Evaluation of Voids in Mineral Aggregate for Hot Mix Asphalt

8/9/2019 Evaluation of Voids in Mineral Aggregate for Hot Mix Asphalt

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EVALUATIONOFVOIDSINTHEMINERALAGGREGATEFORHMA PAVINGMIXTURES

By

PrithviS.Kandhal

AssociateDirectorNationalCenterforAsphaltTechnology

AuburnUniversity,Alabama

SanjoyChakraborty

GraduateStudentNationalCenterforAsphaltTechnology

AuburnUniversity,Alabama

PaperpublishedintheProceedingsoftheCanadianTechnicalAsphaltAssociation(CTAA),VolumeXLI,

November1996


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DISCLAIMER

Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsandpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.

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ABSTRACT

Voidsinthemineralaggregate(VMA),togetherwiththevoidsinthetotalmix(VTM),areconsideredimportantparametersinhotmixasphalt(HMA)mixdesign.ItisbelievedthataminimumVMArequirementisnecessarytoensurethattheHMAmixisnotdeficientinasphaltcement(sothemixisdurable),and/orinVTM(topreventflushingand/orrutting).CurrentVMArequirementsarebasedlargelyupontheworkdoneduring1950to1960.However,theliteraturereviewedaspartofthisstudydidnotindicatetheexistenceofanysignificantrationaldatacorrelatingthedurabilityofHMApavementswiththeminimumVMAvaluesspecifiedformixdesign.

Thisstudywasundertaken(a)toreexaminetherationalebehindtheminimumVMAvalues

currentlybeingused,and(b)toquantifytherelationshipbetweenvariousasphaltfilmthicknessesandtheagingcharacteristicsoftheHMAmix,sothatanoptimumfilmthicknessdesirableforsatisfactorymixdurabilitycouldbeestablished.Theoptimumfilmthicknesscould

thenbeusedtoestablishVMArequirements.Mixespreparedwithasphaltbinderfilmthicknessrangingfromabout4to13microns,weresubjectedtoacceleratedagingusingStrategicHighwayResearchProgram(SHRP)procedurestosimulatebothshortandlongtermaging.Boththeaggregate(RD)andtheasphaltcement(AAM-1)usedinthisstudywereobtainedfromtheSHRPMaterialsReferenceLibrary.Theaged,compactedmixwastestedfortensilestrength,tensilestrainatfailureandresilientmodulus.Theagedasphaltcementwasrecoveredandtestedforpenetration,viscosity,complexbetweenvariousasphaltfilmthicknessesandtheagingcharacteristicsoftheHMAmix,sothatanoptimumfilmthicknessdesirableforsatisfactorymixdurabilitycouldbeestablished.Theoptimumfilmthicknesscouldthenbeusedtoestablishmodulusandphaseangle.Agingindiceswereobtainedfromthesetests,andtherelationshipbetweenfilmthicknessandtheagedmix/agedasphaltcementpropertiesweredeterminedusingregressionanalysis.Fortheparticularaggregate/asphaltcementcombinationusedinthisstudy,itwasfoundthatacceleratedagingwouldoccuriftheasphaltbinderfilmthicknesswaslessthan9-10micronsinanHMAmixturecompactedto8%airvoidcontent.TheminimumVMArequiredtoaccommodatethisoptimumasphaltfilmthicknessand4%airvoidcontent,wasalsocalculated.

KEYWORDS:VMA,voidsinmineralaggregate,hotmixasphalt,HMA,asphaltconcrete,

asphaltpavingmixture,filmthickness,durability.

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EVALUATIONOFVOIDSINTHEMINERALAGGREGATEFORHMAPAVING

MIXTURES

PrithviS.KandhalandSanjoyChakraborty

INTRODUCTION

Theconceptofvoidsinthemineralaggregate(VMA)isconsideredfundamentalinthedesignofdense-gradedhotmixasphalt(HMA)mixtures.ThetermVMAdescribesthatportionofthespaceinacompactedHMApavementorspecimenwhichisnotoccupiedbytheaggregate.CurrentmixdesignproceduresincludingSuperpavearelargelybasedupontheneedforselectingandproportioningthevariousmaterialstomeetcertainvolumetricpropertiessuchasVMA.AminimumVMArequirement(basedonthemaximumnominalsizeoftheaggregate)isusedtoensurethattheHMAmixisnotdeficientinasphaltcement(therebyensuringmixdurability)and/orinthevoidsinthetotalmix(VTM)topreventbleedingorrutting.

TheminimumrequirementsforVMAhavebeenquestionedbymanyresearchersbecausethere

isalackofsignificantresearchdatacorrelatingtheVMAwiththeHMAmixperformanceintermsofdurability.

OBJECTIVES

Thisstudywasundertakentoachievethefollowingobjectives:

1.Reviewavailableliteraturetore-examinetherationalebehindtheminimumVMArequirementscurrentlybeingused.

2.DeterminetheoptimumasphaltbinderfilmthicknessinanHMAmixturetominimizeshortandlongtermagingoftheasphaltbinder,thusprovidingreasonabledurabilityoftheHMAmix.TheoptimumfilmthicknesscanthenbeusedtoestablishminimumVMArequirements.

LITERATUREREVIEW

MinimumVMARequirements

InapaperpresentedtotheHighwayResearchBoardin1956[1],McLeodpointedoutthatthebasiccriteriaforboththedesignandanalysisofpavingmixturesshouldbeonavolumetricbasisandnotonthebasisofweight.Mostspecificationsinthosedaystendedtospecifyarangeofasphaltcontentbyweightalongwithgradingbandsorlimitsfortheaggregate,whichineffectrequiredadesignonthebasisofweight.

McLeod[1]illustratedthevolumetricrelationshipbetweenthetotalasphaltbinder,airvoids

betweenthecoatedaggregateparticles,andthetotalaggregateinacompactedpavingmixture.

HebasedthecompactionrequirementsupontheMarshalltestprocedure,with75blowsoneachsideofthebriquette.HealsorecommendedthattheVMA,whichisthevolumeofvoidsbetweentheaggregateparticles,shouldberestrictedtoaminimumvalueof15%,thevolumeoftheairvoids(withintheVMA)shouldliebetween3and5%,whichinturnrestrictedthevolumeofasphaltcementinthecompactedmixturetoapermissibleminimumof10%byvolume.Therefore,hisproposalforaspecificationofaminimum15%VMA,alongwith5%airvoids,automaticallyestablishesaminimumasphaltcontentofabout4.5%byweight(10%byvolume).Hiscalculationswerebaseduponabulkspecificgravityof2.65fortheaggregateand1.01fortheasphaltcement.Noasphaltabsorptionwasconsideredinthevolumetricanalysis.

AnotherpaperpresentedbyMcLeodin1959[1]totheAmericanSocietyofTestingand

Materials,advocatedtheuseofbulkspecificgravityoftheaggregateforcalculatingboththe

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VMAandtheairvoids.Absorptionoftheasphaltcementintotheaggregatewasalsotakeninto

accountinthevolumetricanalysis.McLeodrecommendedagainthatthelowestpermissibleasphaltcontentinaHMAmixshouldbe4.5%byweight,toensuremixdurability.Thisamountstoabout10%asphaltcementbyvolume.NoHMAperformancedatawerepresentedtosupport

theminimumasphaltcontentof4.5%onwhichtheminimumVMArequirementwasbased.Inthispaper,McLeodalsoproposedarelationshipbetweentheminimumVMAandthenominalmaximumparticlesizeoftheaggregate,whichwasadoptedbytheAsphaltInstitutein1964[3].Hebasedthisrelationshipuponthebulkspecificgravityoftheaggregateandanairvoidscontentof5%forthecompactedmix.However,thebackgrounddataforrelatingtheminimumVMArequirementstothenominalmaximumsizeoftheaggregatewasnotgiven[2].

Duringthelast30yearsorso,mostasphaltpavingtechnologistsdidnotrealizethatthese

minimumVMArequirementswerebasedon5%airvoidcontent(andnot4%airvoidcontentgenerallyusedformixdesign)and75-blowmarshallcompaction.Obviously,theminimumVMArequirementscorrespondingto4%airvoidcontentwouldbe10/0lowerthanthoserecommendedinearliereditionsofAsphaltInstituteMS-2[3].Thiswasrecognizedin1993and

theAsphaltInstituteMS-2wasrevised[4]togiveminimumVMArequirementscorrespondingto3,4,and5%airvoidcontents.TheserevisedminimumVMArequirementshavealsobeenincorporatedinSuperpavemixdesignprocedures.

AsphaltBinderFilmThicknessinDurabilityConsiderations

Itisgenerallyagreedthathighpermeability,highairvoids,andthinasphaltcoatingsontheaggregateparticlesaretheprimarycausesofexcessiveagingoftheasphaltbinderwhichcontributestothelackofdurabilityoftheHMAmixesoftenencounteredinthefield.However,theconceptofan"averagefilmthickness"fordense-gradedasphaltmixturesisnoteasilyunderstood.Howmuchvaliditycanbeassignedtoafilmthickness,calculatedsimplybydividingthetotalsurfaceareaoftheaggregate(obtainedfromitsgradation)bytheeffectiveasphaltcontent?Itishighlyunlikelythatalltheparticlesinamixhavethesamefilmthickness

ofasphaltcoating.Fineaggregateparticlesmayhaveamuchthickercoatingascomparedtothecoarseaggregateparticles,andinfact,forallpracticalpurposes,someveryfineparticlesmightsimplybeembeddedintheasphaltcement/fillermortarsystem.Therefore,theterm"filmthickness"iselusiveanddifficulttodefine.However,forthepurposeofcalculationlaterinthispaper,weshallassumethattheconceptofthe"averagefilmthickness"isindeedvalid,andproceedwiththecalculations.SurfaceareawillbecalculatedusingtheprocedureoutlinedintheAsphaltInstitute'sMS-2[4].Campen,Smith,EricksonandMertz[5]presentedtherelationshipbetweenvoids,surfacearea,filmthicknessandstabilityfordensegradedHMA.Theauthorsrecognizedthatthickerasphaltbinderfilmsproducedmixeswhichwereflexibleanddurable,whilethinfilmsproducedmixeswhichwerebrittle,tendedtocrackandravelexcessively,retardedpavementperformance,andreduceditsusefulservicelife.Onthebasisofthedatatheyanalyzed,averagefilmthicknesses

rangingfrom6to8micronswerefoundtohaveprovidedthemostdesirablepavementmixtures.Theyalsoconcludedthatthefilmthicknessdecreasesasthesurfaceareaoftheaggregateisincreased.However,theasphaltbinderrequirementofamixisnotdirectlyproportionaltoitssurfacearea.Theasphaltbinderrequirementwasfoundtoincreaseasthesurfaceareawasincreased,butataratemuchlowerthanthatguidedbyarelationshipofdirectproportionality[5].

GoodeandLufsey[6]alsodidsomesignificantworkinrelatingasphalthardeningtovoids,

permeabilityandfilmthickness.Theyrecognizedthatthehardeningoftheasphaltbinderinamixwasafunctionofairvoids,filmthickness,temperature,andtime.Onthebasisoftheirworktheyconcludedthataminimumvalueof0.00123for'bitumenindex'(whichcorrespondstoavalueof6micronsofaveragefilmthickness)couldbeincludedasacriterioninallmixdesign

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procedures.The'bitumenindex'wasdefinedaspoundsofasphaltcementpersquarefootof

surfacearea.Theyusedtheconceptofbitumenindextoavoidtheimplicationthatallparticleswerecoatedwiththesameuniformthicknessofasphaltcement.Theirstudyindicatedthatacombinedfactoroftheratiooftheairvoidstothebitumenindexcouldbesatisfactorilyrelated

totheasphaltbinderhardeningcharacteristicsintheHMAmixture.TheysuggestedthattheMarshallmethodofmixdesigncouldbeimprovedbyincorporatingamaximumvalueofvoids-bitumenindexratioinplaceofamaximumvalueofairvoidsalone,andsuggestedavalueof4asthemaximumforthisratio,toensurereasonableresistancetoaging.

KumarandGoetz[7]studiedtheasphaltbinderhardeningasrelatedtoHMApermeabilityand

asphaltfilmthickness.Theystatedthatthebestprocedureforpredictingtheresistanceofhardeningofasphaltbinderinasingle-sizedHMAmixwastocalculatetheratioofthefilmthicknessfactortopermeability.Thefilmthicknessfactorwasdefinedastheratioofthepercentasphaltcontentavailableforcoatingtheaggregatetothesurfaceareaoftheaggregate.Theyindicatedthatfordense-gradedmixtures,theconceptofanaveragefilmthicknessisatbestdubious,ifnottotallyerroneous.Fordense-gradedmixtures,permeabilitywasstatedtobethe

bestmeasureoftheresistancetohardening.However,atthedesignvalueof4%airvoidsasiscommonformostdense-gradedHMAmixtures,theeffectofpermeabilityofthemixwasdeterminedtobequiteinsignificant.

TESTINGPROGRAM

Thistestingprogramwascarriedoutwiththefollowingobjectives:

Toevaluatethechangesinthetheologicalpropertiesoftheasphaltcementduetoaginginrelationtotheasphaltfilmthickness.Bothshortterm(duringHMAproductionandconstruction)andlongterm(duringservicelife)agingwereconsidered.

Todetermineanoptimumrangefortheasphaltfilmthickness,ifpossible,whichwouldminimizeagingoftheasphaltbinder.Thisoptimumfilmthicknesscanthenbe

usedindevelopingminimumVMArequirementsforHMAmixtures.

MaterialUsed

AggregateandasphaltcementsampleswereobtainedfromtheSHRPMaterialReferenceLibrary(MRL).Onlyoneaggregate(SHRPMRLDesignationRD):FrederickLimestonewasusedinthisstudy.Table1givesthephysicalpropertiesofthetotalaggregateobtainedfromSHRPMRL.Table2givesthewashedgradationoftheaggregateusedintheHMAmixture.

Table1.PhysicalPropertiesofRDAggregate(FrederickLimestone)

Property

BulkSpecificGravity

WaterAbsorption,percentL.A.Abrasion(AASHTOT96)

%Wear

FlakinessIndex,percent

SandEquivalent(AASHTOT176)

Value

2.704

0.3

23.4

34.7

69

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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

Anasphaltcement(SHRPMRLDesignationAAM-1)wasusedinthisstudy.ThisasphaltcementwasselectedbecauseithadoneofthehighestpropensitiestoageintheHMAmixbasedontheworkdoneinSHRPA-003AbySosnovskietal[3].ItsphysicalandchemicalpropertiesasobtainedfromSHRParegiveninTable3.

TestProceduresUsed

ThesurfaceareaoftheaggregatewascalculatedusingthesurfaceareafactorsgiveninMS-22

[4].Fortheaggregategradationused(Table2)thesurfaceareawascalculatedtobe27.626ft/lb(5.662m2/kg).

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

Avalueof0.20%asphaltabsorptionwasusedfortheRD-AAM-1combinationasdetermined

andreportedin[9].Thisrequiredsixasphaltcontents(byweightofthetotalmix)asfollows:2.2,3.2,4.2,5.1,6.1and7.1percenttoobtainasphaltfilmthicknessrangingfrom3.7to13.0micronasmentionedabove.

AllsixHMAmixtureswerepreparedatthemixingtemperatureof1433C.Thetesting

sequenceforeachmixisgiveninFigure1.ThelooseHMAmixsamplesweresubjectedtoshorttermagingfollowingSHRP#1025

procedures[10].TheprocessinvolvesagingofthelooseHMAmixinaforceddraftovenfor4hoursatatemperatureof135C.Theloosemixisplacedinabakingpanandspreadtoaneventhicknessthatproducedabout21kg/m2.ThisprocedureisdesignedtosimulatetheagingthatthelooseHMAmixundergoesduringtheconstructionphaseofthepavement.ThreesamplesoftheagedHMAmixweresubjectedtoAbsonmethodofrecoveringasphaltbinder.Therecoveredasphaltbinderwastestedforpenetrationat25Candviscosityat60C.Thecomplexmodulus(G*)andphaseangle(*)werealsodeterminedat64CfortherecoveredasphaltcementusingtheDynamicShearRheometer.Thetemperatureof64Cwasusedbecausejustafterconstruction,ruttingfactor(G*/sin*6)iscriticalathighpavementtemperatures.ThistemperaturewouldbeusedfortestingaSuperpavePG64-34binderaftersubjectingittorolling

thinfilmoven(RTFO)whichsimulatesshorttermaging.

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Table3.PropertiesofAsphaltCement(AAM-1)Used

Property

ORIGINALASPHALTCEMENT

SpecificGravity

Viscosityat60C,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*at20C,kPa

COMPONENTANALYSIS

Asphaltenes(n-heptane)

PolarAromatics

NaptheneAromatics

Saturates

ELEMENTANALYSIS

Nitrogen,%

Sulphur,%

Vanadium,ppmNickel,ppm

Value

0.993

199.2

56964

4.6

51.71.15

0.00516

394.7

744

2.46

3,200

3.9

50.3

41.91.9

0.50

2.40

60.0

29.0

Five100-mmdiameterspecimenswerecompactedfromeachmixtypeaftershorttermaging.Thecompactedspecimenswerepreparedtogiveatargetairvoidcontentlevelof81%.TheCorpsofEngineersGyratoryTestingMachine(GTM)wasusedforthispurpose.Theresilientmodulus(MR)ofallthecompactedspecimenswasdeterminedat25C.Totalnumberofsamplestested=6(filmthicknesses)x5(replicates)=30.

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Figure1.TestSequenceforEachAsphaltContent/FilmThickness

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The30compactedspecimensweresubjectedtolongtermagingfollowingSHRP#1030

procedures[11].Theprocedureconsistsofplacingthecompactedspecimensonarackinaforceddraftovenfor120hours,andatatemperatureof85C.ThisprocedurewasdesignedtosimulatetheagingthatthecompactedHMApavementundergoesduringits5-10yearsservice

life.Anairvoidcontentof81%incompactedspecimensisusedtosimulatecompactionatthetimeofconstruction.Lowerairvoidcontentsmayalsonotprovideinterconnectedvoidswhichareessentialforthisacceleratedagingtest.

Thefollowingtestswereconductedonthecompactedspecimensafterlongtermaging:

1.Resilientmodulus(MR)at25C.2.Tensilestrength(ST),alongwiththestrainatfailure,at25Cusingastrainrateof50

mm(0.05m)perminute.3.Absonrecoveryofagedasphaltbinderfromall30brokenspecimens.Therecovered

asphaltbindersweretestedforpenetrationat25C,viscosityat60C,complexmodulus(G*)at19C,andphaseangle(6)at19C.

ComplexmodulusandphaseangleweremeasuredusingSuperpavetestprocedures[12].The

temperatureof19Cwasusedbecausethefatiguefactor(G*sin*)iscriticalatmidservicepavementtemperaturesaccordingtoSuperpaveperformancegraded(PG)binderspecifications[12].AfterlongtermagingHMAmixesbecomestiffand,therefore,fatiguecrackingbecomestheprimarydistressofconcernaffectingtheHMAdurability.Thetemperatureof19CwouldbeusedtotestaSuperpavePG64-34binderafteragingitinRTFOandpressureagingvessel(PAV).

ANALYSISOFTESTRESULTS

Table4givesthecompactedHMA'sphysicalproperties(suchasresilientmodulusat25Candtensilestrengthat25C)aftershortandlongtermagingcorrespondingtoasphaltfilmthicknessrangingfrom3.7to13.0micron.Table5givestheconventionalproperties(suchaspenetration

at25Candviscosityat60C)oftherecoveredasphaltbindersaftershorttermandlongtermaging.Table6givesSuperpavebinderproperties(suchascomplexmodulusG*)fortheserecoveredasphaltbinders.

Table4.CompactedHMAPropertiesAfterShortandLongTermAging1

ResilientModulusat25C,MPa

FilmThickness(microns)

3.7

5.6

7.4

9.311.1

13.0

AfterSTA

8,184

6,357

4,027

2,9012,572

1,958

2 AfterLTA3

12,293

9,398

5,240

3,7162,696

2,020

LTA/STARatio

1.50

1.48

1.30

1.281.05

1.0312Allreporteddataareaveragesoffivesamples STA=ShortTermAging3LTA=LongTermAging

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Table5.RecoveredConventionalAsphaltBinderPropertiesAfterAging1

Viscosityat60C,Pa.s ViscosityRatio Penetrationat425C,

0.1mm

RetainedPenetration5

FilmThickness After2 After3 AfterSTAAfterLTAAfterSTAAfterLTAAfterSTAAfterLTA

(microns) STA LTA

3.7 1262.1 4744.4 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

7.4 526.1 434.2 2.56 21.19 39.6 27.3 63.9 44.0

9.3 434.6 3940.1 2.12 19.20 43.6 29.0 70.3 46.811.1 276.3 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 55.312Allreporteddataareaveragesofthreesamples STA=ShortTermAging3LTA=LongTermAging 4BasedonviscosityoforiginalasphaltcementmeasuredatNCAT(205.2Pa.s) 5BasedonpenetrationoforiginalasphaltcementmeasuredatNCAT(62)

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Table6.RecoveredSuperpaveAsphaltBinderPropertiesAfterAging1

Complex Complex G*sin*at19C,Modulus,G*at Modulus,G*at Pa

64C,Pa 19C,Pa

FilmThickness(microns)

3.7

5.6

7.4

9.3

11.1

13.0

AfterSTA2

2090

3590

2270

2460

1310

1220

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

1.918E+06

1.787E+06

1.094E+0612Allreporteddataareaveragesofthreesamples STA=ShortTermAging3

LTA=LongTermAgingTheconceptofpolynomialregressionhasbeenusedasatooltofittheobserveddatatocurve,

whichquantifytherelationshipbetweentheindependentandthedependentvariables.Theindependentvariable,inmostcases,istheasphaltfilmthickness,againstwhichareplottedthevaluesoftherecoveredasphaltcementpropertieslikepenetration,viscosity,andcomplexmodulus,orthemeasuredpropertiesofthecompactedHMAmix,liketensilestrengthandresilientmodulus.Therelationshipbetweenthemeasuredpropertiesantithefilmthicknesshasbeenquantifiedformixeswhichhavebeensubjectedtobothshortandlongtermaging.AlldependentvariableswhosevaluesaregiveninTables4,5and6wereanalyzed,thedetailedanalysesaregivenelsewhere[13].Aselectednumberofdependentvariablesarediscussedbelow.

CompactedHMAMixPropertiesTheresilientmodulusofthecompactedHMAspecimenswasmeasuredbothaftershorttermagingandafterlongtermaging(Table4).Theloosemixhadbeensubjectedtoshorttermagingbeforecompaction,i.e.,beforethepreparationofthecompactedsamples.Thecompactedsampleswerethensubjectedtolongtermaging.Resilientmodulustestingwascarriedoutat25Cinthediametralorindirecttensilemode.Figures2and3showtherelationshipbetweenthefilmthicknessandmodulusvaluesaftershorttermandlongtermaging,respectively.Quadraticpolynomialregressiongaveanacceptablemodelforthisrelationshipaspresentedbelow:

AfterShortTermAging

Mrst=2069.9-273.15:+10.53:2

R2=0.99035

where,

Mrst=resilientmodulusaftershorttermaging(ksi)(1ksi=6.895Mpa)

:=filmthicknessinmicrons

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AfterLongTermAging

Mrlt=3267.6-456.75:+17.55:2

R2=0.9896

where,Mrlt=resilientmodulusafterlongtermaging(ksi)

Figure2.AsphaltFilmThicknessvs.ResilientModulusafterShortTerm Aging

ItcanbeseeninFigures2and3thatataboutafilmthicknessof11microns,thefittedcurvetendstoflattenoutanddoesnotchangesignificantlywithincreasingfilmthickness.Also,theslopeofthecurvebecomessteeperasthefilmthicknessfallsbelowavalueofabout9to10microns,whichindicatesthatthestiffness(causedbyaging)oftheHMAmixstartstoincreasequiterapidlywithadecreaseinfilmthicknessbelowabout9-10microns.Also,thereisamarkedsimilaritybetweenthecurvesobtainedaftershorttermandlongtermaging.

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Figure3.AsphaltFilmThicknessvs.ResilientModulusafterLongTerm Aging

SomemoreinformationconcerningthechangeinresilientmodulusvalueswithfilmthicknesscanbeobtainedfromthegraphinFigure4wheretheresilientmodulusvaluesaftershorttermaginghavebeenplottedagainstthecorrespondingmodulivaluesafterlongtermaging.Eachpointonthegraphcorrespondstooneparticularasphaltfilmthicknesswhichdecreasesfromlefttoright(becausetheresilientmodulusvaluesincreasewithdecreaseinfilmthickness).Regressionanalysisleadstoalinearrelationshipinthedataasmodeledbythefollowingequation:

Mrlt=-198.12+1.675Mrst

R2=0.998

TheveryhighvalueofR2indicatesthatalinearmodelalmostexactlyrepresentstherelationship.FromthisitcanbeinferredthatbothshortandlongtermagingoftheHMAmixareaffectedinexactlythesamewaybythefilmthickness.ThismeansthickasphaltfilmsminimizetheagingoftheHMAmixesduringconstructionaswellasduringservicelife.

Asexpected,thetensilestrengthat25Cdecreasesandthetensilestrainatfailureincreasesas

theasphaltfilmthicknessintheHMAmix(afterlongtermaging)isincreasedfrom3.7to13.0micron.

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Figure4.ResilientModulusBeforevs.ResilientModulusAfterLTOA

RecoveredAsphaltBinderProperties

ThemeasuredvaluesofviscosityoftherecoveredasphaltaftershorttermandlongtermaginghavebeenpresentedgraphicallyinFigures5and6,respectively.Forsamplessubjectedtoshorttermaging,regressionanalysisleadstotheestablishmentofaquadraticmodeldefinedbythefollowingequation:

Vst=22069-3268.8:+132.8:2

R2=0.988

where,

Vst=viscosityofasphaltcementaftershorttermaging(poises)(1poise=0.1pa.s)

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Figure5.AsphaltFilmThicknessvs.ViscosityAfterShortTermAging

Figure6.AsphaltFilmThicknessvs.ViscosityAfterLongTermAging13


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Ascanbeseenfromthedataforshorttermaging(Figure5),astheasphaltfilmthickness

decreasesbelowavalueofabout9-10micronsthefittedcurvetendstosteepenindicatinganacceleratedrateofincreaseinviscosity.Ontheotherend,thesamecurveisseentoflattenoutatabout11micronsfilmthickness,whichindicatesthatthefilmthicknesshaslesserandlesser

effectontheagingoftheasphaltcementonceitisincreasedaboveavalueofabout11microns.Forsamplessubjectedtolongtermaging,regressionanalysisfailedtoproduceasatisfactorymodelwhichcouldexplainthenatureoftherelationshipbetweenfilmthicknessandagedviscosity.Thus,noequationisavailabletodefinetherelationship.Instead,thepointsonthegraphhavebeenconnectedtogetherbyasmoothcurve.AscanbeseenfromFigure6,theviscosityincreasesatanacceleratedrateoncetheasphaltfilmthicknessdecreasesbelowavalueofabout10microns.Thenatureofthecurvesobtainedwhentheviscosityratio(Table5)isplottedagainstfilmthickness,isaboutthesameasinthepreviouscasesforshortandlongtermagedconditions[13].Theviscosityratioisdefinedastheratiooftheviscosityoftheagedasphalttotheviscosityoftheunaged/originalasphalt.

Similarrelationshipswerealsoobservedbetweenasphaltfilmthicknessandpenetrationor

retainedpenetrationofasphaltcementsaftershortandlongtermaging[13].

Forasphaltcementsubjectedtoshorttermaging,complexmodulusG*andphaseangle*weremeasuredatatemperatureof64C,whereasforlongtermagedspecimens,thetestingoftheasphaltcementwascarriedoutat19C(Table6).Sincethestiffnessoftheasphaltbinderismorecriticalafterlongtermagingratherthanshorttermagingfromthedurability(orresistancetofatigue)standpoint,thecomplexmodulusG*andfatiguefactor(G*sin*)afterlongtermagingwillonlybepresentedhere.TherelationshipbetweenasphaltfilmthicknessandG*at19CafterlongtermaginghasbeenshowninFigure7.AlthoughtheregressionanalysisofG*at19Candfilmthicknessgaveaquadraticmodelfortheshorttermagedasphaltcement[D],alinearmodel(Figure7)betterexpressestherelationshipbetweenasphaltfilmthicknessandG*at19Cafterlongtermaging,asfollows:

G*lt=3158521-176472.6:

R=0.98

where,

G*lt=complexmodulusofasphaltcementsubjectedtolongtermaging(Pa)

Asexpected,valuesofthecomplexmodulusG*decreasewithincreasingasphaltfilmthickness,

bothforshortandlongtermaging.ThisindicatesthatthepresenceofthickerfilmsofasphaltcementintheHMAmixminimizesagingoftheasphaltbinder.However,itisnotapparentfromthefittedcurvesastowhatrangeofasphaltfilmthicknessmightprovetobeoptimuminminimizingasphaltcementaging.

TherelationshipbetweenG*sin*(fatiguefactor)andfilmthicknesshasbeenpresentedinFigure8,forasphaltcementrecoveredfrommixeswhichhadbeensubjectedtolongtermaging.Alinearmodelwasobtainedasfollows:

GSDlt=1848404-98052.5:

R2=0.98

where,

GSDlt=G*sin*(inpascals)forsamplessubjectedtolongtermaging.

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Figure7.AsphaltFilmThicknessvs.ComplexModulus(G*)AfterLongTermAging

Figure8.AsphaltFilmThicknessvs.G*sin *AfterLongTermAging15


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Asisevidentfromthefittedcurve(Figure8),G*sin*increaseswithdecreaseinthefilmthicknessoftheasphaltbinderintheHMAmix.ThisindicatesthatthelowertheasphaltbinderfilmthicknesspresentinaHMAmix,themoresusceptiblethepavementistofatiguecrackinginthelongterm.SHRPhassuggestedavalueof5000KPaastheupperlimitforG*sin*forasphaltcementsubjectedtoacceleratedaginginthePressureAgingVessel(PAV).PAVagingsimulatestheagingthattheasphaltbinderinaHMApavementundergoesafterabout5-10yearsinservice.Asdiscussedearlier,thisstudyemployedaprocedure(alsodevelopedbySHRP)tosimulatelongtermagingofin-servicepavementsusingcompactedHMAsamples,insteadofjustasphaltcement,asisusedinthePAVtest.Therefore,forasphaltcementrecoveredfromcompactedHMAsamplessubjectedtoacceleratedlongtermlaboratoryagingprocedures,5000kPashouldbeconsideredastheupperlimitforG*sin*.AsisevidentfromthedatapresentedinFigure8,themaximumrecordedvalueofG*sin*isabout1500kPa,whichismuchlessthanthelimitingvalueof5000kPa.ThisindicatesthatthePAVagingofasphaltcementismuchmoresevereinthislimitedlaboratorystudythanthatoccurringincompactedHMAsamplesagedinforceddraftovenat85Cfor120hours.

AirVoidstoBitumenIndexRatioAnalysis

Theconceptoftheratiooftheairvoids(percent)tobitumenindex,asameasureoftheagingsusceptibilityofamix(whateverbeitsgradation),wasdiscussedearlier.GoodeandLufsey[6]hadproposedamaximumvalueof4.0forthisratiowhichtheybelievedwouldpreventpavementdistressbyreducingtheagingoftheasphaltfilmcoatingtheaggregate.Mathematically,whattheystatedwas:

Notingthat:

filmthicknessinmicrons=bitumenindex4870

Thepreviousexpressioncanbereducedtoaminimumfilmthicknessrequirement,varyingwith

theairvoidscontentofthegivenmix,asfollows:Atargetvalueof8%fortheairvoidsinthecompactedHMAspecimenswasusedinthepresent

study,inconformancewiththeacceleratedlongtermagingproceduredevelopedinSHRPA003-AProject.Thiscorrespondstoaminimumfilmthicknessrequirementsof9.74microns(about10microns),basedupontheaboveequation.AscanbeseenfromFigures2and3,thecurvesoftheresilientmoduli(forbothshortandlongtermagingconditions)versusfilmthicknesstendtosteepenasthefilmthicknessdecreasesbelowtherangeof9to10microns.Thisindicatesthattherateofagingoftheasphaltcementisacceleratedwhenthefilmthicknessislessthan9-10microns.Thisacceleratedagingratecanalsobeseenintheplotsofviscosityversusfilmthickness(Figures5and6).Thereforeitcanbeconcluded,onthebasisofthepresentstudy,thatamaximumvalueof4.0forthevoids/bitumenindexratioisindeedreasonable,andmightprovetobeabetterspecificationfordesign,atleastasfarastheagingofasphaltcementisconcerned.

Basedonthedatapresentedinthislimitedstudyofoneasphaltcement/oneaggregate

combination,letusassumethattheoptimumasphaltfilmthicknesstominimizeagingis9

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microns.Thevolumeofasphaltcementbindercanthenbecalculatedbasedonthisoptimumfilm

thicknessandsurfaceareaoftheaggregateintheHMAmixused.Thisvolumeofasphaltcementwhenaddedto4%airvoidcontent(generallyusedinHMAmixdesign)shouldthengivetheminimumVMArequiredforthisHMAmixtoensurereasonabledurability.Itisinterestingto

notethattheminimumVMArequirementforthemixusedinthisstudywouldbecalculatedat15.6%baseduponthisprocedureforafilmthicknessof9micronsandanairvoidscontentof4%.AspertherecommendationsoftheAsphaltInstitute[4],thecorrespondingminimumVMAthathastobeprovidedforthismix(withamaximumnominalsizeof12.5mmaccordingtothenewdefinition)is14percent.Thenominalmaximumsizehasnowbeendefinedasonesievesizelargerthanthefirstsievetoretainmorethan10percent.Therefore,theminimumVMArecommendationoftheAsphaltInstitute(alsoadoptedinSuperpavemixdesign)isabout1.5%lessthanthatneededforobtaininganoptimumasphaltfilmthicknessbasedonthisstudy.Thisdifferenceislikelytovaryifdifferentgradationand/ordifferentasphaltcement/aggregatecombinationsareused.ItisalsointerestingtonotethatMcLeod[14]hadrecommendedin1971toincreaseallminimumVMArequirementsinMS-2by2%toobtaingreaterdurability,althoughwithoutprovidinganysignificantsupportingdata.However,itmaynotbepossibleto

increasetheVMAinmanydense-gradedHMAmixtures.CONCLUSIONSANDRECOMMENDATIONS

Thisstudywasundertaken(a)toreexaminetherationalebehindtheminimumVMArequirementscurrentlybeingused,and(b)toquantifytherelationshipbetweenvariousasphaltbinderfilmthicknessesandtheagingcharacteristicsoftheHMAmixsothatanoptimumasphaltfilmthicknessdesirableforsatisfactorymixdurabilitycouldbeestablished.Thefollowingconclusionsweredrawnandrecommendationsmade:

1.TheliteraturereviewedaspartofthisstudydidnotindicatetheexistenceofanysignificantrationaldatacorrelatingtheperformanceofHMApavementswiththeVMAvaluescurrentlyspecifiedforHMAmixdesigns.

2.Therelationshipbetweentheasphaltfilmthicknessandtheagedproperties(both

shorttermandlongterm)oftheHMAmixtures,suchmtensilestrengthandresilientmodulus,wasquantified.AfairlygoodcorrelationwasobtainedbetweentheasphaltfilmthicknessandtheresilientmodulusoftheagedHMAmixtures.Anoptimumfilmthicknessof9-10micronswasindicatedfromthedata,belowwhichtheHMAmix(compactedto8%airvoidcontent)agedatanacceleratedrate.ThisrangeappearstoconcurwiththeresultsobtainedbyGoodeandLufseyintermsofairvoids/bitumenindexratio.

3.Relationshipswerealsoestablishedbetweentheasphaltfilmthicknessandtheagedasphaltbinderproperties(bothshortandlongterm)suchasviscosity,penetration,andcomplexmodulus.Anoptimumfilmthicknessof9-10micronswasgenerallyindicatedfromthedata,belowwhichtheasphaltbinderagedatanacceleratedrate.ThisfilmthicknesscorrespondstoasphaltbindercontainedinanHMAmixcompactedto8%airvoidcontent.

4.TheminimumVMAfortheHMAmixusedinthisstudywascalculatedtobe15.6%toaccommodateanoptimumasphaltfilmthicknessof9micronsand4%airvoids.ThecorrespondingAsphaltInstituteorSuperpaverecommendationforminimumVMAis14%forthismix(maximumnominalsizeof12.5mm).However,itmaynotbepossibletoachievethedesiredVMA(15.6%)insomedense-gradedHMAmixturesofsimilargradation.

5.Theprecedingconclusionsarebasedononlyoneaggregate/asphaltcementcombination.SHRPA-003AandA-003BProjectshaveindicatedthattheagingphenomenonisinfluencedbytheinteractionbetweentheaggregateandtheasphaltcement.Therefore,theoptimumasphaltfilmthicknessindicatedinthisstudyneedstobeconfirmedbyconductingmorestudiesinvolvingdifferentaggregate/asphaltcementcombinations.TheminimumVMArequirementthencouldbebasedontheoptimumasphaltfilmthicknesswhichgivesreasonabledurabilityoftheHMAmix.

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Volume35,(1956).McLeod,N.W.,"VoidRequirementsforDense-GradedBituminousPavingMixtures,"ASTM,SpecialTechnicalPublication252,(1959).MixDesireMethodsforAsphaltConcreteandOtherHotMixTypes,TheAsphaltInstitute,MS-2,May(1984).MixDesignMethodsforAsphaltConcreteandOtherHotMixTypes,TheAsphaltInstitute,MS-2.SixthEdition,(1993).Carnpen,W.H.,Smith,J.R,Erickson,L.G.andMertz,L.R.,"TheRelationshipsBetweenVoids,SurfaceArea,FilmThicknessandStabilityInBituminousPavingMixtures,"Proceedings,AAPT,Vol.28,(1959).Goode,J.F.andLufsey,L.A.,"Voids,Permeability,FilmThicknessvs.AsphaltHardening,"Proceedings,AAPT,Vol.34,(1965).

Kumar,A.andGoetz,W.H.,"AsphaltHardeningasAffectedbyFilmThickness,VoidsandPermeabilityinAsphalticMixtures,"Proceedings,AAPT,Vol.46,(1977).Sosnovske,D.,AbWahab,Y.,andBell,C.,"TheRoleofAsphaltandAggregateintheAgingofBituminousMixtures,"TransportationResearchRecord1386,(1993).Kandhal,P.S.,andKhatri,M.A.,"RelatingAsphaltAbsorptiontoPropertiesofAsphaltCementandAggregate,"TransportationResearchRecord1342,(1992)."StandardPracticeforShortTermAgingofAsphaltConcreteMixtures,"SHRP#1025,(1992)."StandardPracticeforLongTermAgingofAsphaltConcreteMixtures,"SHRP#1030,(1992)."TheSuperpaveMixDesignSystemManualofSpecifications,TestMethods,andPractices,"SHRP-A-379,(1994).Chakraborty,S.,"EvaluationofVoidsintheMineralAggregateforHMAPaving

Mixtures,"M.S.Thesis,AuburnUniversity,(1994).McLeod,N.W.,"DesigningStandardAsphaltPavingMixturesforGreaterDurability,"Proceedings,CanadianTechnicalAsphaltAssociation,Vol.16,(1971).

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Evaluation of Voids in Mineral Aggregate for Hot Mix Asphalt

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