土木工程外文文献.docx

土木工程外文文献.docx

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土木工程外文文献

MaterialsandStructures

© RILEM 2010

10.1617/s11527-010-9700-y

OriginalArticle

Impactofcrackwidthonbond:

confinedandunconfinedrebar

David W. Law1 ,Denglei Tang2,Thomas K. C. Molyneaux3andRebecca Gravina3

（1） 

SchooloftheBuiltEnvironment,HeriotWattUniversity,Edinburgh,EH144AS,UK

（2） 

VicRoads,Melbourne,VIC,Australia

（3） 

SchoolofCivil,EnvironmentalandChemicalEngineering,RMITUniversity,Melbourne,VIC,3000,Australia

David W. Law

Email:

D.W.Law@hw.ac.uk

Received:

14 January 2010  Accepted:

14 December 2010  Publishedonline:

23 December 2010

Abstract  

Thispaperreportstheresultsofaresearchprojectcomparingtheeffectofsurfacecrackwidthanddegreeofcorrosiononthebondstrengthofconfinedandunconfineddeformed12and16 mmmildsteelreinforcingbars.ThecorrosionwasinducedbychloridecontaminationoftheconcreteandanappliedDCcurrent.Theprincipalparametersinvestigatedwereconfinementofthereinforcement,thecoverdepth,bardiameter,degreeofcorrosionandthesurfacecrackwidth.Theresultsindicatedthatpotentialrelationshipbetweenthecrackwidthandthebondstrength.Theresultsalsoshowedanincreaseinbondstrengthatthepointwhereinitialsurfacecrackingwasobservedforbarswithconfiningstirrups.Nosuchincreasewasobservedwithunconfinedspecimens.

Keywords  Bond - Corrosion - Rebar - Cover - Crackwidth - Concrete

1 Introduction

Thecorrosionofsteelreinforcementisamajorcauseofthedeteriorationofreinforcedconcretestructuresthroughouttheworld.Inuncorrodedstructuresthebondbetweenthesteelreinforcementandtheconcreteensuresthatreinforcedconcreteactsinacompositemanner.However,whencorrosionofthesteeloccursthiscompositeperformanceisadverselyaffected.Thisisduetotheformationofcorrosionproductsonthesteelsurface,whichaffectthebondbetweenthesteelandtheconcrete.

Thedeteriorationofreinforcedconcreteischaracterizedbyageneralorlocalizedlossofsectiononthereinforcingbarsandtheformationofexpansivecorrosionproducts.Thisdeteriorationcanaffectstructuresinanumberofways;theproductionofexpansiveproductscreatestensilestresseswithintheconcrete,whichcanresultincrackingandspallingoftheconcretecover.Thiscrackingcanleadtoacceleratedingressoftheaggressiveagentscausingfurthercorrosion.Itcanalsoresultinalossofstrengthandstiffnessoftheconcretecover.Thecorrosionproductscanalsoaffectthebondstrengthbetweentheconcreteandthereinforcingsteel.Finallythecorrosionreducesthecrosssectionofthereinforcingsteel,whichcanaffecttheductilityofthesteelandtheloadbearingcapacity,whichcanultimatelyimpactupontheserviceabilityofthestructureandthestructuralcapacity[12,25].

Previousresearchhasinvestigatedtheimpactofcorrosiononbond[2–5,7,12,20,23–25,27,29],withanumberofmodelsbeingproposed[4,6,9,10,18,19,24,29].Themajorityofthisresearchhasfocusedontherelationshipbetweenthelevelofcorrosion（masslossofsteel）orthecurrentdensitydegree（corrosioncurrentappliedinacceleratedtesting）andcrackwidth,orontherelationshipbetweenbondstrengthandlevelofcorrosion.Otherresearchhasinvestigatedthemechanicalbehaviourofcorrodedsteel[1,11]andthefrictioncharacteristics[13].However,littleresearchhasfocusedontherelationshipbetweencrackwidthandbond[23,26,28],aparameterthatcanbemeasuredwithrelativeeaseonactualstructures.

Thecorrosionofthereinforcingsteelresultsintheformationofironoxideswhichoccupyalargervolumethanthatoftheparentmetal.Thisexpansioncreatestensilestresseswithinthesurroundingconcrete,eventuallyleadingtocrackingofthecoverconcrete.Oncecrackingoccursthereisalossofconfiningforcefromtheconcrete.Thissuggeststhatthelossofbondcapacitycouldberelatedtothelongitudinalcrackwidth[12].However,theuseofconfinementwithintheconcretecancounteractthislossofbondcapacitytoacertaindegree.Researchtodatehasprimarilyinvolvedspecimenswithconfinement.Thispaperreportsastudycomparingthelossofbondofspecimenswithandwithoutconfinement.

2 Experimentalinvestigation

2.1 Specimens

Beamendspecimens[28]wereselectedforthisstudy.Thistypeofeccentricpulloutor‘beamend’typespecimenusesabondedlengthrepresentativeoftheanchoragezoneofatypicalsimplysupportedbeam.Specimensofrectangularcrosssectionwerecastwithalongitudinalreinforcingbarineachcorner,Fig. 1.An80 mmplastictubewasprovidedatthebarunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhancedduetoa（transverse）compressiveforceactingonthebaroverthislength.

Fig. 1 Beamendspecimen

Deformedrebarof12and16 mmdiameterwithcoverofthreetimesbardiameterwereinvestigated.Duplicatesetsofconfinedandunconfinedspecimensweretested.Theconfinedspecimenshadthreesetsof6 mmstainlesssteelstirrupsequallyspacedfromtheplastictube,at75 mmcentres.

Thisrepresentsfourgroupsofspecimenswithacombinationofdifferentbardiameterandwith/withoutconfinement.Thespecimenswereselectedinordertoinvestigatetheinfluenceofbarsize,confinementandcrackwidthonbondstrength.

2.2 Materials

Themixdesignisshown,Table 1.ThecementwasTypeIPortlandcement,theaggregatewasbasaltwithspecificgravity2.99.ThecoarseandfineaggregatewerepreparedinaccordancewithAS1141-2000.MixingwasundertakeninaccordancewithAS1012.2-1994.Specimenswerecuredfor28 daysunderwethessianbeforetesting.

Table 1 Concretemixdesign

Material

Cement

w/c

Sand

10 mmwashedaggregate

7 mmwashedaggregate

Salt

Slump

Quantity

381 kg/m3

0.49

517 kg/m3

463 kg/m3

463 kg/m3

18.84 kg/m3

140 ± 25 mm

Inordertocomparebondstrengthforthedifferentconcretecompressivestrengths,Eq. 1isusedtonormalizebondstrengthfornon-corrodedspecimensashasbeenusedbyotherresearcher[8].

（1）

where

isthebondstrengthforgrade40concrete,τexptlistheexperimentalbondstrengthandfcistheexperimentalcompressivestrength.

ThetensilestrengthoftheΦ12andΦ16 mmsteelbarswasnominally500 MPa,whichequatestoafailureloadof56.5and100.5 kN,respectively.

2.3 Experimentmethodology

Acceleratedcorrosionhasbeenusedbyanumberofauthorstoreplicatethecorrosionofthereinforcingsteelhappeninginthenaturalenvironment[2,3,5,6,10,18,20,24,27,28,30].Thesehaveinvolvedexperimentsusingimpressedcurrentsorartificialweatheringwithwet/drycyclesandelevatedtemperaturestoreducethetimeuntilcorrosion,whilemaintainingdeteriorationmechanismsrepresentativeofnaturalexposure.Studiesusingimpressedcurrentshaveusedcurrentdensitiesbetween100 μA/cm2and500 mA/cm2[20].Researchhassuggestedthatcurrentdensitiesupto200 μA/cm2resultinsimilarstressesduringtheearlystagesofcorrosionwhencomparedto100 μA/cm2[21].Assuchanappliedcurrentdensityof200 μA/cm2wasselectedforthisstudy—representativeofthelowerendofthespectrumofsuchcurrentdensitiesadoptedinpreviousresearch.However,cautionshouldbeappliedwhenacceleratingthecorrosionusingimpressedcurrentastheaccelerationprocessdoesnotexactlyreplicatethemechanismsinvolvedinactualstructures.Inacceleratedteststhepitsarenotallowedtoprogressnaturally,andtheremaybeamoreuniformcorrosiononthesurface.Alsotherateofcorrosionmayimpactonthecorrosionproducts,suchthatdifferentoxidationstateproductsmaybeformed,whichcouldimpactonbond.

Thesteelbarsservedastheanodeandfourmildsteelmetalplateswerefixedonthesurfacetoserveascathodes.Sponges（sprayedwithsaltwater）wereplacedbetweenthemetalplatesandconcretetoprovideanadequatecontact,Fig. 2.

Fig. 2 Acceleratedcorrosionsystem

Whentherequiredcrackwidthwasachievedforaparticularbar,theimpressedcurrentwasdiscontinuedforthatbar.Thespecimenwasremovedforpullouttestingwhenallfourlocationsexhibitedthetargetcrackwidth.Averagesurfacecrackwidthsof0.05,0.5,1and1.5 mmwereadoptedasthetargetcrackwidths.Thesurfacecrackwidthwasmeasuredat20 mmintervalsalongthelengthofthebar,beginning20 mmfromtheendofthe（plastictube）bondbreakerusinganopticalmicroscope.Thelevelofaccuracyinthemeasurementswas±0.02 mm.Measurementsofcrackwidthweretakenonthesurfacenormaltothebardirectionregardlessoftheactualcrackorientationatthatlocation.

Bondstrengthtestswereconductedbymeansofahandoperatedhydraulicjackandacustom-builttestrigasshowninFig. 3.TheloadingschemeisillustratedinFig. 4.Aplastictubeoflength80 mmwasprovidedattheendoftheconcretesectionunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhancedbythereactive（compressive）force（actingnormaltothebar）.Thespecimenwaspositionedsothatanaxialforcewasappliedtothebarbeingtested.Therestraintsweresufficientlyrigidtoensureminimalrotationortwistingofthespecimenduringloading.

Fig. 3 Pull-outtest,16 mmbarunconfined

Fig. 4 Schematicofloading.Note:

onlytestbarshownforclarity

3 Experimentalresultsanddiscussion

3.1 Visualinspection

Followingtheacceleratedcorrosionphaseeachspecimenwasvisuallyinspectedforthelocationofcracks,meancrackwidthandmaximumcrackwidth（Sect. 2.3）.

Whileeachspecimenhadameantargetcrackwidthforeachb