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Square concrete columns strengthened with carbon fiber reinforced plastics sheets at low temperatures

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Square concrete columns strengthened with carbon fiber reinforced plastics sheets at low temperatureswith,fiber,Fiber

    Square concrete columns strengthened with

    carbon fiber reinforced plastics sheets at

    low temperatures

    J.Cent.SouthUniv.Techno1.(2009)16:08350840

    DoI:10.1007/s1l77l-009-0l38-8

    Squareconcretecolumnsstrengthenedwith

    carbonfiberreinforcedplasticssheetsatlowtemperatures

    MAQinyong(~芹永),LUXiao.yu(卢小雨)

    (1.SchoolofCivilEngineeringandArchitecture,

    AnhuiUniversityofScienceandTechnology,Huainan232001,China;

    2.ScienceSchool,AnhuiUniversityofScienceandTechnology,Huainan232001,China) Springer

    Abstract:Twenty

    onesquareconcretecolumnswereconstructedandtested.Thetestingresultsindicatethatbondedcarbonfiber

    reinforcedplastics(CFRP)sheetscanbeusedtoincreasethestrengthandimprovetheserviceabi1it,rofdamagedconcretecolumnsat

    1owtemperatures.Thefailureofthespecimens.inmostcases.takesplacewithinthemiddlehalfofthecolumns.Andthefailureof

    strengthenedcolumnsissuddenandexplosive.TheCFRPsheetsincreaseboththeaxialloadcapacityandtheultimateconcrete

    compressivestrainofthecolumns.Theultimateloadsofstrengthenedcolumnsat

    l0,0and10?increaseaveragelybv9.O9%,

    6.63%and17.83%.respectively,ascomparedwiththoseofthecontrolspecimens.Theaxialcompressivestrengthofstrengthened

    columnsisrelatedtothecuringtemperatures.Theimprovementofaxialcompressivestrengt

hdecreaseswithreducingtemperature.

    andwhenthetemperaturedropstoacertainvalue,theimprovementincreaseswithfallingtem

    perature.

    Keywords:carbonfiberreinforcedplastics(CFRP)sheet;squareconcretecolumn;stress--st

    rainmodel

    lIntrOductiOn

    Applicationsofcarbonfiberreinforeedplastics (CFr~)sheetsincivilengineeringaregrowing[3],

    butoneoftheareaswhereknowledgeis1imitedisthe behaviourofconcretecolumnsstrengthenedwithCFRP sheetsatlowtemperatures.Toovercomethisdeficiency, testswereconductedtoevaluatetheeffectivenessand feasibilityofbondedCFRPsheetsonthebehaviourof strengthenedsquareconcretecolumnssubjectedtoroom andlowtemperatures.Theresearchprovidesthe foundationofsciencefortheapplicationofcarbonfiber strengtheninginwinterweather.

    Sofartherehavebeenonlyafewreviewsrelatedto theefrectoflowtemperatureontheperformanceof strengthenedconcretebeams.CHAJESetalf45]fcIund

    thattheenvironmenta1exposurecanlcadtochangein failuremodecomparedtonon.exposedspecimens.While NAAMAN61andGREENetal78]foundthatthe

    failuremodeofs~engthenedbeamstestedatlow temperaturewasthesameasthatofbeamsloadedat roomtemperature.GREENetal[78],H0Aetal[9],

    FINCHetaln01,N0LLETetal111,M0HAMADetal

    121andTU131foundthatFRPsheetscanbeeffectively bondedtoconcreteunderwinterweatherandthe ultimate1padatfreezingtemperaturecanbeincreasedas

    comparedtothecontrolbeams.AROCKIASAMYeta1 [14]andBAUMERTetal[15]foundthatflexure

    behaviourofstrengthenedspecimenswasnotaffectedby thereducedtemperatureexposure.

    2Experimental

    2.1Columnsdescriptionanddesign

    Twenty.onesquarecolumnswithl501Tmlinwidth and550mminlengthwereconstructedandtested.A11 specimenswereloadedinaxia1compressionuntilfailure. DetailsofthetestspecimenareshowninFig.1.Thetest specimensweredividedintoninegroups.T'able1gives thedetailsofthetestspecimens.

    2.2Materialproperties

    Themechanicalpropertiesoftheconcreteand

    reinforcementatroomtemperaturewereobtainedfrom ancillarytests.Theconcretemixtureproportionin masswas1.00(cement):1.35(fineaggregate):2.21(coarse aggregate).Thewater-cementratiowas0.39andthe cementcontentwas484.85kg/m.OrdinaryPortland cementandwellroundedgradedgravelwiththe

    maximumaggregatesizeof15mlnwereused.

    ThemechanicalpropertiesoftheresinandCFRP wereobtainedfromthedescriptionsprovidedbythe manufacturers.Thelowtemperaturestrengthproperties ofthereinforcementwerenotdetermined.duetothelack ofappropriatelowtemperaturetestfacilitiesand Foundationitem:Projiect(04043076)supportedbytheOutstandingYouthFoundationforSc

    ientificandTechnologicalResearchofAnhuiProvince,China; Project(2007jq1035)supportedbytheScientificResearchProjectsforYoungCollegeTeach

    ersofAnhuiProvince,China

Receiveddate:2008——09——23;Accepteddate:2009——06——17

    Correspondingauthor:LUXiaoyu,Doctoralcandidate;Tel:+865546668286;E

    mail:luxy@aust.edu.ca

    836J.Cent.SouthUniv.Techno1.(2009)l6:08350840

    06150

    (a)

    Fig.1Sketchofreinforcement

    (b)Verticalview

    (b)

    (Unit:ram):(a)Topview;

    apparatus.Thepropertiesofconcrete,reinforcement, resinepoxyandCFRPsheetsarepresentedinTables2——

    5,respectively.

    2.3Columnfabrication

    Thesteelcageswereassembledandcarefully placedintheforms.Acarewastakennottodamagethe straingaugesmountedonthereinforcementbars.The Table1Mainoarametersofeachcolumn

    concretemixturewasslowlypumpedintotheformsto preventsegregation,andvibratorswereusedtovibrate theconcretecarefultopreventvoidsfromforming. Slumptestsweretakenonsitetoensurethequalityof concrete.Theconcretecolumnswerecoveredwith burlapsheetsandmoistcuredforaperiodof2d.Then thecolumnswereremovedfrointheironformwork.At thetimeofcastingthecolumns,sixstandardcubes f150mm×150mm×150ram).werecastfromthesame mixtureforancillaryteststodeterminethecompressive s~engthoftheconcrete.

    Theconcretecolumns,aftercuring,werestriped,

    sharpedgeswereground,andthenthespecimenswere carefullysandblasted.Athinlayerofpenetratingprimer wasappliedtotheconcretespecimenandallowedtocure for2h.Voidsanddeformitiesonthesurfaceofthe specimenswerefilledusingaligningepoxyandallowed tocurefor2h.Themaceratingepoxy,consistingofresin andhardener,wasappliedtotheconcretespecimen.The CFRPsheetswerethenapplieddirectlytothesurfaceof thespecimens.providingunidirectiona1lateral confinementinthehoopdirection.Finally.fourwood platestiedthespecimenswrappedwithCFRPsheets. GroupNo.Ultimateload/MNCubecompressivestrength/MPaGroupNo.Ultimateload/M

    NCubecompressivestrength/MPa

    1.O91748.51.137950.6

    11.093048.621.076947.9

    1.038046.11.144049.8

    Table3Mechanicalpropertiesoflongitudinalsteelrebars J.Cent.SouthUniv.Techno1.(2009)16:08350840837

    Table4Mechanicalpropertiesofresin

    哪拄Pm.…仃

    3.8376.22.8043.1

    Thewoodplateswereremovedafterthespecimenswere curedatdifferentlowtemperaturesfor7d.

    2.4Testsetupandinstrument

    Ahydraulictestingmachinewithacapacity0f 2?【Nappliedtheaxia11oad.Priortotheactualtest.the columnswere1oadedinitiallytoasmallpre.1oadand thenunload.Thepurl:}oseofthisstepwastostabilizethe column.Therecordsofthestraingaugeswerethen broughtbacktozero.Theloadwasappliedgradually

througha1oadcontrolmode.Thecolumnswere

    subjectedtoastaticmonotonic1oadandtestedunti1the ultimateloadwasattained.Four2mmlongstraingauges

    weremountedatmiddlelocationsoneachlongitudinal steelrebartomeasurethelongitudinalstrainof longitudinalsteelrebar.Six20mmlongstraingauges

    weremountedatmiddlelocationsonCFRPsheetsto measurethelateralstrainofCFRPsheet.Eight50mm. 1ongstraingagesweremountedatmiddlelocationson theconcretetomeasurethelongitudinaland1atera1strain oftheconcrete.A11thestraingageswerelocatedon centralsectionofthespecimens.

    3Experimentalresults

    3.1ultimateloadandstrainresults

    Theresultsofultimateloadandstrainatfailureare giveninTlab1e6.Theultimate1oadsofstrengthened columnsat10.0and10?increaseaveragelyby

    9.09%,6.63%and17.83%,respectively,ascompared withthoseofthecontrolspecimens.

    Theultimateloadsofstrengthenedcolumnsat-5 and5?increaseaveragelyby25I42%and28.96%. respectively,ascomparedwiththoseofthecontro1 specimens.Theexperimenta1resultsshowthatusing CFRPsheetscaneffectivelyincreasetheultimateloads andultimatestrainofstrengthenedcolumns,regardless oftheircuringtemperatures.

    3.2Generalobservationsofexperimentaltests AscanbeseeninFig.2,largelongitudinalcracks aredevelopedunderloadingthatleadstothecomplete disintegrationofthecontrolspecimen.Andpiecesof

brokenconcretearefound.

    Crackingnoisesstart

    tofailure,indicatingthe

    fromthedilatedconcrete

    tobeheardatmomentsprior

    occurrenceofstresstransfer

    tothejacket.Thefailureis

    suddenandexplosive.andtheCFRPsheets,inmost cases.ruptureinthemiddleportionofthespecimens. TheCFRPsheetsrapturestartsatoneofthecomers withinthemidheightofthespecimensandprogressesup anddownalongthespecimen.Piecesofbrokenconcrete arefound.Thebrokenspecimensshowagoodcontact betweentheCFRPsheetsandtheconcrete.indicating thatnode.bondingtakesplace.Acompletedilationof thecoreconcreteisobvious.asshowninFig.2. 3.3Load--straincurves

    Thetypicalload-straincurvesofvariousconcrete columnsareshowninFig.3.

    Fig.2Failurepa~emsofspecimens:(a)1_3;(b)2-3;(c)8-1 838J.Cent.SouthUniv.Techno1.f2009)16:08350840

    g

    -J

    Z

    0

    

    Strain/l03

    Strain/l0

    Z

    0

    Z

    

    0

    Strain/103

    Strain/l03

    Strain/l0

    Fig.3Load--traincurvesofvariousconcrete columns:(a)3;(b)2-3;(c)3-2;(d)4-3;

    (e)5-1;(f)6-1;(g)7-2;(h)8-1;(i)9-1

    Z)I\0

    Z)l\勺砖0

    J.Cent.SouthUniv.Techno1.(2009)16:08350840839 Table6Ultimateloadandstrainresults

    3.4Influenceoftemperatureonstrengtheningeffect Fig.4showsthattheaxialcompressivestrengthof strengthenedcolunmsiSrelatedtothecuring temperatures.Theimprovementofaxialcompressive strengthdecreaseswithreducingtemperature,andwhen thetemperaturedropstoacertainvalue,the improvementincreaseswithfallingtemperature.The regressiveequationandcoefficientofcorrelationare v=O.6267ff+4.1333t+62.667.R=0.6001.

    Temperature/~C

    Fig.4Influenceoftemperatureonstrengtheningeffect 3.5Influenceoftemperatureonaxialstrain Fig.5showsthattheaxialstrainofstrengthened columnsiSrelatedtothecuringtemperatures.The improvementofaxialstraindecreaseswithreducing temperature.Whenthetemperaturedropstoacertain

valuetheimprovementincreaseswithfallingtempera

    ture.Theregressiveequationandcoefficientofcorrela

    tionareV=7.5167ff-20.867l756.3.R=0.1799.

    Temperature/~C

    Fig.5Influenceoftemperatureonaxialstrain 3.6Mechanismoflowtemperatureonstrengthening efrect

    Themainreasonfortheimprovementofaxial compressivestrengthdecreasingwithreducing temperatureiSthattheresinbecomesbrittleatlOW temperature,whichleadstounevenloadingoftheCFRP sheet,especiallyfortheCFRPsheetsatthecomers. Therefore,thefailureofCFRPsheetsatthecornersiS alwaystensilefailureatfirst.ButwhentemperatureiS 1owerthanthefreezingpointofwaterandthendrops form12to10?.concreteshowsasmallincreasein strengthwithincreasingtime.Whentemperatureislower than12?.concretedoesnotappeartogainstrength withtime.BROWNEandBAMFOIHf161foundthat thestrengthofconcreteincreasedattemperature1ower thanl1?.Sotheimprovementincreaseswithfalling temperature.

    4Conclusions

    (1)Thefailureofthespecimens,inmostcases, takesplacewithinthemiddlehalfofthecolumns.And thefailureofstrengthenedcolumnsissuddenand explosive.

    r21TheCFRPsheetsincreaseboththeaxialload capacityandtheultimateconcretecompressivestrainof thecolumnsatlOWtemperatures.

    (31Theultimate1oadsofstrengthenedcolumnsat

    l0,0and10?increaseaveragelyby9.09%,6.63%

    and17.83%.respectively,ascomparedwiththoseofthe 0/J1?001x0cg8c

    —自\000=l0?砖.Iul

    840J.Cent.SouthUniv.Techno1.(2009116:08350840

    contro1specimens.Theultimateloadsofstrengthened columnsat-5?and5?increaseaveragelybv25.42%

    and28.96%,respectively,ascomparedwiththatofthe controlspecimens.

    (4)Comparisonofroomandlowtemperaturetest resultsindicatesthattheaxialcompressivestrengthof strengthenedcolumnsisrelatedtothecuring temperatures.

    (5)Theimprovementofaxialcompressivestrength decreaseswithreducingtemperature,andwhenthe temperaturedropstoacertainvalue,theimprovement increaseswithfallingtemperature.

    References

    1]ZHAOTong,XIEJianNewtechnologyofCFRPstrengthened concretestructure[M].Tianjin:TianjinUniversityPress,2001(in

    Chinese)

    [2]TENGJG,cHENJsMITHsLLINLFRPstrengthenedRc structures[M].LIRong,TENGJingguang,GULei,transI.Beijing: ChinaBuildingIndustryPress,2001(inChinese) [3]HEXue-jun,ZHOUChaoyang,LIYihui,xuLingLaggedstrain oflaminatesinRCbeamsstrengthenedwithfiber-reinforcedpolymer

    J1JournalofCentralSouthUniversityofTechnology,2007,l4(3):

    432-435.

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