工程管理外文翻译原文+译文.docx

工程管理外文翻译原文+译文.docx

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工程管理外文翻译原文+译文

ConcreteConstructionmatter

T.Pauly,M.J.N.Priestley

Abstract

Viewedintermsofacceptedpractices,concreteconstructionoperationsleavemuchtobedesiredwithrespecttothequality,serviceability,andsafetyofcompletedstructures.Theshortcomingsoftheseoperationsbecameabundantlyclearwhenamagnitude7.6earthquakestrucknorthernPaki-stanonOctober8,2005,destroyingthousandsofbuildings,damagingbridges,andkillinganesti-mated79,000people.Theunusuallylowqualityofconstructionoperationsprevalentwasamajorcauseoftheimmensedevastation.

Keywords:

ConcretePlacingCuringConstructionTechnology

PlacingConcrete

Ifconcreteisplacedinthesurface,thesur-faceshouldbefilledwithwatersufficientlytopreventitfromabsorbingtheconcreteofitswater.Iffreshconcreteistobeplacedonornearbytoconcretethathassolidified,thesurfaceoftheplacedconcreteshouldbecleanedabsolutely,preferablywithahigh-pressureairorwaterjetorsteel-wirebrushes.Thesurfaceshouldbewet,butthereshouldbenomuchwater.Alittlequantityofcementgroutshouldbebrushedoverthewholearea,andthenfollowedimmediatelywiththeapplicationofa1/2-inLayerofmortar.Thefreshconcreteshouldbeplacedonoragainstthemortar.

Inordertodecreasethedisintegrationre-sultingfromcarriageafteritisplaced.Thecon-creteshouldbeplacedasnearlyasprobablyinits

finalpoint.Itshouldbeplacedinlayerstopermituniformcompaction.Thetimeintervalbetweentheplacingoflayersshouldbelimitedtoassureperfectbondbetweenthefreshandpreviouslyplacedconcrete.

Inplacingconcreteindeeperpatters,aves-selshouldbeusedtolimitthefreefalltonotover3or4ft,inordertopreventconcretedisintegra-tion.Thevesselisapipemadeoflightweightmetal,havingadjustablelengthsandattachedtothebottomofahopperintowhichtheconcreteisdeposited.Asthepattersarefilled,sectionsofthepipemayberemoved.

Immediatelyaftertheconcreteisplaced,itshouldbecompactedbyhandpuddingorame-chanicalvibratortoeliminatevoids.Thevibratorshouldbeleftinonepositiononlylongenoughtoreducetheconcretearoundittoaplasticmass;thenthevibratorshouldbemoved,ordisintegra-

tionoftheaggregatewilloccur.Ingeneral,thevibratorshouldnotbepermittedtopenetrateconcreteinthepriorlift.

Themainlyadvantageofvibratingisthatitpermitstheuseofadrierconcrete,whichhasahigherstrengthbecauseofthereducedwatercontent.Amongtheadvantagesofvibratingcon-cretearethefollowing:

1.Thedecreasedwaterpermitsareductioninthecementandfineaggregatebecauselesscementpasteisneeded.

2.Thelowerwatercontentdecreasesshrinkageandvoids.

3.Thedrierconcretedecreasesthecostoffinishingthesurface.

4.Mechanicalvibrationmayreplacethreetoeighthandpuddles.

5.Thelowerwatercontentincreasesthestrengthoftheconcrete.

6.Thedriermixturepermitstheremovalofsomepattersmorequickly,whichmayreducethecostofpatters.

CuringConcrete

Ifconcreteistogainitsmaximumstrengthandotherdesirableproperties,itshouldbecuredwithadequatemoistureandatafavorabletem-perature.Failuretoprovidetheseconditionsmayresultinaninferiorconcrete.

Theinitialmoistureinconcreteisadequatetohydrateallthecement,provideditisnotshouldreplacethemoisturethatdoesevaporate.Thismaybeaccomplishedbymanymethods,suchasleavingthepattersinplace,keepingthesurfacewet,orcoveringthesurfacewithaliquidcuringcompound,whichcomesbeingtoawater-tightmembranethatpreventstheescapeoftheinitialwater.Curingcompoundsmaybeappliedbybrushesorpressuresprayers.Agallonwillcover200to300sqft.

Concreteshouldbeplacedatatemperaturenotlessthan40ormorethan80°F.Alowertem-peraturewilldecreasetherateofsetting,whilea

highertemperaturewilldecreasetheultimatestrength.

PlacingConcreteinColdWeather

Whentheconcreteisplacedduringcoldweather,itisusuallynecessarytopreheatthewater,theaggregate,orbothinorderthattheini-tialtemperaturewillassureaninitialsetandgaininstrength.Preheatingthewateristhemostef-fectivemethodofprovidingthenecessarytem-perature.Forthispurposeawaterreservoirshouldbeequippedwithpipecoilsthroughwhichsteamcanbepassed,orsteammaybedischargeddirectlyintothewater,severaloutletsbeingusedtogivenbetterdistributionoftheheat.

Whenthetemperaturesofthemixturesareknown,somespecificchartsmaybeusedtocal-culatethetemperatureofconcrete.Astraightlinepassallthreescales,passingthrougheverytwoknowntemperatures,willassurethedetermina-tionofthethirdtemperature.Ifthesurfaceofsandisdry,thefactlinesofthescalesgivingthetemperatureofconcreteshouldbeused.However,ifthesandcontainsabout3percentmoisture,thedottedlinesshouldbeused.

Specificationsusuallydemandthatfreshlyplacedconcreteshallbekeptatatemperatureofnotlessthan70°Ffor3daysor50°Ffor5daysafteritisplaced.Somepropermethodmustbeprovidedtokeepthedemandedtemperaturewhenthecoldweatherisestimated.

Reinforcingsteelsforconcrete

Comparedwithconcrete,steelisahighstrengthmaterial.Theusefulstrengthofordinaryreinforcingsteelsintensionaswellascompres-sion,i.e.,theyieldstrength,isabout15timesthecompressivestrengthofcommonstructuralcon-crete,andwellover100timesitstensilestrength.Ontheotherhand,steelisahigh-costmaterialcomparedwithconcrete.Itfollowthatthetwomaterialsarethebestusedincombinationiftheconcreteismadetoresistthecompressivestressesandthecompressiveforce,longitudinalsteelreinforcingbarsarelocatedclosetotheten-sionfacetoresistthetensionforce.,andusuallyadditionalsteelbarsaresodisposedthattheyre-sisttheinclinedtensionstressesthatarecausedbytheshearforceinthebeams.However,rein-forcementisalsousedforresistingcompressiveforcesprimarilywhereitisdesiredtoreducethecross-sectionaldimensionsofcompressionmembers,asinthelower-floorcolumnsofmulti-storybuildings.Evenifnosuchnecessityexits,aminimumamountofreinforce-mentisplacedinallcompressionmemberstosafeguardthemagainsttheeffectsofsmallaccidentalbendingmomentsthatmightcrackandevenfailanunre-inforcedmember.

Formosteffectivereinforcingaction,itisessentialthatsteelandconcretedeformtogether,i.e.,thattherebeasufficientlystrongbondbe-tweenthetwomaterialstoensurethatnorelativemovementsofthesteelbarsandthesurroundingconcreteoccur.Thisbondisprovidedbytherela-tivelylargechemicaladhesionwhichdevelopsatthesteel-concreteinterface,bythenaturalroughnessofthemillscaleofhot-rolledrein-forcingbars,andbythecloselyspacedrib-shap-edsurfacedeformationswithwhichreinforcingbarsarefurnishedinordertoprovideahighde-greeofinterlockingofthetwomaterials.

Steelisusedintwodifferentwaysincon-cretestructures:

asreinforcingsteelandasprestressingsteel.reinforcingsteelisplacedintheformspriortocastingoftheconcrete.Stressesinthesteel,asinthehardenedconcrete,arecausedonlybytheloadsonthestructure,exceptforpossibleparasiticstressesfromshrinkageorsimilarcauses.Incontrast,inpriestessesconcretestructureslargetensionforcesareappliedtothereinforcementpriortolettingitactjointlywiththeconcreteinresistingexternal.

Themostcommontypeofreinforcingsteelisintheformofroundbars,sometimescalledrebars,availableinalargerangeofdiameters,from10to35mmforordinaryapplicationsandintwoheavybarsizesoff44and57mmthesebarsarefurnishedwithsurfacedeformationsforthepurposeofincreasingresistancetoslipbe-tweensteelandconcreteminimumrequirementsforthesedeformationshavebeendevelopedinexperimentalresearch.Differentbarproducersusedifferentpatterns,allofwhichsatisfytheserequirements.

Weldingofrebarsinmakingsplices,orforconvenienceinfabricatingreinforcingcagesforplacementintheforms,mayresultinmetal-lurgicalchangesthatreducebothstrengthandductility,andspecialrestrictionsmustbeplacedbothstrengthandductility,andspecialrestric-tionsmustbeplacedbothonthetypeofsteelusedandtheweldingprocedurestheprovisionsofASTMA706relatespecificallytowelding.

Inreinforcedconcretealong-timetrendisevidenttowardtheuseofhigherstrengthmateri-als,bothsteelandconcrete.

Reinforcingbarswith40ksiyieldstress,almoststandard20yearsago,havelargelybeenreplacedbybarswith60ksiyieldstress,bothbecausetheyaremoreeconomicalandbecausetheirusetendstoreducecongestionofsteelintheforms.

TheACICodepermitsreinforcingsteelsuptoFy=80ksi.SuchhighstrengthsteelsusuallyyieldgraduallybuthavenoyieldplateauinthissituationtheACICoderequiresthatatthespeci-fiedminimumyieldstrengththetotalstrainshallnotexceed0.0035thisisnecessarytomakecur-rentdesignmethods,whichweredevelopedforsharp-yieldingsteelswithayieldplateau,appli-cabletosuchhigherstrengthsteels.thereisnoASTMspecificationfordeformedbarsmaybeused,accordingtotheACICode,providingtheymeettherequirementsstatedunderspecialcircumstancessteelinthishigherstrengthrangehasitsplace,e.g.,inlower-storycolumnsofhigh-risebuildings.

Inordertominimizecorrosionofrein-forcementandconsequentspellingofconcreteunderseverexposureconditionssuchasinbridgedeckssubjectedto