道路工程施工概况英文.docx
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道路工程施工概况英文.docx
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道路工程施工概况英文
Overview
Constructionqualityiscrucialtothelong-termpavementperformance.Constructionfactorssuchassurfacepreparation,placement,jointconstructionandcompaction/consolidationhaveanoverwhelmingeffectonpavementperformance,whichcannotbeignoredorcompensatedforinmixorstructuraldesign.
Compaction
CompactionistheprocessbywhichthevolumeofairinanHMAmixtureisreducedbyusingexternalforcestoreorienttheconstituentaggregateparticlesintoamorecloselyspacedarrangement.ThisreductionofairvolumeproducesacorrespondingincreaseinHMAdensity(Robertsetal.,1996[1]).
Figure1:
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]).
CompactionMeasurementandReporting
CompactionreducesthevolumeofairinHMA.Therefore,thecharacteristicofconcernisthevolumeofairwithinthecompactedpavement,whichistypicallyquantifiedasapercentageofairvoidsinrelationtototalvolumeandexpressedas“percentairvoids”.Percentairvoidsiscalculatedbycomparingatestspecimen’sdensitywiththedensityitwouldtheoreticallyhaveifalltheairvoidswereremoved,knownas“theoreticalmaximumdensity”(TMD)or“Ricedensity”afterthetestprocedureinventor.
AlthoughpercentairvoidsistheHMAcharacteristicofinterest,measurementsareusuallyreportedasameasureddensityinrelationtoareferencedensity.Thisisdonebyreportingdensityas:
∙PercentageofTMD(or“percentRice”).Thisexpressionofdensityiseasytoconverttoairvoidsbecauseanyvolumethatisnotasphaltbinderoraggregateisassumedtobeair.Forexample,adensityreportedas93percentRicemeansthatthereare7percentairvoids(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.
Figure2:
CoreExtraction
Figure3:
PavementCore
Figure4:
ThinLiftNuclearDensityGauge
Figure5:
TakingaNuclearDensityReading
FactorsAffectingCompaction
HMAcompactionisinfluencedbyamyriadoffactors;somerelatedtotheenvironment,somedeterminedbymixandstructuraldesignandsomeundercontractorandagencycontrolduringconstruction(seeTable1).
Table1:
FactorsAffectingCompaction
EnvironmentalFactors
MixPropertyFactors
ConstructionFactors
Temperature
Aggregate
Rollers
*Groundtemperature
*Gradation
*Type
*Airtemperature
*Size
*Number
*Windspeed
*Shape
*Speedandtiming
*Solarflux
*Fracturedfaces
*Numberofpasses
*Volume
*Liftthickness
AsphaltBinder
Other
*Chemicalproperties
*HMAproductiontemperature
*Physicalproperties
*Hauldistance
*Amount
*Haultime
Foundationsupport
ANoteontheTimeAvailableforCompaction
HMAtemperaturedirectlyaffectsasphaltbinderviscosityandthuscompaction.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.
∙Ensurethematiscompactedtothedesiredairvoidcontentbeforecessationtemperatureisreached.Thiscanbedonebycalculatingthetimeittakesthemattocoolfrominitialtemperaturetocessationtemperature.Allcompactionmustbeaccomplishedwithinthis“timeavailableforcompaction”.
MultiCool,developedbyProfessorVaughnVoellerandDr.DavidTimm,isaWindowsbasedprogramthatpredictsHMAmatcooling.MultiCoolcanbeusedtopredictthetimeavailableforcompactionandisavailableontheNationalAsphaltPavementAssociation’sAGuideforHotMixAsphaltPavementCD-ROMorfordownloadat:
∙UniversityofCaliforniaPavementResearchCenter(http:
//www.ucprc.ucdavis.edu/SoftwarePage.aspx)
∙NationalAsphaltPavementAssociation(http:
//www.asphaltpavement.org/index.php?
option=com_content&task=view&id=178&Itemid=273)
CompactionEquipment
TherearethreebasicpiecesofequipmentavailableforHMAcompaction:
(1)thepaverscreed,
(2)thesteelwheeledrollerand(3)thepneumatictireroller.EachpieceofequipmentcompactstheHMAbytwoprincipalmeans:
1.ByapplyingitsweighttotheHMAsurfaceandcompressingthematerialunderneaththegroundcontactarea.Sincethiscompressionwillbegreaterforlongerperiodsofcontact,lowerequipmentspeedswillproducemorecompression.Obviously,higherequipmentweightwillalsoincreasecompression.
2.Bycreatingashearstressbetweenthecompressedmaterialunderneaththegroundcontactareaandtheadjacentuncompressedmaterial.Whencombinedwithequipmentspeed,thisproducesashearrate.Loweringequipmentspeedcandecreasetheshearrate,whichincreasestheshearingstress.Highershearingstressesaremorecapableofrearrangingaggregateintomoredenseconfigurations.
ThesetwomeansareofcompactingHMAareoftenreferredtocollectivelyas“compactiveeffort”.
SteelWheelRollers
Steelwheelrollers(seeFigures6and7)areself-propelledcompactiondevicesthatusesteeldrumstocompresstheunderlyingHMA.Theycanhaveone,twooreventhreedrums,althoughtandem(2drum)rollersaremostoftenused.Thedrumscanbeeitherstaticorvibratoryandusuallyrangefrom35to85inchesinwidthand20to60inchesindiameter.Rollerweightistypicallybetween1and20tons(seeFigures5and6).
Somesteelwheelrollersareequippedwithvibratorydrums.Drumvibrationaddsadynamicloadtothestaticrollerweighttocreateagreatertotalcompactiveeffort.Drumvibrationalsoreducesfrictionandaggregateinterlockduringcompaction,whichallowsaggregateparticlestomoveintofinalpositionsthatproducegreaterfrictionandinterlockthancouldbeachievedwithoutvibration.Asageneralrule-of-thumb,acombinationofspeedandfrequencythatresultsin10–12impactsperfootisgood.At3000vibrations/minutethisresultsinaspeedof2.8–3.4mph.
Figure6:
SteelWheelRollers
Figure7:
SteelWheelRollers
PneumaticTireRollers
Pneumatictirerollersareself-propelledcompactiondevicesthatusespneumatictirestocompacttheunderlyingHMA.Pneumatictirerollersemployasetofsmoothtires(notread)oneachaxle;typicallyfourorfiveononeaxleandfiveorsixontheother.Thetiresonthefrontaxlearealignedwiththegapsbetweentiresontherearaxletogivecompleteanduniformcompactioncoverageoverthewidthoftheroller.Compactiveeffortiscontrolledbyvaryingtirepressure,whichistypicallysetbetween60and120psi(TRB,2000[10]).Inadditiontoastaticcompressiveforce,pneumatictirerollersalsodevelopakneadingactionbetweenthetiresthattendstorealignaggregatewithintheHMA.Becauseasphaltbindertendstostickmoretocoldtiresthanhottires,thetireareaissometimesinsulatedwithrubbermattingorplywoodtomaintainthetiresnearmattemperaturewhilerolling(seeFigures8and9).
Figure8:
PneumaticTireRoller
Figure9:
PneumaticTires
Comp
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