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Founded in 1966, The Concrete Society brings together all with
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Guide to evaluation and repair of concrete structures in the
Arabian Peninsula
Concrete Society Special Publication CS 137
Kay, T and Walker, M
0 2002 The Concrete Society
ISBN 0 946691 94 0
Keywords
Arabian Peninsula, concrete, contracts, corrosion, durability,
environmental conditions, materials characteristics, materials
selection, reinforcement corrosion, repair, site practice,
specification, standards, testing.
Reader interest
Engineers and technologists involved in evalu- ating and
identifying causes of deterioration of concrete structures in the
Arabian Peninsula and similar hot climates and selecting appro-
priate repair techniques.
I
Classification Availability Unrestricted Content Guidance on
concrete performance, testing and
repairs Status Committee guided User Civil, structural and
materials engineers
Published by:
The Concrete Society, Century House, Telford Avenue, Crowthorne,
Berkshire RG45 6YS, UK Tel: +44 (0) 1344 466007; Fax: +44 (0) 1344
466008; E-mail [email protected]; www.concrete.org.uk
in collaboration with: The Bahrain Society of Engineers, P.O.
Box 835, Manama, Bahrain Tel: 00 973 727100; Fax: 00 973 729819;
E-mail:[email protected]; www.mohandis.org
Design and production: Siriol Bowman and Jon Webb
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Guide to evaluation and repair of concrete structures in the
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Prepared for a Joint Working Group of The Concrete Society and
The Bahrain Society of Engineers
Ted Kay and Mike Walker
Published by The Concrete Society
in collaboration with the Bahrain Society of Engineers
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This document has been prepared by co-authors Ted Kay and Mike
Walker with the support of the Joint Working Group, The Concrete
Society and the Bahrain Society of Engineers.
Dr Ted Kay*
Mike Walker*
Dr Eng Habib M Zein AI Abideen
Miss Zahra Salman A1 Aboodi
Dr Jameel Al-Alawi
Dr Phi1 Bamforth
Nick Clarke*
Prof Peter Fookes
Dr Magdi M Khalifa
Dr Adil bin Abdul Aziz AI Kindy
Dr Abdulghafoor Qasemi
Adman Abdul Rehman Sharafi
Hisham Shehabi
Jim Sokolowski
*Editorial Group
Halcrow Group Ltd, and Visiting Professor, Queens University of
Belfast
The Concrete Society
Deputy Minister for Public Works & Housing, Saudi Arabia
Ministry of Public Works & Housing, United Arab Emirates
Bahrain Society of Engineers
Taylor Woodrow Construction Ltd
The Concrete Society
Consulting Engineering Geologist
Ministry of Public Works & Housing, Saudi Arabia
Muscat Municipality, Sultanate of Oman
Dubai Municipality
Dubai Municipality
Bahrain Society of Engineers
Halcrow International Partnership, Dubai
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Grateful acknowledgements are extended to those many
organisations particularly in the Arabian Peninsula, listed below,
for their support and assistance. The expertise of their staff was
important and the work could not have been undertaken without the
financial assistance that they provided.
Authorities
Bahrain Ministry of Works and Agriculture
Bahrain Ministry of Housing, Municipal Affairs and
Environment
Dubai Municipality
Muscat Municipality
Saudi Arabia, Ministry of Public Works
UAE, Ministry of Public Works and Housing
Operating Companies (see following page)
Carillion - AI-Futtaim Tarmac
Consolidated Contractors Company
Elkem Materials
Fosroc
Grace Construction Products
Halcrow International Partnership
RMC Group Services
In addition to The Concrete Society and the Bu-a in Society of
Engineers, L..: following Engineering Societies an Organi- sations
have supported this work:
The Kuwait Society of Engineers
The Forum of Qatari Engineers
The UAE Engineering Society
Contact was also established with the Saudi Arabia Engineering
Committee and with colleagues in Muscat who are represen- tatives
of the Oman engineering fraternity.
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C~U~OO~OOU - AO [F~tUsh UZIUODUZIU L ~ ~ O U & I J U ! ~ S
D~V~S~OUU, is one of the largest providers of independent materials
testing and consultancy services in the Middle East. Facilities
exist for a wide range of tests on construction materials and the
labo- ratory undertakes research projects, ground investigations
and materials evaluation. Additionally environmental services are
offered which include air-quality monitoring and ground-con-
tamination investigations.
e-mail: [email protected]
C @ ~ ~ ~ d i d a U ~ d CUDGUU~XUCDUS C t ~ m p a ~ ~ y (CCC)
has been established for 50 years and is based in Athens, Greece.
CCC is the largest multi-disciplinary engineering and construction
company in the Arabian Peninsula and is certified to IS0 9000. The
company also operates extensively in the Eastern Mediter- ranean,
through the whole of Africa and also the CIS States. Affiliated
companies are located in the USA, England and Italy. CCC is placed
at No. 1 in the Middle East, No. 7 in the IndustriaLPetroleum
sector and No. 17 in the world in the 2001 ENR rankings of
international contractors.
www. ccc . gr
EkSrrnO MZI%eUiaOS markets Elkem Microsilica, an ultrafine
powder created in ferrosilicon and silicon production. Microsilica
is used throughout the world, wherever concrete durability is the
prime concern. Elkem Materials is part of Elkem ASA and was set up
in 1982 to promote and market microsilica via sales offices, agents
and distributors in most regions of the globe. With its advanced
research and devel- opment facilities, the company is committed to
improvements through product technology and developing new
solutions and better products for its customers. This is part of
Elkem Materials' role in protecting the environment, both in
production and where its products are used.
www.concrete.elkem.com
IFOSk'OC is one of the world's foremost manufacturers of
advanced technology products for construction, and recognised
market leader in concrete repair and protection. In the Middle and
Near East it has eight manufacturing plants and over 30 offices.
The company has a strong reputation in many product fields, and its
success is built on a combination of market knowledge, technical
resource, manufacturing expertise and problem-solving capabilities.
Fosroc's commitment to total quality and environmental management
systems is reflected in its IS0 9002 and 14001 certification.
Fosroc develops close working partnerships with international
contractors, specifiers
and operators, with a strong emphasis on after-sales service.
Its products help maximise the use of local materials and improve
productivity on site, while meeting safety standards and the
demands of new construction and operational techniques.
www.fosroc.com
Grace CapUUSfhUd~OUU PUOdUdS, a core business of W R Grace and
Co., is a world leader in the construction industry. Grace's
concrete admixtures, cement additives, and speciality building
materials, which include waterproofing and fire- proofing products,
strengthen and protect the world's most important structures. For
more than 40 years Grace Construction Products has improved the
quality of reinforced concrete by developing value-added admixtures
recognised for their strength, durability enhancement and chloride
resistance. Grace is committed to raising standards through
development of inno- vative products and practices to meet the
needs and demands for high-quality concrete. Through its affiliate
in the Arabian Gulf, Emirates Chemicals LLC, Grace has contributed
and supported the publication of this Guide.
www.graceconstruction.com
HdCUOW is an independent provider of infrastructure-based
business solutions. Specialising in the transport, water and
property sectors, it offers professional consultancy resources for
the planning, design and supervision of development on a global
basis. The company regularly provides consulting engineering
services to government departments, public sector authorities and
utilities, industrial and commercial companies, international
funding agencies, financial institutions and private individuals.
Halcrow has been at the forefront of research and development of
durable concrete structures in the Middle East for over a quarter
of a century.
www. halcrow.com
RMC GuapUg se[IpIkeS is the representative office of the RMC
Group in the UAE. Its operations include RMC Topmix, sup- plying
ready-mixed concrete to the Dubai and Sharjah markets with five
batch plants, over 60 transit mixers and 10 pumps. All products
meet strict quality standards and are IS0 9001 accredited. RMC
Supermix operates three batching plants in Abu Dhabi, supplying
numerous prestigious projects. Falcon Cement markets Duracem ground
granulated blastfurnace slag, produced at the purpose-built
facility at Jebel Ali. Gulf Quarries in the Hajjar mountains uses
state-of-the-art technology to produce high-quality aggregate for
use in Dubai Municipality.
www.rmctopmix.com
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Members of the Joint Working Group Acknowledgements Sponsoring
Organisations List of Tables List of Figures
Sketch map ~ Preface
iv 2.15 V 2.16 v i 2.17 X
X
xi xii
3.1
101 ONUWODMCUOON . . . . . . . . . . . . page 1 3.2 1.1
Background 1.2 Purpose and scope
3.3 1
2. I Introduction
UWE PROBLEMS. . . . . . . . . . . . page3 3 3.4
I 2.2 Factors affecting the type and rate of I deterioration
4
2.3 Timescale for appearance of defects 4
I
2.4 Relationship between damage and deterioration 4
2.5 Corrosion of reinforcement 4 2.5.1 Introduction 4 2.5.2
Characteristics of carbonation-induced
and chloride-induced corrosion 6 2.6 Sulfate attack 7 2.7
Physical salt weathering 8 3.5
' I
2.8 Deterioration related to aggregate properties 2.8.1
Introduction 2.8.2 Aggregate shrinkage and swelling 2.8.3 Aggregate
softening 2.8.4 Alkali-silica reaction
2.9 Abrasion, erosion and cavitation 2.10 Cracking occumng
during or soon after
construction 2.10.1 Introduction 2.10.2 Plastic shrinkage
cracking 2.10.3 Plastic settlement cracking 2.10.4 Early thermal
contraction cracking 2.10.5 Crazing Long-term drying shrinkage
cracking Cracks induced by temperature changes Other features
related to construction
2.1 1 2.12 2.13 2.14 Weathering of structures
8
8 m 8
4.1 4.2
8 8 9 4.3
4.4
9 9 9
10 I1 4.5 I1 12 12 12 14
Bacteriological attack Staining Lime leaching
14
14 14
ONVESUOGAUOONS . . . . . . . . . . . page 15
Introduction Preliminary survey - planning and preparations
Preliminary inspection 3.3.1 Introduction 3.3.2 Inspection 3.3.3
Results Additional information and initial diagnosis of causes of
deterioration 3.4.1 Introduction 3.4.2 Existing records 3.4.3
Design 3.4.4 Materials 3.4.5 Construction 3.4.6 History of the
structure 3.4.7 3.4.8
Main investigation
Current and future use of the structure Initial diagnosis of
causes of deterioration
15
16 17 17 17 18
20 20
20 20 20 21
21 21
21 21
UESUUNG . . . . . . . . . . . . . . . . page 22 Introduction
Selection of sampling and test locations Access Sampling and
testing 4.4.1 Types of test 4.4.2 Test locations 4.4.3 Samples
4.4.4 Reinstatement of test sites In-situ testing 4.5.1
Introduction 4.5.2 Surface hardness 4.5.3 Ultrasonic pulse velocity
4.5.4 Near-surface strength 4.5.5 Reinforcement depth and
position
22 22 23 23 23
23 24 26 26
26 26 26 27 27
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4.6
H 5.1 5.2
5.3 5.4
6.1 6.2
6.3
6.4
4.5.6 Carbonation depth 4.5.7 Half-cell potential 4.5.8
Resistivity 4.5.9 Surface absorption Laboratory testing 4.6.1
Introduction 4.6.2 Core testing for strength 4.6.3 Dust samples
4.6.4 Petrographic examination 4.6.5 Water permeability
28 28 29 29 30 30 30 30 30 30
. . . . . . . . . . . . . . page31 Objectives Interpretation of
non-destructive test results 5.2.1 General 5.2.2 Carbonation depth
5.2.3 Chloride profiles 5.2.4 Concrete strength and density 5.2.5
Ultrasonic pulse velocity 5.2.6 Initial surface absorption 5.2.7
Electrical potential mapping 5.2.8 Resistivity Use of results
Predicting the future behaviour of a structure
31 31 31 31 31 32 33 33 33 34 34 35
Introduction Reinstatement with concrete or mortar 6.2.1
Introduction 6.2.2 6.2.3 Breaking out 6.2.4 Treatment of
reinforcement 6.2.5 Bonding aids 6.2.6 Mortar repairs 6.2.7 Repairs
in shutters 6.2.8 Sprayed concrete 6.2.9 Incorporation of
sacrificial anodes 6.2.10 Quality control and testing Coatings for
concrete 6.3.1 6.3.2 Materials 6.3.3 Application 6.3.4 Quality
control and testing Crack injection 6.4.1 6.4.2 Materials
Areas of application and limitations
Areas of application and limitations
Areas of application and limitations
36 36 36 37 37 38 40 40 42 44 45 46 46 46 46 47 48 48 48 48
6.5
6.6
6.7
6.8
7.1 7.2 7.3
7.4 7.5
8.1 8.2 8.3
6.4.3 Method of injection 6.4.4 Leaking cracks 6.4.5 Quality
control and testing Corrosion inhibitors 6.5.1 Corrosion-inhibiting
admixtures 6.5.2 Migrating corrosion inhibitors 6.5.3
6.5.4 6.5.5
Cathodic protection 6.6.1 Introduction 6.6.2 Sacrificial anode
systems 6.6.3 Impressed current systems 6.6.4 Quality control and
testing Re-alkalisation 6.7.1 Introduction 6.7.2 Quality control
and testing Chloride extraction (desalination) 6.8.1 Introduction
6.8.2 Quality control and testing
Areas of application and limitations of MCIs Method of
application of MCIs Quality control and testing of MCIs
48 49 49 49 50
50 ~
50 50
50 50 50 50 51 51 51 51 52 52 52 52
and protection systems Electrochemical systems Comparing costs
and performance
NS . . . . . . . . . . . . . . . . .page 53 Expected performance
53 Temporary works 53 I Performance of non-electrochemical repair
I
. .
Introduction European standards Specifications 8.3.1
Introduction 8.3.2 General matters 8.3.3 Access 8.3.4 Surface
cleaning 8.3.5 8.3.6 Concrete removal 8.3.7 Reinforcement 8.3.8
Reinstatement 8.3.9 Materials 8.3.10 Trial repairs 8.3.11
Testing
Survey and location of defects
53 54 54
I
. page 55 ~
55 55 55 55 55 56 56 57 57 57 57 58 58 58
1
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Con tents
8.3.12 Surface preparation for protective treatment 59
8.3.13 Application of protective treatment 59 8.3.14 Resin
injection 59 8.3.15 Cathodic protection 60 8.3.16 Electrochemical
chloride extraction
and re-alkalisation 60 8.4 Bills of quantities and measurement
60
APPENDIX U Less common testing Uechniqnnes. . . . . A 1.1
Introduction Al.2 In-situ testing
A I .2.1 Crack movement Al.2.2 Ground-probing radar A1.2.3
Corrosion rate A1.2.4 Moisture A1.2.5 Radiography A 1.2.6
Thermography A1.2.7 Fluid transport properties
A1.3 Laboratory testing
page 63 63 63 63 63 63 64 64 64 65 65
A 1.3.1 Water absorption 65 A1.3.2 Gas permeability 65 A1.3.3
Gas diffusion 66 A1.3.4 Chloride diffusion 66 A1.3.5 Bulk chloride
diffusion 66 A1.3.6 The value of measuring transport
properties 66 A 1.3.7 Concrete porosity 66
APPENDIX 2
of siUane and s~loxane . . . . . . . . . . . . page 68
DQdQlrM!liMliUUg depth Of pQIIIQdradiQn
A2.1 Dye penetration test 68 A2.2 Capillary rise test 68
REFERENCES. . . . . . . . . . . . . . . . . .page 73
DMBflECU INDEX . . . . . . . . . . . . . . . page 77
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Guide to evaluation and repair of concrete structures in the
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Table 2. I
Table 2.2
Table 2.3 Table 2.4 Table 3. I
Table 3.2
Table 4.1
Table 4.2: Table 4.3:
Table 5.1 :
Table 5.2: Table 5.3:
Table 5.4:
Common forms of deterioration in concrete structures.
Typical times of appearance of various types of defect (after
Fookes, 1976). page 5 Damage related to cover and bar diameter:
pnge 6 Some construction defects. page 13 Examples of
classification of defects (after RlLEM TC 104). page 18 Strength
classes for normal-weight concretes, from BS EN 206. page 20
Relative numbers of readings necessary for various tests (from
Bungey and Millard, 1996 and BS 1881: Part 207, 1992). page 22
Principal test methods. page 24 Tests and techniques for various
types of structure.
Chloride contents (% by weight of cement) at 3 years.
Interpretation of ISAT results. page 33 Relationship between
potential and risk of corrosion. page 34 Relationship between
resistivity and corrosion rate.
page 5
page 25
page 33
page 34
Table 6. I : Table 6.2:
Features of methods of breaking out concrete. page 38
Considerations in the choice between bonding aids and soaking
the substrate for different repair types.
Advantages and disadvantages of sprayed concrete.
page 4 I
page 44 Table 6.3:
Table AI . 1 : Values of water permeability. page 65
Table A I .2: Relationship between sorptivity and concrete
quality.
Table A I .3: Values of gas permeability. page 66
Table A I .4: Values of gas diffusion coeficients. page 66
Table A I .5: Diffusion coeficients. page 66
Table A I .6: Classi3cation of porosity. page 67
Table A3. I : European Standards dealing with protection and
repair methods and products. page 69
Table A3.2: Protection and repairprinciples from BS EN 1504:
Part 9. page 70
Table A3.3: Performance testing requirements for structural and
non-structural repair mortars or concretes from BS EN 1504: Part 3.
pnge 70
Table A3.4: Requirements for hand-applied mortar from BS EN
1504: Part 10. page 71
Table A3.5: European CEN Standards for test methods and repair
materials. puge 72
page 65
Figure 2.1 :
Figure 2.2:
Figure 2.3:
Figure 2.4:
Figure 2.5:
Figure 2.6:
Figure 3.1 :
Figure 3.2:
Figure 3.3:
Figure 4.1 :
Figure 4.2:
Figure 4.3:
Figure 4.4:
Figure 4.5:
Figure 4.6:
Figure 4.7:
Figure 5.1 :
Moisture movement. page 4
Examples of intrinsic cracks in hypothetical structure (from
Concrete Society TR22, based on Fookes, 1976). page 10
Formation of plastic shrinkage cracks. page 10
Formation of plastic settlement cracks. pnge I I Formation of
plastic shrinkage cracks in columns.
Drying shrinkage cracks on wall (vertical lines indicate the
position of reinforcement). page 12
Flow chart - from routine inspection to solution. page 15
The whole assessment process. page 16
An example of a preliminary inspection summary form. page 19
Method of obtaining dust samples. page 25
Applications of UPV measurement. (T = transmitter; R = receiver)
page 27
Covermeter being used on vertical surface. page 27
Half- cell potential equipment (courtesy of Wexham
Developments). page 28
Half-cell potential measurements. page 28
Typical map of equal potential contours (values in mV). page
29
Typical layout of resistivity test array. page 29
Typical chloride profiles. page 32
/.'age II
Figure 5.2: Bestfit to measured values. page 33
Figure 5.3: Interpretation of indirect UPV measurement. p q e
33
Figure 5.4: The effect of moisture and chloride content on
corrosion rate (after Browne, 1982). page 34
Figure 6.1 : Undercutting of repair edges. page 37 Figure 6.2:
Rounding of repair corners to facilitate compaction.
puge 38
Figure 6.3: Method of injecting resin into anchor holes. page
39
Figure 6.4: Ways in which bonding aids may affect passage of
cathodic protection current. page 4 I
Figure 6.5: Some features of shutters for repairs. page 43
Figure 6.6: Funnel and hose used for flowing concrete. page 43
Figure 6.7: Flow through test for flowing concrete. page 44 Figure
6.8: Method of spraying for vertical repairs. From ACI
506R-90, reapproved 1995. Guide to shotcrete. page 45
Figure 6.9: Method of injection using packers. page 49
Figure 6.10: Schematic of cathodic protection system. pccge 5 I
Figure 6.1 I : Schematic of the re-alkalisation process. page 51
Figure 6.12: Schematic of the chloride extraction process. page
52
Figure AI. 1 : Demec gauge with digital read-out (courtesy
of
Figure A I .2: Typical ground-probing radar equipment
(courtesy
Figure A I .3: Layout of typical radiography system for
concrete.
Figure A I .4 Equipment for in-situ permeability testing
(courtesy
Wexham Developments). page 63
of Aperio). page 63
page 64
of Wexham Developments). page 65
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The Concrete Society and the Bahrain Society of Engineers have
enjoyed a close collaboration for many years, particu- larly their
joint involvement with the International Con- ferences on the
Repair and Maintenance of Reinforced Concrete in the Arabian Gulf.
Following the fifth conference in 1997 it was agreed that a review
document should be prepared outlining the basic steps for
evaluating the con- dition of a concrete structure and giving
details of available, and workable, repair systems and methods.
For this purpose two authors, Ted Kay and Mike Walker, both with
close working knowledge of structural performance as well as repair
and maintenance in the Arabian Peninsula, have prepared this Guide
in collaboration with a Joint Working Group centred on The Concrete
Society and the Bahrain Society of Engineers.
A Draft for Comment was issued to delegates to the Sixth
International Conference in November 2000 and to the sup-
porting authorities and organisations acknowledged pre- viously.
Comments on the draft were considered by the Joint Working Group
and incorporated into this final document.
Following an introduction, the Guide is in eight main chap-
ters. The first chapter covers the problems encountered in the
Region, and deterioration processes. The following three chapters
deal with inspection, tests and the evaluation of the present and
future behaviour of a structure. Chapter six con- tains guidance on
repair techniques, their appropriateness and application. The two
final chapters deal with selection of repair options, and with
contract documents for repair works.
The Guide also has three appendices, the first providing more
information on some of the tests described in the main text,
particularly less common testing techniques. The second outlines
the tests for silane penetration, and, for information on current
developments, the third summarises developing European standards
for concrete repair.
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SKETCH MAP
ed Sea \, t I -,B Height range (approximately)
2000to4000m Gulf of Aden
Fcv3
Some principal cities not shown. Boundaries shown are not meant
to be dejnitive.
E5- kalla
Arabian Sea
Health and safcty
Construction activities, particularly on construction sites,
have significant health and safety implications, either as the
result of the activities themselves, or from the nature of the
materials and chemicals used. This Guide does not endeavour to
cover health and safety issues relevant to the construction of
reinforced concrete in the Arabian Peninsula comprehensively,
although specific points to note are mentioned where appropriate in
the text. Readers should consult other specific published guidance
relating to health and safety in construction.
I Quarries and pits and associated processing areas can be
dangerous. Visitors should take appropriate safety precautions and
be escorted by I quarry staff.
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Construction of large civil engineering projects and major
buildings in the Arabian Peninsula region has generally taken place
only in the last 30 to 40 years. During that time, many huge
projects, including harbours, airports, sewage treatment works,
power and desalination plants and industrial facilities, and whole
cities of modern buildings with their associated infrastructure
have been constructed. This work posed a sub- stantial challenge to
the construction industry and, like any undertaking of this
magnitude, has not been without prob- lems. These have been
extensively documented elsewhere and the causes can be summarised
as follows:
there was no previous experience anywhere in the world of
concrete construction at such high temperatures often in a salty
environment and therefore no established technology
there were few local resources in terms of a concrete industry,
concrete materials supply or labour force; local quarries and
borrow pits were opened up but some of these sources were only
marginally suitable for high-quality concrete production
in some regions there was little potable water for making or
curing concrete
many aspects of the local environment are not conducive to the
production of high-quality concrete and are aggres- sive to
concrete structures in service; key factors in terms of
aggressivity are a saline water table close to the ground surface
and the widespread occurrence of salty soils.
As a result of these factors, in combination and with others,
many concrete structures have not been as durable as expected. This
has left a legacy of repair and protection of concrete structures,
which has been required for some time, needs to be undertaken now,
and which will continue into the foreseeable future.
All concrete structures deteriorate with time, though the rate
of deterioration is affected by many factors. The result is a
change in the performance of the structure, which may affect its
behaviour under normal working conditions or its struc- tural
safety.
The starting point in most cases is that deterioration is
evident because of visible signs of damage, such as cracking or
excessive deflections; these signs may be identified during routine
inspection of the structure, during maintenance or during
day-to-day operation. Detailed inspections of struc- tures are
often carried out when ownership or use changes, e.g. when an hotel
or apartment block is changed into offices. Sometimes the
deterioration may be too severe to justify
retaining the affected structure or component, but generally
repair and protection can add many useful years to its life. Repair
and protection measures must be selected to meet the needs
identified as part of a structured process that involves inspection
and testing, determining the causes of deteriora- tion and
assessing the probable future behaviour of the struc- ture. The
cost of repair and protection must also be assessed in relation to
the value of the structure over its remaining useful life and the
costs of the alternative of demolition and replacement. Carrying
out a detailed investigation to identify the underlying causes of
defects is always necessary so that a clear remedial strategy can
be determined.
A sound understanding of the underlying causes of the dete-
rioration or lack of durability of a structure is necessary before
setting out on a protection and repair programme. The changes that
constitute deterioration can be the result of several factors,
including the design of the structure, the standard of workmanship
during construction, the materials used, the action of the
environment during construction and in service, and the loads
acting upon the structure. Fortu- nately, many deterioration
processes lead to characteristic visible features such as
distinctive crack patterns. This means that visual inspection often
plays an important part when starting an investigation. An early
visual inspection can be extremely useful both in setting the scene
and in providing information to help in developing a full
investigation pro- gramme. Other useful pointers are the time at
which a par- ticular type of defect became apparent and the fact
that some structural types or locations are likely to lead to a
particular form of deterioration. Typical timescales for the
various dete- rioration processes and the types of structure that
are most at risk are outlined in this Guide.
This report has been written as a guide to the whole process of
concrete repair from the first realisation that there may be a
problem through to the mechanics of various repair and protection
options. Information is provided on each step on the way. The
emphasis is inevitably on repair of structures undergoing corrosion
of reinforcement because this is by far the commonest cause of
deterioration in the Region.
This Guide complements the Guide to the construction of
reinforced concrete in the Arabian Peninsula (The Concrete
Society/ClRIA, 2002), which is published simultaneously.
This Guide describes the major causes and processes of dete-
rioration likely to be encountered in the Arabian Peninsula Region.
These include:
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in-built problems such as unsatisfactory design, poor
workmanship, the use of unsuitable aggregates or cement, or
inappropriate concrete mix design features that appear during
construction or soon afterwards the action of the local environment
on the completed structure.
Deterioration may have a single cause but usually several
factors are involved.
Damage caused by fire or structural problems such as over-
loading or impact damage are not covered specifically but some
repair techniques described are equally applicable to these
situations. Concrete Society Technical Report 33 (The Concrete
Society, 1990) is a useful guide in the case of fire- damaged
concrete structures.
Three stages in the assessment process are essential for a suc-
cessful repair and protection contract:
Investigation of the present condition of the structure Analysis
of the results of the investigation, leading to a clear
understanding of the causes of deterioration and an assessment of
future performance Consideration of the available options,
including repair and protection, partial or full demolition and
replacement, and selection of the most appropriate solution.
The methods described in this Guide are suitable for cast in-
situ and precast concrete structures that are unreinforced,
reinforced or prestressed. Where one of these forms has been found
to be susceptible to a particular deterioration process, this is
noted in the text. The durability of a structure depends on the
influence of the local environment and reference is made to the
common exposure zones encountered in the Region. The
micro-environment can also have a significant effect on performance
of individual elements of a structure, for example where local
wetting occurs in an otherwise dry situation. This needs to be
taken into account when planning and assessing the results of
investigations and considering repair and protection options.
The local environment in the Arabian Peninsula Region is
extremely aggressive to concrete and its reinforcement. Any- one
dealing with repair and protection of a concrete structure needs to
understand how this environment is acting on the structure under
consideration and why this action has resulted in the deterioration
that can be seen. Only then can rational and informed decisions be
taken on how to restore the structure to a satisfactory condition
and protect it against deterioration in the future. This Guide
therefore starts by describing the common forms of deterioration
encountered in the Region and how the environment influences these
deterioration processes.
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The difficulties that surround the concrete construction pro-
cess in the Arabian Peninsula and the destructive effects of the
aggressive environment on concrete structures in service have been
well documented. See, for example, Guide to the construction of
reinforced concrete in the Arabian Peninsula (The Concrete
Society/CIRIA, 2002), and the Proceedings of the series of
conferences on Deterioration and repair of rein- forced concrete in
the Arabian Gulfheld regularly since 1985 and published by the
Bahrain Society of Engineers. However, it is worth summarising the
principal features as they will be relevant to many structures
under investigation.
The rapid and widespread development in the Region during the
late 1960s and 1970s it presented an unusual situation to designers
and materials engineers. The average, ambient and maximum
temperatures were higher than in most other regions where concrete
construction was undertaken, the soils and groundwater contained
much higher salt concentrations, there was no high volume aggregate
industry, no local cement production, in some areas there were only
limited supplies of potable water, no local concrete supply
industry and above all, little time to develop a relevant local
concrete technology. This being the case, designers adopted design
standards from Europe and America which were not necessarily
appropriate in the local situation; contractors had to rely on
cement and reinforcement imported by sea, some-times as deck cargo
and sometimes with prolonged waiting periods offshore because of
congestion in the ports; marginal local sources of aggregate had to
be used; contractors had to import mainly unskilled labourers and
train them in the art of concrete production and construction on
the job; little or no water was available for curing concrete.
It is not surprising, therefore, that a survey of reinforced
concrete marine structures in the late 1970s predicted a life of
between 15 and 20 years. Sadly, this has turned out to be an
accurate prediction in many cases. The whole concrete construction
scene in the Arabian Peninsula is now vastly different to that in
the 1960s and 1970s but the changes have been gradual and the
requirement to repair older concrete structures will continue into
the future. Modem concrete spe- cifications should result in a
product that is much more durable than in earlier projects.
However, the local environ- ment is still highly aggressive to
concrete and reinforcement and life expectancy cannot be as great
as for similar structures in Europe, America or even the Far East.
Structure owners need to be vigilant, to undertake routine
inspections, to maintain protective systems, and to undertake
timely repairs.
The main durability problems encountered in the Region are
cracking and spalling due to reinforcement corrosion: the key
factors causing these are outlined below.
Climate
The very high temperatures accelerate deterioration re- actions.
They also make concrete production and placing dif- ficult, not
only because of loss of water from the mix by evaporation but also
because at higher temperatures mixes are less workable as hydration
proceeds faster and mixes tend to stiffen more quickly. There may
be a temptation to add water to mixes but this will reduce their
durability and their ability to protect the reinforcement. High
temperatures in association with high humidity make working
outdoors ex- tremely uncomfortable and exhausting. Workers and
super- visors tire and lose concentration, and less attention is
then paid to the detail needed to produce high-quality concrete.
Good workmanship, particularly in achieving the design cover to
reinforcement and well-compacted and cured concrete, can be a major
factor in durability. High rates of evaporation during the day and
hot dry winds mean that concrete dries out quickly and is
susceptible to cracking. Curing systems suffer rapid evaporation
and curing may become ineffective for long periods. The cover
concrete, which is most affected by curing and which gives direct
pro- tection to the reinforcement, may be the most permeable
concrete in an element. Large variations in temperature from day to
night and the increases in temperature that result from intense
solar radiation can cause cracking on exposed surfaces.
Materials
Cement and reinforcement of good quality are produced in the
Arabian Peninsula and are now widely available through- out the
Region. However, cement and clinker are still im- ported into some
parts of the Region from various sources. Major variations in their
characteristics can be experienced.
High-quality aggregates are available in particular locations
and are exported to other countries in the Region. However, because
of high transport costs or expediency, aggregates from sources
local to the project may be used. Some local sources may be
contaminated with salts, leading to corrosion of reinforcement when
used in concrete, or be mineralogi- cally unsound, leading to
deterioration of the concrete itself. Some rock types may form
poorly shaped aggregate, and rock may be processed inappropriately
to produce flaky or elongated particles. Aggregate of poor shape
requires more water to produce workable concrete and hence the
concrete is of lower quality and less durable.
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Environment and exposure conditions
Much of the development in the Region has been concen- trated in
the coastal areas. These areas usually have a highly saline water
table close to the surface which can extend tens of kilometres
inland. Capillary rise in granular soils brings moisture from the
water table to the surface. The moisture evaporates leaving salts
behind. The ground surface can be heavily contaminated by salt
which is taken up into the atmosphere by wind as dust. This salty
dust is then deposited on buildings and other structures. This
dust, in combination with moisture from condensation, leaks or
discharge from air-conditioning units, can permeate into concrete
and can attack it.
Structures that are founded in salty soils and the capillary
rise zone are particularly vulnerable to salt. If the concrete
below ground is not properly tanked, moisture rises through the
concrete, carrying salts with it. Once above ground, the mois- ture
moves towards the warm outer surface and evaporates, leaving behind
the salt in concentrated form. The process is shown in Figure 2.1.
Many problems with concrete structures are first observed at or
just above ground level. Similar moisture movements can occur in
water-retaining structures and other structures where one face is
in contact with water.
2.2 FACTORS AFFECnNC THE TYPE AND RATE OF DETERIORATION
Most concrete deterioration processes depend on the presence of
water, the moisture state within the concrete being more important
than that of the surrounding atmos- phere. Concrete takes in water
from the environment more rapidly than it loses it and so the
average internal humidity in the concrete is generally higher than
the average external humidity.
All chemical reactions are accelerated by increases in tem-
perature. In general terms, an increase in temperature of 10C
causes a doubling in the rate of reaction. Putting this another
way, the time taken to reach a particular condition is halved by a
rise in temperature of 10C, all other factors being equal. It is
understandable therefore, that deterioration of concrete
Evaporation
Figure 2.1 : Moisture movement.
structures can be two or three times faster than in most parts
of Europe.
The types of deterioration that occur in a concrete structure
are often related to its environment. The deterioration pro- cesses
likely to occur in various types of structure are sum- marised in
Table 2.1. The list only indicates the deterioration that may
occur; as outlined above there may be many other contributory
factors.
2.3 TIMESCALE FOR APPEARANCE OF DEFECTS
The time of appearance of a defect is one indicator of its
cause. Table 2.2 indicates the likely times for various types of
defect.
2.4 RELATIONSHIP BETWEEN DAMAGE AND DETERIORATION
Many processes can cause concrete structures to deteriorate and
can lead to defects. Anyone involved in determining the causes of
defects has to have knowledge of these underlying processes. The
necessary background information on the principal types of
deterioration is given in this Chapter.
An investigation should lead to the identification of various
types of fault and mechanisms of deterioration in the structure. It
is important to distinguish between those due to an earlier event
(e.g. a construction defect) and those symp- tomatic of an on-going
problem (e.g. cracking of the concrete due to reinforcement
corrosion). Some faults may not be detrimental to the structure at
the time of inspection but may cause problems in the future (e.g.
cracks developing during construction may be a path by which
chlorides can reach the reinforcement and lead to corrosion). In
addition, the signifi- cance of a particular type of deterioration
will depend on the type of structure and the location of the
affected element within the structure.
As noted earlier, many forms of deterioration give rise to
characteristic defects and the visual appearance of the defect can
be extremely helpfil in the diagnosis process. In the Sections
covering the individual processes that follow, the characteristic
defects are described, in italics, as an aid to diagnosis,
particularly at the preliminary stage.
2.5 CORROSION OF REINFORCEMENT
2.5.1 Introduction
Reinforcement in concrete does not naturally rust as it is
shielded from the external environment by the thickness of the
cover concrete. It is also protected by the alkaline envi- ronment
provided by the hydrated cement. The large amount of calcium
hydroxide in the pore solution of concrete gives a pH of about 12.5
and the small amounts of sodium and potassium present in the cement
push this to a higher value.
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Marine structures
Table 2. I: Common forms of deterioration in concrete
structures.
Chlorides
Sulfates
Corrosion of reinforcement particularly in the splash zone
Salt weatheringnoss of concrete section particularly in splash
zone
Possible cause of deterioration Type of structure
Bridges and highway structures
Deterioration
Marine creatures
Chlorides from environment (dust and condensation)
Leaching/mechanical damage below water level
Corrosion of reinforcement particularly on upper deck surfaces
and substructures where water leaks from above
Section
2.5
~~
Buildings
Corrosion of reinforcement on facades. Interior concrete
carbonates but is unlikely to lead to corrosion unless there is a
source of moisture
May be deposited in dust on facades. Can then be carried into
concrete by condensation or leakage from air-conditioning units
Carbonation
Chlorides
2.6,2.7
2.7,2.15
Buried structures or structures in contact with the ground
Sdfates Loss of concrete section
Chlorides
Salt weathering
Corrosion of reinforcement, particularly just above ground
level
Loss of concrete surface and later loss of section
2.5
~~ ~
Ground slabs Chlorides
2.5
~
Industrial plants (including sewage treatment works)
2.5
Chlorides
2.6
Chlorides
Sulfates Tunnels
Trafficked areas Abrasion
Corrosion of reinforcement - will occur first near leaking
joints
Loss of concrete section
Loss of concrete surface
2.5
2.7
Corrosion of reinforcement in top mat because chlorides are
drawn through from the ground or chloride-contaminated soil or dust
is blown onto them.
2.5
Corrosion of reinforcement anywhere that saline water comes into
contact with reinforced concrete; corrosion unlikely below water
level where there is permanent contact with water and concrete
remains saturated
2.5
2.5
2.6,2.15
2.9
2.15 Sewer pipes Sulfates Loss of section by acid attack after
sulfates converted to sulfuric acid by bacteria
Table 2.2: Typical times of uppearunce of various types of
defect (after Fookes, 1976).
Typical time of appearance Type of defect
Plastic settlement cracks*
Plastic shrinkage cracks*
Construction defects*
Crazing
Early thermal contraction cracks*
Long-term drying shrinkage cracks
Chemical attack (including sulfate attack)
Damage due to temperature movements (seasonal)
Alkali-silica reaction
Reinforcement corrosion
Section
Ten minutes to three hours
Thirty minutes to six hours
On removal of formwork
One to seven days - sometimes much later
One day to two or three weeks
Several weeks or months
Few months up to several years depending on nature of the
materials
Probably up to a year, but may be longer
Several years
Several years, but may be much shorter
2.10.3
2.10.2
2.13
2.10.5
2.10.4
2.1 I
2.6, 2.7
2.12
2.8.4
2.5
In these conditions a protective oxide layer is formed and
maintained on the surface of the steel. This is generally referred
to as a passive layer. Loss of this passive condition can occur as
a result of: carbonation of the concrete, ex- cessive
concentrations of chlorides present in the concrete materials
during construction, ingress of chlorides into finished concrete
(commonly sodium chloride from seawater, groundwater or soil), or a
combination of carbonation and
I chlorides.
Carbonation is the reaction of carbon dioxide in the envi-
ronment with calcium hydroxide in the cement. The results of the
reaction are calcium carbonate and a lowering of the pH to about 9.
Carbonation starts at the concrete surface which is in contact with
the atmosphere. As carbon dioxide pene- trates into the concrete a
carbonated layer develops at the surface. The 'layer' within the
concrete at which concrete changes from carbonated to uncarbonated
is known as the carbonation front. Over time the carbonation moves
inwards
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and eventually reaches the reinforcement. As noted above, the pH
drops from 13 to about 9 over a few millimetres. It is possible to
distinguish between carbonated and uncarbonated concrete on a
freshly exposed surface using a simple test, such as the
phenolphthalein indicator described in Section 4.5.6. This enables
the risk of corrosion of reinforcement to be assessed. At a pH at
or below 9, the protective oxide layer on the surface of the
reinforcement cannot be maintained and corrosion of the reinforcing
bar becomes possible if moisture and oxygen can gain access.
The reaction of carbon dioxide and calcium hydroxide requires
moisture, so in very dry concrete carbonation will be slow. In
saturated concrete the moisture presents a barrier to the
penetration of carbon dioxide and again carbonation rates will be
low. The most severe condition for carbonation is when there is
sufficient moisture for the reaction to occur, but not enough to
act as a barrier. This usually occurs when the concrete is exposed
to atmospheric relative humidity in the range 50-70% or when the
conditions vary from wet to dry. Wetting and drying alternately
allows ready ingress of water vapour and oxygen, providing all the
conditions required for corrosion. Relative humidities within the
critical range occur daily throughout the year in coastal locations
and from November to March even at sites that are a considerable
distance from the coast.
The passive layer on reinforcement is not static but is in
dynamic equilibrium. The passive layer is continually broken down
and re-established in the highly alkaline conditions within
uncarbonated concrete. Chloride ingress does not cause a reduction
in background pH, but inhibits the mech- anism by which the
protective oxide layer is maintained. Chlorides penetrate concrete
in solution and are able to pene- trate most rapidly in dry
environments that are infrequently wetted. Examples are the splash
zone above high tide level on marine structures, dry dock floors
and walls, areas which are infrequently washed down with saline
water, and areas in process plant where saline water occasionally
leaks. Alterna- tively, chloride may be present in the concrete
from the start, having been inadvertently added to the mix as a
contaminant of one of the mix constituents, for example, in beach
sand or from stockpiles that have been contaminated by groundwater
or wind-blown salty dust.
Corrosion of metal is an electro-chemical process, with dif-
ferent reactions occurring at anodic and cathodic sites. A ready
supply of moisture and oxygen to the cathodes is re- quired to fuel
the process. Reinforced concrete can deteriorate if the steel
reinforcement corrodes. The corrosion products (rust) form at the
anodes and usually occupy a much larger volume than the uncorroded
steel. This expansion exerts bursting stresses on the concrete and
can cause cracking and spalling of the cover. In very rare and
special circumstances in which the supply of oxygen is restricted,
black rust forms which does not produce expansion. This anaerobic
corrosion causes little or no disruption to the concrete.
A very small amount of corrosion of the reinforcement - a layer
less than 0.1 mm thick on the surface of the bar - can cause the
concrete to crack. This amount of corrosion rep-
resents only about 2.5% of the cross-sectional area of a typical
16 mm bar. Hence evidence of corrosion will be visible long before
the loss of steel cross-section is likely to affect structural
integrity. However, there may still be a risk of injury or damage
caused by loose concrete spalling from the structure.
Characteristic defects
Damage caused by reinforcement corrosion is ojien charac-
terised by cracks running parallel to the steel, which may cause
spalling, particularly at external corners of members such as beams
and columns. Corrosion of reinforcement can also result in
delamination of the surface ofslabs or walls. If the bars are
widely spaced and close to the surface (in relation to their
diameter), cracking will be the most likely form of damage. Deeper
bars, and bars close together; tend to cause the concrete to
delaminate as the cracks can combine before they reach the surface.
The nature of damage is thus a function of the bar size and
spacing, and the cover As a guide, the type of damage can be
related to the ratio ofcover to bar diameter; as indicated in Table
2.3. When corrosion is very far advanced the whole of the cover
concrete may become severely cracked and almost disintegrated.
2.5.2 Characteristics of carbonation-induced and
chloride-induced corrosion
The characteristics of corrosion resulting from carbonation and
chloride ingress differ in one principal respect. Carbona-
tion-induced corrosion tends to be general, though isolated bars
with low cover can lead to local problems. Chloride- induced
corrosion is characterised by local, rapidly corroding areas of
bars. There may be areas of intense localised cor- rosion on bars
while only a few centimetres away the bar surface is in perfect
condition. Acting within the concrete, chlorides (in combination
with water, oxygen and the rein- forcing steel) form and drive an
electrical corrosion cell with anode and cathode sites at different
positions on the rein- forcement. The anodes corrode whilst the
cathodes are unaf- fected. Once delamination has occurred, oxygen
and moisture can gain access to the reinforcement along the plane
of delamination and hence the whole mat may take on a more or less
uniformly corroded appearance.
An essential difference between chloride-induced and carbo-
nation-induced corrosion is the propagation period, that is, the
time between corrosion starting and damage occurring. Carbonation
occurs most rapidly in moderate humidity envi- ronments (relative
humidity 50-70%) but at this humidity the corrosion will occur very
slowly (for example inside build- ings) and will be insignificant
in relation to the life of the
Table 2.3: Damage related to cover and bar diamete,:
Cover bar diameter Likely type of damage on a flat surface
I 1 I Cracks and possible local spalling I 2 I Larger cracks and
risk of delamination I 3 I Delamination
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structure. However, cycles of wetting and drying can give rise
to a situation where carbonation can result in corrosion. Under
exposure to chlorides, however, the conditions that lead to the
most rapid chloride penetration, i.e. wetting and drying or high
humidity (90-95%), are also the conditions leading to the most
rapid corrosion. While there may be a long period of corrosion
propagation in carbonated concrete, if chlorides are present the
propagation period is likely to be relatively short. An exception
is when chlorides are included in the concrete mix and the concrete
is subsequently exposed to a low humidity environment. Corrosion
begins on the externally exposed (wetted) surfaces but not on the
internal (dry) surfaces.
The relationship between corrosion and chlorides is complex. In
uncontaminated concrete the passive layer on the surface of the
reinforcement is continually breaking down and being rein- stated.
If chlorides enter the concrete from outside, the chloride
concentration at the reinforcement increases over time. Chlorides
disturb the equilibrium of the process under way at the surface of
the reinforcement. They interrupt the process to an extent that is
dependent on the amount of chloride present. It is generally
believed that the ratio of chloride to hydroxyl ions in the pore
water is critical in relation to depassivation, but for practical
reasons chloride concentrations are most com- monly measured and
reported as a percentage by weight of binder in the concrete. At
low chloride concentrations the process of reinstatement of the
passive layer is slowed down and eventually a concentration is
reached at which the rate of reinstatement falls below the rate of
breakdown. At this stage complete depassivation occurs.
An additional difficulty in assessing the corrosion risk asso-
ciated with chlorides is that macro-cells (i.e. corrosion cells
with the anode and cathode separated by several metres) can be
formed by variations in chloride content. Steel in concrete with a
chloride content higher than the assumed threshold at which
corrosion can be initiated may be protected locally if an anode has
developed at a nearby site where the chloride content is even
higher. The reinforcement corrodes preferen- tially at this anodic
site and the neighbouring steel is protected.
Characteristic defects
Deterioration of concrete due to sulfate attack takes two forms:
9 Expansive formation of ettringite and/or gypsum in the
hardened concrete causes cracking and exfoliation. Hydrated
cementing compounds soften and disintegrate to a crumbly mass with
substantial loss of surface con- crete. This is due to direct
attack on the cement com- pounds by sulfates or by their
decomposition when calcium hydroxide is removed by its reaction
with the sulfates.
,
Either or both of these mechanisms can occur; depending on the
temperature, types and concentrations of sulfate in solu- tion and
the composition of the concrete.
Most sulfates are potentially harmful to concrete. Sulfates
occur naturally in soils, rocks and groundwater and in many parts
of the Arabian Peninsula there are very high concen- trations of
sulfates in the groundwater and the surface soils, particularly in
coastal regions and the adjoining sabkhas. Gypsum (calcium sulfate)
may be present as a contaminant in some aggregates.
The literature on sulfate attack is complex and confusing and
there is no consensus on some of the mechanisms involved.
In the Arabian Peninsula, probably the commonest form of attack
is from calcium sulfate in the groundwater or soil, which attacks
concrete to form ettringite in the hardened cement paste. This
formation generates stresses in the cement paste and, as a result,
cracks develop in the concrete until the outer surface
disintegrates. This disintegration exposes fresh areas to attack.
There are such high concentrations of sulfate in the soil and
groundwater that there is still plenty available to further attack
the affected area. Disintegration can be very rapid.
The form of attack varies, depending on how long it has been
taking place, the moisture conditions, temperature, concrete type
and so on. In many cases, spectacular deterioration and
disintegration of the concrete that is in a terminal condition
destroys the evidence of the earlier phases of the attack.
Examination of concrete below ground presents special prob- lems
because removal of the soil inevitably disrupts many of the areas
that have been attacked and special care is needed to gain an
accurate picture of the attack.
The external appearance of sulfate attack varies considerably:
Sulfate attack may lead to heaving and cracking in foun- dations
and ground floors on or in fill or soil containing sulfates. Heave
is due to the formation of expansive sul- fates within the
concrete.
During the early stages of attack by sodium sulfate and calcium
sulfate, the physical effect is that the outer layers of the
concrete exfoliate. This is accompanied by cor- rosion of the
reinforcement and a change of the hardened paste around aggregates
to a soft condition, with depo- sition of salt on surfaces and in
exfoliation cracks.
Concrete attacked by sodium sulfate, and in the long term by
calcium sulfate, is eventually reduced to a soft crumbly material;
but when magnesium sulfate is the main agent, the concrete remains
hard but becomes expanded and cracked. Magnesium sulfate is present
in seawater, but seawater is not generally regarded as being
aggressive to concrete because, in the presence of sodium chloride,
there is an inhibition of, or retarding action on, the expan- sive
reaction.
Another much rarer form of attack is through the formation of
thaumasite as a result of the reaction between calcium sili- cates
in the cement, calcium carbonate from limestone aggre- gates or
fillers, and sulfates, usually from external sources. The
conditions generally considered necessary for its for- mation
(Hartshorn and Sims, 1998) are:
Low temperature (less than 15OC)
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Guide to evaluation and repair of concrete structures in the
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Consistently high relative humidity
The thaumasite form of sulfate attack is unlikely in the Region
because of the cold conditions required for thaumasite for- mation.
However, it could theoretically occur in cold stores and similar
environments with continuous low temperatures.
Supplies of calcium, silicate, sulfate and carbonate
Initial reactive alumina (0.4 to 1 .O%).
Characteristic defects
Initially, physical salt weathering causes flakes of cement
paste and possibly $ne aggregate to become detached from the
surface. npically, this form of attack occurs from the ground
surface upwards fo r a f ew tens of centimetres, depending on the
chemistry of the soil, the concrete and the capillary rise
conditions. Eventually the surface may be eroded to a depth of 20
or 30 mm, leaving the aggregate standing proud.
I f the aggregate is more susceptible to this form of weathering
than the cement paste (e.g. porous and friable materials like some
weaker limestones and weathered gabbro) small discs of paste
between the aggregate pieces and the outside surface may be blown
off due to the expansive action within the aggregate. Such features
are called pop-outs. Eventually, the fragmentation of the aggregate
itself may lead to its complete disintegration, leaving a void in
the surface of the concrete.
Attack by salt weathering is designated in the literature as
physical salt weathering to distinguish it from chemical
weathering. The mechanism is similar to freeze-thaw disinte-
gration of concrete in that crystals (usually salts of sulfates and
possibly chlorides) develop in the pores of the concrete close to
its surface. The crystal growth exerts pressure in the pores
leading to local tensile stresses in the cement paste.
When capillary rise occurs, salt from groundwater or damp soil
is transported in solution vertically up through the concrete
member. Above ground level, the moisture is drawn to the surface
and evaporates, leaving crystals of salt growing in the
near-surface pores and resulting in significant surface erosion.
This form of attack is common just above ground level and may also
occur in marine structures and in process plants where seawater is
used for cooling or where parts of the structure are frequently
splashed by salty water.
2.8.1 Introduction
Most rocks used as aggregates are not affected by the sur-
rounding cement paste, but a few exhibit changes that can affect
the performance of the concrete. Some of these changes may occur
very rapidly while others may continue
for several decades. Some of the significant changes that may
take place within the concrete due to the materials used are
considered in the following sections.
2.8.2 Aggregate shrinkage and swelling
Characteristic defects
Aggregate swelling will lead to pop-outs on the surface and the
formation of three-legged cracks radiating from a point (also known
as Isle of Man cracks, as their appearance resembles the symbol of
the island on the west coast of Britain). Eventually individual
aggregate pieces may disin- tegrate leaving pock marks on the
surface. /f most of the aggregate in a concrete member is subject
to swelling, the tensile forces generated may crack the paste and
lead to general breakdown of the rnembev.
Shrinkage of clay-rich particles is very common although these
do not usually form a large proportion of an aggregate. Some clays
are particularly prone to expansion on wetting and shrinkage on
drying. Clay can be a contaminant of the aggregate or a natural
part of it. For example, instances have been reported of shrinkage
of natural aggregates that initially appear to be strong.
2.8.3 Aggregate softening
Characteristic defects
Aggregate softening will lead to pop-outs on the surface and the
formation of three-legged Isle of Man cracks. Individual aggregate
particles may disintegrate leaving pock marks on the surface
Softening can take place when an aggregate contains weak or
porous particles. In addition, aggregates composed of, or
containing large proportions of, clay minerals are also soft and
porous. When such rocks are present in hardened con- crete, the
concrete will exhibit greater volume changes on wetting and drying
than similar concrete containing non- swelling aggregate. Softening
also facilitates the ingress of aggressive substances such as
chlorides.
2.8.4 Alkali-silica reaction
It should be noted that alkali-silica reaction (ASR) is rare in
the Arabian Peninsula.
Characteristic defects
The main external evidence fo r damage to concrete due to
alkali-silica reaction is cracking. In the early stages, cracks are
centred on individual reacting aggregate pieces near the surface
and may give rise to three-legged Isle of Man crack patterns. In
unrestrained concrete the cracks develop with a random
distribution, often referred to as map cracking, where a network
offine cracks is bounded by a few larger cracks. When the expansive
forces are restrained, f o r example by reinforcement or by applied
loads in a column, the pattern of cracking is modified. Cracks tend
to run parallel to the direction of restraint, i.e. parallel to the
rein- forcement or vertically in a column. The cracks may exude
gel, which shows up as white deposits on the surface. There
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The problems
may be pop-outs where individual pieces of aggregate have
expanded and disintegrated just beneath the surface. This pattern
of cracking should not be confused with plastic shrinkage cracking
which may appear early in the life of the structure or drying
shrinkage cracks which may appear after some months or years. The
earliest time at which cracking due to ASR has been observed in
structures in the UK has been about Jive years after casting. With
some aggregates, cracking may not appear until a much longer time
has elapsed. There may be evidence of expansion of whole members
such as the closing of expansion joints.
Very f ew cases of alkali reactivity have been confirmed in the
Region, nonetheless the possibility of ASR should be con- sidered
when undertaking condition surveys.
Alkali-silica reaction occurs when the alkaline pore fluid and
the minerals in some aggregates react together to form a calcium
alkali-silicate gel. This gel takes up water, pro- ducing a volume
expansion that can disrupt the concrete, see Digest 330 (Building
Research Establishment, 1999). ASR will only cause damage to
concrete when all three of the fol- lowing factors are present:
sufficiently high alkalinity
sufficient moisture.
If any one of these factors is absent, then damage from ASR will
not occur.
Much work has been carried out on the structural effects of
ASR-induced expansion. In general it has been found that there is
little effect on structural performance. Indeed it has been
suggested that structural performance can be enhanced in some
situations; where the reaction causes some com- pression in the
concrete, this can offset some of the tensile stresses caused by
loading. However, the structural signifi- cance of the expansion
will depend on the type of structure, the effectiveness of the
reinforcement detailing and the exact location within the
structure. More importantly, cracking induced by ASR can reduce the
long-term durability of com- ponents, particularly through
corrosion of reinforcement.
A working party of the Institution of Structural Engineers has
made recommendations on assessing structures damaged by ASR
(Institution of Structural Engineers, 1992).
a critical amount of reactive silica in the aggregate
Characteristic defects
Abrasion results in localised or general depressions in a con-
crete surface. The abraded area usually has a rougher surface
texture.
Abrasion is the wearing away of the surface of the concrete.
Localised abrasion of floors may result from foot or vehicular
traffic. A common problem is the abrasion caused by the wheels of
fork-lift trucks. Abrasion may occur if
traffic is permitted to move on concrete slabs too early before
they have gained sufficient maturity.
Erosion may be caused by air or waterbome particles, such as the
erosion of coastal structures by the action of sand and pebbles
carried by the waves. Erosion will only occur if the abrading
medium is harder than the concrete.
In structures such as spillways and in tunnels and pipes with a
high flow rate, pitting of the surface due to cavitation may
occur.
2.10.1 Introduction
Cracks and crack patterns have different characteristics
depending on the underlying cause. For example, plastic shrinkage
cracks (see Section 2.10.2) tend to have an irregular pattern over
the structure while cracks due to cor- rosion (see Section 2.5)
will follow the lines of the rein- forcement.
The presence of cracks can influence the behaviour and dura-
bility of a concrete member. They can reduce the shear capa- city
of a section or provide a path by which moisture, oxygen, carbon
dioxide, chlorides etc can penetrate into the concrete surrounding
the reinforcement; in time this may result in reinforcement
corrosion. These aspects are covered in more detail in Section
2.5.
Concrete Society Technical Report 22 Non-structural cracks in
concrete (The Concrete Society, 1994) provides infor- mation on the
most common forms of intrinsic cracks in concrete. Figure 2.2
illustrates most of the types of crack likely to be experienced in
the lifetime of a concrete structure. Different types of crack may
occur at different times in the life of a concrete member. So as
well as crack patterns, a knowledge of the time of first appearance
of cracks is helpful in diagnosing causes of cracking. Crack types
and the time at which they are most likely to develop are listed in
Table 2.2.
2.10.2 Plastic shrinkage cracking
In the period just after it has been placed, the cement paste in
a concrete mix is still plastic and has little strength, and water
is able to move relatively freely in what is still a mixture.
Water, which is the least dense component of the mixture, and tends
to move upwards towards the surface as heavier materials move down
under gravity during compaction. This upward movement of water is
known as bleeding. Evapo- ration of water occurs at the surface,
becoming more rapid at high temperatures andor low humidity,
particularly in windy conditions, which can occur at any time of
the year in the Arabian Peninsula. If evaporation occurs faster
than bleed- ing, there is a net loss of water from the surface
layer of concrete leading to a reduction in volume. The surface
layer of concrete tries to shrink but is restrained by underlying
layers that are not subject to the same reduction in volume.
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Guide to evaluation and repair of concrete structures in the
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The result of the restraint is that tensile stresses are
developed in a zone near the surface. As the concrete is still in a
plastic state and of very low strength, irregular tension cracks
develop. The process is illustrated in Figure 2.3, the upper part
showing initiation and the lower the condition after a few hours.
(a) Initiation Characteristic defects
Plastic shrinkage cracks tend to be I to 2 mm wide and are
Evaporation
t 1 W 1 5 1 5
typically 300 to 500 mm long and 20 to 50 mm deep, though, in
some extreme circumstances, they may extend through the full depth
of a member The pattern of cracks is random but may be influenced
by the direction in which finishing opera- tions have been carried
out. As the cracks form in concrete when the paste is still in a
plastic state, they run through the paste rather than through
pieces of aggregate. These cracks can form in both unreinforced and
reinforced concrete.
2.10.3 Plastic settlement cracking
(b) A,,er a few hours
The upward movement of water described in Section 2.10.2 can
also lead to plastic settlement cracks. As the water moves upwards,
the denser constituents of the mixture move down- ward. This
downward movement may be obstructed by the top layer of
reinforcement or by the shuttering. The plastic concrete may arch
over the top of individual reinforcing bars, bringing the surface
into tension. Regular cracks over the bars often occur in
conjunction with voids under the bars, as shown in Figure 2.4; the
upper part shows initiation and the lower the condition after a few
hours. If the top layer of bars is closely spaced, the whole
surface layer of the concrete may hang up on the reinforcement
while the concrete below settles. This can lead to a horizontal
discontinuity beneath the bars, resulting in loss of bond and a
loss of the layer of concrete that would protect the bars against
corrosion.
Characteristic defects
The patterns of cracks associated with plastic settlement depend
on what is obstructing the downward movement. Most
Figure 2.3: Formation of plastic shrinkage cracks.
commonly, movement is restrained by reinforcement. The cracks
occur on the top surface and usually follow the line of the
uppermost bars, giving a series of parallel cracks: there may also
be shorter cracks over the bars running transversely. Cracks are
typically 1 mm wide and can run from the sui$ace to the bars, see
Figure 2.4. The settlement may also result in visible undulations
in the su$ace with high points over the top reinforcing bars. The
concrete can also be supported by the shuttering, causing restraint
to the concrete in connected members. This typically happens at
mushroom heads on columns, as illustrated in Figure 2.5, but can
also occur at other locations, such as under spacer blocks. Cracks
at mushroom heads of columns are generally horizontal, I mm wide
and can cross the full section. As these cracks form at very early
age they pass through the cement paste and not through aggregate
particles.
sion bending cracks Cracks at kicker join
Type of cracklng
Plastic settlement
Plastic shrinkage
Early thermal contraction
Long-term drying shrinkage
Crazing
Corrosion of reinforcement
Alkali-silica reaction
Figure 2.2: Examples of intrinsic cracks in hypothetical
structure (from Concrete Society TR22, based on Fookes, 1976).
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The problems
(a) Initiation
Crack
(b) After a few hours
figure 2.4: Formation of plastic settlement cracks.
2.10.4 Early thermal contraction cracking
The hydration reaction between cement and water that takes place
in concrete generates heat. The amount of heat gener- ated and the
rate at which it is generated depend on the amount and type of
cement and its fineness. The peak tem- perature reached is
dependent upon the cement type and content, initial temperature of
the concrete, ambient con- ditions, geometry of the member and type
of formwork. The high ambient and concrete temperatures encountered
in the Region speed up the reaction, resulting in a more rapid tem-
perature rise. Slabs have a large exposed surface area through
which the concrete can lose heat. Members with large cross-
sectional areas can develop higher internal temperatures than those
with smaller section, as the loss of heat through the top and side
surfaces has greater effect. Timber formwork pro- vides more
insulation than steel and so higher peak temper- atures may be
reached when timber formwork is used.
As concrete heats up it expands. If there is any restraint to
this expansion, for example from previous pours, such as when a
wall stem is concreted on a base slab or foundation, com- pressive
stresses will be generated in the young concrete. These stresses
are low, due to the low elastic modulus of the young concrete, and
are generally relieved by creep. Once the peak temperature has been
reached, at say 12 to 18 hours after
Figure 2.5: Formation of plastic shrinkage cracks in
columns.
placing (much later for very thick members), the concrete starts
to cool and reduce in volume. Restraint to this thermal contraction
will result in the development of tensile stresses. At this stage,
the concrete is more mature and has less capacity for relief of
strain by creep. The Young's modulus is greater and hence the
stresses generated are higher. The concrete is still relatively
weak in tension and the stresses caused by restraint to thermal
contraction can cause cracking.
Cracks may also be caused by differential temperatures in thick
members. When the surface layer cools, movement is restrained by
the core of the member, which is still at a higher temperature, and
cracks may form in the surface. When the temperature through the
member eventually becomes uniform, the surface cracks usually
close. In large members, there will tend to be a series of cracks
across the short direction in plan and elevation and possibly a
series of com- plimentary cracks in the long direction. Cracks will
tend to be wider near to edges and corners where heat is lost from
two or more faces.
Differential thermal contraction cracks are inevitable in some
situations, such as when a member is cast onto a previously poured
and hardened foundation. These cracks are antici- pated and
reinforcement is provided to control them and limit their width.
Even so, some cracks wider than the nominal design width can occur.
They do not necessarily mean that there are shortcomings in the
design or workmanship. Cracks of this type often occur in the walls
of water-retaining structures and may cause initial leakage. The
leakage will tend to reduce with time because of autogenous healing
of the cracks.
Characteristic defects
A classic case of early thermal contraction cracking is that
occurring in some walls poured on strip footings that were cast
several days earliez The stiflstrip footings restrain the thermal
movements in the wall. Cracks may form in the wall, starting at the
base and running approximately vertically. They will usually pass
right through the wall section. Cracks near the end of bays may be
inclined at approximately 45". Crack spacing and width will depend
on the amount of rein- forcement provided. Because these cracks
form a fer hardening but before full strength is achieved, they
generally run entirely through the paste and not through the
aggregate.
2.10.5 Crazing
Crazing can occur both on exposed surfaces and on surfaces in
contact with formwork. It occurs either when there is a change in
properties close to the surface or a high moisture content
gradient. The type of formwork is also important, as it can affect
the permeability of the formed concrete surface. Steel and plastic
formwork faces