土木工程外文文献及翻译.docx
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土木工程外文文献及翻译
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).
Whileeachspecimenhadameantargetcrackwidthforeachbar,variationsinthiscrackwidthwereobservedpriortopullouttesting.Thisisduetocorrosionandcrackingbeingadynamicprocesswithcrackspropagatingatdifferentrates.Thus,whileindividualbarsweredisconnected,oncethetargetcrackwidthhadbeenachieved,corrosionandcrackpropagationcontinued(tosomeextent)untilallbarshadachievedthetargetcrackwidthandpullouttestsconducted.Thisresultedinarangeofdataforthemaximumandmeancrackwidthsforthepullouttests.
Thevisualinspectionofthespecimensshowedthreestagestothecrackingprocess.Theinitialcracksoccurredinaveryshortperiod,usuallygeneratedwithinafewdays.Afterthat,mostcracksgrewataconstantrateuntiltheyreached1 mm,3–4 weeksafterfirstcracking.Aftercrackshadreached1 mmtheythengrewveryslowly,withsomecracksnotincreasingatall.Fortheconfinedandunconfinedspecimensthesurfacecrackstendedtooccuronthesideofthespecimens(asopposedtothetoporbottom)andtofollowthelineofthebars.Inthecaseoftheunconfinedspecimensingeneraltheseweretheonlycrackwhileitwascommoninthecasesofconfinedspecimenstoobservecracksthatwerealignedverticallydowntheside—adjacenttooneofthelinks,Fig. 5.
Fig. 5 Typicalcrackpatterns
Duringthepull-outtestingthemostcommonfailuremodeforbothconfinedandunconfinedwassplittingfailure—withtheinitial(pre-test)crackscausedbyth