BSEN12504-4:2021

BSIStandardsPublication

Testingconcreteinstructures

Part4:Determinationofultrasonicpulsevelocity

bsi.

BSEN12504-4:2021BRITISHSTANDARD

Nationalforeword

ThisBritishStandardistheUKimplementationofEN12504-4:2021.ItsupersedesBSEN12504-4:2004,whichiswithdrawn.

TheUKparticipationinitspreparationwasentrustedtoTechnicalCommitteeB/517/1,Concreteproductionandtesting.

Alistoforganizationsrepresentedonthiscommitteecanbeobtainedonrequesttoitscommitteemanager.

Contractualandlegalconsiderations

Thispublicationhasbeenpreparedingoodfaith,howeverno

representation,warranty,assuranceorundertaking(expressor

implied)isorwillbemade,andnoresponsibilityorliabilityisorwillbeacceptedbyBSIinrelationtotheadequacy,accuracy,completenessor

reasonablenessofthispublication.Allandanysuchresponsibilityandliabilityisexpresslydisclaimedtothefullextentpermittedbythelaw.

Thispublicationisprovidedasis,andistobeusedatthe

recipient'sownrisk

Therecipientisadvisedtoconsiderseekingprofessionalguidancewithrespecttoitsuseofthispublication.

Thispublicationisnotintendedtoconstituteacontract.Usersareresponsibleforitscorrectapplication.

◎TheBritishStandardsInstitution2021

PublishedbyBSIStandardsLimited2021

ISBN9780539053500

ICS91.100.30

CompliancewithaBritishStandardcannotconferimmunityfromlegalobligations.

ThisBritishStandardwaspublishedundertheauthorityofthe

StandardsPolicyandStrategyCommitteeon31July2021.

Amendments/corrigendaissuedsincepublication

DateTextaffected

EUROPEANSTANDARD

NORMEEUROPEENNE

EUROP?ISCHENORM

EN12504-4

July2021

ICS91.100.30SupersedesEN12504-4:2004

EnglishVersion

Testingconcreteinstructures-Part4:Determinationofultrasonicpulsevelocity

Essaispourbétondanslesstructures

-Partie4:Déterminationdelavitessedepropagationdesultrasons

PrüfungvonBetoninBauwerken-Teil4:

BestimmungderUltraschall-Impulsgeschwindigkeit

ThisEuropeanStandardwasapprovedbyCENon30May2021.

CENmembersareboundtocomplywiththeCEN/CENELECInternalRegulationswhichstipulatetheconditionsforgivingthisEuropeanStandardthestatusofanationalstandardwithoutanyalteration.Up-to-datelistsandbibliographical

referencesconcerningsuchnationalstandardsmaybeobtainedonapplicationtotheCEN-CENELECManagementCentreortoanyCENmember.

ThisEuropeanStandardexistsinthreeofficialversions(English,French,German).Aversioninanyotherlanguage

madebytranslationundertheresponsibilityofaCENmemberintoitsownlanguageandnotifiedtotheCEN-CENELECManagementCentrehasthesamestatusastheofficialversions.

CENmembersarethenationalstandardsbodiesofAustria,Belgium,Bulgaria,Croatia,Cyprus,CzechRepublic,Denmark,Estonia,Finland,France,Germany,Greece,Hungary,Iceland,Ireland,Italy,Latvia,Lithuania,Luxembourg,Malta,

Netherlands,Norway,Poland,Portugal,RepublicofNorthMacedonia,Romania,Serbia,Slovakia,Slovenia,Spain,Sweden,Switzerland,TurkeyandUnitedKingdom.

EUROPEANCOMMITTEEFORSTANDARDIZATIONCOMITEEUROPEENDENORMALISATION

EUROP?ISCHESKOMITEEFURNORMUNG

CEN-CENELECManagementCentre:RuedelaScience23,B-1040Brussels

◎2021CENAllrightsofexploitationinanyformandbyanymeansreservedRef.No.EN12504-4:2021:E

worldwideforCENnationalMembers

2

BSEN12504-4:2021

EN12504-4:2021(E)

ContentsPage

Europeanforeword iii

1Scope 4

2Normativereferencess 4

3Termsanddefinitions 4

4Principle 4

5Apparatus 4

5.1General 4

5.2Performancerequirements 5

5.3Transducers 5

6Procedures 5

6.1Determinationofpulsevelocity 5

6.1.1Factorsinfluencingpulsevelocitydetermination 5

6.1.2Transducerarrangement 5

6.1.3Pathlengthmeasurement 6

6.1.4Couplingthetransducersontotheconcrete 7

6.1.5Measurementoftransittime 7

7Expressionofresult 7

8Testreport 8

9Precision 8

AnnexA(informative)Determinationofpulsevelocity-indirecttransmission 9

AnnexB(informative)Factorsinfluencingpulsevelocitydetermination 11

AnnexC(informative)Correlationofpulsevelocityandcompressivestrength 14

Bibliography 16

iii

BSEN12504-4:2021

EN12504-4:2021(E)

Europeanforeword

Thisdocument(EN12504-4:2021)hasbeenpreparedbyTechnicalCommitteeCEN/TC104"Concreteandrelatedproducts",thesecretariatofwhichisheldbySN.

ThisEuropeanStandardshallbegiventhestatusofanationalstandard,eitherbypublicationofanidenticaltextorbyendorsement,atthelatestbyJanuary2022,andconflictingnationalstandardsshallbewithdrawnatthelatestbyJanuary2022.

Attentionisdrawntothepossibilitythatsomeoftheelementsofthisdocumentmaybethesubjectofpatentrights.CENshallnotbeheldresponsibleforidentifyinganyorallsuchpatentrights.

ThisdocumentsupersedesEN12504-4:2004.

Incomparisonwiththepreviousedition,thefollowingchangeshavebeenmade:

-optiontouseequipmentutilizingtransversewaves.

Thisdocumentisoneofaseriesontestingconcrete.

EN12504,Testingconcreteinstructures,consistsofthefollowingparts:

一Part1:Coredspecimens—Taking,examiningandtestingincompression;

一Part2:Non-destructivetesting—Determinationofreboundnumber;

一Part3:Determinationofpull-outforce;

一Part4:Determinationofultrasonicpulsevelocity.

ThisdocumentisbasedontheInternationalStandardISO1920-7,Testingofconcrete-Part7:Non-destructivetestsonhardenedconcrete.Itisrecognizedthattheultrasonicpulsevelocitydeterminedusingthisdocumentisaconventioninasmuchthatthepathlengthoverwhichthepulsetravelsisnotalwaysstrictlyknown.

Themeasurementofpulsevelocitycanbeusedforthedeterminationoftheuniformityofconcrete,thepresenceofcracksorvoids,changesinpropertieswithtimeandinthedeterminationofdynamicphysicalproperties.Thesesubjectswereconsideredtobeoutsidethescopeofthisdocument,butsomeinformationisgiveninAnnexBandmoreinformationcanbefoundinthetechnicalliterature.ThemeasurementcanalsobeusedtoestimatethestrengthofinsituconcreteelementsorspecimensgiveninEN13791,Assessmentofinsitucompressivestrengthinstructuresandprecastconcretecomponents.However,itisnotintendedasanalternativetothedirectmeasurementofthecompressivestrengthofconcrete.

Anyfeedbackandquestionsonthisdocumentshouldbedirectedtotheusers'nationalstandardsbody.AcompletelistingofthesebodiescanbefoundontheCENwebsite

AccordingtotheCEN-CENELECInternalRegulations,thenationalstandardsorganisationsofthefollowingcountriesareboundtoimplementthisEuropeanStandard:Austria,Belgium,Bulgaria,Croatia,Cyprus,CzechRepublic,Denmark,Estonia,Finland,France,Germany,Greece,Hungary,Iceland,Ireland,Italy,Latvia,Lithuania,Luxembourg,Malta,Netherlands,Norway,Poland,Portugal,Republic

ofNorthMacedonia,Romania,Serbia,Slovakia,Slovenia,Spain,Sweden,Switzerland,TurkeyandtheUnitedKingdom.

4

BSEN12504-4:2021

EN12504-4:2021(E)

1Scope

Thisdocumentspecifiesamethodforthedeterminationofthevelocityofpropagationofpulsesofultrasoniclongitudinalwavesorultrasonictransversewavesinhardenedconcrete,whichisusedforanumberofapplications.

2Normativereferences

Thefollowingdocumentsarereferredtointhetextinsuchawaythatsomeoralloftheircontentconstitutesrequirementsofthisdocument.Fordatedreferences,onlytheeditioncitedapplies.Forundatedreferences,thelatesteditionofthereferenceddocument(includinganyamendments)applies.

EN206,Concrete-Specification,performance,productionandconformity

3Termsanddefinitions

Forthepurposesofthisdocument,thetermsanddefinitionsgiveninEN206andthefollowingapply.ISOandIECmaintainterminologicaldatabasesforuseinstandardizationatthefollowingaddresses:

—IECElectropedia:availableat

/

—ISOOnlinebrowsingplatform:availableat

/obp

3.1

transittime

timetakenforanultrasonicpulsetotravelfromthetransmittingtransducertothereceivingtransducer,passingthroughtheinterposedconcrete

NOTETransittimeisreferredtoastimeofflightinENISO5577.

3.2

onset

leadingedgeofthepulsedetectedbythemeasuringapparatus

3.3

risetime

timefortheleadingedgeofthefirstpulsetorisefrom10%to90%ofitsmaximumamplitude

4Principle

Apulseoflongitudinalortransversevibrationsisproducedbyanultrasonictransducerheldincontactwithonesurfaceoftheconcreteundertest.Aftertraversingaknownpathlengthintheconcrete,thepulseofvibrationsisconvertedintoanelectricalsignalbyasecondultrasonictransducerandanelectronictimingdeviceenablesthetransittimeofthepulsetobemeasured.

5Apparatus

5.1General

Theultrasonictestequipmentconsistsofanelectricalpulsegenerator,apairoftransducers,anamplifierandanelectronictimingdeviceformeasuringthetimeintervalelapsingbetweentheonsetofapulsegeneratedatthetransmittingtransducerandtheonsetofitsarrivalatthereceivingtransducer.

Areferencebarorprismisusedtozerotheinstrumentortoprovideadatumforthevelocitymeasurement.

NOTE1Thisistypicallynecessarywhentheuserchangesthelengthofthecablesbeingused.

BSEN12504-4:2021

5

EN12504-4:2021(E)

Threeformsoftheelectronictimingdevicesareavailable:

a)anoscilloscopeonwhichthefirstfrontofthepulseisdisplayedinrelationtoasuitabletimescale;

b)anintervaltimerwithadirectreadingdigitaldisplay;

c)anA-scandisplaybuiltdirectlyintotheinstrument.

NOTE2AnoscilloscopeorintegratedA-scandisplayprovidesthefacilityformonitoringthewaveformofthepulse,whichcanbeadvantageousincomplextestingsituationsorinautomaticsystemmeasurements.

5.2Performancerequirements

Theultrasonictestequipmentshallconformtothefollowingperformancerequirements:

-Itshallbecapableofmeasuringtransittimesinthereferencebarorprismtoalimitdeviationof±0,1μs.

-Theelectronicexcitationpulseappliedtothetransmittingtransducershallhavearisetimeofnotgreaterthanone-quarterofitsnaturalperiod.Thisistoensureasharppulseonset.

-Thepulserepetitionfrequencyshallbelowenoughtoensurethattheonsetofthereceivedsignalisfreefrominterferencebyreverberations.

Theultrasonictestequipmentshallbecapableofdeterminingthetransittimeofthefirstfrontofthepulsewiththelowestpossiblethreshold,eventhoughthismaybeofsmallamplitudecomparedwiththatofthefirsthalfwaveofthepulse.

Theultrasonictestequipmentshallbeusedwithintheoperatingconditionsstatedbythemanufacturer.

5.3Transducers

Thenaturalfrequencyofthetransducersshouldnormallybewithintherange20kHzto150kHz.

NOTEForlongitudinalwaves,frequenciesaslowas10kHzandashighas200kHzcansometimesbeused.Highfrequencypulseshaveawelldefinedonset,but,astheypassthroughtheconcrete,theybecomeattenuatedmorerapidlythanpulsesoflowerfrequency.Itis,therefore,preferabletousehighfrequencytransducers(60kHzto200kHz)forshortpathlengths(downto50mm)andlowfrequencytransducers(10kHzto40kHz)forlongpathlengths(uptoamaximumof15m).Transducerswithafrequencyof40kHzto60kHzarefoundtobeusefulformostapplications.Forultrasonicpulse-echomeasurementsusingtransversewavesthetransducerwithafrequencyof40kHzto60kHzisalsotypical,butinthiscasethepathlengthislimitedtoamaximumofapproximately2,5m.

6Procedures

6.1Determinationofpulsevelocity

6.1.1Factorsinfluencingpulsevelocitydetermination

Inordertoprovideadeterminationofpulsevelocity,whichisreproducible,itisnecessarytotakeintoaccountvariousfactorswhichcaninfluencethemeasurements.ThesearesetoutinAnnexB.

6.1.2Transducerarrangement

Althoughthedirectioninwhichthemaximumenergyispropagatedisatrightanglestothefaceofthetransmittingtransducer,itispossibletodetectpulseswhichhavetravelledthroughtheconcreteinsomeotherdirection.Itisthereforepossibletomakemeasurementsofpulsevelocitybyplacingthetwotransducersonoppositefaces(directtransmission),oronadjacentfaces(semi-directtransmission),orthesameface(indirectorsurfacetransmission;pulse-echotransmission)(seeFigure1)ofaconcrete

6

BSEN12504-4:2021

EN12504-4:2021(E)

structureorspecimen.Inthecaseofpulse-echotransmission,thereceiverdetectspulseswhichhavetravelledthroughtheconcretetotheoppositesurfaceandhavebeenreflectedbacktothefirstsurface.Whereitisnecessarytoplacethetransducersonoppositesurfacesbutnotdirectlyoppositeeachothersucharrangementshallberegardedasasemi-directtransmission(seeFigure1b).

NOTE1Theindirecttransmissionarrangementistheleastsensitiveandisusedwhenonlyonefaceoftheconcreteisaccessibleorwhenthequalityofthesurfaceconcreterelativetotheoverallqualityisofinterest.

NOTE2Thesemi-directtransmissionarrangementisusedwhenthedirectarrangementcannotbeused,forexampleatthecornersofstructures.

NOTE3Pulse-echotransmissionisanalternativetoindirecttransmissionwhenonlyonefaceoftheconcreteisaccessible.Thesensitivityiscomparablewithdirecttransmission.

a)

b)

c)

a)Directtransmissionb)Semi-directtransmissionc)Pulse-echotransmissionKey

Risthereceivertransducer

Tisthetransmittertransducer

Figure1—Transducerpositioning

6.1.3Pathlengthmeasurement

Fordirecttransmission,thepathlengthistheshortestdistancebetweenthetransducers.Themeasurementofthepathlengthshallberecordedtothenearest1%ofthemeasureddistance.

BSEN12504-4:2021

7

EN12504-4:2021(E)

Forsemi-directtransmission,itisgenerallyfoundtobesufficientlyaccuratetotakethepathlengthasthedistancemeasuredfromcentretocentreofthetransducerfaces.Theaccuracyofpathlengthisdependentuponthesizeofthetransducercomparedwiththecentretocentredistance.

Withindirecttransmission,thepathlengthisnotmeasured,butaseriesofmeasurementsismadewiththetransducersatdifferentdistancesapart(seeAnnexA).

Forpulse-echotransmission,thepathlengthistwotimesthedistancefromthesurfacewherethetransducerisplacedandtheoppositesurface.Thepathlengthshallberecordedasfordirecttransmission.

6.1.4Couplingthetransducersontotheconcrete

Thereshallbeadequateacousticalcouplingbetweentheconcreteandthefaceofeachtransducer.Formanyconcretesurfaces,thefinishissufficientlysmoothtoensuregoodacousticalcontactbytheuseofacouplingmediumsuchaspetroleumjelly,grease,softsoapandkaolin/glycerolpasteandbypressingthetransduceragainsttheconcretesurface.

Repeatedreadingsofthetransittimeshouldbemadeuntilaminimumvalueisobtained,indicatingthatthethicknessofthecouplanthasbeenreducedtoaminimum.

Whentheconcretesurfaceisveryroughanduneven,theareaofthesurfaceshouldbesmoothedandlevelledbygrinding,orbytheuseofaquick-settingepoxyresin.

NOTE1Specialtransducersareavailableforuseonveryroughsurfaces.

NOTE2Pulse-echotransducerscanutilizedrypointcontactelementswhichdonotrequireacouplant.Inthiscase,itistypicalforseveraltransducerelementstobeconnectedinparalleltoprovidesufficientsignalstrength.

6.1.5Measurementoftransittime

Usingtheultrasonictestequipmentthetransittimeshallbedeterminedinaccordancewiththemanufacturer'sinstruction(see5.2).

7Expressionofresult

Fordirect,semi-directandpulse-echotransmissionsthepulsevelocityshallbecalculatedfromtheformula:

where

Visthepulsevelocity,inkm/s;

Listhepathlength,inmm;Tisthetransittime,inμs.

Forthevelocitybyindirecttransmission,seeAnnexA.

Theresultantdeterminationofthepulsevelocityshallbeexpressedtothenearest10m/s

8

BSEN12504-4:2021

EN12504-4:2021(E)

8Testreport

Thetestreportshallincludethefollowing:

-areferencetothisdocument,includingitsyearofpublication;—identificationoftheconcretestructureorspecimenstested;

-locationofperformanceofthetest;

-dateofthetest;

-descriptionoftheconcreteincludingmixproportions(ifknown);

-ageofconcrete,attimeoftest(ifknown);

-temperatureoftheconcrete,attimeoftest(whenappropriate,seeB.3);—typeandmakeofapparatusused,including:

一dimensionsofcontactareatransducers;

-naturalpulsefrequencyoftransducers;

-anyspecialcharacteristics;

—transducerarrangementsandtransmissionmethod(sketch,whenappropriate);

-detailsofreinforcingsteelorductsinthevicinityofthetestareas(ifknown);

-surfaceconditionsandpreparationattestpoints;

-measuredvaluesofpathlength,(fordirect,semi-directandpulse-echotransmission)includingthemethodofmeasurement;

-calculatedvaluesofpulsevelocityalongeachpath;

-adeclarationbythepersontechnicallyresponsibleforthetest,thatitwascarriedoutinaccordancewiththisdocument;

-anydeviationfromthemethodssetoutinthedocument.

9Precision

Therearenoprecisiondataavailableforthistest.

9

BSEN12504-4:2021

EN12504-4:2021(E)

AnnexA

(informative)

Determinationofpulsevelocity-indirecttransmission

A.1Withindirecttransmissionthereissomeuncertaintyregardingtheexactlengthofthetransmissionpathbecauseofthesignificantsizeoftheareasofcontactbetweenthetransducersandtheconcrete.Itis,therefore,preferabletomakeaseriesofmeasurementswiththetransducersatdifferentdistancesaparttoeliminatethisuncertainty.

A.2Todothis,thetransmittingtransducershallbeplacedincontactwiththeconcretesurfaceatafixedpointxandthereceivingtransducershallbeplacedatfixedincrementsxnalongachosenlineonthesurface.Thetransmissiontimesrecordedshouldbeplottedaspointsonagraphshowingtheirrelationtothedistanceseparatingthetransducers.AnexampleofsuchaplotisshowninFigureA.1.

A.3Theslopeofthebeststraightlinedrawnthroughthepoints(1/tan?)shallbemeasuredandrecordedasthemeanpulsevelocityalongthechosenlineontheconcretesurface.Wherethepointsmeasuredandrecordedinthiswayindicateadiscontinuity,itislikelythatasurfacecrackorsurfacelayerofinferiorqualityispresent(seeB.Z)andavelocitymeasuredinsuchaninstanceinunreliable.

Risthereceivertransducer

Tisthetransmittertransducerttransittime

BSEN12504-4:2021

EN12504-4:2021(E)

10

Xidistancebetweentrasnmitterandreceivertransducers中angleofslope

xdistancefromtransmittertransducer

FigureA.1-Exampleofthedeterminationofpulsevelocitybyindirect(surface)transmission

11

BSEN12504-4:2021

EN12504-4:2021(E)

AnnexB

(informative)

Factorsinfluencingpulsevelocitydetermination

B.1General

Inordertoprovideadeterminationofpulsevelocitywhichisreproducibleandwhichdependsessentiallyonthepropertiesoftheconcreteundertest,itisnecessarytoconsiderthevariousfactorswhichcaninfluencepulsevelocityanditscorrelationwithvariousphysicalpropertiesoftheconcrete.

B.2Moisturecontent

Themoisturecontenthastwoeffectsonthepulsevelocity,onechemical,andtheotherphysical.Theseeffectsareimportantintheproductionofcorrelationsfortheestimationofconcretestrength.Betweenaproperlycuredstandardcubicalorcylindricalspecimenandastructuralelementmadefromthesameconcrete,therecanbeasignificantpulsevelocitydifference.Muchofthedifferenceisaccountedforbytheeffectofdifferentcuringconditionsonthehydrationofthecement,whilesomeofthedifferenceisduetothepresenceoffreewaterinthevoids.Itisimportantthattheseeffectsarecarefullyconsideredwhenestimatingconcretestrength(seeAnnexC).

B.3Temperatureoftheconcrete

Variationsoftheconcretetemperaturebetween10℃and30℃havebeenfoundtocausenosignificantchangeofvelocitieswithouttheoccurrenceofcorrespondingchangesinstrengthorelasticproperties.Correctionstopulsevelocityvaluesshouldbemadeonlyfortemperaturesoutsidethisrangeusingguidanceintherelevantliterature.

B.4Pathlength

Thepathlengthoverwhichthepulsevelocityisdeterminedshouldbelongenoughnottobesignificantlyinfluencedbytheheterogeneousnatureoftheconcrete.Itisrecommendedthat,exceptfortheconditionsstatedinB.5,theminimumpathlengthshouldbe100mmforconcreteinwhichthenominalmaximumsizeofaggregateis20mmorlessand150mmforconcreteinwhichthenominalmaximumsizeofaggregateisbetween20mmand40mm.Thepulsevelocityisnotgenerallyinfluencedbychangesinpathlength,althoughtheultrasonictestequipmentcanindicateatendencyforvelocitytoreduceslightlywithincreasingpathlength.Thisisbecausethehigherfrequencycomponentsofthepulseareattenuatedmorethanthatlowerfrequencycomponentsandtheshapeoftheonsetofthepulsebecomesmoreroundedwithincreaseddistancetravelled.Thus,theapparentreductionofpulsevelocityarisesfromthedifficultyofdefiningexactlytheonsetofthepulseandthisdependsontheparticularmethodusedforitsdetermination.Thisapparentreductioninvelocityisusuallysmallandwellwithinthetoleranceoftransittimemeasurementaccuracygivenin5.2,butparticularcareneedstobetakenwhentransmittingoverlongpathlengths.

B.5Shapeandsizeofspecimen

Thevelocityofshortpulsesofvibrationsisindependentofthesizeandshapeofspecimeninwhichtheytravel,unlessitsleastlateraldimensionislessthanaminimumvalue.Belowthisvalue,thepulsevelocitymaybereducedappreciably.Theextentofthisreductiondependsmainlyontheratioofthewavelengthofthepulsevibrationstotheleastlateraldimensionofthespecimenbutisinsignificantiftheratioislessthanunity.TableB.1givestherelationshipbetweenthelongitudinalpulsevelocity

12

BSEN12504-4:2021

EN12504-4:2021(E)

intheconcrete,thetransducerfrequencyandgivesrecommendationsfortheminimumpermissiblelateraldimensionofthespecimenwhenmeasuringwithlongitudinalwaves.

Iftheminimumlateraldimensionislessthanthewavelengthoriftheindirecttransmissionarrangementisused,themodeofpropagationchangesand,therefore,themeasuredtransittimevelocitywillbedifferent.Thisisparticularlyimportantincaseswhereconcreteelementsofsignificantlydifferentsizesarebeingcompared.

TableB.1—Effectofspecimendimensionsonpulsevelocityoflongitudinalwaves

TransducerfrequencyPulsevelocityinconcrete

km/s

kHzVc=3,50Vc=4,00Vc=4,50

Minimumrecommendedlateralspecimendimensionmm

24

54

82

150

146

65

43

23

167

74

49

27

188

83

55

30

B.6Effectofreinforcingbars

Whenpossible,measurementsincloseproximitytosteelreinforcingbars,paralleltothedirectionofpulsepropagationshouldbeavoided.

Whenanultrasonicpulsetravellingthroughconcretemeetsaconcrete-steelinterface,roughly40%to50%oftheenergyisreflected.Inthecaseofpulseechotransmission,thismaybeusedtoestimatethedepthandlocationofreinforcingbars,metalpipesandtendonducts.

B.7Cracksandvoids

Whenanultrasonicpulsetravellingthroughconcretemeetsaconcrete-airinterface,thereisnegligibletransmissionofenergyacrossthisinterface.Thus,anyair-filledcrackorvoidlyingimmediatelybetweentwotransducerswillobstructthedirectultrasonicbeamwhentheprojectedlengthofthevoidisgreaterthanthewidthofthetransducersandthewavelengthofsoundused.Whenthishappens,thefirstpulsetoarriveatthereceivingtransducerwillhavebeendiffractedaroundtheperipheryofthevoidandthetransittimewillbelongerthaninsimilarconcretewithnovoid.

Dependingonthedistanceseparatingthetransducersitispossibletomakeuseofthiseffectforlocatingflaws,voidsorotherdefectsgreaterthanabout100mmindiameterordepth.Relativelysmalldefectshavelittleornoeffectontransmissiontimes,butequallyareprobablyofminorengineeringimportance.Plottingcontoursofequalvelocityvaluesoftengivessignificantinformationregardingthequalityofaconcreteunit.Examinationofthesignalattenuationmayalsoprovidehelpfulinformation.

Incrackedmembers,wherethebrokenfacesofthemembersareheldtightlytogetherinclosecontactbycompressionforces,theultrasonicpulsesmaypassunimpededacrossthecrack.Asanexample,thiscanoccurincrackedverticalbearingpiles.Ifthecrackisfilledwithliquidwhichtransmitstheultrasonicenergy,e.g.inmarinestructuresorifthecrackispartiallyfilledwithsolidparticles,thecracki