土木工程外文文献及翻译Word文档下载推荐.docx
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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
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