土木工程外文文献及翻译Word文档下载推荐.docx

土木工程外文文献及翻译Word文档下载推荐.docx

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RebeccaGravina3

（1SchooloftheBuiltEnvironment,HeriotWattUniversity,

）Edinburgh,EH144AS,UK

（2VicRoads,Melbourne,VIC,

）Australia

（3SchoolofCivil,EnvironmentalandChemicalEngineering,RMIT

）University,Melbourne,VIC,3000,Australia

DavidW.Law

Email:

D.W丄aw@hw.ac.uk

Received:

14January2010Accepted:

Abstract

Thispaperreportstheresultsofaresearchprojectcomparingtheeffectofsurfacecrackwidthanddegreeofcorrosiononthebondstrengthofconfinedandunconfineddeformed12and16mnmildsteelreinforcingbars.ThecorrosionwasinducedbychloridecontaminationoftheconcreteandanappliedDCcurrent.Theprincipalparametersinvestigatedwereconfinementofthereinforcement,thecoverdepth,bardiameter,degreeofcorrosionandthesurfacecrackwidth.Theresultsindicatedthatpotentialrelationshipbetweenthecrackwidthandthebondstrength.Theresultsalsoshowedanincreaseinbondstrengthatthepointwhereinitialsurfacecrackingwasobservedforbarswithconfiningstirrups.Nosuchincreasewasobservedwithunconfinedspecimens.Keywords:

bond;

corrosion;

rebar;

cover;

crack

1Introduction

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

Thedeteriorationofreinforcedconcreteischaracterizedbyageneralorlocalizedlossofsectiononthereinforcingbarsandtheformationofexpansivecorrosionproducts.Thisdeteriorationcanaffectstructuresinanumberofways;

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

Previousresearchhasinvestigatedtheimpactofcorrosionon

bond[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.

2Experimentalinvestigation

2.1Specimens

Beamendspecimens[28]wereselectedforthisstudy.Thistypeofeccentricpulloutor‘beamend'

typespecimenusesabondedlengthrepresentativeoftheanchoragezoneofatypicalsimplysupportedbeam.Specimensofrectangularcrosssectionwerecastwithalongitudinalreinforcingbarineachcorner,Fig.1.An80mm

plastictubewasprovidedatthebarunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhancedduetoa（transverse）compressiveforceactingonthebaroverthislength.

mndiameterwithcoverofthreetimes

bardiameterwereinvestigated.Duplicatesetsofconfinedandunconfinedspecimensweretested.Theconfinedspecimenshadthreesetsof6mmstainiesssteelstirrupsequallyspacedfromtheplastictube,at75mmcentres.

Thisrepresentsfourgroupsofspecimenswithacombinationofdifferentbardiameterandwith/withoutconfinement.Thespecimenswereselectedinordertoinvestigatetheinfluenceofbarsize,confinementandcrackwidthonbondstrength.

2.2Materials

Themixdesignisshown.Table1.ThecementwasTypeIPortiandcement,theaggregatewasbasaltwithspecificgravity2.99.ThecoarseandfineaggregatewerepreparedinaccordaneewithAS1141-2000.MixingwasundertakeninaccordaneewithAS1012.2-1994.

Specimenswerecuredfor28daysunderwethessianbeforetesting.

Table1Concretemixdesign

Material

Cement

w/c

Sand

10mmwashedaggregate

7mmwashedaggregate

Salt

Slump

Quanti

381kg

0.4

517

kg

463kg

18.84

k

140±

25

ty

/m3

9

/m3J

g/m3

mm

Inordertocomparebondstrengthforthedifferentconcrete

compressivestrengths,Eq.1isusedtonormalizebondstrengthfor

non-corrodedspecimensashasbeenusedbyotherresearcher[8].

（1）

whereisthebondstrengthforgrade40concrete,texptlistheexperimentalbondstrengthandfcistheexperimentalcompressivestrength.

Thetensilestrengthofthe①12and①16mmsteelbarswas

nominally500MPa,whichequatestoafailureloadof56.5and

100.5kN,respectively.

2.3Experimentmethodology

Acceleratedcorrosionhasbeenusedbyanumberofauthorstoreplicatethecorrosionofthereinforcingsteelhappeninginthenaturalenvironment[2,3,5,6,10,18,20,24,27,28,30].These

haveinvolvedexperimentsusingimpressedcurrentsorartificialweatheringwithwet/drycyclesandelevatedtemperaturestoreducethetimeuntilcorrosion,whilemaintainingdeteriorationmechanismsrepresentativeofnaturalexposure.Studiesusingimpressedcurrentshaveusedcurrentdensitiesbetween100卩A/cm2and500mA/cm2

[20].Researchhassuggestedthatcurrentdensitiesupto

200卩A/cm2resultinsimilarstressesduringtheearlystagesofcorrosionwhencomparedto100卩A/cm2[21].Assuchanappliedcurrentdensityof200卩A/cm2wasselectedforthis

study—representativeofthelowerendofthespectrumofsuchcurrentdensitiesadoptedinpreviousresearch.However,cautionshouldbeappliedwhenacceleratingthecorrosionusingimpressedcurrentastheaccelerationprocessdoesnotexactlyreplicatethemechanismsinvolvedinactualstructures.Inacceleratedteststhepitsarenotallowedtoprogressnaturally,andtheremaybeamoreuniformcorrosiononthesurface.Alsotherateofcorrosionmayimpactonthecorrosionproducts,suchthatdifferentoxidationstateproductsmaybeformed,whichcouldimpactonbond.

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

Fig.2Acceleratedcorrosionsystem

Wheritherequiredcrackwidthwasachievedforaparticularbar,theimpressedcurrentwasdiscontinuedforthatbar.Thespecimenwasremovedforpullouttestingwhenallfourlocationsexhibitedthetargetcrackwidth.Averagesurfacecrackwidthsof0.05,0.5,1and1.5mmwereadoptedasthetargetcrackwidths.Thesurfacecrackwidthwasmeasuredat20mmintervalsalongthelengthofthe

bar,beginning20mmfromtheendofthe（plastictube）bondbreakerusinganopticalmicroscope.Thelevelofaccuracyinthemeasurementswas±

0.02mm.Measurementsofcrackwidthweretakenonthesurfacenormaltothebardirectionregardlessoftheactualcrackorientationatthatlocation.

Bondstrengthtestswereconductedbymeansofahandoperatedhydraulicjackandacustom-builttestrigasshowninFig.3.The

loadingschemeisillustratedinFig.4.Aplastictubeoflength

80mmwasprovidedattheendoftheconcretesectionunderneaththetransversereactiontoensurethatthebondstrengthwasnotenhancedbythereactive（compressive）force（actingnormaltothe

bar）.Thespecimenwaspositionedsothatanaxialforcewasappliedtothebarbeingtested.Therestraintsweresufficientlyrigidto

ensureminimalrotationortwistingofthespecimenduringloading.

3Experimentalresultsanddiscussion

3.1Visualinspection

Followingtheacceleratedcorrosionphaseeachspecimenwasvisuallyinspectedforthelocationofcracks,meancrackwidthandmaximumcrackwidth（Sect.2.3）.

Whileeachspecimenhadameantargetcrackwidthforeachbar,variationsinthiscrackwidthwereobservedpriortopullouttesting.Thisisduetocorrosionandcrackingbeingadynamicprocesswithcrackspropagatingatdifferentrates.Thus,whileindividualbarsw