A Seminar Report on Significance of Non-Destructive Testing of Concrete.

.~..

REPOR-:f

SIGNIFICANCE OF NON - DESTRUCTIVE

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~(. I I'.}. 26

TESTING OF CONCRETE

AP;'UEC fv1ECHAIliCSDEPARTMENT

/"IV ~IVNAl. ~ Oi..Lf{;~ \,It ENf:INEER,I'4!';' ..f Chl'jOLu r

ftlC T ~9~ ~n.,

ACKNOWLEDGEMENT

: ta,ke thlS "'pport mity to express m;' sincere tha,nks a,nd deep sense of

grati tude to my guide Hiss Vaishal1 Patel Department of Applied tlechanics, Sardar

"alla,bhbhai Regional College of EngHreering &:Technology, Surat "nthout whose

guidance and encourageroent this seminar mould not have been fulfilled.

Ja,ln Niktlll R.

BE. IV CI"IL"

CONTENT

1.0 UITRODUCTION

2, u t-[E71-iODSFORNIT OF CONCRETE

L.,. SUPFACEHAFiDNESSHETHOD

2.1.1 INDENTATIONMETHOD

:.1.2 REBOI~1DMETHOD

2.: IBR1..TIONOR D't1lliIIC METHOD

:.2.1 RESONANTFREQUENCYMETHOD

2.2,2 PULSEvELOSIT:METHOt

iA SONICPULSEVELOCIT, METHOD

B\ ULTRASONICPU:"SEVELOC:""',HETHOD

2.3 RADIO-ACTI''E ORNUCLE,a.R!lETHO:

2,4. J. HAGNETI: HETHODS

2. it.: :~IELECTRJ."liEA.SURE11Et;;'S

2.4.3 ELECTRICALRESISTI'IT: TES:

2.5 n'THERETHOD;;:;

2.5 1 PENETRa.TIONMETHOD

2.5.2 PULLOUtMETrlOD

2.5.3 ACOUSTI~EMISSlv~TES:

2.6 COHBINEDMETHODS

2,'" CONCLUSION

REFERENCES.

I

_S.EMINAR '99;

1.0 INTRODUCTION

Concrete is a composite material made from cement, fine aggregate, coarseaggregate and water. A large number of variables are involved in the production ofconcrete that affect the properties of the resulting concrete. It is important to knowthe properties of concrete in its final form in the structure proper. The conventionalmethod of ascertaining the quality of concrete is to test a number of controlspecimens.This, at best, can give the potential quality of concrete as mixed.

It is desirable to have some method of testing the concrete in the structurewithout damaging it. This is possible with the help of Non-DestructiveTesting (NDT).NDT can be defined as examiningmaterial or products to determine their fitness forcertain purpose or functional requirement without impairingtheir desirable properties.

--NDT methods of testing concrete can not be expected to yield absolute values

of strength. Therefore these methods attempt to measure some other property ofconcrete from which an estimate of its strength. Its durability and its elasticparameters is obtained.

NDT is a quality assurance managementtool which can give impressive resultswhen used correctly. It requires an understanding of the various methods available,their capabilities and limitations. knowledge of the relevant standards andspecifications for performing the tests.

This paper discuss various NDT methods which are most commonly used inthe testing of concrete. These methods include Surface methods, Vibration orDynamic methods, Radio-active or Nuclear methods, Electro-Magnetic methods andsome other methods.

2.0 METHODS FOR NOT OF CONCRETE

The various techniques that can be used for NDT of concrete are listed below :1. SURFACE METHODS: (I ) Hardness: (a) Indentation

(b) Rebound(ii) Water absorption

2. VIBRATION OR DYNAMICMETHODS: (i) Resonancefrequency method(ii) Pulsevelocity method

(a) Sonic(b) Ultrasonic

3. RADIOACTIVEMETHODS: (i) x-ray or gamma -ray transmission(ii) gamma-ray back scatter(HI, Neutronmoderation & scattering(iV) ActiVation analysis

Applied Mechanics Department

S. V. Regional College of Engineering & Technology, SURA T.

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4. ELECTROMAGNETICMETHODS: (j) Magnetic methods(ii) For Dielectric properties(iii) Conductivity! Resistivity

5. OTHER METHODS: (!i PenetrationTest(Ii) Pull out(iii)Cutoff(iv) Acoustic emission

6. COMBINEDMETHODS.

2.1 SURFACE HARDNESS METHOD

2.1.1 INDENTATION METHOD

In this method a certain amount of force is applied to the surface of thematerial to be tested through a suitable indenter forming a permanent impression.The size of this mdentation is measured and that indicates the hardness. Smaller theindentation, harder the material using the same indenter and the same force.

The dlstnbutlon of stress and strain around the actuai indentation-the contactzone-have been analysed in detail indicating that a definite thickness below thesurface is affected, about 2 times the size of the indentation.

A calibration curve can be established for a material like concrete relating thesiZeof indentationand the strength of concrete.

T'" . d t d " \1\1"1"." ~ t t. .

t Ilie In en.er use. are (Ii v~!,,~ums .es mg pIS 0,(II~ FranK spring hammer

(lit) Elnbeck pendulum hammer

2.1.:! REBOUND METHOD

The S\IVISSEngineer Ernst Schmidt developed rebound hammer in 1940.Rebouna hammer consists of a spring-controlled hammer mass that slides on aplunger with a tubular rousing.The plunger retracts against a spring when passedaga'"st the concrete surface and this spring is automatically released when fullytensioned causing t'1e I1arnmern"tassto impact against the concrete through theplunger When sprin~ controllec.mass rebounds, it takes with it a rider which slidesalong 2 scale and IS risible.. IIou~... a sMa:! window in the side of the casing. The'"'cer ca- be "eld 11pos'''OJ' G' ..,e sr-ale tJy depressHlg the locking button. Theolunger IS "'''e<;s~'~SI'"O:-'g''" ar d steaa"y against the concrete at right angle to itssurface,untlll tne spnng loadeo .1ass IS tnggerea from ,ts locked position. The scale"eading is .u".Clmas the rebound number and is an arbitrary measure. since itdeperds or the energ.. s~orec"I r""e~. ter sprirg artc en the mass used



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This test is most suitable for concrete in the 20-60 N/mm2 strength range. as1881 part 202 recommends 12 readings taken over an area not exceeding 300 mm2,with the impact points not less than 20mm from each other or from an edge.

A calibration curve can be established relating theRebound No. & compressive strength.

Rebound No. & flexural strength.Rebound No. & modulusof elasticity.Rebound No. & wall thickness.

FACTOR AFFECTING SURFACE HARDNESS METHOD (LIMITATIONS

(I )(ii )(iii )(IV )(V )

(VI J(vii)

Smoothnessof surface under test.Size, Shape & rigidity of the specimenAge of test specimen.Surface & internal moisture condition of concrete.Tvpe of coarse aggregate.Type of cement_Carbonation of concrete surface.

APPLICATIONS OF SURFACE HARDNESS METHODS (SIGNIFICANCE ..

(i ) Checkingthe uniformity of concrete qualityfll, Comparinga given concrete with a specified requirement.(iii) Approximltv estimation of strength.(iv) Abrat,ionresistance classification

\n recent years an elec~ronicversion of the schimidt concrete test hammerhas been introduced. ThiS Instrument delivers much more precise information onstrength than the scale instrumentused to date. This is because the electronic versionautomatically compansates for vanous factors such as direction of impact, formfactor and age of the concrete.

2.2 VIBRATION OR DYNAMICMETHOD

2.2.1 RESONANT FREQUENCYMETHOD

It is based upon the determination of the fundamental resonentfrequency of vibration of a specimen, the continuous vibration being generatedelectromechanically.This freque,lcy was read accurately from the graduated scale ofthevariabledrM.1gaudiooscillatorusuallykn",wn as a sonometer.

This methods al e used for the purpose of calculatingyoung'smodulusof elasticity and rigidity, fo, determiningthe poisson's ratio.



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SEMINAR '99'" ~

2.2.2 PULSE VELOCITY METHOD

This method subdivided mto two parts:

(a) SONIC PULSE VELOCITY METHOD

This involvesmeasurement of the time of travel of longitudinal orcompressional 'Navesgenerated by a slllgie impact hammer blow or repetitive blow's.In this method the galvanometer is used for measurent of time interval. cathode-ray-oscilloscope are also used for this purpose.

tb) ULTRASONIC PULSE VELOCITY METHOD

The ultrasonicpulse velocity method consists of measuringthe timeof travel of an ultrasonic pulse passing through the concrete to be tested. The pulsegenerator circuit consists of electronic circuit!)' for generating pulses of voltage and atransducer for transforming these electronic pulses into wave bursts of mechanicalenergy having vibrationIrequencie~..1the range of 15 t050 KHz. contact with the concrete is made through asuita':)i!ecoP;" 19 Tec'um.t4, s!mil8r transducer is coupled to the concrete at ameasUleo d1sta"Ice "tom the first. This transducer receive pulse by changingmechanical energy into electronic energy of the same frequency. TI1isway time oftravel aT pulse ''3 mea';lo:-edelectronically. The path length between transducersc"idee. ~y:, e "'l"1e"'ITtrave ~ives the average velocity of wave propagation.

Recommendation for the use of this method are given in as 1881:part 203.The wave velocity depends upon the elastic properties and mass of the medium and~eatlce IT t"'e f'Y'ass 8"'d "elo,-,ty of wave propagationare known it is possible toassess the elastIc properties. I=orthis method generally "Portable Ultrasinic Non -Dest:-uctlVeDiqitallndicatlng Tester" (PUNDIT) instrumentused.

T'1e basic circuitf"l-,requirements are shovm in figure :

AS PER FIG. NO. .

T!"Iere are three basic ways 111which the transducers may be arranged.

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f.j SEMINAR '99~~.- ~

FACTOR AFFECTING MEASUREMENT OF PULSE VELOCITY

(i) Smoothnessof contact surface under test(il ) Influenceof pathlengthon pulsevelocity.(iii) Temperature of concrete.<Iv Moisture condition of concrete.(V Presence of reinforcing steel.

APPLICATION ! SIGN~FICANCE).

(i) Establishinguniformity of concrete.(ii ) Pulsevelocity is related to elastic -modulus.(iii) Estimationof strength of concrete.(iv) Studies on durability of concrete.,v) Measurement & detection of cracks.(vi) Determinationof time for removal of formwork.(llin Inspectionof reinforcedconcreteflexturalmembers.

2.3. RADIO-ACTIVEOR NUCLEAR METHOD

The basic princIple of radiographic inspection is that the object to beexaminedis placed In the path of a beam of radiation from an X-ray and gamma- raysource. A recording medium, usuallyfilm is placed close to object being examined buton the opposite side from the beam source. some of the radiation will be absorbed bythe object but some wiUtravel through the object and impinge on the film, producing alatent image. \lVhen the film has been developed there will be an area of differentimage density, which corresponds to the flaw In the material. This shadow may be oflessor or greater density than the surrounding image, depending on the nature of thedefect and its relative absorptive characteristics.

X-ray or GaMma-ray interact INith any substance, causing a reduction in theirintensiN ~ differential at~enuatlon of such radiation can indicate differential propertiesof the materia, - espacially the density-which can be used for NOT.

The basic relatiol' used IsI = IC' Be '-I"t I

I = Intensity of radiation after passing through a thickness.::f material

!0= 1'1ltia:intensit~ of radiation.t'! = Linear absorption coefficient (connected with density).B = Build-up factor.



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POWER. SOURCE

Fit;. NO.3

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~S.E_M.. .I.N_.A..R._' 9_9~

SIGNIFICANCE

Theitwestigations includedthe study ofII) Direct observations of the arrangements of aggregate of particles, with special

reference to spacing and thickness of paste films separating aggregateparticles

(it ) Threedimensiona!observatIon"f enclosedair \iOids from bothentrapedandentrained air.

(ill) Theeffectof segregation.(iv) The Presenceof cracks regardless of their origin.(V) X-radiography are significantto show variations in density and to locate

reinforcing bars.(vi) Gamma-radigraphy has been used for determiningthe position and condition

of reinforcement, voids in concrete, voids in the grouting of posttensionedprestressed concrete

LIMITATIONS

(I )(Ii, 1

It tends to be an expensivetechnique compared with other NOTtest methods.F~r those method considerable space is needed for a radiography laboratory,includinga dark room for film processing.The operation cost for radiography are also high.The setting -up time for radiography is often lengthy.!t is dangerous highvoltage equipment.The equipment requires skilled person for its operation.

(III)

\IV )

(v )(Vi \

2.J ELECTRO - MAGNETIC METHODS

2.4. atMAGNETIC METHODS (For location of reinforcement and cover)

The patchometer ana co\/er methods are magnetic devices and are based onthe principle that the presence of stee! affects the field of a highly permeable V-shaped magt'et'c core or. Nhlcr wo cnils are mounted. An alternating current ISpassed through one of these coils and the current induced in the other coil isl'Y'easureo.The inducted current ISaffected by the presence and proximity of the steelbars, so this can be used for determiningthe size of the bars and cover of concrete.fAS PER FIG. ...~O.3}

III heavllv Ieinforced sectiof1~, the effect of secondary reinforcement can notbe eleminareo eriU the.satisfac.tufy determination O'~the cover to steel is practicallyimpossible



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2.4.2. DIELECTRIC MEASUREMENTS (For moisture content)

Dielectric properties of hardened concrete change with change in its moisturecontent. This approach is based on measurement of the dielectric constant anddissipation factor. The properties of a capacitor formed by two parallel conductiveplates depend upon the ~haracter of the separating medium. The dielectric constantsdefined as the ratio of capacitances of the same plates when separated by themedium under test J and by a vaccum. When a potential difference IS applied to theplates, opposite charges will accumulate, and if the separating medium is ideal thesewill remain constant and no current will flow. In practice, electron drift will occur and a'conduction' current flows, and the ratio of this current to the initial charging current isthe dissipation factor. When these measurement is carried out at frequency range of10-100MHz, the effect 01 dissolved salts and faulty contact with electrodes areminimised.

2.4.3. ELECTRICALRESISTIVITYTEST (For thickness of conc. pavement)

Electrical resstivity tests have been used for soil testing for many years andnow, has been developed for applicationto in-situ concrete. In this test four electrodsare placed In a straight line on, or just below, the concrete surface at equal spacing~as shown in fig 4. a low frequency alternating electrical current is passed between thetwo outer electrodes whilst the voltage drop between the inner electrodes ismeasured.The apparent resistivity is calculated as :

R=JrISV/IS=Electrode spacing.V=Voltage drop.'=Current.

The resistivity is usuallyexpressed in ohm-cm.

The electrode spacings are varied and a change of slope of theresistivity/spacing piot will occur as a proportion of the current flows through the basematerial. A concrete pavementhas a resistivity characteristic that usually differs fromthat of the underlying subgrade iayers, thus a change in the slope of the resistivityversus depti" curve is used to estimate the depth of concrete pavement.

The electrical resistivity method can also be used for estimating the position ofsteel reinforcement.

The resistivity Of concrete is highly dependent on its moisture and salt contentand its temperature, which is the limitation of this method.



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2.5. OTHERMETHODS

2.5.1. PENETRATION(For compressivestrength)

The technique ot firing steel nails or bolts into a concrete surface to providefixings is well established, and It is known that the depth of penetration is influencedby the strength of the concrete. A strength determination method based on thisapproach, uSinga specially designed bolts and standerdized explosive cartridge isknbNnas the windsor prob test.

The windsor prob equipment consists of a powder-actuated gun or driver,hardened alloy probes, loaded cartndges, depth gage for measuring penetration ofprobes and other related equipment. The probe is driven into the concrete by thefiring of a precision powder charge. The exposed lengths of the individualprobes aremeasured by a calibrated depth gage The manufacturer of the windsor probequipment has published calibration tables relating exposed length of the probe withcompressive strength of concrete.

This method cause some localized damage but damage is sufficiently small tocause no loss in structural performance. The results are affected by the type ofaggregate and so proper calibration with particular type of aggregate is required.

2.5.2 PULL OUT TEST

A pull out test measures the force required to pull out from the concrete aspecially shaped rod whose enlarged end has been cast into that concrete. Thestronger the concrete. the more ISthe force required to pull out. The ideal way to usepullout test in the field would be to incorporate assemblies in the structure. Thesestandard specimens could then be pulled out at any point of time. The force requireddenotes the strength of concrete. Another way to use pullout test in the field would beto cast one or t vo large blocksof concrete incorporating pullout assemblies. Pullouttest could then be per''Jrmed to assess the strength of concrete. The damage to theconcrete surface must be required. The pullout tests do not measure strength in theinterior of mass concrete.

2.5.3. ACOUSTIC EMISSION

Acoustic emission are small amplitude elastic stress waves created bylocalized deformations in concrete at points being strained beyond their elastic limit.During the deformation process, kinetic energy is related to propagate rapid elasticwaves throughout the specirT1er. At the surface they are detected as smalldisplacements by transducer,Jpositioned on the surface of the test specimens. Thevariations in the time 0: arrival of stress waves at each sensor position are used tolocate the source of deformation



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The detected acoustic emissions are then amplified, selectively filteredprocessed and then channeled to either a magnetic tape recorder or tc a speciallydeveloped digital computer for recording and analvsls. \.Nhichshows in Fig. NO.5.

These techniques have been used to study the rate of cracking and thepresence and gro\l~1hof fatigue cracks ;n metals.

The equipment available commercially is very expensive and proper testmethods have yet to be developed.

2.8. COMBINED METHODS

Because of the limitations of the individualmethods as discussed so far It isfelt that a better picture can emerge and more reliable interpretation can be obtainedby usingtwo or more methods Of NDT in combination.

One of the many objectives of non-destructive methods of testing concrete isto estimate the compressive strength of concrete in structures. To predict thecompressive strength of in-situ concrete more accuratelyI investigators have tried toapply more than one nondestructi\le test method at the same time.

Some of the combined methods used in the laboratory or field are describedbp-Ioy,. .

ii \ D~rra"'lc modulus of elasticity and damping constant(determined by resonallcetests )(II l Ultrasonic pulse velocity and damping constant.(Hi) IItrasulliC'pulse \ 'eloci~) ad pulse at~er"Jation(IV \ Ultrasonicpulsevelosit)'-andreboundnumber.

In this last test approach, ultrasonic pulsevelosit)'-measurements aretaken on conclere specimens or ill-S'tU concrete. At the same time the surfacehardness is determined by rr'ealls ot the schmidt rebound hammer. The pulsevelosity and rebound number are then combined to obtain a multiple linear regressionequation witn compressive strength as the dependent variable. the regressionequations tl)us developed appear to give a somewhat higher degree of accuracy inthe predictic'1 of compressive strength. The regressior' equations developed byweibenga are of the following form .

Log S;;AV+BR-C.S;; Cube compo strength t<N/cm2V;;pulse velosity m/Sec.R;;Rebound number.A, BIC=Constant



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2.7. CONCLUSIOill

rHe conventional testing of control cubes canJ at best ~ ,ndicate the potentialquality of concrete assummg that the composition IS same as that going Into thestructure Two mOle steps"compaction and curing- can be different for cubes and thestructure. so tile USe u; NOT can give a better understanding of the quality of;ol1cre£eHI lS finalv~sitlor

Anotner Qreat advantage IS the possibility of repeating the NDT 111case ot anydoub~. ThiS IS not possible in case of destructive testing. So there IS no scope forrt'\ar"i'u~~ ~~:.

These met'"tod$ are basicaliy applicable to both metals as well as non me[als.HO'vvever1/ gere. t:.. ~"e emphasis in the case of metals is on locating the local~e~ects \r 1isl"'ol1tir".:i;ies. :... case ::/ "'')t'\crete, on the hand, the emphasis ..:; :).1eS(~ing tf'\e grcss prQf)ert:es c,. ;:ua'ity Of concrete This is because the concrete isalways full of featUres that: can be cailed as defects.

F"O 1"1 t;1e bas'c structure Ofthe materia! it can be expected that any change inore p:-opert: :":)JiC affect a" {..~e: properties as \/eillhis ,s the main reason wnyIndireCt tests are useful. In Otner -NOms, It is deslrabie to determine the quality of tt,eentire ilolume o~concrete III tel liS Jf the property i eqUired for end use strength I:Igerera.. ;hlS is jJst net possible USing conventional destructIve tests With NOT it IS

possible to CO'/ert"'e er t!re lol~r"'e, with appropriate tests

\Nhen estimating say strengtn trom anyone ot tne NDT methods I the questionof accuracy IS always raised. "lilt..flproper correlation, It may be passable to get tne"esults Nell with 511".In difficult situations alsc., it would still be possIble to estimatetre resultsWlt';::--ahoL:~25°fr)accuracy



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(.,iSEMINAR '99~'" ~

REFERENCES..

... ~',";~I'I:'J':.""':~ ,- ~:"""= ~ p~ :PEPTIE~"F ,..".,If'P E~E P ~JD

CO'l'=FE"~~ ~lA- ",''3 'I;: ""E"'ALS (ASTFII~1984.

J h. HUN0t: i ES rfl~(-; ,F COt,iCRETE IN STRUCTURES, SuRREy~i~' iEPS.T'a RES': .E'.. r:P ~98Q.

... ,,1/ I...HO"qJ.\,TES riNG LfARDEHrD:rJ~CRE"'E '. ~"C~lDESTRUCTI\tE .l~ETuODS ,11':'\A.MERL~IlN";iJ"'CRETE I"JST.MONOG~AP~ SE~IESi 1976.

f{.G. L/rv1AYt=, rHE INDIA'! t.ONCRETE JOURNAL (JANUARY 1993,

CIVIL ENGINEERING & CCNS I RU~TION REVIEWJOURrlAL (AUG03T~998

M.5. Sf-/£TTY r;()"'CJ~E,E T-~I-I!!c:.L~&YJ 5. cHAf'JO Af'lO CO. f rD'J j.99b.-' - - - -



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A Seminar Report on Significance of Non-Destructive Testing of Concrete.

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