UNIT 5 TESTING OF HARDENED

Structure
5.1 Introduction
Objectives
5.2 Surface Hardness Methods
5.2.1 Indentation Principle
5.2.2 Rebound Principle
5.3 Penetration Techniques
5.3.1 Windsor Probe
5.3.2 PNR Tester

i 5.4 Pullout Tests

5.5 Resonant Frequency Method
/! 5.6 Ultrasonic Pulse Velocity Method
Pulse Attenuation Method
Pulse Echo Method
Combined Methods
5.9.1 Dynamic Modulus of Elasticity and Llamping Constant
5.9.2 Lntrasonic Pulse Velocity and Rebound Nurnher
5.9.3 Pulse Velocity and Pulse echo Method
5.9.4 Pulse Velocity and Core Drilling
Radioactive Method
Nuclear Methods
5.11.1 Neutron Mositure Gauges
5.11.2 Neutron Activation Analysis
Magnetic Methods
Electrical Methods
5.13.1 Determination of Corrosion of Reinforcement Bar
5.13.2 Determination of Thickness of Concrete Pavement
5.13.3 Determination of Moisture Content of Hardened Concrete
5.13.4 Electrical Resistivity Method for Estimation of Compressive Strength

Acoustic Emission Technique

Summary
Key Words
Answers to SAQs
-- -

5.1 INTRODUCTION
Non-destructive testing methods have been in use for about four decades now. They are
now considered a powerful tool for evaluating,existing concrete structures with regard to
their strength and durability, apart from assessment and control of quality of hardened
concrete. In certain cases the investigation of crack depth, micro cracks and progressive
deterioration are also studied by this method. However, at present no Indian Standard
Code exists on thcse methods.
Non-destructive testing methods are relatively simple to perform, but the analysis and
interpretation of their test results is not easy. In these methods the specimens are not
loaded to failure and as such the strength inferred or estimated cannot be expected to yield
absolute values of strength. The non-destructive testing methods attempt to measure some
other properties of concrete from which an estimate of its strength, durability and elastic
parameters are obtained. Some such properties are hardness, penetration, rebound,
resonant frequency, response to gamma rays and acoustic emission etc.
The non-destructive test methods could be classified broadly as :
a) Surface Hardness Methods,
Tests for Concrete and c) Pullout Test,
MiscellaneousMaterials
d) Resonant Frequency Method,
e) Ultrasonic Pulse Velocity Method,
f) Combined Methods,
g) Radioactive and Nuclear Methods,
h) Magnetic and Electrical Methods, and
i) Acoustic Emission Techniques.
Objectives
By the end of this unit, you should be able to :
explain the need for non-destructive test,
explain the limitation of non-destructive test, and
describe the suitability of different tests for different purposes.

5.2 SURFACE HARDNESS METHODS

There are two principles using which surface hardness tests can be carried out. These are,
Indentation principle and rebound principle.
5.2.1 Indentation Principle
The known methods employing this principle are, William's testing pistol, Frank spring
hammer and Einbeck pendulum hammer. The underlying principle essentially consists of
impacting the surface of concrete in a standard manner using a given mass aclivated by a
given energy and measuring the size of indentation. There is a little apparent theoretical
relationship between the strength of the concrete and its hardness so measured.
The relation established for indentation principle by Williams testing pistol is

where,
f,= compressive strength, and
Z = the curved surface of indentation.
Uses
They are simple methods md provide large number of readings in a short time. It is
claimed that the strength of concrete can be predicted with an accuracy of k 20 - 30%.
However, the measurement of the size of indentation is a tedious job and strains eyes for
large scale use.
5.2.2 Rebound Principle
The only known instrument using this principle for concrete testing is the test hammer by
Schmidt.
In this method the hammer is held against the surface be tested and pressed gently to
release the spring loaded mass from its locked position to cause impact and the mass
rebounds carrying a rider with it along an index which is read to the nearest whole number
and average of such 10 readings is designated as rebound number.
The various factors affecting rebound number are :
i) smoothness of surface under test,
ii) size and rigidity of test specimen,
iii) age of test specimen,
iv) surface and internal moisture condition of the concrete,
v) type of coarse aggregate,
vi) type of cement, and
. n..rknn.,t;~n,.f n-,r,.mt,, n..rfnnn
 Testing of Hardened
The relation between compressive strength and rebound number fits linear regression, Concrete-I I
f,=A+BN
where,
f, = compressive strength ( ~ l m m ~ ) ,
N = rebound number, and
A and B = constants, by which a calibration curve can be established.

Irexpensive, simple in operation and large number of readings can be obtained quickly and
c Ileaply permitting a rapid assessment of the correlated property.
Limitations
Estimation of strength of concrete by Schmidt hammer with an accuracy o f f 15 to 20% +
may be possible only for specimens cast cured and tested under identical conditions as
those from which the calibration curves are establised. When little information about the
concrete is available, then the possible error may be upto f 25%.
Uses
This method is generally used for :
i) checking the uniformity of concrete quality with respect to the mean quality
in statistical terms of measured headness,
ii) comparing a given concrete with a reference to statistical terms of a specific
requirement,
iii) approximate indication of concrete strength, and
iv) acceptance testing in precast concrete industry.

SAQ 1
Differentiate between indentation and rebound principles?

PENETRATION TECHNIQUES
The known instruments in this technique are Simbi Hammer, Spit Pins, Windsor Probe and
PNR Tester.

5.3.1 Windsor Probe

% Iconsists of powder actuated gun or driver, hardened alloy probes, loaded cartridges and
k p t h gauge. The probe has a diameter of 6.3 mm, length of 79.5 mm and a frusto conical
point on the front. The rear of the probe is threaded and screws into a probe driving head
which is 12.6 mm in diameter and fits snuggly into the bore of the drive. The probe is
driven by firing a powder charge that develops an energy of 79.5 kg-m. The exposed
length of the probe is measured by a calibration depth gauge.
Cylinder once probed shall not be used to get compressive strength. It is reported to give
115 % for high.strength concrete when compared to unprobed companion cylinders at an
age of 28 days. The probe leaves a minor disturbance for the depth it penetrated, which can
be manageable in large structures.
f, = A W compressive strength in kg/cm2.
where, W = Penetration of the probe in mm and A and B are constants.
Advantages
The windsor probe equipment is simple and within the grasp of laboratory technician.
Equipment is well made and needs little maintenance. The system has a number of built-in
safety features that prevent accidental discharge.
Tests for Concrete and Use and Limitations
Miscellaneous Materials
It should not be expected to yield absolute strength values. It provides an excellent means
for determining the relative strength of concrete in the same structure or in different
structures without extensive calibration with specific concretes.
5.3.2 PNR Tester
This tester consists of a spring loaded hammer encased within its body that drives a small
pin into the concrete. The hammer can grip a pin of length of 30.5 rnrn having a diameter
of 3.56 mm and a tip machined at an angle of 22.5'. The hammer is activated when it is
tightened against the spring by rotating and forcing its handle through the loading bolt. The
spring stifmess is 49.7 Nlmm and the apparatus is built to store about 108 N - mm of
energy. For testing, the equipment is kept on the concrete surface. The hammer is attached
with pin and tightened to buildup 108 N-mm of energy, then released to penetrate the
concrete and the penetration of the pin is measured with the help of a dial gauge
(Figure 5.1).

Figure 5.1 :PM1 Tater

A linear relationship exists between pin penetration and strength given by:
C=A+BP ...(5.2)

where,
A and B = constants,
P = pin penetration in mm, and
C = compressive strength in kg/ cm2.
Uses of PNR Tester
i) Useful in operations of prestressing and form removal.
ii) A linear relationship can be established between strength and penetration.
iii) It is easy to conduct and inexpensive.

5.4 PULLOUT TESTS

The various types of pullout tests tried out are :
a) Lok Test also known as Standard Pullout Assembly.
b) Break off Tester (TNS Tester).
c) Internal Fracture Test.
d) Epoxy Grouted Bolt.
e) Internal Pullout Test.
f) Pulloff Test.
In this unit, we will only examine the Lok Test, among the Pullout Tests.
 -
Testing of Hardened
-
5.4.1 Standard Pullout Assembly or Lok Test Concrete-I I
This pullout test measures with a hollow tension 1a111,Lhe force required to pullout from
the concrete, a specially shaped stecl rod whose enlarged end has been cast into that
concrete. The pullout force required is measured using a dynamometer. The concrete is
subjected to a complex three-dimcnsional state of stress, that is tension, tension and
compression. Tensile stresses are directed in the circumferential and the radial direction,
while the compressive stress is directed parallel to a line from the disk to the reaction ring.
The pullout strength is of the order of 20% of the compressive strength (Figure 5.2).

CROSS SECTION
Load cell
s1it

TOP VIEW

Figure 5.2 :LoL Test

Field Use
Pullout assemblies can be incorporated in the formwork for critical structural members and
then tested for strength at required age. Another way is to cast large blocks separately of
the same mix as that of the members and care should be taken to give compaction and
curing similar to that in the structure.
Advantages and Limitations
The main advantage is that, the pullout test measures directly shear strength of concrete in
the structure. The equipment is simple to assemble and operate. The equipment is safe and
the testing can be done in the field in a mattcr of minutes.
The rcsults of tests are reproducible with an acceptable degree of accuracy and do correlate
with compressive strength of concrete.
log f, = B0 + Bl log P ...(5.3)

where,
f, = cube compressive strength (kg/cm2),
P = pullout strength (kg/cm2),
Bo = intercept of the line, and
B, = slope of the line.
Pullout tests are superior lo the other NDT methods like rebound hammer and windsor
probe because of the greater depth and volume of concrete tested. The major drawback is
that, the tests are to be planned in advance and the damage to the concrete surface must be
repaired.
SAQ 2
What are the advantages and limitations of Lok test?
Tests for Concrete and
Miscellaneous Materials

This method is based upon the determination of the fundamental resonant frequency of
vibration of a specimen, the continuous vibration being generated electromechanically.
The equipment essentially consists of vibration generating section and vibration sensing
section. A specimen can be vibrated in three modes, which are longitudinal Resonance,
Transverse Resonance and Torsional Resonance. ASTM C 215-60 gives the details of
equipment, specimen and testmg procedures in detail.
This method is uscful in studying :
i) deterioration of concrete in freeze thaw cycling, and
ii) corrosion of concrete in aggressive media.
The test can be used to determine the dynamic modulus of elasticity given by:

where ,
W = weight of the specimen,
C = constant, and
n = resonance frequency.
Dynamic modulus of elasticity so determined is free from the effect of creep and is larger
than the static modulus, the ratio being of the order of 1.3 to 1.7 depending on the material
and mix parameters.
It is useful for computing damping constant, Q, given by

wherc,
fo = resonance frequency of vibration, and
f, and[, = frequencies on either side of resonance at which amplitude is 0.707 that
at resonance.
Also useful in determination of damage due to fire exposure.
The major limitations of these methods are :
a) These tests can normally be carried out only on small size specimens in a
laboratory rather than on structural members in the field.
b) The equations for the calculation of dynamic modulus involve shape factor
corrections. This necessarily limits the shape of the specimen to cylindrical
or prismatic types. Any deviation from the standard shape can render the
application of shape factor correction rather complex. This method is more
useful for laboratory work and not for site work.

5.6 ULTRASONIC PULSE VELOCITY METHOD

The ultrasonic pulse velocity method consists of measuring the time of travel of an
ultrasonic wave passing through the concrete. The instrument essentially consists of pulse
generator and receiver circuit. Pulses are generated by piezo electric crystals. Pulse
generator circuit converts an AC signal to wave of mechanical energy having vibration
frequencies in the range ot 20 kHz to 150 kHz and repeating at a rate of not less than
10 pulses1 sec nor more than 150 pulse/ sec. Probe contact with the surface of concrete is
made through a suitable coupling medium to avoid any air entraping, thus causing loss of
acoustic energy at the interface. A similar probe is coupled to the concrete at the opposite
face which changes mechanical energy'into an AC signal of the same frequency. Thc time
of travel between the initial onset and reception of the pulse is measured electro11ic;ally.
There are three instruments using different frequency of probes commercially available.
They are ultrasonic concretc tester (150 kHz), PUNDIT (50 kHz) and soniscope (20 kHz).
The path length between transducers divided by the time of travel gives the average
velocity of wave propagation. .
where, Testing of Hardened
Concrete-I1
V = UPV (m/ sec), and
t = Time of travel (sec).
Pulse velocity, 'V ' is related to the elastic modulus (E), density (p) and the poissons ratio
CL by

The techniques of measuring pulse velocity through concrete are :

1) smoothness of contact surface under test,
ii) temperature of concrete,
iii) moisture condition of concrete,
iv) presence of reinforcing steel,
v) age of concrete,
vi) water - cement ratio,
vii) type of aggregate,
viii) presence of calcium chloride,
ix) air entrainment, and
X) stress level.
Thert: are several applications of this method, which are :
i) establishing uniformity of concrete,
ii) dqtermination of modulus of elasticity,
iii) determination of setting characteristics of concrete,
iv) studies on durability of concrete,
v) measurement and detection of cracks,
vi) determination of time of removal of formwork and the time of carrying out
prestressing operation, and
vii) inspection of RCC members in a building.

5.7 PULSE ATTENUATION METHOD

The process that causes energy loss in an acoustic wave due to scattering and absorption is
called attenuation. Attenuation method involves measurement of the amplitude of the
wavefornl by virtue of its higher sensitivity to the extent of cracking, which is found to be
a better indicator of crack growth than pulse velocity method. The damage growth is
inferred from the reduction in amplitude of ultrasonic waveforms transmitted through the
specimen during the test.

5.8 PULSE ECHO METHOD

In thls method a transient stress pulse is introduced into a test object by a point impact. At
the opposite face they get reflected and propagate back into the test object. Thus, a
transient resonance condition is set up by multiple reflections of waves between the top
surface and internal flows or external boundaries. Depth T of the reflecting interface can be
calculated by the equation:

where,
Cp= ultrasonic pulse velocity in d s e c ,and
fp = frequency of the wave.
This method suffers from the limitations, that it reqMres htgh degree of experience and
very sophisticated time measuring devices.
Tests for Concrete and
Misce~~aneous Materials 5.9 COMBINED METHODS
In order to predict the compressive strength of cast-in-situ concrete very accurately, efforts
have been made to combine different NDT methods. Some popular combinations are now
discussed.
5.9.1 Dynamic Modulus of Elasticity and Damping Constant
These two properties obtained from resonance method discussed earlier can be combined
to estimate compressive strength. The accuracy of prediction was within an error o f f 5%
and it was claimed that the above accuracy wasgossible without howledge of age, mix or
moisture content of the concrete.
5.9.2 Ultrasonic Pulse Velocity and Rebound Number
This is the most popular combined method in the field. Both the methods are easy to
perform at site a i d one can collect large number of data in small interval of time. This is
the most economical combination. The pulse velocity and rebound number can be
combined to obtain multiple linear regression equation with compressive strength as
dependent variable in the form:
log S = AV + BR - C ...(5.9)

where,
S = Cube compressive strength (W/ cm2),
V = UPV (m/ sec),
R = Rebound number, and
A, B and C = constants.
Facaoaru has published the most comprehensive data on the use of this combined method.
He has also developed calibration charts for standardized concrete mixes from which, by
knowing the pulse velocity and the rebound number the compressive strength can be read
off. A correction factor in the form is applied :
CT = C, Cd C, C, C, ...(5.10)

where,
C, = Total correction,
C, = Influence of type of cement,
C, = Influence of dosage of cement,
C, = Type of aggregate,
C, = Maximum size of the aggregate, and
C, = Influence of fine fractions of the aggregate.
5.9.3 Pulse Velocity and Pulse Echo Method
Combination of these two methods is of recent origin, which is very useful in finding
depth of pile foundation, to locate point of crack, loose compaction in foundations, etc.
First, pulse velocity in the material is found out by pulse velocity method taking
measurements on similar concrete which is later used in pulse echo method to locate depth
of crack and many other applications.
5.9.4 Pulse Velocity and Core Drilling
This is a well known combination and is used to solve disputes in the field which may arise
between two parties due to many reasons. Firstly, the disputed member is thoroughly scanned
by making grids and analysed for its uniformity and strength. The point where it is giving less
velocity, is marked out and three cores are taken and tested for compressive strength; from
which one can solve the dispute..
Other combinations may be, pulse velocity and magnetic method, pulse velocity and corrosion
measurements, etc. Like this, various combinations can be used at site depending on what one
is evaluating for. However, the use of more than two combined methods on the same structural
concrete cannot he recommended hecause of economy, time requirement and doubts as to the
i n r r o ~ c oi n t h o I P ~ * ~ W I PnI ,f nrollir-tinn n f r - n r n n r ~ r c ic ~t r~~~n o t h
 Testing of Hardened
5.10 RADIOACTIVE METHOD Concrete-11

In radioactive method, there are two distinct fields, namely radiography and radiometry
Let us understand them.
Radiography
X-rays and Ganlrna rays, both components of the high energy electromagnetic spectrum,
penetrate matter but undergo attenuation in the proccss. The degree of attenuation depends
on the kind of matter traversed, its thickness and wavelength of radiation.
X-ray source used for radiography on concrete have energies ranging from 30 kev to
125 kev. In this range, attenuation depends on atomic number of the absorber. Gamma ray
source used is of the range of 0.3 mev to 1.38 mev. In this range of energy, attenuation for
unit thickness is roughly proportional to the density of the absorber.
I Radiometry
Ganuna rays of a suitable radio-isotope are made to pass through concrete and the intensity
of emerging radiation is observed by radiation detectors, such as Geiger or scintillation
counters and measured by associated electronic apparatus.
Idmitations and Usefulness
Radioactive methods are potentially very attractive as these can be used to determine the
location of remforcement, density and honeycombing in structural concrete. X radiography
has a serious limitation due to the necessity of costly and high voltage equipment. Apart
from a research tool in laboratory, X radiography offers little scope for use in the field,
since very large area is to be radiographed in concrete.
Gamma radiography is being accepted for use in the field. The equipment is relatively
portable and the rumling cost is negligiblc. Concrete upto thickness oC45 cm can be
examined economically. Above this thickness, the long exposure time makes the method
uneconomical. High initial cost and safcty aspects are the other li~rfilations.
Giln~naradiometry method is burdened with expensive detecting equipment. The
equipment is 11otonly expensive but requires skilled personnel for its operation.

5.11 NUCLEAR METHODS

In this method two principal techniques have been reported, namely Neutron moisture
gauges for deternlining water content and Neutron activation analysis for the
tieternlination of cement content.
5.1 1.1 Neutron Moisture Gauges
containiilg materials act as an excellent moderators for fast neutrons depending
Hydrc~:~crl
on the ;I I ' 1 writ of hydrogen present in the material. Thus, counting down the slowed down
~~eutrons ~ I L CUIC~ llleasure of the hydrogen content in the matrix. Isotopic neutron sources
are generally used in moisture gauges. The commonly used isotope sources are of the
, ( a , q) type that produce neutrons indirectly by the interaction of alpha particles from thc
dccay o l an alpha emitting isotope such as radium with beryllium.
5.1 1.2 Neutron Activation Analysis
This method is still largely undeveloped and little published data is available. This method
depends on the fact that most elements become radioactive on neutron bombardment.
These radio-isotopes decay to a stable ground state with the emission of energy in form of
bcla and I or gamma radiation of characteristic energies and half-life factors which allows
characterisation of thc radio-isotope and hence of its stable parent nuclide. The method is
almost always performed on a comparative basis. Generally, standard of the element being
determined 1s activated under exaclly the same conditions as the unknown sample. Then a
c.ompariso11o l the activity induced in the known weight of the reference sample to the
aclivity of the same isotope in Ule same under investigation allows the weight of the
element of interest to be calculated. The major potential use of these methods lies in the
detemnation of cemenl content of fresh or Hardened concrete which contain high calcium
iuld s~lica.
Tests for Concrete and
Miscellaneous Materials 5.12 MAGNETIC METHODS
Magnetic devices are based on the principle that presence of steel aff&ts the filed of
electromagnet. The device consists of a highly permeable U-shaped magnetic core on
which two coils are mounted. An alternating current is passed through one of these coils
and the current induced in the other coil is measured. The induced current depends upon
the mutual inductance of the coils and the nearness of the steel bars. A moving coil meter
measures the induced current. The concrete must have low magnetic permeability so as not
to affect the readings.
The instrument is used by placing the probe over the reinforced concrete under test and
moving it about, until a reading is obtained on the dial. The reinforcing bar has to be
parallel to the length of the probe. Depending on the diameter of the bar the dial reading
gives directly the cover to the reinforcement.
Limitations
Magnetic methods give satisfactory results in lightly reinforced members. Bars running
parallel to probe at a distance of 2 or 3 times the cover will influence the reading.
Calibration is done for mild steel and when any other steel is used, self calibration is
required to be done.

ELECTRICAL METHODS
Electrical methods are gaining increasing acceptance as a tool for evaluation of in-situ
concrete to determine reinforcement corrosion and thickness of concrete pavements. They
also offer potential for determining moisture content and its penetration through hardened
concrete. Some of the applications of electrical methods are now discussed.
5.13.1 Determination of Corrosion of Reinforcement Bar
Galvanic corrosion cells are formed in concrete when corrosion of reinforcement takes
place. This can be detected by measuring the half-cell potential of the reinforcements with
respect to a reference electrode. Copper-copper sulphate or the Silver-silver chloride half
cells are most commonly used. These are placed on the moist concrete surface and
connected to an exposed reinforcement. The potential of embedded reinforcement is
measured relative to the half - cell reference electrode through a high impedance volt
meter in the circuit. The moist concrete functions as an electrolyte. The corrosion activity
is empirically related to the measured potential difference in the circuit.
The half-cell potential measurement is suitable in field, as equipment is portable. It is
applicable to concrete members regardless of their size and depth of concrete cover. In
practice half-cell potential measurements are obtained on predetermined grids over the
surface of the concrete. Plots of equipotential contours are constructed to identify areas of
possible corrosion activity in a concrete member.
5.13.2 Determination of Thickness of Concrete Pavement
The electrical methods have also beell used to determine the thichess of goncrete
pavements. Principle underlying is that, a material under test offers resistance to the
passage of an electric current when the latter is made to pass through it. A concrete
pavement has a resistivity characteristic that usually differs from that of the underlying
subgrade layers, thus chahge in the slope of the resistivity versus probe space curve is used
to estimate the depth of concrete pavement.
5.13.3 Determination of Moisture Content of Hardened Concrete
Dielectric properties of hardened concrete changes with change in its moisture content.
The dielectric constant of any material is the ratio of the capacitance formed by two plates
with the material between them to the capacitance of the same plates separated by the same
distance in a vacuum. The results of the study at 50 Hz - 25 kHz indicated that the
dielectric constant values for the pastes were significantly higher than those of concrete
and the paste made with different type of cements had different values. The dielectric
constant of all the specimens decreased with age.
It is reported that dielectric constants measured at frequencies of 10 - 100 MHz, the effects
of conductance caused by dissolved salts and faulty contacts with electrodes were
minimized. At these frequencies dielectcic constant of water was 78 and of other
constituents of concrete ranged from 2-5 only.
 Activity ! Testing of Hardened
Concrete-11
AIMIL Sales and Agencies Pvt. Ltd., 131, 15th Cross, Malleswararn,
Bangalore-560 003 are sales and service agents of AIMIL who deal in some NDT
equipment as given below
a) The James Pullout System
b) The Windsor Prode
c) UST Ultrasonic Tester
I
1 You may write to the firm or other firms and collect more information on NDT methods
1 and supplement your knowledge.
L

5.13.4 Electrical Resistivity Method for Estimation of

Compressive Strength
It has been reported that. there is a linear relationship between resistivity and compressive
strength 'and that resistivity can be used to continuously monitor strength development in
concrete. Porosity cdmalso be assessed by measuring resistivity in the dry as well as in the
water saturated condition. An important observation is that a highly porous concrete in
spite of its low strength can have a very high resistivity value. It is observed that the
resistivity increases exponentially with time and also strength gain with age, so it can be
used to monitor strength gain.

5.14 ACOUSTIC EMISSION TECHNIQUE

Acoustic waves are small amplitude elastic waves created by localised deformtion in
concrete. At the surface these waves are detected by sensors and are amplified. The
variations in the time of arrival of stress waves at each position are used to locate the
source of deformation. The method is useful in monitoring progressive cracking. The
major drawback of this method is that the observahons can be made only during a period
of increasing deformation and stress.

5.15 SUMMARY
Non-destructive tests can be satisfactorily used in quality control assurance of concrete.
However, they need skilled handling. The results need to be interpreted carefully as they
are dependent upon several parameters and appropriate calibration. The correlation
between the measured property and strength is different for different concretes and
therefore it must be determined for the particular concrete under examination. Some of the
methods are still in their infancy and development stage while in others the equipment is
expensive and requires skilled persomiel. The interpretation of test data depends
considerably on the skill and experience of the operator and it should be interpreted by a
professional who is having sufficient field experience. Many firms are now existing in
major cities which give consultancy work in non-destructive testing. We have seen that
each method has its own special procedure and equipment and is suitable for measuring a
certain parameter. While choosing a particular method its advantages and limitations
should be kept in view.

5.16 KEY WORDS

Attenuation : Scattering and absorption of an acoustic wave.
NDT : Non-destructive testing.

5.17 ANSWERS TO SAOs

Refer preceding text for answers.