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

8/9/2019 Effect of Asphalt Film Thickness on Short and Long Term Aging of Asphalt Paving mixtures

1/20

EFFECTOFASPHALTFILMTHICKNESSONSHORTANDLONGTERMAGINGOFASPHALTPAVING MIXTURES

By

PrithviS.Kandhal

SanjoyChakraborty

PaperpublishedintheTransportationResearchBoard,TransportationResearchRecord1535,1996

277TechnologyParkway Auburn,AL36830


2/20

EFFECTOFASPHALTFILMTHICKNESSONSHORTANDLONG TERMAGINGOFASPHALTPAVINGMIXTURES

By

PrithviS.Kandhal

AssociateDirectorNationalCenterforAsphaltTechnology

AuburnUniversity,Alabama

SanjoyChakaraborty

GraduateStudentAuburnUniversity,Alabama

PaperpublishedintheTransportationResearchBoard,TransportationResearchRecord1535,1996


3/20

DISCLAIMER

Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsandpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.

i


4/20

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

ii


5/20


6/20

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.

2


7/20

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.

3


8/20

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.

4


9/20

Kandhal&Chakraborty

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

5


10/20

Kandhal&Chakraborty

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.

6


11/20

Kandhal&Chakraborty

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

7


12/20

Kandhal&Chakraborty

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

8


13/20

Kandhal&Chakraborty

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:

9


14/20


15/20

Kandhal&Chakraborty

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.

11


16/20

Kandhal&Chakraborty

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)

12


17/20

Kandhal&Chakraborty

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.

13


18/20

Kandhal&Chakraborty

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:

14


19/20

Kandhal&Chakraborty

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.

15


20/20

Kandhal&Chakraborty

4.Theprecedingconclusionsarebasedononlyoneaggregate/asphaltcement

combination.SHRPA-003AandA-003BProjectshaveindicatedthattheagingphenomenonisinfluencedbytheinteractionbetweentheaggregateandtheasphaltcement.Therefore,theoptimumasphaltfilmthicknessindicatedinthisstudyneeds

tobeconfirmedbyconductingmorestudiesinvolvingdifferentaggregate/asphaltcementcombinations.

REFERENCES

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

"MixDesignMethodsforAsphaltConcreteandOtherHotMixTypes,"TheAsphaltInstitute,MS-2.SixthEdition,1993.W.H.Campen,J.R.Smith,L.G.Erickson,andL.R.Mertz,"TheRelationshipsBetweenVoids,SurfaceArea,FilmThicknessandStabilityInBituminousPavingMixtures,"Proceedings,AAPT,Vol.28,1959.J.F.GoodeandL.A.Lufsey,"Voids,Permeability,FilmThicknessvs.AsphaltHardening,"Proceedings,AAPT,Vol.34,1965.

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

Mixtures,"M.S.Thesis,AuburnUniversity,1994.

16