土建外文翻译word版.docx

土建外文翻译word版.docx

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土建外文翻译word版

Residualbondstrengthbetweensteelbarsandconcreteafterelevatedtemperatures

A.FerhatBingölRüstemGül

CivilEngineeringDepartment,AtatürkUniversity,Erzurum,Turkey

ARTICLE

Articlehistory

Receivedinrevisedform

Revised28March2009.

Accepted3April2009.

Availableonline6May2009.

Keywords

Concrete;

Elevatedtemperatures;

Pull-out;

Bondstrength;

Coolingregimes

Abstract

Theeffectsofelevatedtemperaturesandcoolingregimesontheresidual（aftercooling）bondstrengthbetweenconcreteandsteelbarsareinvestigated.Forthisstudy,ribbedsteelbarsof8mmdiameterareembeddedintoC20andC35concreteblockswithembedmentlengthsof6,10and12cm.Unsealedspecimensareheatedto12differenttemperaturesrangingbetween50and700°Candthencooledinwaterorinair.Pull-outtestsarecarriedoutonthespecimens,andtheeffectsofelevatedtemperaturesontheresidualbondstrengthareinvestigatedbycomparingtheresultsagainstunheatedspecimens.

Increasesinbondstrengthareobservedfortemperaturesupto150°C;however,thereisdecreaseforallothertemperatures.Theeffectofthecoolingregimeislesspronouncedfortheconcrete-barbondstrength.Moreover,itisconcludedthatconcrete-barbondstrengthincreaseswiththeincreaseincompressivestrengthofconcreteandembedmentlengthofthebar.

1.Introduction

Reinforcedconcrete,oneofthemostwidelyusedconstructionmaterialsforavarietyofstructures,isacompositematerialconsistingofreinforcingbarsinahardenedconcretematrix.Concreteisuniqueforitsversatilityandlargecapacitytoresistcompressivestresses.However,itslowtensilecapacitymakesitimperativetoincorporateanothermaterialcapableofresistingandtransmittingtensilestresses.SteelisrecognisedworldwideasthemostcompetentreinforcingmaterialinstructuralconcreteduetoitshightensilecapacityandsimilarrateofthermalexpansionandPoisson'sratioasofconcrete[1].

Moreattentionhasbeenpaidtothemechanicalpropertiesofconcreteathightemperatureortotheresidualpropertiesofconcreteafterexposuretohightemperatures.Concretemaybeexposedtoelevatedtemperaturesduringafireorwhenitisclosertofurnacesandnuclearreactors.Itsmechanicalpropertiessuchasstrength,modulusofelasticityandvolumedeformationdecreaseandthisresultsinstructuralqualitydeteriorationofconcrete[2],[3],[4],[5],[6],[7]and[8].Ofparticularimportancearelossinstrengthandelasticmodulus,crackingandspalling,ductility,andlossofbondwithanysteelreinforcement[3]and[9].Anassessmentofthedegreeofdeteriorationoftheconcretestructureafterexposuretohightemperaturescanhelpengineerstodecidewhetherastructurecanberepairedratherthanrequiredtobedemolished[10].

Deteriorationinmechanicalpropertiesofconcreteuponheatingmaybeattributedtomaterialfactorsandenvironmentalfactors.Materialfactorsarepropertiesofaggregate,propertiesofcementpasteandaggregate–cementpastebondandtheirthermalincompatibilitybetweeneachother.Environmentalfactorscanbelistedasheatingrate,durationofexposuretomaximumtemperature,coolingrate,loadingconditionsandmoistureregime[11]and[12].Siliceousaggregatescontainingquartzmaycausedistressinconcreteatabout573°Csincethetransformationofquartzfromαtoβformisassociatedwithasuddenexpansionof0.85%[11].

Thestructureofconcretematerialcanbeapproximatelyclassifiedintomicrolevel（lessthan1μm）,mesolevel（between1μmand1cm）,andmacrolevel（greaterthan1cm）.Forconcretesubjectedtohightemperature,withtheincreaseintemperature,strengthandYoung'smodulusdecreaseatmacrolevel,internalstructuresdegenerateandmicrodefectsdevelopatmicroandmesolevels[13].

Theeffectoffireandhightemperatureonthebehaviourandpropertiesofreinforcedconcretestudiesincludescompressivestrength,modulusofelasticity,shearmodulus,thermalconductivity,specificheatandcreepofconcretealongwithmodulusofelasticityandcoefficientofthermalexpansionandtensilestrengthofreinforcingsteel.Informationonthebondbetweenconcreteandsteelislimited.Instudyingbondstrength,pull-outtestsareappliedandaverageloadsareusedtocalculateaveragebondstrength[14].Somepreviousworkwasconductedonthebondbetweensteelbarsandconcrete,thebondbetweenfibresandconcreteandthebondinotherfibre-reinforcedcomposites[15],[16],[17],[18],[19],[20]and[21].

Mostresearchdataofresidualpropertiesofconcreteafterexposuretohightemperaturewereobtainedunderconditionsofnaturalcooling,whichshoulddifferobviouslyfromcoolingregimesinarealfire,wherewatersprayingisusuallyusedforfireextinguishingandconsequentlythermalshockisinducedtoconcrete.Ithasbeenreportedthatwatercoolingcausedmoreseveredecreaseinstrengthcomparedtonaturalcooling.Therefore,theeffectofcoolingregimesonthemechanicalpropertiesofconcreteisofgreatconcern[22],[23],[24]and[25].

Asaresultofthesereasonssummarisedabove,theeffectsofelevatedtemperaturesandcoolingregimesaftertheheatingprocessontheresidualbondstrengthbetweenconcreteandsteelbarsareexaminedinthisexperimentalinvestigation.Normalstrengthconcretemixtureswiththeinitialcompressivestrengthsof20and35MPa,whicharecommonlyusedinbuildingsinTurkey,aretestedthroughoutthestudy.Reinforcementbarsareembeddedintoconcretespecimenswith3differentembedmentlengthsof6,10and16cm.Specimensarecooledeitherinwaterorinair,afterheatingprocesses.

2.Materialsandmethods

ASTMTypeI,PortlandCement（PC）,fromAşkaleCementFactoryinErzurum,Turkey,wasusedinthisinvestigation.Naturalaggregate（NA）withamaximumsizeof16mmwasobtainedfromAltunkentregioninErzincan,Turkey.Aggregateusedinthisstudywasriversandandgravelanditwassiliceousaggregate.ThechemicalcompositionofPCissummarisedinTable1,physicalandmechanicalpropertiesofPCaregiveninTable2andthepropertiesofaggregateareshowninTable3.Thediameteroftheribbedsteelbarswas8mmandthepropertiesofthebarsarelistedinTable4.

Twodifferentconcretemixeswithinitialcompressivestrengthsof20and35MPaareproducedinlaboratory-typemixerwiththecapacityof60dm3.Ribbedsteelbars8mmindiameterareembeddedintoconcreteblockswiththeembedmentlengthsof6,10and16cm.Foreachgroup,sixsamplesof100mmdiameterand200mmheightcylindersarepreparedforalltemperaturevalues.Threeofthemarecooledinwaterandtheotherthreespecimensarecooledinlaboratoryconditionsafterheating.Thespecimensareplacedinawatertankat23±2°C,24hafterthecastingandcureduntilthe26thday.Attheendof28daysofwatercuring,thespecimensaredriedandheateduptotargettemperaturesandthencooledtoroomtemperatureeitherinairgraduallyorinwaterrapidly.Pull-outtestsareconducted24hafterthespecimensarecooledtoroomtemperatureandtheresidualbondstrengthsaredetermined.Thepull-outtestiscommonlyusedtomeasurethesteel–concretebondstrength[26],becauseitiseasieraccordingtoothertestmethodsandsuitableforcomparingrelativebondproperties[27].Thepull-outtestmachine,usedinthisstudy,isshowninFig.1.

Forinvestigatingtheeffectofhightemperaturesoverthebondstrengthbetweenconcrete–steelbars,thetemperaturevaluesarechosenas50,100,150,200,250,300,350,400,450,500,600and700°Cinthisstudy.Alsounheatedspecimensareproducedandtestedforcomparingwiththeheatedspecimens.Fortheheatingprocessalaboratory-typefurnace,capacityof1000°C,isused.Therateoftemperatureincreaseofthefurnaceis12–20°C/min.Afterheatingspecimensuptothereferencetemperature,thefurnacetemperatureiskeptconstantfor3h.Allgroupsarecooledtoroomtemperatureeitherinairinlaboratoryconditionsorinwater,andtheeffectsofcoolingregimesareinvestigated.Foreachgroup3specimensaretestedandtheaverageofthesespecimens’testresultsisrecordedasthegroup'sresidualbondstrength.

3.Resultsanddiscussion

3.1.Bondstrengthbetweenconcrete–steelbarsexposedtoelevatedtemperatures

Aftertheeffectoftheelevatedtemperaturesinarangeof50–700°C,thebondstrengthsareshowninFig.2andFig.3forC20andC35concretes,respectively.ThechangeratiosofthebondstrengthsbytheeffectofelevatedtemperaturesarealsopresentedinTable6,Table7andTable8,andarediscussedbelow.

Increasesinresidualbondstrengthsareobservedupto150°Cforallembedmentlengths.Thisisthoughttobeduetotheincreaseinresidualcompressivestrengthforthesametemperatures.Thehighestincreaseamountsof6cmembedmentlengthare14%and12%forC20andC35concretes,respectively.Thesevaluesarebothtakenfromthespecimensthatareheatedto50°Candthencooledinwater.Above150°C,bondstrengthdecreaseswithincreaseintemperature.Forthetemperatureof700°C,bondstrengthsarerecordedasminimumvalues.C20sampleslost89%andC35sampleslost76%oftheirinitialbondstrengthsforthistemperature.Effectofcoolingregimesisnotdistinguishedfor6cmembedmentlength.Thisisconsideredtobeduetotheinsufficiencyofembedmentlength.Steelbarsslippedfromtheconcreteatlowpull-outstrengthsforbothair-andwater-cooledspecimens.

For10cmembedmentlength,bondstrengthsofC20specimensincreased10%and9%for50and100°C,butthentherearedecreasesforallothertemperatures.Bon