土木工程外文文献及翻译

1、精品Word精品Word文档欢迎下载精品资料精品资料Word欢迎使用精品资料精品资料Word欢迎使用外文文献：MaterialsandStructuresRILEM201010.1617/S11527-010-9700-yOriginalArticleImpactofcrackwidthonbond:confinedandunconfinedrebarDavidW.Law1,DengleiTang2,ThomasK.C.Molyneaux3andRebeccaGravina3(1SchooloftheBuiltEnvironment,HeriotWattUniversity,)Edinburg

2、h,EH144AS,UK(2VicRoads,Melbourne,VIC,)Australia(3SchoolofCivil,EnvironmentalandChemicalEngineering,RMIT)University,Melbourne,VIC,3000,AustraliaDavidW.LawEmail:D.W. HYPERLINK mailto:Lawhw.ac.uk Lawhw.ac.ukReceived:14January2010Accepted:14December2010Publishedonline:23December2010AbstractThispaperrepo

3、rtstheresultsofaresearchprojectcomparingtheeffectofsurfacecrackwidthanddegreeofcorrosiononthebondstrengthofconfinedandunconfineddeformed12and16mmmildsteelreinforcingbars.ThecorrosionwasinducedbychloridecontaminationoftheconcreteandanappliedDCcurrent.Theprincipalparametersinvestigatedwereconfinemento

4、fthereinforcement,thecoverdepth,bardiameter,degreeofcorrosionandthesurfacecrackwidth.Theresultsindicatedthatpotentialrelationshipbetweenthecrackwidthandthebondstrength.Theresultsalsoshowedanincreaseinbondstrengthatthepointwhereinitialsurfacecrackingwasobservedforbarswithconfiningstirrups.Nosuchincre

5、asewasobservedwithunconfinedspecimens.Keywords:bond;corrosion;rebar;cover;crackwidth;concreteIntroductionThecorrosionofsteelreinforcementisamajorcauseofthedeteriorationofreinforcedconcretestructuresthroughouttheworld.Inuncorrodedstructuresthebondbetweenthesteelreinforcementandtheconcreteensuresthatr

6、einforcedconcreteactsinacompositemanner.However,whencorrosionofthesteeloccursthiscompositeperformanceisadverselyaffected.Thisisduetotheformationofcorrosionproductsonthesteelsurface,whichaffectthebondbetweenthesteelandtheconcrete.Thedeteriorationofreinforcedconcreteischaracterizedbyageneralorlocalize

7、dlossofsectiononthereinforcingbarsandtheformationofexpansivecorrosionproducts.Thisdeteriorationcanaffectstructuresinanumberofways;theproductionofexpansiveproductscreatestensilestresseswithintheconcrete,whichcanresultincrackingandspallingoftheconcretecover.Thiscrackingcanleadtoacceleratedingressofthe

8、aggressiveagentscausingfurthercorrosion.Itcanalsoresultinalossofstrengthandstiffnessoftheconcretecover.Thecorrosionproductscanalsoaffectthebondstrengthbetweentheconcreteandthereinforcingsteel.Finallythecorrosionreducesthecrosssectionofthereinforcingsteel,whichcanaffecttheductilityofthesteelandtheloa

9、dbearingcapacity,whichcanultimatelyimpactupontheserviceabilityofthestructureandthestructuralcapacity12,25.Previousresearchhasinvestigatedtheimpactofcorrosiononbond2一5,7,12,20,23一25,27,29,withanumberofmodelsbeingproposed4,6,9,10,18,19,24,29.Themajorityofthisresearchhasfocusedontherelationshipbetweent

10、helevelofcorrosion(masslossofsteel)orthecurrentdensitydegree(corrosioncurrentappliedinacceleratedtesting)andcrackwidth,orontherelationshipbetweenbondstrengthandlevelofcorrosion.Otherresearchhasinvestigatedthemechanicalbehaviourofcorrodedsteel1,11andthefrictioncharacteristics13.However,littleresearch

11、hasfocusedontherelationshipbetweencrackwidthandbond23,26,28,aparameterthatcanbemeasuredwithrelativeeaseonactualstructures.Thecorrosionofthereinforcingsteelresultsintheformationofironoxideswhichoccupyalargervolumethanthatoftheparentmetal.Thisexpansioncreatestensilestresseswithinthesurroundingconcrete

12、,eventuallyleadingtocrackingofthecoverconcrete.Oncecrackingoccursthereisalossofconfiningforcefromtheconcrete.Thissuggeststhatthelossofbondcapacitycouldberelatedtothelongitudinalcrackwidth12.However,theuseofconfinementwithintheconcretecancounteractthislossofbondcapacitytoacertaindegree.Researchtodate

13、hasprimarilyinvolvedspecimenswithconfinement.Thispaperreportsastudycomparingthelossofbondofspecimenswithandwithoutconfinement.ExperimentalinvestigationSpecimensBeamendspecimens28wereselectedforthisstudy.Thistypeofeccentricpulloutorbeamendtypespecimenusesa精品Word精品Word文档欢迎下载精品Word精品Word文档欢迎下载精品资料精品资料W

14、ord欢迎使用精品资料精品资料Word欢迎使用bondedlengthrepresentativeoftheanchoragezoneofatypicalsimplysupportedbeam.Specimensofrectangularcrosssectionwerecastwithalongitudinalreinforcingbarineachcorner,Fig.1.An80mmplastictubewasprovidedatthebarunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhanceddue

15、toa(transverse)compressiveforceactingonthebaroverthislength.tupbar4batlombn1300mmbpnckdTOPVIEWFormwoikindLcinforceincfttFig.1Beamendspecimenbaroverthislength.tupbar4batlombn1300mmbpnckdTOPVIEWFormwoikindLcinforceincfttFig.1BeamendspecimenDeformedrebarof12and16mmdiameterwithcoverofthreetimesbardiamet

16、erwereinvestigated.Duplicatesetsofconfinedandunconfinedspecimensweretested.Theconfinedspecimenshadthreesetsof6mmstainlesssteelstirrupsequallyspacedfromtheplastictube,at75mmcentres.Thisrepresentsfourgroupsofspecimenswithacombinationofdifferentbardiameterandwith/withoutconfinement.Thespecimensweresele

17、ctedinordertoinvestigatetheinfluenceofbarsize,confinementandcrackwidthonbondstrength.MaterialsThemixdesignisshown,Table1.ThecementwasTypeIPortlandcement,theaggregatewasbasaltwithspecificgravity2.99.ThecoarseandfineaggregatewerepreparedinaccordancewithAS1141-2000.MixingwasundertakeninaccordancewithAS

18、1012.2-1994.Specimenswerecuredfor28daysunderwethessianbeforetesting.Table1ConcretemixdesignMaterialCemenTw/cSand10mmwashedaggregate7mmwashedaggregateSaltSlumpQuantity381kg/m30.49517kg/m3463kg/m3463kg/m318.84kg/m314025mmInordertocomparebondstrengthforthedifferentconcretecompressivestrengths,Eq.1isuse

19、dtonormalizebondstrengthfornon-corrodedspecimensashasbeenusedbyotherresearcher8.whereisthebondstrengthforgrade40concrete,texptlistheexperimentalbondstrengthandfcistheexperimentalcompressivestrength.Thetensilestrengthofthe012and16mmsteelbarswasnominally500MPa,whichequatestoafailureloadof56.5and100.5k

20、N,respectively.ExperimentmethodologyAcceleratedcorrosionhasbeenusedbyanumberofauthorstoreplicatethecorrosionofthereinforcingsteelhappeninginthenaturalenvironment2,3,5,6,10,18,20,24,27,28,30.Thesehaveinvolvedexperimentsusingimpressedcurrentsorartificialweatheringwithwet/drycyclesandelevatedtemperatur

21、estoreducethetimeuntilcorrosion,whilemaintainingdeteriorationmechanismsrepresentativeofnaturalexposure.Studiesusingimpressedcurrentshaveusedcurrentdensitiesbetween100|jA/cm2and500mA/cm220.Researchhassuggestedthatcurrentdensitiesupto200pA/cm2resultinsimilarstressesduringtheearlystagesofcorrosionwhenc

22、omparedto100pA/cm221.Assuchanappliedcurrentdensityof200pA/cm2wasselectedforthisstudyrepresentativeofthelowerendofthespectrumofsuchcurrentdensitiesadoptedinpreviousresearch.However,cautionshouldbeappliedwhenacceleratingthecorrosionusingimpressedcurrentastheaccelerationprocessdoesnotexactlyreplicateth

23、emechanismsinvolvedinactualstructures.Inacceleratedteststhepitsarenotallowedtoprogressnaturally,andtheremaybeamoreuniformcorrosiononthesurface.Alsotherateofcorrosionmayimpactonthecorrosionproducts,suchthatdifferentoxidationstateproductsmaybeformed,whichcouldimpactonbond.Thesteelbarsservedastheanodea

24、ndfourmildsteelmetalplateswerefixedonthesurfacetoserveascathodes.Sponges(sprayedwithsaltwater)wereplacedbetweenthemetalplatesandconcretetoprovideanadequatecontact,Fig.2.Fig.2AcceleratedcorrosionsystemWhentherequiredcrackwidthwasachievedforaparticularbar,theimpressedcurrentwasdiscontinuedforthatbar.T

25、hespecimenwasremovedforpullouttestingwhenallfourlocationsexhibitedthetargetcrackwidth.Averagesurfacecrackwidthsof0.05,0.5,1and1.5mmwereadoptedasthetargetcrackwidths.Thesurfacecrackwidthwasmeasuredat20mmintervalsalongthelengthofthebar,beginning20mmfromtheendofthe(plastictube)bondbreakerusinganoptical

26、microscope.Thelevelofaccuracyinthemeasurementswas0.02mm.Measurementsofcrackwidthweretakenonthesurfacenormaltothebardirectionregardlessoftheactualcrackorientationatthatlocation.Bondstrengthtestswereconductedbymeansofahandoperatedhydraulicjackandacustom-builttestrigasshowninFig.3.Theloadingschemeisill

27、ustratedinFig.4.Aplastictubeoflength80mmwasprovidedattheendoftheconcretesectionunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhancedbythereactive(compressive)force(actingnormaltothebar).Thespecimenwaspositionedsothatanaxialforcewasappliedtothebarbeingtested.Therestraintsweresuffic

28、ientlyrigidtoensureminimalrotationortwistingofthespecimenduringloading.Ihndj.K：kLjodCcllFig.3Pull-outtest,16mmbarunconfinedIhndj.K：kLjodCcllFig.3Pull-outtest,16mmbarunconfined|TransverseReactionLoadJReactionFig.4Schematicofloading.Note:onlytestbarshownforclarityExperimentalresultsanddiscussion3.1Vis

29、ualinspectionFollowingtheacceleratedcorrosionphaseeachspecimenwasvisuallyinspectedforthelocationofcracks,meancrackwidthandmaximumcrackwidth(Sect.2.3).Whileeachspecimenhadameantargetcrackwidthforeachbar,variationsinthiscrackwidthwereobservedpriortopullouttesting.Thisisduetocorrosionandcrackingbeingad

30、ynamicprocesswithcrackspropagatingatdifferentrates.Thus,whileindividualbarsweredisconnected,oncethetargetcrackwidthhadbeenachieved,corrosionandcrackpropagationcontinued(tosomeextent)untilallbarshadachievedthetargetcrackwidthandpullouttestsconducted.Thisresultedinarangeofdataforthemaximumandmeancrack

31、widthsforthepullouttests.Thevisualinspectionofthespecimensshowedthreestagestothecrackingprocess.Theinitialcracksoccurredinaveryshortperiod,usuallygeneratedwithinafewdays.Afterthat,mostcracksgrewataconstantrateuntiltheyreached1mm,3-4weeksafterfirstcracking.Aftercrackshadreached1mmtheythengrewveryslow

32、ly,withsomecracksnotincreasingatall.Fortheconfinedandunconfinedspecimensthesurfacecrackstendedtooccuronthesideofthespecimens(asopposedtothetoporbo卄om)andtofollowthelineofthebars.Inthecaseoftheunconfinedspecimensingeneraltheseweretheonlycrackwhileitwascommoninthecasesofconfinedspecimenstoobservecrack

33、sthatwerealignedverticallydownthesideadjacenttooneofthelinks,Fig.5.Fig.5Typicalcrackpa卄ernsDuringthepull-outtestingthemostcommonfailuremodeforbothconfinedandunconfinedwassplittingfailure一withtheinitial(pre-test)crackscausedbythecorrosionenlargingunderloadandultimatelyleadingtothesectionfailingexhibi

34、tingspallingofthetopcorner/edge,Fig.6.Howeverforseveraloftheconfined精品Word精品Word文档欢迎下载精品Word精品Word文档欢迎下载精品资料精品资料Word欢迎使用精品资料精品资料Word欢迎使用specimens,asecondmodeoffailurealsooccurredwithdiagonal(shearlike)cracksappearinginthesidewalls,Fig.7.Theappearanceofthesecracksdidnotappeartoberelatedtothepresenceo

35、fverticalcracksobserved(inspecimenswithstirrups)duringthecorrosionphaseasreportedabove.Fig.6Longitudinalcrackingafterpull-outFig.7Diagonalcrackingafterpull-outThebarswereinitially(precasting)cleanedwitha12%hydrochloricacidsolution,thenwashedindistilledwaterandneutralizedbyacalciumhydroxidesolutionbe

36、forebeingwashedindistilledwateragain.Followingthepull-outtests,thecorrodedbarswerecleanedinthesamewayandweighedagain.Thecorrosiondegreewasdeterminedusingthefollowingequation6=；x1QQ%whereG0istheinitialweightofthesteelbarbeforecorrosion,Gisthefinalweightofthesteelbarafterremovalofthepost-testcorrosion

37、products,g0istheweightperunitlengthofthesteelbar(0.888and1.58g/mmfor12and16mmbars,respectively),listheembeddedbondlength.Figures8and9showsteelbarswithvaryingdegreeofcorrosion.Themajorityexhibitedvisiblepitting,similartothatobservedonreinforcementinactualstructures,Fig.9.However,asmallnumberofotherse

38、xhibitedsignificantoverallsectionloss,withamoreuniformlevelofcorrosion,Fig.8,whichmaybeafunctionoftheaccelerationmethodology.Fig.8Corroded12mmbarwithapproximately30%masslossFig.9Corroded16mmbarwithapproximately15%masslossFig.8Corroded12mmbarwithapproximately30%masslossFig.9Corroded16mmbarwithapproxi

39、mately15%massloss3.2BondstressandcrackwidthFigure10showsthevariationofbondstresswithmeancrackwidthfor16mmbarsandFig.11forthe12mmbars.Figures12and13showthedataforthemaximumcrackwidth.4Um廿袖in員ic6心$i.Meancrackwidtli(mm)uCLlNj着扫匸Fig.10Meancrackwidthversusbondstressfor16mmbarsFig.11Meancrackwidthversusbo

40、ndstressfor12mmbarsMaKiEnumcrackwidth(rnni)Fig.12Maximumcrackwidthversusbondstressfor16mmbars精品Word精品Word文档欢迎下载精品Word精品Word文档欢迎下载精品资料精品资料Word欢迎使用精品资料精品资料Word欢迎使用Fig.13Maximumcrackwidthversusbondstressfor12mmbarsThedatashowaninitialincreaseinbondstrengthforthe12mmspecimenswithstirrups,followedbyasign

41、ificantdecreaseinbond,whichisinagreementwithotherauthors12,15.Forthe16mmspecimensanincreaseonthecontrolbondstresswasobservedforspecimenswith0.28and0.35mmmeancrackwidths,however,adecreaseinbondstresswasobservedforatthemeancrackwidthof0.05mm.The12mmbarswithstirrupsdisplayedanincreaseinbondstressofappr

42、oximately25%fromthecontrolvaluestothemaximumbondstress.Anincreaseofapproximately14%wasobservedforthe16mmspecimens.Otherresearchers17,24,25havereportedenhancementsofbondstressofbetween10and60%duetoconfinement,slightlyhighertothatobservedintheseexperiment.Howevertheloadingtechniquesandcoverdepthshaven

43、otallbeenthesame.Variationsinexperimentaltechniquesincludeashorterembeddedlengthandalowercover.Thevariationontheproposedempiricalrelationshipbetweenbondstrength,degreeofcorrosion,barsize,cover,linkdetailsandtensilestrengthpredictedbyRodriguez24hasbeendiscussedindetailinTangetal.28.Theanalysisdemonst

44、ratesthattherewouldbeanexpectedenhancementofbondstrengthduetoconfinementofapproximately25%correspondingtoachangeofbondstrengthofapproximately0.75MPaforthe16mmbars(assessedata2%sectionloss).Forthe12mmbarsthecorrespondingeffectofconfinementisfoundtobeapproximately35%correspondingtoa1.0MPadifferenceinb

45、ondstress.Theexperimentalresults(14and25%,above)are60一70%ofthesevalues.Bothsetsofdataindicatearelationshipshowingdecreasingbondstrengthwith(visiblesurface)crackwidth.Aregressionanalysisofthebondstrengthdatarevealsabetterlinearrelationshipwiththemaximumcrackwidthasopposedtothemeancrackwidth(excluding

46、theuncrackedconfinedspecimens),Table2.Table2Bestfitparameters,crackwidthversusbondstrengthUnconfined12mmConfined12mmUnconfined16mmConfined16mmMeancrackwidthR20.9200.6370.6720.659Slope(m)-3.997-3.653-2.999-8.848Intercept(b)7.5608.1226.4968.746MaximumcrackwidthR20.9370.8550.7140.616Slope(m)-2.719-2.96

47、8-1.815-5.330Intercept(b)7.8058.4036.7079.636Therewasalsoasignificantlybetterfitfortheunconfinedspecimensthantheconfinedspecimens.Thisisconsistentwiththeobservationthatintheunconfinedspecimensthebondstrengthwillberelatedtothebondbetweenthebarsandtheconcrete,whichwillbeaffectedbythelevelofcorrosionpr

48、esent,whichitselfwillinfluencethecrackwidth.Inconfinedspecimenstheconfiningsteelwillimpactuponboththebondandthecracking.CorrosiondegreeandbondstressItisapparentthat(Fig.14)forcorrosiondegreeslessthan5%thebondstresscorrelatedwell.However,asthedegreeofcorrosionincreasedtherewasnoobservablecorrelationa

49、tall.Thiscontrastswiththerelationshipbetweentheobservedcrackwidthandbondstress,whichgivesareasonablecorrelation,evenascrackwidthsincreaseto2and2.5mm.Apossibleexplanationforthisvariationisthatintheinitialstagesofcorrosionvirtuallyallthedissolvedironionsreacttoformexpansivecorrosionproducts.Thisreacti

50、onimpactsonboththebondstressandtheformationofcracks.However,oncecrackshavebeenformeditispossiblefortheironionstobetransportedalongthecrackandoutoftheconcrete.Asthebondhasalreadybeeneffectivelylostatthecrackanyironionsdissolvingatthecrackandbeingdirectly精品Word精品Word文档欢迎下载精品Word精品Word文档欢迎下载精品Word精品Wor

51、d文档欢迎下载精品资料精品资料Word欢迎使用精品资料精品资料Word欢迎使用精品资料精品资料Word欢迎使用transportedoutoftheconcretewillcauseanincreaseinthedegreeofcorrosion,butnotaffectthesurfacecrackwidth.Thelocation,orientationandchemistrywithinthecrackwillcontroltherelationshipbetweenbondstressanddegreeofcorrosion,whichwillvaryfromspecimentospe

52、cimen.Hencethelargevariationsincorrosiondegreeandbondstressforhighlevelsofcorrosion.io-*4s+io-*4s+76&E101214Corrosi&tiEJegrec(%)CL貝j戏岂左puucclFig.14Bondstressversuscorrosiondegree,12mmbars,unconfinedspecimenSignificantlylargercrackwidthswereobservedfortheunconfinedspecimens,comparedtotheconfinedspeci

53、menswithsimilarlevelsofcorrosionandmasslost.Thelargestobservedcrackforunconfinedspecimenswas2.5mmcomparedto1.4mmfortheconfinedspecimens.Thisisasexpectedandisadirectresultoftheconfinementwhichlimitsthedegreeofcracking.EffectofconfinementTheunconfinedspecimensforboth16and12mmbarsdidnotdisplaytheinitia

54、lincreaseinbondstrengthobservedfortheconfinedbars.Indeedtheunconfinedspecimenswithcracksalldisplayedareducedbondstresscomparedtothecontrolspecimens.Thisisinagreementwithotherauthors16,24findingsforcrackedspecimens.IncrackedcorrodedspecimensFangobservedasubstantialreductioninbondstrengthfordeformedba

55、rswithoutstirrups,whileRodriguezobservedbondstrengthsofhighlycorrodedcrackedspecimenswithoutstirrupswereclosetozero,whilehighlycorrodedcrackedspecimenswithstirrupsretainedbondstrengthsofbetween3and4MPa.InuncorrodedspecimensChananotedanincreaseinbondstrengthduetostirrupsofbetween10and20%14.HoweverRod

56、riguezandFangobservednovariationduetothepresenceofconfinementinuncorrodedbars.Thedataisperhapsunexpectedasitcouldbeanticipatedthatthecorrosionproductswouldleadtoanincreaseinbondduetotheincreaseininternalpressures,causedbythecorrosionproductsincreasingtheconfinementandmechanicalinterlockingaroundtheb

57、ar,coupledwithincreasedroughnessofthebarresultinginagreaterfrictionbetweenthebarandthesurroundingconcrete.However,thesepressureswouldthenrelievedbythesubsequentcrackingoftheconcrete,whichwouldcontributetothedecreaseinthebondstrengthascrackwidthsincrease.Apossiblehypothesisisthatduetothelevelofcover,

58、threetimesbardiameter,theeffectofconfinementbythestirrupsisreduced,suchthatithaslittleimpactonthebondstressinuncrackedconcrete.However,oncecrackinghastakenplacetheconfinementdoeshaveabeneficialeffectonthebond.Itmayalsobethatthecompressivestrengthoftheconcretecombinedwiththecoverwillhaveaneffectonthe

59、bondstressesforuncorrodedspecimens.Thedatapresentedherehasacoverofthreetimesbardiameterandastrengthof40MPa,otherresearchrangesfrom1.5tofourtimescoverwithcompressivestrengthsfrom40to77MPa.Comparisonof12and16mmrebarThemaximumbondstressfor16mmunconfinedbarswasmeasuredat8.06MPaandforthe12mmbarsitwas8.43

60、MPa.Thesebothcorrespondedtothecontrolspecimenswithnocorrosion.Theunconfinedspecimensforboththe12and16mmbarsshowednoincreaseinbondstressduetocorrosion.Fortheconfinedspecimensthemaximumbondstressforthecontrolspecimenswere7.29MPaforthe12mmbarsand6.34MPaforthe16mmbars.Themaximumbondstressforbothsetsofco