道路工程施工概況英文

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]

).Inadequatecompactionresultsinapavementwithdecreased

stiffness

,reduced

\o"FatigueCracking"

fatiguelife

,acceleratedaging/decreased

durability

,

rutting

,

raveling

,and

moisturesusceptibility

(Hughes,1984

[7]

;Hughes,1989

[8]

).

CompactionMeasurementandReporting

Compactionreducesthevolumeofairin

HMA

.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,theconstituentasphaltbinderbecomesmoreviscousandresistanttodeformationresultinginasmallerreductioninairvoidsforagiven

\o"Compaction"

compactiveeffort

.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(

)

NationalAsphaltPavementAssociation(

)

CompactionEquipment

TherearethreebasicpiecesofequipmentavailableforHMAcompaction:(1)thepaverscreed,(2)thesteelwheeledrollerand(3)thepneumatictireroller.EachpieceofequipmentcompactstheHMAbytwoprincipalmeans:

ByapplyingitsweighttotheHMAsurfaceandcompressingthematerialunderneaththegroundcontactarea.Sincethiscompressionwillbegreaterforlongerperiodsofcontact,lowerequipmentspeedswillproducemorecompression.Obviously,higherequipmentweightwillalsoincreasecompression.

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

CompactionSequence

HMAcompactionistypicallyaccomplishedbyasequenceofcompactionequipment.Thisallowseachpieceofequipmenttobeusedonlyinitsmostadvantageoussituationresultinginahigherqualitymat(bothindensityandinsmoothness)thancouldbeproducedwithjustasinglemethodofcompaction.Atypicalcompactionsequenceconsistsofsomeorallofthefollowing(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).Pneumatictirerollersaresometimesusedasintermediaterollersbecausetheyprovideadifferenttypeofcompaction(kneadingaction)thanabreakdownsteelwheelvibratoryroller,whichcanhelpfurthercompactthematorattheveryleast,rearrangetheaggregatewithinthemattomakeitreceptivetofurthercompaction.

FinishRoller.Thelastrollerinthesequence(seeFigure11).Itisusedtoprovideasmoothmatsurface.Althoughthefinishrollerdoesapplycompactiveeffort,bythetimeitcomesincontactwiththemat,thematmayhavecooledbelowcessationtemperature.Staticsteelwheelrollersarealmostalwaysusedasfinishingrollersbecausetheycanproducethesmoothestsurfaceofanyrollertype.

Figure10:PavingOperationShowingaSteelWheelBreakdownRollerandaPneumaticTireIntermediateRoller

Figure11:FinishRoller

Traffic.Aftertherollershavecompactedthemattothedesireddensityandproducedthedesiredsmoothness,thenewpavementisopenedtotraffic.Trafficloadingwillprovidefurthercompactioninthewheelpathsofafinishedmat.Trafficmaycompactthematanadditional2to4percentoverthelifeofthepavement.

Footnotes

(?returnstotext)

Roberts,F.L.,Kandhal,P.S.,Brown,E.R.,Lee,D.Y.,andKennedy,T.W.

(1996).

HotMixAsphaltMaterials,MixtureDesign,andConstruction.

NationalAsphaltPavingAssociationEducationFoundation.

Lanham,MD.

?

Scherocman,J.A.andMartenson,E.D.

(1984).

PlacementofAsphaltConcreteMixtures.

PlacementandCompactionofAsphaltMixtures,

F.T.Wagner,Ed.

ASTMSpecialTechnicalPublication829.

AmericanSocietyforTestingandMaterials.

Philadelphia,PA.

pp.3-27.

?

Scherocman,J.A.

(1984,March).

GuidelinesforCompactingAsphaltConcretePavement.

BetterRoads,Vol.54,No.3.

pp.12-17.

?

Geller,M.

(1984).

“CompactionEquipmentforAsphaltMixtures.”

PlacementandCompactionofAsphaltMixtures,

F.T.Wagner,Ed.

ASTMSpecialTechnicalPublication829.

AmericanSocietyforTestingandMaterials.

Philadelphia,P