well be one of the possible strategies. This strategy must be examined as well. This strategy will have no repair or maintenance costs, but it may imply severe inconvenience for the users of the railway.

6.1 Strategy A – Limited/Temporary Repairing Technical and economically description of the strategy.

6.2 Strategy B – Thorough Repair of Bridge component Technical and economically description of the strategy.

6.3 Strategy C – Replacement of Bridge component Technical and economically description of the strategy.

7. Recommendations of follow-up activities

This chapter describes the special inspection engineer's recommendation of future activities.

Normally the recommendation will be to carry out the repair strategy with the lowest present worth, as this should be the optimum thing to do.

However, in some cases the recommendation may be to carry out further, more de-tailed investigations, or to monitor the development of damage for some time before



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making final conclusions on the optimal repair of individual bridge components or the bridge as a whole.

If there is any doubt of the carrying capacity of the bridge recommendation of calcu-lations must be included in this chapter.



APPENDIX A

Background Material



Appendix A page 1

This appendix includes the inventory of the bridge, the previous principal inspection report (if any), previous extended principal inspection reports and special inspection reports regarding the bridge components chosen for NDT-investigations. As the special inspection provides a better knowledge of the damage to the bridge, and the damage may have developed since the previous principal inspection (if any), the report is revised, and possible corrections are made (in hand writing).



APPENDIX B

Selected Drawings



Appendix B page 1

This appendix includes selected drawings of the bridge itself and of the bridge com-ponents of which the NDT-investigations are carried out.



APPENDIX C

Visual Inspection



Appendix C page 1

This appendix includes the registrations from the visual inspection of all the bridge components included in the inspection. General orientation of the bridge and the bridge components under investigation, numbering of elements and damage pattern are most conveniently shown on sketches.









Appendix C page 2



Subject:


Photo 1: Photo 2:


Photo 3: Photo 4:


Photo 5: Photo 6:



APPENDIX D

NDT-method no. 1



Appendix D page 1

Depending on the complexity of the NDT-method a general description of the princi-ples of the method is described in this appendix. This appendix includes the registrations from one of the NDT-methods used in this special inspection. The appendix should include sketches of the areas of measure-ments and of the measuring grid if used e.g. for HCP, Impact-Echo, Impulse Re-sponse (s’MASH) etc. The appendix should also include relevant photos related to the NDT-investigation (of break ups etc.). Always note the dimensions of the bridge component in question. (Diameter of col-umn; width, depth, spacing and length of girders, etc.).




The template for this appendix is copied for all the NDT-methods used in this special inspection.



Appendix D page 2



Subject:

Insert photo

Photo 7:

Insert photo

Photo 8:



APPENDIX E

Economic Analysis



Appendix E page 1

This appendix includes the data from the economic analysis performed at the differ-ent repair strategies.

ndt_manual.pdf

Documents

structural cracks

general structural damage

damage mechanisms

types of damage

registration of damage

shear cracks

assessment of damage

induced cracks