Effect of Asphalt Film Thickness on Short and Long Term Aging of Asphalt Paving mixtures

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    EFFECTOFASPHALTFILMTHICKNESSONSHORTANDLONGTERMAGINGOFASPHALTPAVING MIXTURES

    By

    PrithviS.Kandhal

    SanjoyChakraborty

    PaperpublishedintheTransportationResearchBoard,TransportationResearchRecord1535,1996

    277TechnologyParkway Auburn,AL36830

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    EFFECTOFASPHALTFILMTHICKNESSONSHORTANDLONG TERMAGINGOFASPHALTPAVINGMIXTURES

    By

    PrithviS.Kandhal

    AssociateDirectorNationalCenterforAsphaltTechnology

    AuburnUniversity,Alabama

    SanjoyChakaraborty

    GraduateStudentAuburnUniversity,Alabama

    PaperpublishedintheTransportationResearchBoard,TransportationResearchRecord1535,1996

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    DISCLAIMER

    Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsandpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.

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    ABSTRACT

    Itisgenerallybelievedthatanasphaltpavingmixtureshouldhaveanadequateasphaltfilm

    thicknessaroundtheaggregateparticlestoensurereasonabledurability(resistancetoaging)ofthemixture.Theminimumasphaltfilmthicknessgenerallyrecommendedrangesfromsixtoeightmicrons,However,nosignificantbackgroundresearchdataisavailableintheliteraturetosupporttheserecommendedminimumasphaltfilmthicknesses.Somestatesspecificminimumasphaltfilmthicknessformixdesigns.Thisstudywasundertakentoquantifytherelationshipbetweenvariousasphaltfilmthicknessesandtheagingcharacteristicsoftheasphaltpavingmix,sothatanoptimumfilmthicknessdesirableforsatisfactorymixdurabilitycouldbeestablished.

    Mixespreparedwithasphaltbinderfilmthicknessrangingfromabout4to13microns,were

    subjectedtoacceleratedagingusingStrategicHighwayResearchProgram(SHRP)procedurestosimulatebothshortandlongtermaging.Boththeaggregate(RD)andtheasphaltcement(AAM-1)usedinthisstudywereobtainedfromtheSHRPMaterialsReferenceLibrary.Theaged,compactedmixwastestedfortensilestrength,tensilestrainatfailureandresilientmodulus.The

    agedasphaltcementwasrecoveredandtestedforpenetration,viscosity,complexmodulusandphaseangle.Agingindiceswereobtainedfromthesetests,andtherelationshipbetweenfilmthicknessandtheagedmix/agedasphaltcementpropertiesweredeterminedusingregressionanalysis.Fortheparticularaggregate/asphaltcementcombinationusedinthisstudy,itwasfoundthatacceleratedagingwouldoccuriftheasphaltbinderfilmthicknesswaslessthan9-10micronsinanasphaltpavingmixturecompactedto8%airvoidcontent.

    KEYWORDS:filmthickness,aging,durability,asphaltfilm,asphaltpavingmixtures,

    asphaltconcrete,hotmixasphalt

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    Kandhal&Chakraborty

    GoodeandLufsey(3)alsodidsomesignificantworkinrelatingasphalthardeningtovoids,

    permeabilityandfilmthickness.Theyrecognizedthatthehardeningoftheasphaltbinderinamixwasafl.mctionofairvoids,filmthickness,temperature,andtime.Onthebasisoftheirworktheyconcludedthataminimumvalueof0.00123for'bitumenindex'(whichcorresponds

    toavalueof6micronsofaveragefilmthickness)couldbeincludedasacriterioninallmixdesignprocedures.The'bitumenindex'wasdefinedaspoundsofasphaltcementpersquarefootofsurfacearea.Theyusedtheconceptofbitumenindextoavoidtheimplicationthatallparticleswerecoatedwiththesameuniformthicknessofasphaltcement.Theirstudyindicatedthatacombinedfactoroftheratiooftheairvoidstothebitumenindexcouldbesatisfactorilyrelatedtotheasphaltbinderhardeningcharacteristicsintheasphaltpavingmixture.TheysuggestedthattheMarshallmethodofmixdesigncouldbeimprovedbyincorporatingamaximumvalueofvoids-bitumenindexratioinplaceofamaximumvalueofairvoidsalone,andsuggestedavalueof4asthemaximumforthisratio,toensurereasonableresistancetoaging.

    KumarandGoetz(4)studiedtheasphaltbinderhardeningasrelatedtoasphaltmixpermeability

    andasphaltfilmthickness.Theystatedthatthebestprocedureforpredictingtheresistanceof

    hardeningofasphaltbinderinasingle-sizedasphaltpavingmixwastocalculatetheratioofthefilmthicknessfactortopermeability.Thefilmthicknessfactorwasdefinedastheratioofthepercentasphaltcontentavailableforcoatingtheaggregatetothesurfaceareaoftheaggregate,Theyindicatedthatfordense-gradedmixtures,theconceptofanaveragefilmthicknessisatbestdubious,ifnottotallyerroneous.Fordense-gradedmixtures,permeabilitywasstatedtobethebestmeasureoftheresistancetohardening.However,atthedesignvalueof4/0airvoidsasiscommonformostdense-gradedasphaltpavingmixtures,theeffectofpermeabilityofthemixwasdeterminedtobequiteinsignificant.

    TESTINGPROGRAM

    Thistestingprogramwascarriedoutwiththefollowingobjectives:

    l.Toevaluatethechangesinthetheologicalpropertiesoftheasphaltcementdueto

    aginginrelationtotheasphaltfilmthickness.Bothshortterm(duringasphaltmixproductionandconstruction)andlongterm(duringservicelife)agingwereconsidered.

    2.Todetermineanoptimumrangefortheasphaltfilmthickness,ifpossible,whichwouldminimizeagingoftheasphaltbinder.

    MaterialUsed

    AggregateandasphaltcementsampleswereobtainedfromtheSHRPMaterialReferenceLibrary(MRL).Onlyoneaggregate(SHRPMRLDesignationRD):FrederickLimestonewasusedinthisstudy.Table1givesthephysicalpropertiesofthetotalaggregateobtainedfromSHRPMRL.Table2givesthewashedgradationoftheaggregateusedintheasphaltpavingmixture.

    Anasphaltcement(SHRPMRLDesignationAAM-1)wasusedinthisstudy.Thisasphalt

    cementwasselectedbecauseithadoneofthehighestpropensitiestoageintheHMAmixbasedontheworkdoneinSHRPA-003AbySosnovskietal(5).ItsphysicalandchemicalpropertiesasobtainedfromSHRParegiveninTable3.

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    Kandhal&Chakraborty

    Table1.PhysicalPropertiesofRDAggregate(FrederickLimestone)

    Property Value

    BulkSpecificGravity

    WaterAbsorption,percent

    L.A.Abrasion(AASHTOT96)

    %Wear

    FlakinessIndex,percent

    SandEquivalent(AASHTOT176)

    2.704

    0.3

    23.4

    34.7

    69

    Table2.WashedGradationofAggregateUsedintheHMAMix

    SieveSize(mm)

    12.5

    9.5

    4.75

    2.36

    1.18

    0.6

    0.3

    0.15

    0.075

    PercentPassing

    100

    8963

    4533

    2113

    85

    TestProceduresUsed

    ThesurfaceareaoftheaggregatewascalculatedusingthesurfaceareafactorsgiveninMS-22

    (1).Fortheaggregategradationused(Table2)thesurfaceareawascalculatedtobe27.626ft/lb(5.662m2/kg).

    Asphaltpavingmixtureswerepreparedateachofthefollowingsixeffectiveasphaltfilm

    thicknesses:3.7,5.6,7.4,9.3,11.1,and13.0microns.Thefilmthicknessesoriginallytargetedforexperimentaldesignwere4,6,8,10,12and14microns.However,certainerrorsinthecalculationswerediscoveredafterthemixeshadactuallybeenprepared.Theactualvaluesofasphaltfilmthicknessesusedwerethenrecalculated.

    Avalueof0.20%asphaltabsorptionwasusedfortheRDaggregateandAAM-1asphaltbinder

    combinationasdeterminedandreportedinReference6.Thisrequiredsixasphaltcontents(byweightofthetotalmix)asfollows:2.2,3.2,4.2,5.1,6.1and7.1percenttoobtainasphaltfilmthicknessrangingfrom3.7to13.0micronasmentionedabove.

    Allsixasphaltmixtureswerepreparedatthemixingtemperatureof1433C.Thetesting

    sequenceforeachmixisgiveninFigure1.

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    Kandhal&Chakraborty

    Table3.PropertiesofAsphaltCement(AAM-l)Used

    Property

    ORIGINALASPHALTCEMENT

    SpecificGravityViscosityat60C,Pa.s

    Viscosityat135C,cSt

    Penetrationat25C,0.1mm

    Ductilityat4C,cm

    SofteningPoint(R&B),C

    DynamicShearRheometer(DSR)data:

    G*/sin*at64C,kPa

    TFORESIDUE

    MassChange,%

    Viscosityat60C,Pa.S

    Viscosityat135C,cSt

    RTFORESIDUE

    DSRdata

    G*/sin*at64C,kPa

    PRESSUREAGINGVESSELRESIDUE

    DSRdata

    G*/sin*at64C,kPa

    COMPONENTANALYSIS

    Asphaltenes(n-heptane)PolarAromatics

    NaptheneAromatics

    Saturates

    ELEMENTANALYSIS

    Nitrogen,%

    Sulphur,%

    Vanadium,ppm

    Nickel,ppm

    Value

    0.993199.2

    56964

    4.6

    51.7

    1.15

    0.00516

    394.7

    7442.463,200

    3.9

    50.341.91.9

    0.50

    2.40

    60.0

    29.0

    ThelooseasphaltmixsamplesweresubjectedtoshorttermagingfollowingSHRP#1025procedures(7).Theprocessinvolvesagingofthelooseasphaltmixinaforceddraftovenfor4hoursatatemperatureof135C.Theloosemixisplacedinabakingpanandspreadtoaneventhicknessthatproducedabout21kg/m2.Thisprocedurewasdesignedtosimulatetheagingthatthelooseasphaltmixundergoesduringtheconstructionphaseofthepavement.ThreesamplesoftheagedasphaltmixweresubjectedtoAbsonmethodofrecoveringasphaltbinder.Therecoveredasphaltbinderwastestedforpenetrationat25Candviscosityat60C.Thecomplexmodulus(G*)andphaseangle(*)werealsodeterminedat64CfortherecoveredasphaltcementusingtheDynamicShearRheometer.Thetemperatureof64Cwasusedbecausejustafterconstruction,ruttingfactor(G*/sin*)iscriticalathighpavementtemperatures.ThistemperaturewouldbeusedfortestingaSuperpavePG64-34binderaftersubjectingittorollingthinfilmoven(RTFO)whichsimulatesshorttermaging.

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