道路工程施工概況(英文)

實用文檔OverviewConstruction quality iscrucial tothelong-term pavementperformance. Constructionfactorssuchassurfacepreparation,placement,jointconstructionandcompaction/consolidation haveanoverwhelming effect onpavementperformance, whichcannotbeignoredorcompensatedforinmixorstructuraldesign.CompactionCompactionistheprocessbywhichthevolumeofairinanHMAmixtureisreducedbyusingexternalforcestoreorienttheconstituentaggregateparticlescloselyspacedarrangement.ThisreductionofairvolumeproducesacorrespondingincreaseinHMAdensity(Robertsetal.,1996[1]).

into amoreFigure1:ASteelWheelandaPneumaticTireRollerWorkingSide-by-Side.Compactionisthegreatestdeterminingfactorindensegradedpavementperformance(ScherocmanandMartenson,1984[2];Scherocman,1984[3];Geller,1984[4];Brown,1984[5];Bellet.al.,1984[6];Hughes,1984[7];Hughes,1989[8]).Inadequatecompactionresultsinapavementwithdecreasedstiffness,reducedfatiguelife,acceleratedaging/decreaseddurability,rutting,raveling,andmoisturesusceptibility(Hughes,1984[7];Hughes,1989[8]).CompactionMeasurementandReportingCompactionreducesthevolumeofairinHMA.Therefore,thecharacteristicofconcernisthevolumeofairwithinthecompactedpavement,whichistypicallyquantifiedasapercentageofairvoidsinrelationtototalvolumeandexpressedas“percentairvoids”.Percentairvoidsiscalculatedbycomparingatestspecimen’sdensity實用文檔withthedensityas“theoretical

it wouldtheoretically haveif all theair voidsmaximumdensity ”(TMD)or“Ricedensity ”after

wereremoved,knownthetest procedureinventor.AlthoughpercentairvoidsistheHMAcharacteristicofinterest,usuallyreportedasameasureddensityinrelationtoareferencedonebyreportingdensityas:

measurementsaredensity.ThisisPercentage of TMD(or “percent Rice”). This expression ofdensity iseasytoconverttoairvoidsbecauseanyvolumethatisnotasphaltbinderoraggregate isassumedtobeair. Forexample, adensity reported as93percentRicemeansthatthereare7percentairvoids(100% –93%=7%).Percentageofalaboratory-determineddensity.Thelaboratorydensityisusuallyadensityobtainedduringmixdesign.Percentageofacontrolstripdensity.Acontrolstripisashortpavementsectionthatiscompactedtothedesiredvalueunderclosescrutinythenusedasthecompactionstandardforaparticularjob.Pavementairvoidsaremeasuredinthefieldbyoneoftwoprincipalmethods:Cores(Figures2and3).AsmallpavementcoreisextractedfromthecompactedHMAandsenttoalaboratorytodetermineitsdensity.Usually,coredensityresultsareavailablethenextdayattheearliest.Thistypeofairvoidstestingisgenerallyconsideredthemostaccuratebutisalsothemosttimeconsumingandexpensive.Nucleargauges(Figures4and5).Anucleardensitygaugemeasuresin-placeHMAdensityusinggammaradiation.Gaugesusuallycontainasmallgammasource(about10mCi)suchasCesium-137locatedinthetipofasmallprobe,whichiseitherplacedonthesurfaceofthepavementorinsertedintothepavement.Readingsareobtainedinabout2–3minutes.Nucleargaugesrequirecalibrationtothespecificmixturebeingtested.Usuallynucleargaugesarecalibratedtocoredensitiesatthebeginningofaprojectandatregularintervalsduringtheprojecttoensureaccuracy.Eachcontractingagencyorownerusuallyspecifiesthecompactionmeasurementmethodsandequipmenttobeusedoncontractsundertheirjurisdiction.實用文檔Figure2:CoreExtraction Figure3:PavementCoreFigure4:ThinLiftNuclearDensityGauge

Figure5:TakingaNuclearDensityReadingFactorsAffectingCompactionHMAcompactionisinfluencedbyamyriadoffactors;somerelatedtotheenvironment,somedeterminedbymixandstructuraldesignandsomeundercontractorandagencycontrolduringconstruction(seeTable1).Table1:FactorsAffectingCompactionEnvironmentalFactors MixPropertyFactors ConstructionFactors實用文檔TemperatureAggregateRollers*Groundtemperature*Gradation*Type*Airtemperature*Size*Number*Windspeed*Shape*Speedandtiming*Solarflux*Fracturedfaces*Numberofpasses*Volume*LiftthicknessAsphaltBinderOther*Chemicalproperties*HMAproductiontemperature*Physicalproperties*Hauldistance*Amount*HaultimeFoundationsupportANoteontheTimeAvailableforCompactionHMAtemperaturedirectlyaffectsasphaltbinderviscosityandthuscompaction.AsHMAtemperaturedecreases,theconstituentasphaltbinderbecomesmoreviscousandresistanttodeformationresultinginasmallerreductioninairvoidsforagivencompactiveeffort.Asthemixcools,theasphaltbindereventuallybecomesstiffenoughtoeffectivelypreventanyfurtherreductioninairvoidsregardlessoftheappliedcompactiveeffort.Thetemperatureatwhichthisoccurs,commonlyreferredtoascessationtemperature,isoftenreportedtobeabout175°Ffordense-gradedHMA(ScherocmanandMartenson,1984[9];Hughes,1989[8]).Belowcessationtemperaturerollerscanstillbeoperatedonthemattoimprovesmoothnessandsurfacetexturebutfurthercompactionwillgenerallynotoccur.Mattemperatureiscrucialtoboththeactualamountofairvoidreductionforagivencompactiveeffort,andtheoveralltimeavailableforcompaction.Ifamat’sinitialtemperatureandcool-downrateareknown,thetemperatureofthematatanytimeafterlaydowncanbecalculated.Basedonthiscalculationrollingequipmentandpatternscanbeemployedto:Takemaximumadvantageofavailablerollercompactiveeffort.Rollerscanbeusedwherethematismostreceptivetocompactionandavoidedwherethematissusceptibletoexcessiveshoving.Ensurethematis compactedtothedesired air void content before cessationtemperatureisreached.Thiscanbedonebycalculatingthetimeittakesthemattocoolfrominitialtemperaturetocessationtemperature.Allcompactionmustbeaccomplished within this“timeavailable for compaction”.實用文檔MultiCool,developedbyProfessorVaughnVoellerandDr.DavidTimm,isaWindowsbasedprogramthatpredictsHMAmatcooling.MultiCoolcanbeusedtopredictthetimeavailableforcompactionandisavailableontheNationalAsphaltPavementAssociation’sAGuideforHotMixAsphaltPavementCD-ROMorfordownloadat:UniversityofCaliforniaPavementResearchCenter )NationalAsphaltPavementAssociationCompactionEquipmentTherearethreebasicpiecesofequipmentavailableforHMAcompaction:screed,(2)thesteelwheeledrollerand(3)thepneumatictireroller.ofequipmentcompactstheHMAbytwoprincipalmeans:

(1)thepaverEachpieceByapplyingitsweighttotheHMAsurfaceandcompressingthematerialunderneaththegroundcontactarea.Sincethiscompressionwillbegreaterforlongerperiodsofcontact,lowerequipmentspeedswillproducemorecompression.Obviously,higherequipmentweightwillalsoincreasecompression.2.Bycreatingashearstressbetweenthecompressedmaterialunderneaththegroundcontactareaandtheadjacentuncompressedmaterial.Whencombinedwithequipmentspeed,thisproducesashearrate.Loweringequipmentspeedcandecreasetheshearrate,whichincreasestheshearingstress.Highershearingstressesaremorecapableofrearrangingaggregateintomoredenseconfigurations.ThesetwomeansareofcompactingHMAareoftenreferredtocollectivelyas“compactiveeffort”.SteelWheelRollersSteelwheelrollers(seeFigures6and7)areself-propelledcompactiondevicesthatusesteeldrumstocompresstheunderlyingHMA.Theycanhaveone,twooreventhreedrums,althoughtandem(2drum)rollersaremostoftenused.Thedrumscanbeeitherstaticorvibratoryandusuallyrangefrom35to85inchesinwidthand20to60inchesindiameter.Rollerweightistypicallybetween1and20tons(seeFigures5and6).實用文檔Somesteelwheelrollersareequippedwithvibratorydrums.Drumvibrationaddsadynamicloadtothestaticrollerweighttocreateagreatertotalcompactiveeffort.Drumvibrationalsoreducesfrictionandaggregateinterlockduringcompaction,whichallowsaggregateparticlestomoveintofinalpositionsthatproducegreaterfrictionandinterlockthancouldbeachievedwithoutvibration.Asageneralrule-of-thumb,acombinationofspeedandfrequencythatresultsin10–12impactsperfootisgood.At3000vibrations/minutethisresultsinaspeedof2.8–3.4mph.Figure6:SteelWheelRollers Figure7:SteelWheelRollersPneumaticTireRollersPneumatictirerollersareself-propelledcompactiondevicesthatusespneumatictirestocompact theunderlyingHMA.Pneumatictirerollers employasetofsmoothtires(notread)oneachaxle;typicallyfourorfiveononeaxleandfiveorsixontheother.Thetiresonthefrontaxlearealignedwiththegapsbetweentiresontherearaxletogivecompleteanduniformcompactioncoverageoverthewidthoftheroller.Compactiveeffortiscontrolledbyvaryingtirepressure,whichistypicallysetbetween60and120psi(TRB,2000 [10]).Inadditiontoastaticcompressiveforce,pneumatictirerollersalsodevelopakneadingactionbetweenthetiresthattendstorealignaggregatewithintheHMA.Becauseasphaltbindertendstostickmoretocoldtiresthanhottires,thetireareaissometimesinsulatedwithrubbermattingorplywoodtomaintainthetiresnearmattemperaturewhilerolling(seeFigures8and9).實用文檔Figure8:PneumaticTireRoller Figure9:PneumaticTiresCompactionSequenceHMAcompaction istypically accomplished byasequenceofcompaction equipment. Thisallowseachpieceofequipmenttobeusedonlyinitsmostadvantageoussituationresultinginahigherqualitymat(bothindensityandinsmoothness)thancouldbeproduced with just asingle methodofcompaction. Atypical compaction sequenceconsistsofsomeorallofthefollowing(inorderofuse):Screed.Thescreedisthefirstdeviceusedtocompactthematandmaybeoperatedinthevibratorymode.Approximately75to85percentofTMDwillbeobtainedwhenthemixpassesoutfromunderthescreed(TRB,2000[10]).Rollers.Generallyaseriesoftwoorthreerollersisused.Contractorscancontrolrollercompactionbyvaryingthingssuchasthetypesofrollersused,thenumberofrollerused,rollerspeed,thenumberofrollerpassesoveragivenareaofthemat,thelocationatwhicheachrollerworks,andthepatternthateachrollerusestocompactthemat.Approximately92to95percentTMDwillbeobtainedwhenallrollersarefinishedcompactingthemat.Typicalrollerpositionusedincompactionare:BreakdownRoller.Thefirstrollerbehindthescreed(seeFigure10).Itgenerallyeffectsthemostdensitygainofanyrollerinthesequence.Breakdownrollerscanbeofanytypebutaremostoftenvibratorysteelwheelandsometimespneumatictire.IntermediateRoller.Usedbehindthebreakdownrollerifadditionalcompactionisneeded(seeFigure10).Pneumatictirerollersare實用文檔sometimesusedasintermediaterollersbecausetheyprovideadifferent typeofcompaction (kneading action) thanabreakdownsteelwheelvibratoryroller, whichcanhelpfurthercompactthe matorattheveryleast,rearrangetheaggregatewithinthemattomakeitreceptivetofurthercompaction.FinishRoller.Thelastrollerinthesequence(seeFigure11).Itisusedtoprovideasmoothmatsurface.Althoughthefinishrollerdoesapplycompactiveeffort,bythetimeitcomesincontactwiththemat,thematmayhavecooledbelowcessationtemperature.Staticsteelwheelrollersarealmostalwaysusedasfinishingrollersbecausetheycanproducethesmoothestsurfaceofanyrollertype.Figure 10:PavingOperationShowingaSteelWheelBreakdownRollerandaPneumaticTireIntermediateRollerFigure11:FinishRoller實用文檔Traffic.Aftertherollershavecompactedthemattothedesireddensityandproducedthedesiredsmoothness,thenewpavementisopenedtotraffic.Trafficloadingwillprovidefurthercompactioninthewheelpathsofafinishedmat.Trafficmaycompactthematanadditional2to4percentoverthelifeofthepavement.Footnotes(?returnstotext)1.Roberts,F.L.,Kandhal,P.S.,Brown,E.R.,Lee,D.Y.,andKennedy,T.W.(1996).HotMixAsphaltMaterials,MixtureDesign,andConstruction.NationalAsphaltPavingAssociationEducationFoundation.Lanham,MD.?2.Scherocman,J.A.andMartenson,E.D.(1984).PlacementofAsphaltConcreteMixtures.PlacementandCompactionofAsphaltMixtures,F.T.Wagner,Ed.ASTMSpecialTechnicalPublication829.AmericanSocietyforTestingandMaterials.Philadelphia,PA.pp.3-27.?3.Scherocman,J.A.(1984,March).GuidelinesforCompactingAsphaltConcretePavement.BetterRoads,Vol.54,No.3.pp.12-17.?4.Geller,M.(1984).“CompactionEquipmentforAsphaltMixtures.”Placemen