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8/9/2019 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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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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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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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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Figure1.TestSequenceforEachAsphaltContent/FilmThickness
Five100-mmdiameterspecimenswerecompactedfromeachmixtypeaftershorttermaging.
Thecompactedspecimenswerepreparedtogiveatargetairvoidcontentlevelof81%.TheCorpsofEngineersGyratoryTestingMachine(GTM)wasusedforthispurpose.Theresilientmodulus(MR)ofallthecompactedspecimenswasdeterminedat25C.Totalnumberofsamplestested=6(filmthicknesses)x5(replicates)=30.
The30compactedspecimensweresubjectedtolongtermagingfollowingSHRP#1030
procedures(9).Theprocedureconsistsofplacingthecompactedspecimensonarackinaforceddraftovenfor120hours,andatatemperatureof85C.Thisprocedurewasdesignedtosimulate
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theagingthatthecompactedasphaltpavementundergoesduringits5-10yearsservicelife.An
airvoidcontentof81%incompactedspecimensisusedtosimulatecompactionatthetimeofconstruction.Lowerairvoidcontentsmayalsonotprovideinterconnectedvoidswhichareessentialforthisacceleratedagingtest.Thefollowingtestswereconductedonthecompactedspecimensafterlongtermaging:
1.Resilientmodulus(MR)at25C.
2.Tensilestrength(ST),alongwiththestrainatfailure,at25Cusingastrainrateof50mmperminute.
3.Absonrecoveryofagedasphaltbinderfromall30brokenspecimens.Therecoveredasphaltbindersweretestedforpenetrationat25C,viscosityat60C,complexmodulus(G*)at19C,andphaseangle(*)at19C.
ComplexmodulusandphaseangleweremeasuredusingSuperpavetestprocedures(8).The
temperatureof19Cwasusedbecausethefatiguefactor(G*sin*)iscriticalatmidservicepavementtemperaturesaccordingtoSuperpaveperformancegraded(PG)binderspecifications(8).Afterlongtermagingasphaltpavingmixesbecomestiffand,therefore,fatiguecrackingbecomestheprimarydistressofconcernaffectingtheasphaltmixdurability.Thetemperatureof19CwouldbeusedtotestaSuperpavePG64-34binderafteragingitinRTFOandpressureagingvessel(PAV).
ANALYSISOFTESTRESULTS
Table4givesthecompactedHMA'sphysicalproperties(suchasresilientmodulusat25Candtensilestrengthat25C)aftershortandlongtermagingcorrespondingtoasphaltfilmthicknessrangingfrom3.7to13.0micron.Table5givestheconventionalproperties(suchaspenetrationat25Candviscosityat60C)oftherecoveredasphaltbindersaftershorttermandlongtermaging.Table6givesSuperpavebinderproperties(suchascomplexmodulusG*)fortheserecoveredasphaltbinders.
Theconceptofpolynomialregressionhasbeenusedasatooltofittheobserveddatatocurve,
whichquantifytherelationshipbetweentheindependentandthedependentvariables.Theindependentvariable,inmostcases,istheasphaltfilmthickness,againstwhichareplottedthevaluesoftherecoveredasphaltcementpropertieslikepenetration,viscosity,andcomplexmodulus,orthemeasuredpropertiesofthecompactedasphaltmix,liketensilestrengthandresilientmodulus,Therelationshipbetweenthemeasuredpropertiesandthefilmthicknesshasbeenquantifiedformixeswhichhavebeensubjectedtobothshortandlongtermaging,AlldependentvariableswhosevaluesaregiveninTables4,5and6wereanalyzed,thedetailedanalysesaregivenelsewhere(10).Aselectednumberofdependentvariablesarediscussedbelow.
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Table4.CompactedHMAPropertiesafterShortandLongTermAging 1
Film ResilientModulusat25C,MPa Tensile TensileThickness Strengthat Strain(microns)
AfterSTA2
AfterLTA3
LTA/STARatio
25C,MpaAfterLTA
Failure,%AfterLTA
3.7 8,184 12,293 1.50 1.52 0.44
5.6 6,357 9,398 1.48 1.37 0.55
7.4 4,027 5,240 1.30 1.08 0.66
9.3 2,910 3,716 1.28 0.94 0.74
11.1 2,572 2,696 1.05 0.73 0.93
13.0 1,958 2,020 1.03 0.62 1.241Allreporteddataareaveragesoffivesamples.23STA=ShortTermAgingLTA=LongTermAging
Table5.RecoveredConventionalAsphaltBinderPropertiesafterAging 1
Viscosityat ViscosityRatio4 Penetrationat RetainedFilm 60C,Pa.s 25C,0.1mm Penetration5
Thickness(microns) After
2
STA
After
3
LTA
AfterSTA
AfterLTA
AfterSTA
AfterLTA
AfterSTA
AfterLTA
3.7 1262.14744.4 6.15 23.12 31.3 24.6 50.5 39.7
5.6 809.9 4658.4 3.95 22.70 35.3 25.7 56.9 41.5
7.4 526.1 4347.2 2.56 21.19 39.6 27.3 63.9 44.09.3 434.6 3940.1 2.12 19.20 43.6 29.0 70.3 46.8
11.1 276.3 3063.3 1.35 14.93 54.0 33.6 87.1 54.2
13.0 236.7 2897.6 1.15 14.12 56.6 34.3 91.3 55.31Allreporteddataareaveragesoffivesamples.23
4
5
STA=ShortTermAgingLTA=LongTermAgingBasedonviscosityoforiginalasphaltcementmeasuredatNCAT(205.2Pa.s) BasedonpenetrationoforiginalasphaltcementmeasuredatNCAT(62)
Table6.RecoveredSuperpaveAsphaltBinderPropertiesAfterAging 1
FilmThickness ComplexModulus,ComplexModulus, G*sin*at19C,Pa(microns)
3.7
5.6
7.4
9.3
11.1
13.0
G*at60C,Pa
AfterSTA2
2090
3590
2270
2460
1310
1220
G*at19C,Pa
AfterLTA3
2.50E+06
2.25E+06
1.74E+06
1.53E+06
1.22E+06
1.71E+06
AfterLTA3
1.488E+06
1.339E+06
1.068E+06
0.918E+06
0.787E+06
1.094E+061Allreporteddataareaveragesofthreesamples.23
STA=ShortTermAgingLTA=LongTermAging
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CompactedAsphaltMixProperties
Theresilientmodulusofthecompactedasphaltmixspecimenswasmeasuredbothaftershorttermagingandafterlongtermaging(Table4).Theloosemixhadbeensubjectedtoshortterm
agingbeforecompaction,i.e.,beforethepreparationofthecompactedsamples.Thecompactedsampleswerethensubjectedtolongtermaging.Resilientmodulustestingwascarriedoutat25Cinthediametralorindirecttensilemode.
Figures2and3showtherelationshipbetweenthefilmthicknessandmodulusvaluesaftershort
termandlongtermaging,respectively.Quadraticpolynomialregressiongaveanacceptablemodelforthisrelationshipaspresentedbelow:AfterShortTermAging
Mrst=2069.9-273.15:+10.53:2
R2=0.99035
where,Mrst=resilientmodulusaftershorttermaging(ksi):=filmthicknessinmicrons
Figure2.AsphaltFilmThicknessvs.ResilientModulusAfterShortTerm Aging
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Figure3.AsphaltFilmThicknessvs.ResilientModulusAfterLong TermAging
AfterLongTermAging
Mrlt=3267.6-456.75:+17.55:2
R2=0.9896
where,Mrlt=resilientmodulusaftershorttermaging(ksi)
ItcanbeseeninFigures2and3thatataboutafilmthicknessof11microns,thefittedcurve
tendstoflattenoutanddoesnotchangesignificantlywithincreasingfilmthickness,Also,theslopeofthecurvebecomessteeperasthefilmthicknessfallsbelowavalueofabout9to10
microns,whichindicatesthatthestiffness(causedbyaging)oftheasphaltpavingmixstartstoincreasequiterapidlywithadecreaseinfilmthicknessbelowabout9-10microns,Also,thereisamarkedsimilaritybetweenthecurvesobtainedaftershorttermandlongtermaging.
Somemoreinformationconcerningthechangeinresilientmodulusvalueswithfilmthickness
canbeobtainedfromthegraphinFigure4wheretheresilientmodulusvaluesaftershorttermaginghavebeenplottedagainstthecorrespondingmodulivaluesafterlongtermaging.Eachpointonthegraphcorrespondstooneparticularasphaltfilmthicknesswhichdecreasesfromlefttoright(becausetheresilientmodulusvaluesincreasewithdecreaseinfilmthickness).Regressionanalysisleadstoalinearrelationshipinthedataasmodeledbythefollowingequation:
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Figure5.AsphaltFilmThicknessvs.ViscosityAfterShortTerm Aging
where,
Vst=22609-23268.8:+132.8:2R=0.988
Vst=viscosityofasphaltcementaftershorttermaging(poises)
Ascanbeseenfromthedataforshorttermaging(Figure5),astheasphaltfilmthickness
decreasesbelowavalueofabout9-10micronsthefittedcurvetendstosteepenindicatinganacceleratedrateofincreaseinviscosity.Ontheotherend,thesamecurveisseentoflattenoutatabout11micronsfilmthickness,whichindicatesthatthefilmthicknesshaslesserandlessereffectontheagingoftheasphaltcementonceitisincreasedaboveavalueofabout11microns.
Forsamplessubjectedtolongtermaging,regressionanalysisfailedtoproduceasatisfactorymodelwhichcouldexplainthenatureoftherelationshipbetweenfilmthicknessandagedviscosity.Thus,noequationisavailabletodefinetherelationship.Instead,thepointsonthegraphhavebeenconnectedtogetherbyasmoothcurve.AscanbeseenfromFigure6,theviscosityincreasesatanacceleratedrateoncetheasphaltfilmthicknessdecreasesbelowavalueofabout10microns.
Thenatureofthecurvesobtainedwhentheviscosityratio(Table5)isplottedagainstfilm
thickness,isaboutthesameasinthepreviouscasesforshortandlongtermagedconditions(l0).Theviscosityratioisdefinedastheratiooftheviscosityoftheagedasphalttotheviscosityoftheunaged/originalasphalt.
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Figure6.AsphaltFilmThicknessvs.ViscosityAfterLongTermAging
Similarrelationshipswerealsoobservedbetweenasphaltfilmthicknessandpenetrationorretainedpenetrationofasphaltcementsaftershortandlongtermaging(10).
Forasphaltcementsubjectedtoshorttermaging,complexmodulusG*andphaseangle*weremeasuredatatemperatureof64C,whereasforlongtermagedspecimens,thetestingoftheasphaltcementwascarriedoutat19C(Table6).Sincethestiffnessoftheasphaltbinderismorecriticalafterlongtermagingratherthanshorttermagingfromthedurability(orresistancetofatigue)standpoint,thecomplexmodulusG*andfatiguefactor(G*sin*)afterlongtermagingwillonlybepresentedhere.Thedataobtainedat64CaftershorttermagingwasanalyzedandisreportedinReference10.TherelationshipbetweenasphaltfilmthicknessandG*at19Cafter
longtermaginghasbeenshowninFigure7.AlthoughtheregressionanalysisofG*at19Candfilmthicknessgaveaquadraticmodelfortheshorttermagedasphaltcement(10),alinearmodel(Figure7)betterexpressestherelationshipbetweenasphaltfilmthicknessandG*at19Cafterlongtermaging,asfollows:
G*lt=3158521-176472.6:
R=0.98
where,G*lt=complexmodulusofasphaltcementsubjectedtolongtermaging(Pa)
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Figure7.AsphaltFilmThicknessvs.ComplexModulus(G*)AfterLong
TermAging
Asexpected,valuesofthecomplexmodulusG*decreasewithincreasingasphaltfilmthickness,bothforshortandlongtermaging.Thisindicatesthatthepresenceofthickerfilmsofasphaltcementintheasphaltpavingmixminimizesagingoftheasphaltbinder.However,itisnotapparentfromthefittedcurvesastowhatrangeofasphaltfilmthicknessmightprovetobeoptimuminminimizingasphaltcementaging.
TherelationshipbetweenG*sin*(fatiguefactor)andfilmthicknesshasbeenpresentedinFigure8,forasphaltcementrecoveredfrommixeswhichhadbeensubjectedtolongtermaging.A
linearmodelwasobtainedasfollows:
GSDlt=1848404-98052.5:
R2=0.98
where,GSDlt=G*sin*(inpascals)forsamplessubjectedtolongtermaging.
Asisevidentfromthefittedcurve(Figure8),G*sin*increaseswithdecreaseinthefilmthicknessoftheasphaltbinderintheasphaltpavingmix.Thisindicatesthatthelowertheasphaltbinderfilmthicknesspresentinanasphaltpavingmix,themoresusceptiblethepavementistofatiguecrackinginthelongterm.
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Figure8.AsphaltFilmThicknessvs.G*sin *AfterLongTermAging
SHRPhassuggestedavalueof5000KPaastheupperlimitforG*sin*6forasphaltcementsubjectedtoacceleratedaginginthePressureAgingVessel(PAV).PAVagingsimulatestheagingthattheasphaltbinderinanasphaltpavementundergoesafterabout5-10yearsinservice.Asdiscussedearlier,thisstudyemployedaprocedure(alsodevelopedbySHRP)tosimulatelongtermagingofin-servicepavementsusingcompactedasphaltmixsamples,insteadofjustasphaltcement,asisusedinthePAVtest.Therefore,forasphaltcementrecoveredfromcompactedasphaltmixsamplessubjectedtoacceleratedlongtermlaboratoryagingprocedures,5000kpashouldbeconsideredastheupperlimitforG*sin*.AsisevidentfromthedatapresentedinFigure8,themaximumrecordedvalueofG*sin*isabout1500kpa,whichismuch
lessthanthelimitingvalueof5000kpa.ThisindicatesthatthePAVagingofasphaltcementismuchmoresevereinthislimitedlaboratorystudythanthatoccurringincompactedasphaltmixsamplesagedinforceddraftovenat85Cfor120hours.
AirVoidstoBitumenIndexRatioAnalysis
Theconceptoftheratiooftheairvoids(percent)tobitumenindex,asameasureoftheagingsusceptibilityofamix(whateverbeitsgradation),wasdiscussedearlier,GoodeandLufsey(3)hadproposedamaximumvalueof4.0forthisratiowhichtheybelievedwouldpreventpavementdistressbyreducingtheagingoftheasphaltfilmcoatingtheaggregate.Mathematically,whattheystatedwas:
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Notingthat:
filmthicknessinmicrons=bitumenindexx4870
Thepreviousexpressioncanbereducedtoaminimumfilmthicknessrequirement,varyingwith
theairvoidscontentofthegivenmix,asfollows:Atargetvalueof8%fortheairvoidsinthecompactedHMAspecimenswasusedinthepresentstudy,inconformancewiththeacceleratedlongtermagingproceduredevelopedinSHRPA003-
AProject.Thiscorrespondstoaminimumfilmthicknessrequirementsof9.74microns(about10microns),basedupontheaboveequation.AscanbeseenfromFigures2and3,thecurvesoftheresilientmoduli(forbothshortandlongtermagingconditions)versusfilmthicknesstendtosteepenasthefilmthicknessdecreasesbelowtherangeof9to10microns.Thisindicatesthattherateofagingoftheasphaltcementisacceleratedwhenthefilmthicknessislessthan9-10microns.Thisacceleratedagingratecanalsobeseenintheplotsofviscosityversusfilmthickness(Figures5and6).Thereforeitcanbeconcluded,onthebasisofthepresentstudy,thatamaximumvalueof4.0forthevoids/bitumenindexratioisindeedreasonable,andmightprovetobeabetterspecificationfordesign,atleastasfarastheagingofasphaltcementisconcerned.
Basedonthedatapresentedinthislimitedstudyofoneasphaltcement/oneaggregate
combination,itcanbeconcludedthattheoptimumasphaltfilmthicknesstominimizeaging
rangesfrom9to10microns.CONCLUSIONSANDRECOMMENDATIONS
Thisstudywasundertakentoquantifytherelationshipbetweenvariousasphaltbinderfilmthicknessesandtheagingcharacteristicsoftheasphaltpavingmixsothatanoptimumasphaltfilmthicknessdesirableforsatisfactorymixdurabilitycouldbeestablished.Thefollowingconclusionsweredrawnandrecommendationsmade:
1.Theliteraturereviewedaspartofthisstudydidnotindicatetheexistenceofanysignificantrationaldatacorrelatingthedurabilityofasphaltpavementswiththeasphaltbinderfilmthickness.
2.Therelationshipbetweentheasphaltfilmthicknessandtheagedproperties(bothshorttermandlongterm)oftheasphaltpavingmixtures,suchastensilestrengthand
resilientmodulus,wasquantified.Afairlygoodcorrelationwasobtainedbetweentheasphaltfilmthicknessandtheresilientmodulusoftheagedasphaltpavingmixtures,Anoptimumfilmthicknessof9-10micronswasindicatedfromthedata,belowwhichtheasphaltmix(compactedto8%airvoidcontent)agedatanacceleratedrate.ThisrangeappearstoconcurwiththeresultsobtainedbyGoodeandLufseyintermsofairvoids/bitumenindexratio.
3.Relationshipswerealsoestablishedbetweentheasphaltfilmthicknessandtheagedasphaltbinderproperties(bothshortandlongterm)suchasviscosity,penetration,andcomplexmodulus.Anoptimumfilmthicknessof9-10micronswasgenerallyindicatedfromthedata,belowwhichtheasphaltbinderagedatanacceleratedrate.Thisfilmthicknesscorrespondstoasphaltbindercontainedinanasphaltpavingmixcompactedto8%airvoidcontent.
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4.Theprecedingconclusionsarebasedononlyoneaggregate/asphaltcement
combination.SHRPA-003AandA-003BProjectshaveindicatedthattheagingphenomenonisinfluencedbytheinteractionbetweentheaggregateandtheasphaltcement.Therefore,theoptimumasphaltfilmthicknessindicatedinthisstudyneeds
tobeconfirmedbyconductingmorestudiesinvolvingdifferentaggregate/asphaltcementcombinations.
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A.KumarandW.H.Goetz,"AsphaltHardeningasAffectedbyFilmThickness,VoidsandPermeabilityinAsphalticMixtures,"Proceedings,AAPT,Vol.46,1977.D.Sosnovske,Y.AbWahab,andC.Bell,"TheRoleofAsphaltandAggregateintheAgingofBituminousMixtures,"TransportationResearchRecord1386,1993.P.S.KandhalandM.A.Khatri,"RelatingAsphaltAbsorptiontoPropertiesofAsphaltCementandAggregate,"TransportationResearchRecord1342,1992."StandardPracticeforShortTermAgingofAsphaltConcreteMixtures,"SHRP#1025,1992."TheSuperpaveMixDesignSystemManualofSpecifications,TestMethods,andPractices,"SHRP-A-379,1994."StandardPracticeforLongTermAgingofAsphaltConcreteMixtures,"SHRP#1030,1992.S.Chakraborty,"EvaluationofVoidsintheMineralAggregateforHMAPaving
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