abstract_project asli 2
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CHAPTER-1
INTRODUCTION
To keep a high level of structural safety, durability and performance of the
infras tructure in each country, an eff icient sys tem for early and regular
s tructural assessment is urgently required. The quali ty assurance during
and af ter the cons truct ion o f new s tructu res and af ter recons t ruction
processes and the characterisation of material properties and damage as a
function of time and environmental influences is more and more becoming
a serious concern.
Non-destructive testing (NDT) methods have a large potential to be part of
such a system. NDT methods in general are widely used in several industry
branches. Aircrafts , nuclear facilities, chemical plants, electronic devices
and other safety cri t ical ins tal lat ions are tes ted regularly with fas t and
reliable test ing technolog ies . A varie ty o f advanced NDT methods are
available for metallic or composite materials.
In r ecen t yea rs , inn ov at iv e NDT metho ds , which can b e u sed for the
assessment o f exis ting s tructu res , have become avai lab le fo r concrete
structures, but are still not established for regular inspections. Therefore,
the object ive of th is project is to s tudy the applicabil i ty, performance,
availability, complexity and restrictions of NDT.
The purpose of establishing standard procedures for nondestructive testing
(NDT) o f con crete s truc tu res i s to q ua li fy and q uant ify the mater ia l
propert ies of in-si tu concrete without in trus ively examining the material
properties. There are many techniques that are currently being research for
the NDT of materials today.1.1 Background of the problem
Concrete structures as many other engineering structures are subjected to deterioration that
affect their integrity, stability and safety. Faced with the importance of the damages noted
on the structures, the current choices are directed towards the repair of the existing
structures rather than towards the demolition and construction of new ones. But before any
repair work being done, it is common practice to determine the causes of the deterioration
so that successful repair can be done. Many repair work fail because the exact causes ofthe deterioration was not adequately identified.
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This identification process comprises many methods including non-destructive testing
methods. Non-Destructive Testing is usually undertaken as part of the detailed
investigation to complement the other methods. Sometimes, the conclusions of the
investigation are based essentially on these tests.
First developed for steel, it has not been easy to transfer the NDT technology to the
inspection of concrete (Carino, 1994)[2].
Because of the characteristics of reinforced
concrete the non destructive testing (NDT) of concrete structures is more complex than the
NDT of metallic materials (Rhazi, 2001)[13]
Since the spread of their application in civil engineering, one of their main disadvantages
lies in the processing and interpretation of the data, which is often not trivial (Colombo
and Forde, 2003)[3]
. In order for the NDT to better achieve its role in structural
assessment there must have agreed standards and guidelines on how to do the survey in
the field and interpret the data obtained Unfortunately until now the choice of the best-
fitted technique for a specific case is not simple, the relevance of the measurement process
not guaranteed, and the question of how to cope with measurement results and how to
finally assess the structural properties remains unanswered (Rilem, 2004)[10]
1.2 Statement of the problem
The application of non-destructive testing to concrete structures is sometimes
disappointing. There are many NDT techniques, each based on different theoretical
principles, and producing as a result different sets of information regarding the physical
properties of the structure. These properties, such as velocities, electrical resistance and so
on, have to be interpreted in terms of the fabric of the structure and its engineering
properties.
The interpretation of the data is the most challenging task of the engineer assessing the
structure. The recommendations made based on the interpreted result can be very
significant. Decision on whether a structure is adequate or not, the standard and
specifications are respected or not, and the exact causes of the deterioration, depends on
the outcome of the datas interpretation. It is neither desirable that they lead to the
condemnation of a structure safe or economically repairable building, nor it is admissible
that they provide a false sense of confidence in an otherwise unsafe structure.
Therefore it is vital to study the reliability in of the NDT results of concrete structures.
How NDT results are interpreted? What are the factors affecting these interpretations?
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1.3 Objectives of the study
The objectives of the study are to:
Investigate the reliability of Non-Destructive Testing (NDT) results of concrete
structures,
Determine the factors affecting the interpretation of (NDT) results of concrete
structures.
1.4 Scope of the study
The present work focuses on the study of the reliability of non-destructive testing results
of concrete structures. It will be accomplished by comparing the tests done in laboratory
& the tests conducted on normal hardened concretes o f A double stor ied bu il ding
having two halls v iz. , Hall No.1 and Hall no.2 in Community Hall Kang
Ma i, Di st ri ct : Hosh ia rp ur ; The study will be restricted to the compressive strength of
concrete.
1.5 Limitations of the study
This study will investigate neither human being role in the reliability of NDT nor will it
focus on how to improve the reliability of the NDT testing equipments.
It will be based on the assumptions that the testing equipments are adequate and the testing
operation done with respect to the procedure from the planning of the testing to the
recording of the data.
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CHAPTER-2
LITRATURE REVIEW
The purpose of this project work is to provide an introduction to commonly used NDTmethods for concrete. Emphasis is placed on the principles underlying the various
methods so as to understand their advantages and inherent limitations. Additional
information on the application of these methods is available in IS: 13311; Part-I & II[7-8]
2.1 Historical Background
Some of the first methods to evaluate the in-place strength of concrete were adaptations of
the Brinell hardness test for metals, which involves pushing a high-strength steel ball into
the test piece under a given force and measuring the area of the indentation. In the metals
test, the load is applied by a hydraulic loading system. Modifications were required to use
this type of test on a concrete structure. In1934, Professor K. Gaede in Germany reported
on the use of a spring-driven impactor to supply the force to drive a steel ball into the
concrete (Malhotra, 1976)[6]
. A nonlinear, empirical relationship was obtained between
cube compressive strength and indentation diameter. In 1936, J.P. Williams in Englandreported on a spring-loaded, pistol-shaped device in which a 4-mm ball was attached to a
plunger (Malhotra, 1976)[6]
. The spring was compressed by turning a screw, a trigger
released the compressed spring, and the plunger was propelled toward the concrete. The
diameter of the indentation produced by the ball was measured with a magnifying glass
and scale. In 1938, a landmark paper by D.G. Skramtajev, of the Central Institute for
Industrial Building Research in Moscow, summarized 14 different techniques for
estimating the in-place strength of concrete, 10 of which were developed in the Soviet
Union (D.G. Skramtajev, 1938)[4]
.
2.2 Recent Developments:
In 1995, B. Haselwander reported, safety, serviceability & due proportion in the three
goals of building construction[1]
. The safety must not be neglected & NDT can make a
significant contribution to evaluating building safety. To achieve the second goal i.e.
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serviceability NDT can play an important role in quality assurance. The third goal i.e. due
proportion or beauty does not have NDT does not have same significance.
In 2003, Michael P. Schullar, reported that Recent advances in nondestructive technology
have led to mainstream use of several methods for evaluating the strength of the structure
[11]. Nondestructive approaches such as Rebound Hardness, stress wave transmission,
impact echo, surface penetrating radar, tomography imaging & infrared thermograph are
useful for qualitative condition survey as well as identification of internal features such as
voids or areas of distress .In-situ test methods are also available for determination of
engineering properties.Standardised methods exists for many of the evaluation approaches
& efforts are ongoing for further improve the quality of NDT testing.
In 2003, J. Mat conducted Nondestructive assessment of the actual compressive strength
of concrete, through an experimental program , involving destructive and nondestructive
applied to different concrete mixes[10]
.He established relationships for pulse velocity,
rebound hammer ,pullout test, probe penetration . The results shows good behavior for
some methods, like pulse velocity, rebound hammer. The relationships for various
methods were compared in terms of dimensionless sensitivity, for different strength levels.
The results showed decreasing sensitivity with increasing strength.
In 2004, P. Turgut, in his Research developed the correlation between concrete strength
and UPV values[12]
, the most appropriate curve is found and shown depending on the
correlation between the curve obtained from existing reinforced structures and the curve
obtained from studies on laboratory originated specimens. Depending on these curves the
best fit formula is found as:
Sn=0.3161e1.03v
n
With the formula Sn = 0.3161e1.03V
n, obtained with the correlation of earlier researches'
findings and this study's findings, approximate value of compressive strength in any point
of concrete can be practically found with ignoring the mixture ratio of concrete through
using only longitudinal velocity variable (Vn).
In 2007, Shibli R. M. Khan, established correlation between UPV tests and destructive
tests, and presented them in the form of regression equations that display standard errors
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between 3.2 to 6.7 Mpa[15]
. The proposed relationships can be used for concrete
strength estimation that is normally required in building or structural assessment,
especially with the present trend of constructing modern structures with high performance
concrete.
In 2009, Ehsan Moshtagh and Ali Massumi reported Seismic assessment can be achieved
by NDT tests[3]
. He proposed two methods viz. forced vibration method and ambient
vibration method for seismic assessment of existing buildings, which is the first step
towards rehabilitation of constructions.
In 2009, Jeffrey Wouters in his paper Applications of Impact-Echo for Flaw Detection,
reported that .Impact-echo testing has been highly successful in locating and evaluating
numerous types of flaws in concrete and masonry structures [9]. He suggested that a
thorough understanding of the actual testing parameters and variables will result in more
accurate and successful flaw detection.
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CHAPTER-3
TESTS AND METHODOLGY
The quality of new concrete structures is dependent on many factors such
a s t yp e o f c em en t, t yp e o f a gg re ga te s, w at er c em en t r at io , c ur in g,
environmental condit ions etc. Besides th is, the control exercised during
cons t ruct ion a lso contr ibutes a lot to achieve the des i red qual ity . The
present system of checking slump and testing cubes, to assess the strength
of concrete, in structure under construction, are not sufficient as the actual
s t rength o f the s t ructu re depend on many o ther facto rs such as p roper
compaction, effective curing also.
Considering the above requirements, need of testing of hardened concrete
in new s tructures as well as o ld s tructures , is there to assess the actual
condition of structures. Non-Destructive Testing (NDT) techniques can be
used effectively for investigation and evaluating the actual condition of the
s tructures . These techniques are relat ively quick, easy to use, and cheap
and give a general indication of the required property of the concrete. Thisapproach will enable us to find suspected zones, thereby reducing the time
and cost of examining a large mass of concrete. The choice of a particular
NDT method depends upon the property of concrete to be observed such as
strength, corrosion, crack monitoring etc.
The subsequent testing of structure will largely depend upon the result of
preliminary testing done with the appropriate NDT technique.
The NDT being fast, easy to use at s ite and relatively less expensive can be
used for
(i) Testing any number of points and locations
(ii) Assessing the structure for various distressed conditions
(iii) Assessing damage due to fire, chemical attack, impact, age etc.
( iv )Detec ting c rack s, v oids , f ractures , h on eyco mb s and weak
locations
(v) Assessing the actual condition of reinforcement
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Many of NDT methods used for concrete tes t ing have their orig in to the
test ing o f more homogeneous , meta l lic sys tem. These methods have a
sound scientific basis , but heterogeneity of concrete makes interpretation
of results somewhat d iff icult. There could be many parameters such as
materials , mix, workmanship and environment, which influence the result
of measurements.
Moreover the test measures some other property of concrete (e.g. hardness)
yet the r es ul ts a re interpreted to ass es s the d if fe rent p ro perty o f the
concrete e.g. (strength). Thus, interpretation of the result is very important
and a d i ff icu l t job where general iza t ion i s not possib le . Even though
operators can carry out the test but interpretation of results must be left to
e xp er ts h av in g e xp er ie nc e a nd k no wl ed ge o f a pp li ca ti on o f s uc h
nondestructive tests .
Var ie ty o f NDT metho ds h av e b een d ev elop ed and a re ava il ab le for
invest igat ion and evaluation of d ifferent parameters related to s trength ,
durability and overall quality of concrete. Each method has some strength
and some weakness. Therefore prudent approach would be to use more than
one method in combination so that the s trength of one compensates the
weakness o f the o ther . The various NDT methods fo r test ing concre te
bridges are listed below A. For strength estimation of concrete
(i) Rebound hammer test
(ii) Ultrasonic Pulse Velocity Tester
(iii) Combined use of Ultrasonic Pulse Velocity tester and
rebound hammer test
(iv) Pull off test
(v) Pull out test
(vi) Break off test
B. For assessment of corrosion condition of reinforcement and to determine
reinforcement diameter and cover
(i) Half cell potentiometer
(ii) Resistively meter test
(iii) Test for carbonation of concrete
(iv) Test for chloride content of concrete
(v) Profometer
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(vi) Micro covermeter
C. For detection of cracks/voids/ delaminating etc.
(i) Infrared thermo graphic technique
(ii) Acoustic Emission techniques
(iii) Short Pulse Radar methods
(iv) Stress wave propagation methods
pulse echo method
impact echo method
response method
3.1 NDE Methods in Practice
Visual inspection: The f ir st s tage in the eva lu at io n o f a con crete
s tructure is to s tudy the condition of concrete, to note any defects in the
concrete, to note the presence of cracking and the cracking type (crack
width, depth, spacing, density), the presence of rust marks on the surface,
the presence of voids and the presence of apparently poorly compacted
areas etc. Visual assessment determines whether or not to proceed with
detailed investigation.
The Surface hardness method: This is based on the principle that the
strength of concrete is proportional to its surface hardness. The calibration
chart is val id for a part icular type of cement, aggregates used, mois ture
content, and the age of the specimen.
The penetration technique: This is bas ical ly a hardness tes t , which
p ro vides a q uick means o f d etermining the r elat iv e s tr en gth o f the
concrete. The results of the tes t are influenced by surface smoothness of
concrete and the type and hardness o f the aggregate used . Again, the
calibration chart is valid for a particular type of cement, aggregates used,
moisture content, and age of the specimen. The test may cause damage to
the specimen which needs to be repaired.
The pull-out test: A pullout test involves casting the enlarged end of a
s tee l rod af ter se t t ing o f concre te , to be tes ted and then measur ing the
force required to pull it out. The test measures the direct shear strength of
concrete. This in turn is correlated with the compressive s trength; thus a
measurement of the in-place compress ive s trength is made. The tes t may
cause damage to the specimen which needs to be repaired.
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Ultra-sonic pulse velocity test: This test invo lves measur ing the
velocity o f sound th rough concre te fo r s treng th determinat ion . S ince ,
concrete is a multi-phase material, speed of sound in concrete depends on
t he r el at iv e c on ce nt ra ti on o f i ts c on st it ue nt m at er ia ls , d eg re e o f
compacting, moisture content, and the amount of discontinuities present.
This technique is applied for measurements of composition (e.g. monitor
the mixing materials during construction, to estimate the depth of damage
caused by fire), s trength estimation, homogeneity, elastic modulus and age,
& to check presence of defects , crack depth and th ickness measurement.
General ly, h igh pulse velocity read ings in concrete are ind icat ive o f
concrete of good quality. The drawback is that this test requires large and
expensive transducers. In addition, ultrasonic waves cannot be induced at
right angles to the surface; hence, they cannot detect transverse cracks.
3.2 Introduction to NDE Methods
Concrete technologists practice NDE methods for
(a) Concrete strength determination
(b) Concrete damage detection
3.2(a) Strength determination by NDE methods:Strength determination of concrete is important because its elastic behavior
& service behavior can be predicted from its strength characteristics. The
conventional NDE methods typically measure certain properties of concrete
f rom which an es t imate o f i t s s t reng th and o ther character is t ics can be
made. Hence, they do not directly give the absolute values of strength.
i) The rebound hammer test: The Sch midt r eb ou nd h ammer i s
b as ic al ly a s ur fa ce h ar dn es s t es t w it h l it tl e a pp ar en t t he or et ic al
relationship between the s trength of concrete and the rebound number of
the hammer. Rebound hammers test the surface hardness of concrete, which
cannot be converted directly to compressive strength. The method basically
measures the modulus o f e las t ic i ty o f the near surface concrete . The
principle is based on the absorption of part of the stored elastic energy of
the s pr in g throu gh p last ic d eformation o f the rock s ur face and the
mechan ical waves p ropagat ing th rough the s tone whi le the remaining
elas t ic energy causes the ac tual rebound of the hammer. The d istance
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travelled by the mass, expressed as a percentage of the initial extension of
the spring, is called the Rebound number. There is a considerable amount
of scatter in rebound numbers because of the heterogeneous nature of near
surface properties (principally due to near-surface aggregate particles).
There a re s ev eral f ac to rs o th er than con crete s tr en gth tha t inf lu en ce
rebound hammer tes t results , including surface smoothness and f in ish ,
moisture content, coarse aggregate type, and the presence of carbonation.
Although rebound hammers can be used to estimate concrete strength, the
rebound numbers mus t be corre la ted wi th the compressive s t rength o f
molded specimens or cores taken from the structure.
Rebound Hammer (Schmidt Hammer)
FIG.3.1 REBOUND HAMMER
This is a s imple, handy tool , which can be used to provide a convenient
and rapid indication of the compressive strength of concrete. It consists of
a spring controlled mass that slides on a plunger within a tubular housing.
The schemat ic d iagram showing various parts o f a rebound hammer i s
given as Fig.3.1
The rebound hammer method could be used for
Assessing the likely compressive strength of concrete with the help
of su itable co-re la t ions between rebound index and compress ive
strength.
Assessing the uniformity of concrete
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Assessing the quality of concrete in relation to standard
requirements.
Ass es sing the q ua li ty o f o ne e lement o f con crete in r el at io n to
another.
Thi s metho d can b e u sed with g reater con fidence for d if fe rent ia ting
between the questionable and acceptable parts of the test is classified as a
hardness tes t and is based on the principle that the rebound of an elas t ic
mas s d ep en ds o n the h ardn es s o f the s ur face aga in st which the mas s
impinges . The energy absorbed by the concrete is related to i ts s trength .
Despite its apparent simplicity, the rebound hammer test involves complex
problems of impact and the associated stress-wave propagation.
There is no unique relation between hardness and strength of concrete but
experimental data relat ionships can be obtained from a given concrete.
However, this relationship is dependent upon factors affecting the concrete
surface such as degree of saturat ion, carbonation, temperature, surface
preparat ion and location, and type of surface f in ish . The result is also
affected by type of aggregate, mix proportions, hammer type, and hammer
inclination. Areas exhibiting honeycombing, scaling, rough texture, or high
porosity must be avoided. Concrete must be approximately of the same age,
moisture conditions and same degree of carbonation (note that carbonated
surfaces yield higher rebound values) . I t is clear then that the rebound
number reflects only the surface of concrete. The results obtained are only
representative of the outer concrete layer with a thickness of 3050 mm.
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Principle:
The method is based on the principle that the rebound of an elas t ic mass
depends on the hardness of the surface against which mass strikes. When
the p lu ng er o f r eb ou nd h ammer i s p ress ed aga in st the s ur face o f the
concrete , the sp ring control led mass rebounds and the ex ten t o f such
rebound depends upon the surface hardness o f concrete . The surface
h ar dn es s a nd t he re fo re t he r eb ou nd a re t ak en t o b e r el at ed t o t he
compressive strength of the concrete. The rebound value is read off along a
graduated scale and is designated as the rebound number or rebound index.
The compressive strength may be read directly from the graph provided on
the body of the hammer.
The impact energy required for rebound hammer for different applications
is given below
Table 3.1 Impact Energy of Rebound Hammers
PRECAUTIONS DURING TESTING
Before commencement o f a test , the rebound hammer shou ld be tested
against the test anvil, to get reliable results . The testing anvil should be of
s te el h av in g B ri ne ll h ar dn es s n um be r o f a bo ut 5 00 0 N /m m2 . The
supplier/manufacturer of the rebound hammer should indicate the range of
readings on the anvil suitable for different types of rebound hammer.
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For taking a measurement, the hammer should be held at right angles to the
surface of the s tructure. The tes t thus can be conducted horizontal ly on
v er ti ca l s ur face and v er ti ca lly u pwards o r d ownwards o n h or izon ta l
surfaces
(Fig. 3.2 shows various positions of Rebound Hammer)
If the
situation so demands, the hammer can be held at intermediate angles also,
b ut in each cas e, the r eb ou nd n umber w il l b e d if fe rent for the s ame
concrete.
The following precautions should be observed during testing
The surface should be smooth, clean and dry
The loosely adher ing scale shou ld be rubbed off wi th a g r inding
wheel or stone, before testing
The tes t should not be conducted on rough surfaces result ing fromincomplete compaction, loss of grout, spalled or tooled surfaces.
The po in t o f impact shou ld be a t leas t 20mm away f rom edge o r
shape discontinuity.
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ii) Ultrasonic Pulse Velocity Tester
Ultrasonic instrument is a handy, battery operated and portable instrument
used for assessing elastic properties or concrete quality. The apparatus for
ultrasonic pulse velocity measurement consists of the following (Fig.3.3) (a) Electrical pulse generator
(b) Transducer one pair
(c) Amplifier
(d) Electronic timing device
FIG. 3.3 USPV TESTER
Objective:
The ultrasonic pulse velocity method could be used to establish:
(a) The homogeneity of the concrete
(b) The presence of cracks, voids and other imperfections
(c) Change in the structure of the concrete which may occur with time
(d) The quali ty of concrete in relat ion to s tandard requirement (e) The
quality of one element of concrete in relation to another
(f) The values of dynamic elastic modulus of the concrete
Principle
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The method is based on the p r incip le that the veloci ty o f an u l t rason ic
pulse through any material depends upon the density, modulus of elasticity
and Po issons ra tio o f the mater ia l . Comparat ively h igher velocity i s
obtained when concrete quali ty is good in terms of densi ty , uniformity,
homogeneity etc. The ultrasonic pulse is generated by an electro acoustical
transducer. When the pulse is induced into the concrete from a transducer,
it undergoes multiple reflections at the boundaries of the different material
phases within the concrete. A complex system of stress waves is developed
which includes longitudinal (compression), shear (transverse) and surface
( Re yl ei gh ) w av es . T he r ec ei vi ng t ra ns du ce r d et ec ts t he o ns et o f
long itudinal waves which is the fastest . The veloci ty o f the pulses i s
almost independent of the geometry of the material through which they
pass and depends only on its elastic properties. Pulse velocity method is a
convenient technique fo r invest igat ing s t ructu ral concre te. For good
quali ty concrete pulse velocity wil l be higher and for poor quali ty i t wil l
be less. If there is a crack, void or flaw inside the concrete which comes in
the way of transmission of the pulses, the pulse strength is attenuated and
it passed around the discontinuity, thereby making the path length longer.
Consequently , lower velocit ies are obtained. The actual pulse velocity
obta ined depends p r imari ly upon the mater ia ls and mix p ropor t ions o f concrete. Density and modulus of elasticity of aggregate also significantly
affects the pulse velocity. Any suitable type of transducer operating within
the frequency range of 20 KHz to 150 KHz may be used. Piezoelectric and
magneto-s tr icture types of t ransducers may be used and the lat ter being
more suitable for the lower part of the frequency range.
The e lec t ronic t iming device shou ld be capab le o f measur ing the t ime
interval elapsing between the onset of a pulse generated at the transmitting
transducer and onset of i ts arr ival at receiving transducer. Two forms of
the electronic t iming apparatus are poss ible, one of which use a cathode
ray tube on which the leading edge of the pulse is displayed in relation to
the sui tab le t ime scale , the o ther uses an in terval t imer wi th a d i rec t
reading digital display. If both the forms of timing apparatus are available,
the interpretation of results becomes more reliable.
The ultrasonic pulse velocity has been used on concrete for more than 60
years . Powers in 1938 and Ober t in 1939 were the f i rs t to develop and
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extensively use the resonance frequency method. Since then, u l trasonic
techniques have been used for the measurements of the various properties
of concrete . Also , many in ternat ional commit tees , speci ficat ions and
standards adopted the ultrasonic pulse velocity methods for evaluation of
concrete. The principle of the tes t is that the velocity of sound in a solid
material, V, is a function of the square root of the rat io of i ts modulus of
elasticity, E, to its density, d, as given by the following equation:
Where, g is the gravity acceleration. As noted in the previous equation, the
velocity is dependent on the modulus of elasticity of concrete. Monitoring
modulus of elasticity for concrete through results of pulse velocity is not
normal ly recommended because concrete does not fu lf i ll the physical
req ui rements for the v al id ity o f the equ at io n (2) n ormally u sed for
calculations for homogenous, isotropic and elastic materials
Where V is the wave velocity, is the density, is Poisson's ratio and Ed
is the dynamic modulus of elasticity. On the other hand, it has been shown
that the strength of concrete and its modulus of elasticity are related.
The method s tarts with the determination of the t ime required for a pulse
of v ibrat ions at an ul trasonic frequency to travel through concrete. Once
the velocity is determined, an idea about quality, uniformity, condition and
strength of the concrete tes ted can be at tained. In the tes t , the t ime the
pulses take to travel through concrete is recorded. Then, the velocity is
calculated as:
V = L/ T
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Where V=pulse velocity, L=travel length in meters and T=effective time in
seconds, which is the measured time minus the zero time correction.
From the l i terature review, i t can be concluded that the ul trasonic pulse
velocity results can be used to:
(a) Check the uniformity of concrete,
(b) Detect cracking and voids inside concrete,
(c) Control the quali ty of concrete and concrete products by comparing
results to a similarly made concrete,
(d) Detect condition and deterioration of concrete,
(e) Detect the depth of a surface crack and
(f) Determine the strength if previous data is available.
Factors influencing pulse velocity measurement
The pulse velocity depends on the propert ies of the concrete under tes t .
Various factors which can influence pulse velocity and its correlation with
various physical properties of concrete are as under:
Moisture Content:
The mois tu re content has chemical and physical ef fec ts on the pu lse
velocity . These effects are important to es tablish the correlat ion for the
es t imation of concrete s trength . There may be s ignif icant d ifference in
pulse velocity between a properly cured s tandard cube and a s tructural
element made from the same concrete. This difference is due to the effect
of different curing conditions and presence of free water in the voids. It is
impor tan t tha t these ef fec ts are careful ly cons idered when es timat ing
strength.
Temperature of Concrete:
No significant changes in pulse velocity, in strength or elastic properties
occur due to variations of the concrete temperature between 5 C and 30
C . Cor rect io ns to p ul se v eloc ity measu rements s ho uld b e mad e for
temperatures outside this range, as given in table 3.2 below:
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Path Length:
The path length ( the dis tance between two transducers) should be long
enough not to be s ignif icantly influenced by the heterogeneous nature of
the concrete. I t is recommended that the minimum path length should be
100mm for concrete with 20mm or less nominal maximum size of aggregate
and 150mm for concrete with 20mm and 40mm nominal maximum size of
aggregate. The pulse velocity is not general ly influenced by changes in
path leng th, a l though the e lec t ron ic t iming appara tus may ind icate a
tendency for slight reduction in velocity with increased path length. This is
because the higher frequency components of the pulse are attenuated more
than the lower frequency components and the shapes of the onset of the
pulses becomes more rounded wi th increased d is tance t ravelled. This
app aren t r ed uc tion in v eloc ity i s u su al ly s mall and wel l w ithin the
tolerance of time measurement accuracy.
Effect of Reinforcing Bars:
The pulse velocity in reinforced concrete in v icini ty of rebars is usually
higher than in plain concrete of the same composit ion because the pulse
velocity in s teel is almost twice to that in p lain concrete. The apparent
increase depends upon the p rox imity o f measurement to rebars, the i r
numbers, diameter and their orientation. Whenever possible, measurement
should be made in such a way that s teel does not l ie in or closed to the
direct path between the transducers. If the same is not possible, necessary
corrections needs to be applied. The correction factors for this purpose are
enumerated in different codes.
Shape and Size of Specimen:
The velocity of pulses of vibrations is independent of the size and shape of
specimen, unless its least lateral dimension is less than a certain minimum
value. Below this value, the pulse velocity may be reduced appreciably.
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The extent of this reduction depends mainly on the ratio of the wavelength
of the pulse vibrations to the least lateral dimension of the specimen but it
is insignificant if the ratio is less than unity. Table given below shows the
re la t ionsh ip between the pulse velocity in the concrete , the t ransducer
frequency and the minimum permissible lateral dimension of the specimen.
Table: 3.3 Effect of specimen dimension on pulse transmission.
The use of the ul trasonic pulse velocity tes ter is in troduced as a tool to
monitor basic initial cracking of concrete structures and hence to introduce
a threshold limit for possible failure of the structures. Experiments using
ultrasonic pulse velocity tes ter have been carr ied out , under laboratory
condit ions , on various concrete specimens loaded in compression up to
fai lure. Special p lots , showing the relat ion between the velocity through
concrete and the stress during loading, have been introduced. Also, stress
s tr ai n m ea su re me nt s h av e b ee n c ar ri ed o ut i n o rd er t o o bt ai n t he
corresponding s trains . Results showed that severe cracking occurred at a
stress level of about 85% of the rupture load. The average velocity at this
critical limit was about 94% of the initial velocity and the corresponding
strain was in the range of 0 .0015 to 0 .0021. The sum of the crack widths
has been es t imated us ing special relat ions and measurements . This value
that corresponds to the 94% relative velocity was between 5.2 and 6.8 mm.
Procedure of using Ultrasonic Pulse Velocity Tester
The equipment should be calibrated before starting the observation and at
the end of test to ensure accuracy of the measurement and performance of
t he e qu ip me nt . I t i s d on e b y m ea su ri ng t ra ns it t im e o n a s ta nd ar d
calibration rod supplied along with the equipment. A platform/staging of
sui tab le height should be erected to have an access to the measur ing
locations . The location of measurement should be marked and numbered
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with chalk o r s imi lar th ing p r io r to ac tual measurement (p re decided
locations).
Mounting of Transducers
The direct ion in which the maximum energy is propagated is normally at
right angles to the face of the transmitting transducer, it is also possible to
detect pulses which have t raveled th rough the concrete in some o ther
direction. The receiving transducer detects the arrival of component of the
pulse which ar rives earl ies t . This i s genera l ly the lead ing edge o f the
longitudinal vibration. It is possible, therefore, to make measurements of
pulse velocity by placing the two transducers in the fol lowing manners
(Fig.3.4)
Fig. 3.4 Various Methods of USPV Testing
(
a)
Direct Transmission (on opposite faces)
This arrangement is the most preferred arrangement in which transducers
are kept directly opposite to each other on opposite faces of the concrete.
The t rans fer of e ne rgy be tw ee n t ra nsduc er s i s ma xi mum in t his
arrangement. The accuracy of velocity determination is governed by the
accuracy of the path length measurement. Utmost care should be taken for
accurate measurement of the same. The coolant used should be spread as
thinly as poss ible to avoid any end effects result ing from the different
velocities of pulse in coolant and concrete.(b) Semi-direct Transmission :
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Thi s a rr angem ent i s use d w he n i t is not possi ble to have dir ec t
t rans miss io n (may b e d ue to l imited acces s) . I t i s l es s s en si tive as
compared to direct transmission arrangement. There may be some reduction
i n t he a cc ur ac y o f p at h l en gt h m ea su re me nt , s ti ll i t i s f ou nd t o b e
suff ic ien tly accurate . This ar rangement i s o therwise s imi lar to d irect
transmission.
(c) Indirect or Surface Transmission :
Indirect transmission should be used when only one face of the concrete is
access ible (when other two arrangements are not poss ible). I t is the leas t
s en si tive o ut o f the three a rran gements. For a g iv en p ath l en gth, the
receiving transducer get signal of only about 2% or 3% of amplitude that
produced by direct transmission. Furthermore, this arrangement gives pulse
v eloc ity measu rements which a re u su al ly inf lu en ced b y the s ur face
concrete which is o f ten having d if ferent compos i tion f rom that below
sur fa ce c onc re te . The re for e, the t es t r esul ts may not b e c or re ct
r ep re se nt at iv e o f w ho le m as s o f c on cr et e. T he i nd ir ec t v el oc it y i s
invariably lower than the direct velocity on the same concrete element.
This difference may vary from 5% to 20% depending on the quality of the
concrete . Wherever p ract icable , s i te measurements shou ld be made to
determine th is d ifference. There should be adequate acoustical coupling b etween con crete and the face o f each t rans du cer to ens ure tha t the
ultrasonic pulses generated at the transmitting transducer should be able to
pass into the concrete and detected by the receiving t ransducer wi th
minimum losses. I t i s impor tan t to ensure that the layer o f smooth ing
medium should be as thin as possible. Coolant like petroleum jelly, grease,
soft soap and kaolin/glycerol paste are used as a coupling medium between
transducer and concrete. Special t ransducers have been developed which
impart or pick up the pulse through integral probes having 6mm diameter
tips. A receiving transducer with a hemispherical tip has been found to be
very successful. Other transducer configurations have also been developed
t o d ea l w it h sp ec ia l c ir cu ms ta nc es . I t s ho ul d b e n ot ed t ha t a z er o
adjustment will almost certainly be required when special transducers are
used. Most of the concrete surfaces are sufficiently smooth. Uneven or
rough surfaces shou ld be smoothened using carborundum s tone before
placing of transducers. Alternatively, a smoothing medium such as quick
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setting epoxy resin or plaster can also be used, but good adhesion between
concrete surface and smoothing medium has to be ensured so that the pulse
is propagated with minimum losses into the concrete. Transducers are then
pressed against the concrete surface and held manually. It is important that
on ly a very th in layer o f coup l ing medium separa tes the su rface o f the
con crete f ro m i ts con tact in g t rans du cer. The d is tance b etween the
measuring points should be accurately measured. Repeated readings of the
transit time should be observed until a minimum value is obtained.
Once the u l trasonic pulse impinges on the surface o f the mater ia l , the
maximum energy is propagated at right angle to the face of the transmitting
transducers and bes t results are, therefore, obtained when the receiving
transducer is placed on the opposite face of the concrete member known as
Direc t T rans miss io n. The p ul se v eloc ity can b e measu red b y Direc t
T ra ns mi ss io n, S em i- di re ct T ra ns mi ss io n a nd I nd ir ec t o r Su rf ac e
Transmiss ion . Normal ly, Direct Transmiss ion is p referred being more
rel iable and s tand ardized. (Variou s cod es g iv e cor re la tion b etween
concrete quality and pulse velocity for Direct Transmission only). The size
of aggregates influences the pulse velocity measurement. The minimum
path length should be 100mm for concrete in which the nominal maximum
size of aggregate is 20mm or less and 150mm for aggregate s ize between20mm and 40mm. Reinforcement , i f p resent , should be avoided dur ing
pulse velocity measurements, because the pulse velocity in the reinforcing
bars is usually higher than in plain concrete. This is because the pulse
velocity in steel is 1.9 times of that in concrete. In certain conditions, the
firs t pulse to arr ive at the receiving transducer travels part ly in concrete
and partly in steel. The apparent increase in pulse velocity depends upon
the proximity of the measurements to the reinforcing bars, the diameter and
numbe r of ba rs and the ir ori enta tion w it h r espe ct t o t he pat h of
propagation. It is reported that the influence of reinforcement is generally
small i f the bar runs in the direct ion r ight angle to the pulse path for bar
d iameter less than12 mm. But if percentage of s teel is quite h igh or the
axis of the bars are parallel to direction of propagation, then the correction
factor has to be applied to the measured values.
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The ze ro t ime cor re cti on i s e qua l t o the tr ave l t ime be tw ee n t he
t ransmit ting and receiving t ransducers when they are p ressed f i rmly
together.
Determination of pulse velocity
A pulse o f long itudinal v ib rat ion i s p roduced by an e lec t ro acoust ica l
t ransducer , which is held in con tact wi th one surface o f the concre te
member u nd er t es t. After t ravers in g a k no wn p ath l en gth (L) in the
concrete, the pulse of vibration is converted into an electrical signal by a
second electro-acoustical t ransducer and electronic t iming circuit enable
the transit time (T) of the pulse to be measured. The pulse velocity (V) is
given by V = L / T
Where, V = Pulse velocity, L = Path length, T = Time taken by the pulse to
traverse the path length
Fig.3.5 testing of a beam by USPV Tester
The ultrasonic pulse velocity results can be used:
To check the uniformity of concrete,
To detect cracking and voids inside concrete,
To control the quality of concrete and concrete products by
comparing results to a similarly made concrete,
To detect the condition and deterioration of concrete,
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To detect the depth of a surface crack, and,
To determine the strength if previous data are available.
i i i) Combined use of Rebound hammer and Ultrasonic Pulse
Velocity Method
In v iew o f the r el at iv e l imitat io ns o f e ithe r o f the two metho ds for
predicting the strength of concrete, both ultrasonic pulse velocity (UPV)
and reb ou nd h ammer metho ds a re s omet imes u sed in combina tion to
a ll ev ia te t he e rr or s a ri si ng o ut o f i nf lu en ce o f m at er ia ls , m ix a nd
environmental parameters on the respective measurements. Relat ionship
between UPV, rebound hammer and compressive strength of concrete are
ava il ab le b as ed o n l ab oratory t es t s pecimen. Bet te r accuracy o n the
es timat ion o f concrete s t rength i s achieved by use o f such combined
m et ho ds . H ow ev er , t hi s a pp ro ac h a ls o h as t he l im it at io n t ha t t he
established correlations are valid only for materials and mix having same
propor t ion as used in the t r ia ls. The int r ins ic d if ference between the
lab oratory t es t s pecimen and in- si tu con crete ( e. g. s ur face t ex tu re ,
moisture content, presence of reinforcement, etc.) also affect the accuracy
of test results .
Combination of UPV and rebound hammer methods can be used for the
ass es smen t o f the q ua li ty and l ik ely compres sive s tr en gth o f in- si tu
concrete. Assessment of l ikely compress ive s trength of concrete is made
from the rebound indices and th is is taken to be indicat ive of the entire
mass on ly when the overa l l qual ity o f concre te judged by the UPV is
go od . W he n t he q ua li ty a ss es se d i s me di um , t he e st im at io n o f
compress ive s trength by rebound indices is extended to the entire mass
o nl y o n t he b as is o f o th er c ol la te ra l m ea su re me nt e .g . s tr en gt h o f
con trol led cub e s pecimen, cemen t con tent o f h ardened con crete b y
chemical analysis or concrete core testing. When the quality of concrete is
poor , no assessment of the s trength of concrete is made from rebound
indices.
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iii) Acoustic emission technique: This technique utilizes the elastic
waves generated by plastic deformations, moving dislocations, etc. for the
analysis and detection of structural defects .
However, there can be mult ip le travel paths available from the source to
the s en so rs . A ls o, e lect ri ca l inter fe rence o r o th er mechanica l n oi ses
hampers the quality of the emission signals.
iv) Impact echo test: In this technique, a stress pulse is introduced at
the surface o f the s tructu re , and as the pulse p ropagates th rough the
structure, it is reflected by cracks and dislocations.
Through the analys is of the reflected waves , the locations of the defects
can b e est imated . The main d rawb ack o f thi s t echn iq ue i s tha t i t i s
insensitive to small s ized cracks
3.2(b) Damage detection by NDE methods:
Global techniques : These techniques rely on global structural response for
damage iden ti f ica tion . Thei r main d rawback is tha t s ince they re ly on
global response, they are not sensi t ive to localized damages . Thus , i t is
poss ible that some damages which may be present at various locations
remain un-noticed.
Local techniques: These techniques employ localized s tructural analys is ,
for damage detection. Their main drawback is that accessories like probes
and fixtures are required to be physically carried around the test s tructure
for data recording. Thus, it no longer remains autonomous application of
the technique. These techniques are often applied at few selected locations,
by the instincts/experience of the engineer coupled with visual inspection.
Hence, randomness creeps into the resulting data.
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CHAPTER-4
RESULTS AND DISCUSSION
4.1 CALIBRATION TESTSPROCEDURE:
The procedure that was fo llowed dur ing exper iments cons isted o f the
following steps:
1. Various concrete mixes were used to prepare standard cubes of 150-mm
side length.
2. Concrete cubes of unknown history made under site conditions were also
brought from various sites for testing.
3 . All cubes were immersed under water for a minimum period of 24 hr .
before te sting.
4. Just before testing, the cubes were rubbed with a clean dry cloth in order
to obtain a saturated surface dry sample.
5 . Once drying was complete, each of the two opposite faces of the cube
was prepared for the rebound hammer test as described in the
specifications.6. The cubes were positioned in the testing machine and a slight load was
applied. The rebound number was obtained by taking three measurements
on each of the four faces of the cube. The rebound hammer was horizontal
in all measurements.
7 . Once the rebound hammer tes t was complete, each of the two surfaces
was p repared fo r the u l trasonic pulse velocity test as descr ibed in the
specif icat ions . Care was taken so that there was no effect of the notches
p roduced by the hammer . The t ime was measured on each o f the two
opposing surfaces and the average was recorded.
8. Once nondestructive testing on each cube was completed, the cube was
loaded to failure and the maximum load was recorded.
9. Results were plotted as shown in Figures.
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4.2 REBOUND HAMMER TEST
The operation of rebound hammer is shown in the fig.4.1. When the plunger of rebound
hammer is pressed against the surface of concrete, a spring controlled mass with a constant
energy is made to hit concrete surface to rebound back. The extent of rebound, which is a
measure of surface hardness, is measured on a graduated scale. This measured value isdesignated as Rebound Number (rebound index). A concrete with low strength and low
stiffness will absorb more energy to yield in a lower rebound value.
FIG. 4.1 OPERATION OF REBOUND HAMMER
PREPARATION OF SPECIMEN:
6 cubes were cast, targeting at different mean strengths. Further, the cubes
were cured for d ifferent number of days to ensure availabil i ty of a wide
range of compressive s trength at tained by these cubes . Size of each cube
was 150150150 mm.
TESTING OF SPECIMEN:
1 0 reading s ( rebo un d n umbers ) were o btained for each cub e, a t
different locations on the surface of the specimen.
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The cube was divided into grid blocks of equal spacing and 10 points were
marked at equal intervals for taking the Rebound Hammer test.
a) The cubes were then given a load of 7 N/mm^2 (as specified by the IS
CODE 13311) in the Compress ion Test ing Machine and the Rebound
Values were obtained.
b) The Pred ic ted Compressive S t rength as p red icted by the Rebound
Hammer was ca lcu la ted f rom the char t g iven on Rebound Hammer
FIG. 4.2 CHART FOR CALCULATING. PREDICTED
COMPRESSIVE STRENGTH FROM REBOUND
NUMBER GIVEN ON REBO UND HAMMER
c) Average of rebound numbers and standard deviations were calculated using
Equations 1 and 2 respectively as:
Where fa is the average of rebound numbers, fi is the rebound number,
And n is the total impact number and S is the standard deviation.
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TEST IS CONSIDERED RELIABLE IF THE STANDARD DEVIATION OF TEN
READINGS IS NOT MORE THAN THE FOLLOWING:
REBOUND VALUE 15 30 45
STANDARD DEVIATION 2.5 3.0 3.5
d) The cubes were then loaded up to their ultimate stress and the Breaking
Load was obtained.
Table 4.1 Rebound Number & Comparative Hardness
Fig.4.3 Components of a Rebound Hammer used in the Project
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Fig.4.4 Rebound Hammer Testing of a Specimen
The fol lowing tables l is ts the Rebound numbers (rebound index), Mean
Rebound Value, Standard Deviation, the Dead Load on the specimen at the
time of testing, the Breaking Load, the Predicted Compressive Strength as
predicted by the Rebound Hammer and the actual Compressive Strength as
obtained by the Compression Testing Machine.
SAMPLE NO. 1
S.No. R.No.
1 19
2 25
3 23
4 22
5 23
6 22
7 22
8 22
9 23
10 22
Mean 22.3
S.D 1.49
Dead Load
= 150 KN
Breaking load = 247 KN
f(ck) N/mm^2 14.2 N/mm^2
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(Predicted)
f(ck) N/mm^2
(Actual)
11.0 N/mm^2
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Table No. 4.2 a
SAMPLE NO. 2
S.No. R.No.
1 192 20
3 19
4 20
5 19
6 20
7 19
8 20
9 19
10 22
Mean 19.7
S.D 0.94
Dead Load
= 150 KN
Breaking load = 311.5 KN
f(ck) N/mm^2
(Predicted)
13.2
N/mm^2
f(ck) N/mm^2
(Actual)
13.8
N/mm^2
Table No. 4.2 b
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SAMPLE NO. 3
S.No. R.No.
1 24
2 25
3 264 26
5 26
6 25
7 25
8 24
9 25
10 25
Mean 25.1
S.D 0.73
Dead Load
= 150 KN
Breaking load = 346.5 KN
f(ck) N/mm^2
(Predicted)
18.8 N/mm^2
f(ck) N/mm^2(Actual)
15.3 N/mm^2
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Table No. 4.2 c
SAMPLE NO. 4
S.No. R.No.
1 42
2 42
3 41
4 42
5 42
6 42
7 43
8 43
9 42
10 42
Mean 42.2
S.D 0.63
Table 4.2(d)
Dead Load
= 150 KN
Breaking load = 830 KN
f(ck) N/mm^2
(Predicted)
42.6 N/mm^2
f(ck) N/mm^2
(Actual)
36.88 N/mm^2
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SAMPLE NO. 5
S. No. R. No.
1 36
2 37
3 37
4 39
5 40
6 40
7 41
8 40
9 40
10 41
Mean 39.1
S.D 1.79
Dead Load
= 150 KN
Breaking load = 710 KN
f(ck) N/mm^2
(Predicted)
36.2 N/mm^2
f(ck) N/mm^2
(Actual)
31.5 N/mm^2
Table No. 4.2 e
SAMPLE NO. 5
S.No. R.No.
1 38
2 38
3 37
4 37
5 38
6 38
7 378 37
9 38
10 38
Mean 37.6
S.D 0516
Dead Load
= 150 KN
Breaking load = 760 KN
f(ck) N/mm^2
(Predicted)
39.7
N/mm^2
f(ck) N/mm^2
(Actual)
33.8
N/mm^2
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Table No. 4.2 f
The following graph is obtained between the Predicted Compressive Strength by the
Rebound Hammer and the Actual Compressive Strength:
Fig. 4.5 Calibration Graph for Rebound Hammer with its Equation
4.3 Ultrasonic Pulse Velocity Test
PREPARATION OF SPECIMEN
9 cubes were cast , ta rgeting a t d if ferent mean s t rengths. Fur ther , the
cubes were cured for different number of days to ensure availability of a
wide range of compressive strength attained by these cubes. Size of each
cube was 150150150 mm.
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TESTING OF SPECIMEN:
3 readings of Ultrasonic Pulse Velocity (USPV) were obtained for each
cube.
The cubes were then g iven a load o f 7 N/mm 2 (as speci f ied by the IS
CODE 13311) in the Compress ion Test ing Machine and the USPV were
obtained.
The cubes were then loaded up to their ultimate stress and the Breaking
Load was obtained.
The fol lowing table l is ts the USPV in each specimen with their mean
velocity, the Dead Load, the Breaking Load and the actual Compressive
Strength as obtained by the Compression Testing Machine.
Fig.4.6 Zeroing of the Transducers
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Fig.4.7 USPV Tester used in the Project
Fig. 4.8 Compression Testing of a Specimen
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OBSERVATIONS
S.
No.
V1 V2 V3 V Breakin
g
f (ck)
N/mm2
m/sec m/sec m/sec m/sec IR
LOAD
LOAD (Actual)
1 2825 2916 2913 2884.6 150 562.5 25
2 3350 3585 3218 3384 150 669.8 29.77
3 3625 3632 3218 3491 150 720 32
4 4219 4213 4007 4146 150 841.5 37.4
5 4411 4444 4117 4324 150 875.2 38.9
6 4625 4525 4417 4522 150 893.2 39.7
Table 4.3 USPV Testing Results
The following graph is obtained between the Compressive Strength and
the Ultrasonic Pulse Velocity:
Fig. 4.9 Graph obtained for USPV Testing
Thi s gr aph ca n now a ls o be use d t o appr oxi ma te ly pr edic t the
Compressive Strength of Concrete.
Although it gives fairly approximate results but it should be verified with
some other tests like the Rebound Hammer test.
The quality of concrete in terms of uniformity can be assessed using the
guidelines given in table below:
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S . N o. P ul se V el oc it y b y C ro ss P ro bi ng
in
m/s
Concrete Quality
Grading
1 Above 4500 Excellent
2 Btween3500-4500 Good3 Between3000-3500 Medium
4 Below 3000 Doubtful
Table 4.5 USPV Criterion for Concrete Quality Grading
4.4 Study of Effect of Reinforcement on the Rebound Values and Pulse
Velocities
To Study the ef fec t o f re in forcement on the Rebound Values and the
Ultrasonic Pulse Velocities:
a) Two Beams were cas t of the fol lowing dimensions:
Length = 1.8 m Breadth = .2 m Depth = .25 m
b) Grade of Concrete Used: M20 and M25
c) The po in ts where the re in forcements ex is ted were known so the
testing was done in two stages :
By avoiding the impact of reinforcements or by trying to minimize its impact.
By undertaking the effect of reinforcements or by maximizing its Impact.
A comparative analys is is then made to know the effect of
Reinforcement on the tests
OBSERVATIONS
Grade of concrete used: M 20
Rebound No. Ultra Sonic Pulse Velocity
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S.
No.
WithoutReinforcement
WithReinforcement
WithoutReinforcement
WithReinforcement
1st end 29 30 2861 3155
Quarter Length 28 29 2941 3053
Mid Span 30 31 2991 3075
Length 28 29 2800 2908
2
nd
Length 29 29 2925 3224Table No.4.6 (a) Testing of Beam (M20) for effect of Reinforcement
Grade of concrete used: M 25
Rebound No. Ultra Sonic Pulse Velocity
S.
No.
WithoutReinforcement
With
Reinforcement
WithoutReinforcement
With
Reinforcement
1st end 36 36 3161 3688
Quarter Length 36 37 3141 3374
Mid Span 35 37 3191 3488
Length 39 41 3322 3778
2 nd Length 39 39 3257 3722
Table No.4.6 (b) Testing of Beam (M25) for effect of Reinforcement
The maximum variation obtained for Rebound Hammer is 3.6%, where as
in case of Ultrasonic Pulse Velocity the maximum variat ion is 16.1%.
Therefore variations are well within tolerable limits .
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4.5 TESTING OF HALL NO. 1 AND HALL NO. 2
Tests were conducted on some of the Columns, Beams and Slabs of Hall
No . 1 and H al l No. 2 f or the as se ssme nt of th ei r qua lit y. Theobservations, results and discussions have been tabulated below:
Fig4.10 community hall at Village: Kang Mal, District: Hoshiarpur
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4.5 (a) TESTING OF COLUMNS
HALL NO.1
COLUMN NO.1
Rebound
Value
Mean USPV
m/s
Quality
fc k
N/mm2
Remarks
BOTTOM
28
29
29
28.67 3313 22.5
Testing was done
over the plaster.
Medium qualityconcrete
MIDDLE
13
14
14
13.67 Over
Range
13.4
Void between
pl aste r & c olumn
face ind icated b y a
peculiar sound when
struck softly by iron
rod
TOP
15
15
15
15.33 Over
Range
14.1
Void between
pl aste r & c olumn
face ind icated b y a
peculiar sound when
struck softly by iron
rod
TABLE NO.4.7
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HALL NO.1
COLUMN NO.2
Rebound
Value
Mean USPV
m/s
Quality
fck
N/mm2
Remarks
BOTTOM
32
33
33
32.67 3754 31.3
Good Quality
concrete
MIDDLE
30
3230
30.67 3531 30.1
Good Quality
concrete
TOP
31
31
30
30.67 3255 30.1
Good Quality
concrete
TABLE NO.4.8
HALL NO.1
COLUMN NO.3Rebound
Value
Mean USPV
m/s
Quality
fc k
N/mm2
Remarks
BOTTOM
36
36
36
36 3744 33.9
Good Quality
concrete
MIDDLE
34
35
35
34.67 3825 33
Good Quality
concrete
TOP
33
34
35
34 3614 32.8
Good Quality
concrete
TABLE NO.4.9
In hal l no.2 , columns are made of brick masonry so th is test i s not
applicable here
4.5 (b) TESTING OF BEAMS
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HALL NO.1
BEAM NO.1
Rebound
Value
Mean USPV
m/s
Quality
fc k
N/mm2
Remarks
1ST
SUPPORT
40
40
37
39 4468 39.9
Good Quality
concrete
MID
SPAN
32
32
34
32.67 3455 29.9
Medium Quality
concrete
2ND
SUPPORT
34
34
35
34.33 3480 30.8
Medium Quality
concrete
TABLE NO.4.10
HALL NO.1
BEAM NO.2
Rebound
Value
Mean USPV
m/s
Quality
fc k
N/mm2
Remarks
1ST
SUPPORT
33
35
32
33.33 3655 30.5
Good Quality
concrete
MID
SPAN
34
35
36
35 3845 31.6
Good Quality
concrete
2ND
SUPPORT
34
37
34
35 3440 31.6
Good Quality
concrete
TABLE NO.4.11
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HALL NO.1
BEAM NO.3Rebound
Value
Mean USPV
m/s
Quality
fck
N/mm2
Remarks
1ST
SUPPORT
43
39
36
39.33 4505 35.5
Good Quality
concrete
MID
SPAN
38
45
37
40 4533 36.1
Excell ent Quality
concrete. Proper
compact io n may b e
the reason
2ND
SUPPORT
45
49
45
46.33 4861 41.2
Excellent Quality
concrete. It is the
junction of three beams
& a column so heavy
reinforcement &
compaction is indicated
TABLE NO.4.12
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HALL NO.2
BEAM NO.1
Rebound
Value
Mean USPV
m/s
Quality
fc k
N/mm2
Remarks
1ST
SUPPORT
26
28
27
27 2620 20.3
Doubtful Quality
Requires Attention
MID
SPAN
25
27
27
26.33 2729 20
Doubtful Quality
Requires Attention
2ND
SUPPORT
29
28
24
27 2645 20.3
Doubtful Quality
Requires Attention
TABLE NO.4.13
HALL NO.2
BEAM NO.2
Rebound
Value
Mean USPV
m/s
Quality
fc k
N/mm2
Remarks
1ST
SUPPORT
31
31
34
32 2620 29.8
Medium Quality
concrete
MID
SPAN
33
32
32
32.33 2729 29.8
Medium Quality
concrete
2ND
SUPPORT
35
34
37
35.33 2645 31.1
Good Quality
concrete
TABLE NO.4.14
HALL NO.2
BEAM NO.3
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Rebound
Value
Mean USPV
m/s
Quality
fck
N/mm2
Remarks
1ST
SUPPORT
28
28
29
28.33 1955 23.8
USPV is low, there is
separat ion o f p las ter
from the beam or
internal voids &
Cracks .Requires
Attention
MID
SPAN
30
32
30
30.67 3233 25.1
Medium Quality
concrete
2ND
SUPPORT
29
33
31
31 3534 25.3
Medium Quality
concrete
TABLE NO.4.15
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4.5 (C) TESTING OF SLABS
Hall No.1
Observat ions were taken on the top roof s lab i . e . 2 nd f loor roof s lab
because the G.F. & F.F. do not have exposed surface due to application
of tiles. The 2 n d floor roof slab has been plastered to protect it from rain,
sun & others extreme conditions & to give a smooth finish .
SLAB BETWEEN A-B
Rebound
Value
Mean USPV
m/s
Quality
fc k
N/mm2
Remarks
EDGES
38
39
39
38.67 4257 34.3
Good quality concrete
MID
SPAN
ALONG
EDGES
36
35
35
35.67 3966 32.7
Good Quality
concrete
CENTRE
OF SLAB
34
34
34
34 3850 31.8
Good Quality
concrete
TABLE NO.4.16
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HALL NO.1
SLAB BETWEEN D-E
Rebound
Value
Mean USPV
m/s
Quality
fc k
N/mm2
Remarks
EDGES
33
30
28
30.33 4122 26.2
H igh USPV may be
due to heavy tors ion
s te el . O ve ra ll g oo d
quality concrete
MID
SPAN
ALONG
EDGES
32
31
31
31.33
3890 27.2
Good Quality
concrete
CENTRE
OF SLAB
29
30
30
29.67 2855 25.0
Little low may be due
to improper shuttering
o f s labs & imp ro per
compaction
TABLE NO.4.17
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HALL NO.1
SLAB BETWEEN E-F
Rebound
Value
Mean USPV
m/s
Quality
fc k
N/mm2
Remarks
EDGES
30
33
28
29.67 4005 26.4
Good quality concrete
MID
SPAN
ALONG
EDGES
28
28
27
27.33 3825 25.6
Good Quality
concrete
CENTRE
OF SLAB
30
29
27
28.67 3988 26
Good Quality
concrete
TABLE NO.4.18
HALL NO.2 (On 1 s t floor slab)
SLAB BETWEEN 3-4
Rebound
Value
Mean USPV
m/s
Quality
fck
N/mm2
Remarks
EDGES
47
45
44
45.33 5710 47.9
Excellent quality
concrete
MID
SPAN
ALONG
EDGES
49
48
44
47 5764 49.3
Excellent Quality
concrete
CENTRE
OF SLAB
53
51
48
50.67 6229 52.6
Excellent Quality
concrete
TABLE NO.4.19
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HALL NO.2
SLAB BETWEEN 4-5Rebound
Value
Mean USPV
m/s
Quality
fck
N/mm2
Remarks
EDGES
47
42
44
44.33 5612 47.8
Excellent quality
concrete
MID
SPAN
ALONG
EDGES
58
47
58
51 5562 52.4
Excellent Quality
concrete
CENTRE
OF SLAB
38
36
40
38 4079 40.5
Excellent Quality
concrete
TABLE NO.4.20
HALL NO.2
SLAB BETWEEN 5-6Rebound
Value
Mean USPV
m/s
Quality
fck
N/mm2
Remarks
EDGES
45
46
50
47 5846 51.9
Excellent quality
concrete
MID
SPAN
ALONG
EDGES
45
45
47
45.67 5760 49.1
Excellent Quality
concrete
CENTRE
OF SLAB
42
39
33
38 4832 39.5
Excellent Quality
concrete
TABLE NO.4.21
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Fig No. 4.11 Rebound Hammer Testing of a Column in Hall No.1
Fig No. 4.12 Rebound Hammer Testing of a Slab in Hall No.1
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Fig No. 4.13 USPV Testing of a Column in Hall No.1
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4.6 Interpretation of Results
From the above discussion following interpretation can be made:
4.6.1 IN CASE OF REBOUND HAMMER:
1. After obtaining the correlat ion between compressive s trength and
rebound number, the strength of structure can be assessed.
2. In general, the rebound number increases as the strength increases.
3. Rebound Number is affected by a number of parameters i .e. type
o f cemen t, typ e o f agg rega te , s ur face con di tion and moisture
content of the concrete, curing and age of concrete, carbonation of
concrete surface etc.
4. The reb ou nd ind ex i s ind icat iv e o f compres sive s tr en gth o f
con crete u p to a l imited d ep th f ro m the s ur face . The interna l
cracks, flaws etc. or heterogeneity across the cross section will not
be indicated by rebound numbers.
5. The estimation of strength of concrete by rebound hammer method
can no t b e h eld to b e v ery accurate and p ro bable accuracy o f
prediction of concrete strength in a structure is 25 percent.6. If the relationship between rebound index and compressive strength
can be found by tests on core samples obtained from the structure
or standard specimens made with the same concrete materials and
mix proportion, then the accuracy of results and confidence thereon
gets greatly increased.
7. The Rebound hammers showed erratic result when the compressive
s t reng th was below 15 N/mm2 . Abo ve 1 5 N/mm2 the predicted
compres sive s tr en gth v ar ied a lmos t l in ea rly w ith the actua l
compressive strength.
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4.6.2 IN CASE OF ULTRASONIC PULSE VELOCITY
TESTER:
1. The u lt rason ic pulse velocity o f concrete can be re la ted to i t s
density and modulus of elasticity.
2. It depends upon the materials and mix proportions used in making
concrete as well as the method of p lacing, compacting and curing
of concrete.
3. If the concrete is not compacted thoroughly and having
segregation, cracks or f laws, the pulse velocity wil l be lower as
compare to good concrete, al though the same materials and mix
proportions ar e used.4. The actual value o f the pu lse veloci ty in concre te depends on a
number of parameters, so the criterion for assessing the quality of
concrete on the basis o f pulse veloci ty i s valid to the genera l
extent.
5. The assessment of quality becomes more meaningful and reliable,
when tests are conducted on different parts of the structure, which
ha ve be en bui lt a t t he sa me t ime w it h si mil ar mat er ia ls,
construction practices and supervision and subsequently compared.
6 . The qual ity o f concrete i s usual ly speci f ied in terms of s treng th
and i t i s the re fo re , s omet imes h elpful to u se u lt raso nic p ul se
velocity measurements to give an estimate of strength.
7 . The relationship between ultrasonic pulse velocity and s trength is
affected by a number of factors including age, curing condit ions,
moisture condition, mix proportions, type of aggregate and type of
cement.
8. The assessment of compressive strength of concrete from ultrasonic
p ul se v eloc ity v alues i s n ot accurate b ecau se the cor re la tion
between u l trasonic pulse veloci ty and compressive s t rength o f
con crete i s n ot v ery c lear . Becau se the re a re l arge n umber o f
p arameter s inv olved, which inf lu en ce the p ul se v eloc ity and
compressive strength of concrete to different extents. However, if
detai ls of material and mix proport ions adopted in the part icular
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s tructure are available, then es t imate of concrete s trength can be
mad e b y estab li sh in g s ui table cor re la tion b etween the p ul se
velocity and the compressive strength of concrete specimens made
with such materia l and mix p ropor t ions, under environmental
conditions similar to that in the structure.
9 . The es timated s trength may vary from the actual s trength by 20
percent.
10.T he c or re la ti on o bt ai ne d f or a p ar ti cu la r g ra de m ay n ot b e
appl icab le fo r concrete o f another g rade o r made wi th d if ferent
types of material.
11. At some places over plaster in rounded columns USPV gave
no results or indicated that the velocity was out of range. In such a
p lace the rebound value was a lso very low. This p lace gave a
unique sound on striking softly with a hard material like iron which
clear ly indicated a void between the concrete o f p i llar and i t s
plastering.
4.6.3 GENERAL TRENDS IN COLUMNS AND BEAMS:
A general trend was obtained in the columns . The trend was such that
towards the base of the column the tests always showed a higher quality
of concrete i .e. , higher compressive strength. The compressive strength
goes on decreasing as we go up towards the roof
Fig No. 4.14 Variation of Strength with increase in Height of Column
A graph has been plot ted with increas ing height agains t the predicted
compressive s trength . I t is evident from the graph that the compress ive
strength goes on decreasing with increase in height of column.
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The reason for this variation is better compaction at the base. Since all
the weight of the column acts at the base higher compaction is achieved
and a lso better compact ion faci l it ies are avai lab le near the base and
process compaction becomes difficult as we go up.
No such regular trend was observed for beams or slabs.
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CHAPTER-5
CONCLUSION
Considerable engineering judgment i s needed to p roperly evaluate a
measurement. Mis interpretat ion is poss ible when poor contact is made.
For example, in some cases i t may not be poss ible to identify severely
corroded reinforcing bar in poor quality concrete. However, it is possible
to identify poor quality concrete which could be the cause of reinforcing
bar problems. The poor quali ty concrete al lows the ingress of moisture
and oxygen to the reinforcing bars, and hence corrosion occurs. Presently
the s ys tem i s l imited to p en et ra tion d ep th s o f 3 00 mm. Res earch i s
ongoing to develop a system that can penetrate to a depth of 3000mm or
more.
W he n v ar ia ti on i n p ro pe rt ie s o f c on cr et e a ff ec t t he t es t r es ul ts ,
(especial ly in opposite d irect ions) , the use of one method alone would
not be sufficient to study and evaluate the required property. Therefore,
the use of more than one method yields more reliable results .
For example, the increase in mois ture content of concrete increases the
ultrasonic pulse velocity but decreases the rebound number.
Hence, us ing both methods together wil l reduce the errors produced by
using one method alone to evaluate concrete. Attempts have been done to
corre la te rebound number and u lt rason ic pulse velocity to concretes trength. Unfortunately , the equation requires previous knowledge of
concrete constituents in order to obtain reliable and predictable results .
The Schmidt hammer provides an inexpensive, s imple and quick method
of obtaining an indicat ion of concrete s trength , but accuracy of 15 to
20 per cent is poss ible only for specimens cas t cured and tes ted under
cond it ions fo r which calib ra t ion curves have been es tabl ished . The
results are affected by factors such as smoothness of surface, s ize and
shape of specimen, mois ture condit ion of the concrete, type of cement
and coarse aggregate, and extent of carbonation of surface.The pulse velocity method is an ideal too l fo r es tab lishing whether
concrete is uniform. It can be used on both existing structures and those
under construction. Usually, i f large differences in pulse velocity are
found within a structure for no apparent reason, there is strong reason to
presume that defect ive or deteriorated concrete is present . Fair ly good
correlation can be obtained between cube compressive strength and pulse
velocity. These relations enable the strength of structural concrete to be
predicted within 20 per cent, provided the types of aggregate and mix
proportions are constant.
In summary, u l trasonic pulse velocity tes ts have a great potential for
concrete control , part icularly for es tablishing uniformity and detect ing
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cracks or defects . I ts use for predict ing s trength is much more l imited,
owing to the large number of variables affect ing the relat ion between
strength and pulse velocity.
The d ev ia tion b etween actua l r es ul ts and p redicted res ul ts may b e
attributed to the fact that samples from existing structures are cores andthe crushing compress ive cube s trength was obtained by us ing various
corrections introduced in the specifications. Also, measurements were not
accurate and representative when compared to the cubes used to construct
the plots. The use of the combined methods produces results that lie close
to the true values when compared with other methods. The method can be
extended to tes t exis t ing s tructures by taking direct measurements on
concrete elements.
Unlike other work, the research ended with two simple charts that require
no previous knowledge of the consti tuents of the tes ted concrete. The
method presented is s imple, quick, rel iable, and covers wide ranges of
con crete s tr en gths . The metho d can b e eas ily app li ed to con crete
specimens as well as existing concrete structures. The final results were
compared with previous ones from literature and also with actual results
obtained from samples extracted from existing structures.
5.1 Recommendations
Since the advantages of the nondestructive testing are its rapidity and non destructivity,
priority should be given to develop correlation relationship by laboratory specimens. For
future work, emphasis should be put on establishing correlation relationship onspecimens cured and vibrated in different conditions. Different mixes targeting the same
strength should also be considered.
As the age of the specimens has a major influence on the nondestructive parameters,
relationships should be developed to relate reading on samples of different ages.
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REFERENCES
1. B. Haselvader , Non Destructive Testing in Civil Engineering from
Contractors point of view in International symposium in Non Destructive
Concrete in Civil engineering 26-28.08.1995 ,pp(1-3)
2. Carino N.J. (1994), Non Destructive testing of concrete , History &
Challenges ,(pp13-17)
3. Colombo ,S. Forde (2003), Alternative method of AE Data processing (pp17-
21)
4. Estimating in situ strength of concrete D.G. Skramtajev of central
institute of Industrial Building Research, Mascow, 1938, (pp 37-69)
5. Ehsan Mohtagh and Ali Massumi Seismic Assessment of R.C. buildings by
estimation of effective parameters on seismic behavior using non destructive
testing in structural design of tall buildings Vol.5-Dec.2009,(pp 789-793)
6. Handbook on Non Destructive Testing of Concrete (second edition),1976
by V.M. Malhotra and N.J. Carino (pp119-126)
7. IS: 13311, (Part-1)-1992, Non Destructive testing of concrete methods of
test, Ultra Sonic Pulse Velocity.
8. IS:13311, ( Part-2)-1992, Non Destructive testing of concrete methods of
test Rebound Hammer
9. Jeffery Wouter Application of Impact Echo for flaw detection Civil
Engineering Department, Faculty of Engineering, Hashemite University,
Zarqa 13115, Jordan (pp 39-43)
10. J. Mat , Non Destructive testing for assessment of compressive strength of
High Compressive Concrete in civil engineering journal vol.15,issue sept.-
oct. 2003,vol.15,(pp 452-459)
11.Micheal P. Schullar PE, Non-destructive testing and demerge assessment
of structures in Progress of Structural engineerin