NDT of bridges:introduction

7/25/2019 NDT of bridges:introduction

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Savitribai Phule Pune University

Non Destructive testing of

PSC Girders in Indian

Railways

A Dissertation Under the guidance of

Asst. Prof. JM Patekari

By

Naan Gu!ta

Submitted in partial fullmentof the requirements for the degree of

Master of Technology


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Acknowledgeents

! "ould li#e to e$press my appreciation to Asst% Prof% &M Pate#ari' for his

valuable support' advice and guidance during this pro(ect "or#% My

special appreciation goes to my family ) my family has been a consistent

source of love' care' support and a*ection' a person could ever as# for% !

"ould li#e to than# administrative and technical sta* members of !+!,-.%

/inally' ! than# the almighty 0od' "ithout blessings of "hose' nothing

"ould be possible%

A"stract

.on)destructive testing is gaining grounds today% !n this era of quic#

decision ma#ing staying aloof from technological developments can

become a nemesis to decay for an organi1ation% !ndian rail"ays hence

should adopt this ne" technology and put it in to day to day use% This "ill

require standardi1ation of equipment and procedures% !t "ill also require

ma#ing our structure suitable for .DT% Through this pro(ect ! "ant to

contribute in ma#ing .DT a regular feature of routine maintenance of

structural assets of !ndian rail"ays%

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#iterature studied

0uidelines on .on)destructive testing of bridges

BS 3 45' August' 2446' +DS7

8andboo# on .on)destructive Testing of ,oncrete

9%M% Malhotra' .icholas ,arino

:0uidelines /or !nspection' Maintenance And +ehabilitation 7f ,oncrete

Bridges:' page no% 2)4' +DS7%

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Contents

Particulars Page Nu"er

IN$R%D&C$I%N ;

ND$ '%R S$R(NG$)

ASS(SM(N$ %' C%NCR($(

!.T+7DU,T!7.

Bridges form the means to connect the distant areas of the nation. Indian railways have more

than one lac bridges and majority of them have already completed their codal life. The

conversion of track to different gauges and increase in speeds and axle loading of trains has

also increased the need of studying existing bridges. Various types of projects in this context

has also been undertaken some of them are acoustic emission, under water inspection, non

distractive testing, instrumentation of bridges etc. The present method of bridge inspection is

mostly visual and is not capable to assess hidden defects in structure. But this method is

highly subjective and leads to unreasonable imbalance in gauging of the health of the bridges.

ence their exist a need to !uantify the inspection and reduce the subjectivity.

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The ever decreasing cost of test and increasing cost of a delayed decision had made "#T an

indispensable preposition. Today it involves lesser expense, preparation or damage as

compared to traditional methods. The choice of particular "#T method depends upon the

property of concrete to be observed such as strength, corrosion, crack monitoring etc. Though

there are some limitations of these test methods. $ven then subse!uent testing of structure will

largely depend upon the results of preliminary. Thereby justifying the expenses involved in

further testing.

"#T techni!ues not only provide fair idea about the relative strength and overall !uality of

concrete in structure but also help in deciding whether more rigorous tests like load testing,

core drilling etc. at selected location are re!uired or not. The objective of a non% destructive

test is to obtain an estimate of the re!uired property of material by measuring certain

parameters which are empirically related to its strength.

Purpose of Non-destructive Tests:

The non%destructive evaluation techni!ues are being increasingly adopted in concrete

structures for the following purposes&

'i( $stimating the in%situ compressive strength

'ii( $stimating the uniformity and homogeneity

'iii( $stimating the !uality in relation to standard re!uirement

'iv( Identifying areas of lower integrity in comparison to other parts

'v( #etection of presence of cracks, voids and other imperfections

'vi( )onitoring changes in the structure of the concrete which may occur with time

'vii( Identification of reinforcement profile and measurement of cover, bar diameter, etc.

'viii( *ondition of pre%stressing+reinforcement steel with respect to corrosion'ix( *hloride, sulphate, alkali contents or degree of carbonation

'x( )easurement of $lastic )odulus

'xi( *ondition of grouting in pre%stressing cable ducts

(xii) Monitoring changes in the structure of the concrete, degree of corrosion in steel

reinforcement, change in geometry of structure etc.

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pportunities:

. -ower )oney cost

. -ower Time cost

!hallenges:

. *orrelation with strength and durability.

". #ystematic testing

Monitoring of structures using systematic testing is my topic of focus in this project

Types of Non $estructive Tests:

/ccording to their use, non%destructive e!uipment can be grouped as under&

i( 0trength estimation of concrete

ii( *orrosion assessment and monitoring

iii( #etecting defects in concrete structure

iv( -aboratory tests

The tests I will like to make use of in my project falls under following heads

@

NDT FOR CORROSION ASSESSMENT

Corrosion Analyer

Resistivity Meter

NDT FOR LOCATING CRACK AND ITS GROWTH

Crack Detection Microsco!e

(ddy Current Meter

Infrared $/eral Iager


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,8APT-+ 2>.DT /7+ ST+-.0T8 ASS-SM-.T 7/ ,7.,+-T-

%. &e'ound ammer

It is used to assess the compressive strength of concrete in existing structures. 1hen plunger

of rebound hammer is pressed against the surface of the concrete, the spring controlled mass

rebounds 2 extent of such rebound depends on the surface hardness which is co% related to

compressive strength of *oncrete. The compressive strength can be read directly from the

graph provided on the body of the hammer.

This method is based on the principle that the rebound of an elastic mass depends on the

hardness of the surface against which the mass impinges. 3ebound ammer consists of a

spring%controlled mass that slides on a plunger within a tubular housing. 1hen the plunger is

pressed against the surface of the concrete, the spring controlled mass rebounds and the

extent of such rebound depends upon the surface hardness and, therefore, the rebound is

related to the compressive strength of the concrete. The rebound value is read along a

graduated scale and is designated as the 4 rebound number4. The compressive strength cab

be read directly from the graph provided on the body of the hammer. #epending upon the

impact energy, these are classified into four types i.e. ", -, ) and 5. Type " test hammer

having impact energy of . "%m and are suitable for grades of concrete from )6 to )76.

Type 5 is suitable for grades of concrete below )6. Type - test hammer is suitable forlightweight concrete or small and impact sensitive part of the structure. Type ) test hammer

is generally recommended for heavy structure and mass concrete.


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Re"ound )aer

The rebound hammer method provides a convenient and rapid indication of the compressive

strength of concrete by means of establishing a suitable correlation between the rebound

number and the strength of concrete. 3ebound hammer directly gives the average

compressive strength of the tested location. The compressive strength is in "+mm. 8nit is

inter changeable to 3, 9, psi and :g+cm.

nternals of re'ound hammer

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The rebound number increases as a strength increases but it is also influenced by a number of

other factors like type of cement and concrete; surface condition and moisture content ,age of

concrete and extent of carbonation on concrete surface . as such the estimation of strength of

concrete by rebound hammer method cannot be considered to be very accurate and probable

accuracy of prediction of concrete strength in a structure is < 6 =. If the relationship

between rebounding number and compressive strength can be checked by test on core

samples obtained from the structure of standard specimens made with the same concrete

materials and mix proportion then the accuracy of the result and confidence there on are

greatly increased . it can then be used with greater confidence for differentiating between the

!uestionable and acceptable parts of a structure or for relative comparison between two

different structures.

". *ltrasonic Pulse +elocity Meter :

It is used for assessment of !uality of concrete in existing structure based on its density.

9uality of concrete is considered as good in terms of its density, homogeneity and uniformity.

This is based on the principle that the velocity of an ultrasonic pulse through any material

depends upon the density, modules of elasticity and 5oisson>s ratio. *omparatively higher

velocity is obtained when concrete !uality is good in terms of density, uniformity,

homogeneity,

&ltrasonic !ulse 0elocity Meter

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5ulse Velocity measurements can be used to assess the homogeneity of concrete,

presence of cracks, voids etc., !uality of concrete relative to standards re!uirements.

8ltrasonic pulse velocity measurements are influenced by surface condition, moisture

content, temperature of concrete, path length, shape and si?e of member and

presence of reinforcing bars. The method is complex and re!uires skill to obtain

usable results, which can often provide excellent information regarding condition of

concrete.

using the special e!uipment the ultrasonic pulse is produced by a transducer held in

contact with one surface of concrete members 8nder test .after traversing a known

path length '-( in the concrete, the pulse of vibration is converted into an electrical

signal by the second transducer held in contact with the other surface of the concrete

members at the predetermined place and an electric timing circuit enables the

transmit time'T( of the pulse to be measured. the pulse velocity is given by

V @L

T in km+sec.

There are three possible ways of measuring pulse velocity.

i( #irect transmission

ii( 0emi direct transmission

iii( Indirect transmission 'surface probing(

out of the three methods , the direct transmission method is considered to be the best.

Ance the ultrasonic pulse impinges on the surface of the material, the maximum energy is

propagated at right angles to the face of the transmitting transducer, and best results are

therefore obtained when the receiving transducer is placed on the opposite face of the

concrete member. This is called #irect TransmissionC or *ross 5robingC. In many

situations, the two opposite faces of the structural member may not be accessible for

measurements. In such cases, the transmitting and receiving transducers are placed on the

same face of the concrete member. This is called 0urface TransmissionC. 0urface


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transmissionC is not so efficient as #irect TransmissionC, because the signal produced at the

receiving transducer has an amplitude of only to D = of that produced by *ross 5robingC,

and the test results may vary from 6 to E= depending upon the !uality of concrete under

test.

In view of inherent variability in the test results, sufficient number of readings are taken by

dividing the structural member under test in suitable grid markings of DExDEcm and in some

cases even smaller. $ach junction point of the grid becomes a point of observation.

$a"le 1 Guidelines for assessing condition of concrete "ased on !ulse velocity

0ince actual values of the pulse velocity obtained depend on a number of parameters, any

criterion for assessing the !uality of concrete on the basis of pulse velocity as given in the

above table can be considered as satisfactory only to a general extent. owever, when the

comparison is made amongst different parts of the structure, which have been built at thesame time with similar materials, construction practices and supervision, the assessment of

!uality becomes more meaningful and reliable.

The assessment of compressive strength of concrete from ultrasonic pulse velocity values is

not ade!uate because the statistical confidence of the correlation between the ultrasonic pulse

velocity and the compressive strength of the concrete is not very high. 8ltrasonic 5ulse

Velocity test can also be used for measuring depth of crack.

'igure 1 Mec/anis for !ulse velocity

. indsor Pro'e:

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1indsor 5robe is used to find out the compressive strength of existing concrete structure.

This e!uipment gives compressive strength of concrete by driving a steel probe into the

surface of concrete. The lower the depth of probe penetration, higher the compressive

strength of concrete. /0T), * FED%F has standardi?ed this e!uipment+test procedure. The

underlying principle of this penetration resistance techni!ue is that for standard test

conditions, the penetration of probe into the concrete is inversely proportional to the

compressive strength of the concrete. In other words, larger the expose length of the probe,

greater the compressive strength of concrete.

Windsor Probe

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+indsor !ro"e sc/eatics

This e!uipment consists of a power%activated gun or driver unit, hardened alloy probe, loaded

art%ridge and a measuring instrument such as depth gauge etc. The probes are G.D6mm in

diameter and H.6mm in length. -arger diameter probes 'H.7mm( are also available for

testing lightweight concrete. 5robe is threaded in to the probe%driving head and fired into the

concrete using a template. The driver utili?es a standard power cartridge. The power level can

be reduced when testing low strength concrete by locating the probe at a fixed position within

the driver barrel. Two types of templates are provided with the e!uipment e.g. single probe

template and a three probe triangular template. $xposed length of probe is correlated to the

compressive strength of concrete.

The 1indsor 5robe is basically a hardness tester and provides an excellent means of

determining the relative strength of concrete in the same structure or relative strengths in

different structures. The test is not expected to determine the absolute values of strength of

concrete in the structure. The method may be used to assess the uniformity of in%situ

concrete, to delineate ?ones or regions of poor !uality or deteriorated concrete in the structure

and to indicate changes with time in characteristics of concrete, when forms and shoring may

be removed.

The precision of 1indsor probe measurement has been found to vary with the maximum si?e

of aggregates in concrete. The penetration of the probe in to the concrete is affected by the;


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hardness of the aggregates. Therefore, it is desirable to prepare separate calibration curve for

the type of aggregate used in the concrete under investigation. There are re!uirements of

minimum edge distance, probe spacing and member thickness. If the minimum recommended

dimensions are not complied with, there can be danger of splitting of members. The

penetration techni!ue is considered almost non%destructive as the damage to concrete made

by G mm probes is only local, which has to be made good. The test has the advantage over

rebound hammer test as the measurement is made not on the surface of the concrete but in

depth .

. !ore !utter :

It is used for estimation of physical characteristics of masonry + concrete by extracting core

samples from the structure. The maximum depth of the core which can be cut is 7DE mm. The

diameter of cutters is 6mm and HFmm.

'igure , Core Cutter

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/ core is usually cut by means of rotary cutting tool with diamond bit . 1ater supply is also

very necessary to lubricate the cutter. The core which is extracted from existing structure is

used for various testingJs like compressive strength, crac/ depth, 0ater a'sorption etc.

The core will be tested for compressive strength and at least three cores shall be tested for

acceptable accuracy. Tests should be conducted as per I0 & 6G%6, I0 & K 6 2 I0 &

76G K EEE.

=% Permeability Tester>

Permeability tester

This instrument is used for determination of the air permeability of cover concrete. This

operates under vacuum 2 used on concrete structure. It permits a rapid 2 non destructive

measurement of !uality of the cover concrete with respect to its durability.

0ignificance of permeability in addition to compressive strength in accessing !uality of

concrete has become more important due to increase instances of corrosion in reinforce

cement concrete. The rate at which the air from concrete cover me extracted is a measure of

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permeability of concrete .this method can be used to access the resistance of concrete to

carbonation, penetration of aggressive ions and !uality of grout in post tension ducts.

It operates under vacuum and can be used at the building site and also in the laboratory. the

essential features of the method of measurement are a two *hamber vacuum cell and pressure

regulator which ensures an air flow at right angles to the surface and into the inner chamber.

#ry 0urface without cracks should be selected for test .it should we ensured that inner

chambers should not be located above the reinforcement bar. pressure loss is calibrated from

time to time and after a large change in temperature and pressure. D to G measurements of

electrical resistance of the concrete and its mean value is taken for the measurement of

coefficient of permeability . this test permits the calculation of the permeability *oefficient

kT on the basis of theoretical model.

In case of dry concrete, the results in good agreement with the laboratory methods, such as

Axygen permeability , capillary suction, chloride penetration and others .

9uality of cover concrete Index kT 'E%Gm(

Very Bad 6 L E

Bad 7 .E % E

"ormal D E. K .E

Mood E.E K E.

Very Mood N E.E$a"le , $/e 2uality class of t/e cover concrete is deterined fro k$

The humidity, a main influence on the permeability, is compensated by additional measuring

the electrical resistance O of the concrete .with kT and O the !uality class is obtained from a

nomogram.

1. +ideo 2ore scope :

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Those who are familiar with maintenance procedures know that there are three types of

maintenance in any facility& preventive maintenance, corrective maintenance and predictive

maintenance. 1e normally follow established procedures in each of these types, since each

one targets a different aspect of maintenance. Borescope inspections are an integral part of

procedures for preventive maintenance, along with such routine tasks.

,8APT-+ 5,7++7S!7. ASS-SSM-.T

1. CORROSION ANALYZER :


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Corrosion analyzer is based on electro chemical process to detect corrosion in the

reinforcement bar of the structure. The instrument measures the potential and the

electrical resistance between the reinforcement and the surface to evaluate the

corrosion activity as well as the actual condition for the cover layer during testing.

The electrical activity of the steel reinforcement and concrete leads them to be

considered as one half of weak battery cell with the steel acting as one electrode

and concrete as electrolyte. The name half cell surveying derives from the fact that

the one half of the battery cell is considered to be the steel reinforcing bar and the

surrounding concrete. The electric potential of a point on the surface of steel

reinforcing bar can be measured comparing its potential with that of copper -

copper sulphate reference electrode on the surface. In field it is achieved by

connecting a wire from one terminal of a voltmeter to the reinforcement and another

wire to the copper sulphate reference electrode.

Corrosion Analyser

This risk of corrosion is evaluated by means of the potential gradient obtained. The

higher the gradient, the higher risk of corrosion. AT! C " #$% prescribes a half

potential method for detection of reinforcement corrosion.

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2. RESISTIVITY METER:

This instrument is used to measure the electrical resistance of concrete cover for

corrosion status of reinforced bars. &ower the electrical resistance, the more is the

probability of corrosion of metal.

It is based on the principle that the corrosion of steel in concrete is an electro-chemical process, which generates a flow of current and can dissolve metals. The

lower the electrical resistance, the more readily the corrosion current flows through

the concrete and the greater is the probability of corrosion.

The limits of possible corrosion are related with resistivity as under' -

(ith )* kcm ++++ corrosion is improbable24


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(ith # to )* kcm ++++ corrosion is possible

(ith # kcm+++ corrosion is fairly certain

where, rho resistivity

/esistivity !eter is used to measure the electrical resistance of the cover concrete.

(ith the graphical display of the ma0or values, it is possible to determine the spots in

the concrete structure where corrosion may occur. The combination of resistance

measurement by /esistivity !eter and potential measurement by Resistivit Meter

Corrosion Analyzing Instrument described below furthermore improves the

information about the corrosion condition of the rebar.

,8APT-+ ;.DT /7+ D-TA!S 7/ +-!./7+,-M-.T ST--

1. PRO!OMETER

5rofometer is a portable battery operated e!uipment used for measuring the depth of cover

concrete , location and si?e of the steel reinforcement embedded in concrete. The e!uipment

is useful for investigating the structures where drawings are not available .the e!uipment

consist of data logger , diameter probe and calibration blocks. The e!uipment has sufficient

memory store the scanned data. The meter needle is ?eroed and the probe moved over the

concrete surface and rotated to obtain a maximum reading and this position correspond to the

location of reinforcement bar. It is used for 'a( measuring concrete cover 'b( detecting

reinforcing bar 'c( determine bar si?e and direction .

In heavily reinforced section, however, the effect of secondary reinforcement cannot be

eliminated completely . "evertheless, this e!uipment give fair idea about average thickness

of cover with maximum duration of < 6 mm .

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&sing Profoeter

,8APT-+ =.DT /7+ 7,AT!.0 ,+A, A.D !TS 0+7CT8

%. !rac/ $etection Microscope

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'igure 3 Crack Detection Microsco!e

This is the pocket si?e e!uipment used crack width measurement of concrete member,

masonry and other type of structures. for measurement of crack width a simple small hand%

held microscope having graduated scale marked on the lens known as 4crack comparer4 may

be used. where greater accuracy of measurement of crack is re!uired. transducer or

extensometer or strain gauges can be used.

#epth of crack can be measured either by 5ulse Velocity Techni!ue '/0T) *%6H( A3 by

taking cores from concrete. *ontinuous monitoring and recording of crack movements for 7

hours maybe re!uired for separating cracks caused due to temperature effects from that due to

o load effects.

". 3ddy !urrent Meter:

$ddy current metre to predominantly used in detecting the cracks in the metal structures.

availability of cracks disrupt the flow of eddy current. /vailability of cracks disrupt the flow

of eddy current this disruption is measured to know the Plaws+*racks+Voids etc. $ddy current

meter can be used in the field for detection of Plaws+*racks+Voids in the metal structures i.e.

0teel girder bridges, PABJs etc. in the field.

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'igure 4 (ddy Current Meter

. nfrared magery

'igure 5 Infrared t/eral iager . source6 www.w7ne8co7usa.co

Infrared is an energy similar to visible light but with a longer wavelength. Infrared energy is

invisible to the human eye, however, while visible light energy is emitted by objects only at a

very high temperature, infrared energy is emitted by all objects at average temperatures.

0ince, thermal imagers sense infrared energy which varies with temperature of objects image

generator provider a thermal signature of these objects. this image cannot be displayed on

standard video monitor. infrared energy from object a focus by optics onto an infrared detector.

The infrared information is then passed to sensor electronics for image processing. The signalprocessing circuitry translates infrared detector data into an image that can be viewed on a

standard video monitor.

The Thermographer measures the temperature of a target. you have to put the proper data into

the camera in order to get accurate temperature measurements. this means that setting up your

camera before an inspection is important.

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The thermal imager uses I3 Pusion technology, which simultaneously captures both a digital

photo as well as infrared image and fuses them together making it easier to identify features

and taking the mystery out of I3 image analysis. simply scroll through the different viewing

modes to better identify trouble areas in full I3 thermal, picture % in % picture or automatic

blend visual and thermal imagers.

0witch on the camera. Insert the gb memory card, allows users to save more than DEEE screen

images or EE I3 fusion images. focus the lens at target by manually rotating lens until the

image is in focus . 5ress the -evel and 0pan button to automatically set the cameras

temperatures level and span. 5ress the same button again to properly scale the image .press and

hold the same button until the I3 fusion blend level control box appears on the display screen.

Tap the Trigger button to retain settings. Tap the Trigger button once to pause the live image.

5ress and hold the trigger button for seconds to save the image.

,8APT-+ ?#IMIT/- 8-T3/0A"I* #I0T/"*$ )$/083I"M T$0T$3

'igure - DIGI$A# $RAS%NIC DIS$ANC( M(AS&RING $(S$(R

There is a large population of *oncrete+ 0teel bridges on Indian 3ailways. 0ometimes it is

very difficult to measure unreachable second point of the girder length of the bridges with

the help of measuring tape. #igital 8ltrasonic )easuring Tool is a measuring device that can

carry several measuring operations such as the length, surface area and volume of

unreachable surfaces with the use of ultrasonic waves. Its measuring angle is E.G metres to

2=


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E metres. An switching the unit it is automatically in the operation mode 4-ength

)easurement4 and measurements can be recorded.

#uring the measuring procedure ,a -aser indicated 'H% point laser( is also activated, which

indicates if the unit is pointed at the desired target 0urface. The laser points are arranged

circular and outline the measured surface. If the ambient light conditions are too bright , the

visibility can be increased by using the laser spectacles.

The running 5eriod of the conical expanding 8ltrasonic waves used for measurement of

distance. The respective measuring surface is )arked by the laser indicator. The

measurements can only be carried out on targets with even and smooth surface .

It can be used to measure the distance of unreachable points. It directly give the surface area

and volume of a rectangular structures by measuring the re!uired dimensions.

2?

utcome of 4iterature &evie0

Present literature on .DT in !ndia covers most of the aspects

of .DT and is being satisfactorily used to gauge structural

strength in case of ne"ly constructed or distressed

structures% The ma(or eld of action of !ndian rail"ays is

routine maintenance of e$isting structures% The present

literature is silent on this aspect% More over the structural

design in !ndia does not ta#e into account the needs of .DT

during the lifespan of the structure% !f these needs are met

NDT of bridges:introduction

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