道路工程施工概况英文.docx

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