waste materials in hot mix asphalt: an overview
TRANSCRIPT
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WASTEMATERIALSINHOTMIX ASPHALT-ANOVERVIEWBy
PrithviS.Kandhal
PaperpublishedinASTM,SpecialTechnicalPublication1193,1993
277TechnologyParkway Auburn,AL36830
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WASTEMATERIALSINHOTMIXASPHALT-ANOVERVIEW
By
PrithviS.KandhalAssistantDirector
NationalCenterforAsphaltTechnologyAuburnUniversity,Alabama
PaperpublishedinASTM,SpecialTechnicalPublication1193,1993
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DISCLAIMER
Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsandpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.
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ABSTRACT
Numerouswastematerialsresultfrommanufacturingoperations,serviceindustries,sewagetreatmentplants,householdsandmining.Legislationhasbeenenactedbyseveralstatesinrecentyearstoeithermandatetheuseofsomewastematerialsortoexaminethefeasibilityofsuchusage.Thehotmixasphalt(HMA)industryhasbeenpressuredinrecentyearstoincorporateawidevarietyofwastematerialsintoHMApavements.Thishasraisedthefollowinglegitimateconcerns:(a)engineeringconcernssuchaseffectontheengineeringproperties(forexample,strengthanddurability),impactonproduction,andfuturerecyclability;(b)environmentalconcernssuchasemissions,fumes,odor,leaching,andhandlingandprocessingprocedures;and(c)economicconcernssuchaslifecyclecosts,salvagevalue,andlackofmonetaryincentives.
Thewastematerialscanbroadlybecategorizedasfollows:(a)industrialwastessuchascellulose
wastes,woodlignins,bottomashandflyash;(b)municipal/domesticwastessuchasincineratorresidue,scraprubber,wasteglassandroofingshingles;and(c)miningwastessuchascoalmine
refuse.AgeneraloverviewofprecedingwastematerialsincludingtheresearchworkdoneinthepastandtheirpotentialforuseinHMApavementsisgiveninthispaper.
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WASTEMATERIALSINHOTMIXASPHALT-ANOVERVIEW
PrithviS.Kandhal
Numerouswastematerialsresultfromeveryaspectofsocietyincludingmanufacturing,serviceindustries,sewagetreatmentplants,householdsandmining.Thedisposalofwasteproductsisprimarilydoneasfollows:
(a)Landfills
(b)Incineration,and(c)Recyclinginotherproducts
However,problemsarebeingexperiencedbecauseoftheinsufficientcapacityoflandfills,air
pollutionassociatedwithincinerators,andlimitedalternativesforrecycling.Legislationhasbeenenactedbyseveralstatesinrecentyearstoeithermandatetheuseofsomewastematerials
ortoexaminethefeasibilityofsuchusage.About450millionmegagrams(Mg)ofhotmixasphalt(HMA)areproducedintheUnitedStatesatacostofabout$12billion.TheHMAindustryhasbeenpressuredinrecentyearstoincorporateawidevarietyofwastematerialsintoHMApavements.Thishasraisedthefollowinglegitimateconcerns( 1).
ENGINEERINGCONCERNS
Thefollowingconcernsmustbeaddressedfromtheengineeringviewpoint.
PropertiesofHMA
Sincethewastematerialwillreplaceand/ormodifythepropertiesofasphaltcementbinderand/orHMA,itwillaffecttheengineeringproperties(suchasstrengthanddurability).
Therefore,theHMAcontainingthewastematerialmustbereevaluatedthoroughlyandcarefullybothinthelaboratoryandthefield.
ImpactonProduction
SomewastematerialsrequireachangeintheHMAproductionequipmentand/orprocessesand,therefore,theproductionislikelytobeaffected.
FutureRecyclability
ItisnotknowninmanyinstanceswhethertheHMAcontainingawastematerialcanbeeffectivelyrecycledinthefuturewithoutanyproblems.Forexample,HMAcontainingsignificantamountsofgroundtirerubberhasnotbeenrecycledasyet.Suchrecyclingmaypose
airpollutionproblems.
ConsistentQuality
Thewastematerialmayvaryinquality(chemicalorphysicalproperties)and,thus,affecttheengineeringpropertiesanddurabilityofthehotmixasphalt.
ENVIRONMENTALCONCERNS
Thefollowingconcernsmustbedealtwithfromanenvironmentalviewpoint,especiallywhenverytightemissionandenvironmentalcontrolsarealreadyinplaceonHMAproductionfacilities.
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Emissions/Fumes/Odor
Itisquitelikelythatsomewastematerialswillhaveemissions/fumes/odorproblemsbecauseHMAisproducedathightemperatures.Theseproblemsmightalsobefacedwhenrecycling
HMAcontainingthewastematerialinthefuture.
Leaching
Component(s)ofwastematerialsmaybesusceptibletoleachingfromHMApavementsandthuscausestormwaterand/orgroundwaterpollution.
HandlingandProcessingProcedure
Theseproceduresarenotfullydevelopedforallwastematerials.SomewastematerialscouldbehazardousandcouldposehealthrelatedriskstoworkersintheHMAindustry.
ECONOMICCONCERNS
Fromaneconomicviewpoint,thefollowingconcernsmustalsobeaddressed.
Price
MandateduseofwastematerialsislikelytoraisethepriceofHMA.Forexample,virginaggregatesaregenerallyavailablelocallyatlowercostscomparedtowastematerialssuchaswasteglasswhichmustbehauled.
LifeCycleCosts
LifecyclecostsneedtobedeterminedforHMApavementscontainingwastematerials.Itis
quitepossiblethatthewastematerialmaynotaffecttheinitialengineeringpropertiesoftheHMApavementbutreduceitsservicelife.
DisposalCosts
IftheHMAcontainingthewastematerialscannotberecycled,thenitsdisposalcostsneedtobedetermined.
SalvageValue
SalvagevaluesofHMAcontainingthewastematerialsarenotknown.
LackofIncentive
AtthepresenttimethereisalackofmonetaryincentivestousethewastematerialsinHMA.
CATEGORIZATIONOFWASTEMATERIALS
Thewastematerialscanbroadlybecategorizedasfollows( 2).SomewastematerialswhichhavebeenusedexperimentallyorroutinelyinHMAarealsogivenforeachcategory.
(a)IndustrialWastes
CelluloseWastesWoodLigninsBottomAsh
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FlyAsh(b)Municipal/DomesticWastes
IncineratorResidueSewageSludgeScrapRubberWasteGlassRoofingShingles
(c)MiningWasteCoalMineRefuse
By-productsfromindustrialprocesses,suchasblastfurnaceslags,steelslagsandsulphurwere
consideredwastematerialsatonetime.However,theseproductsarenowgenerallyusedonaroutinebasisand,therefore,willnotbediscussed.AdetaileddiscussionofsomeselectedwastematerialsusedinHMAfollows.
CELLULOSEWASTES
Cellulosewastesincludeagriculturalwastes(cropresidues,forestingwastes),manufacturingwastes(foodprocessing,woodandpaperindustry),andurbanrefuse(municipalsolidwaste,manufacturingplanttrash).Apyrolysisprocesshasbeenusedoconvertcellulosicwastetoabinder(3).However,theresultingbinderwasnotsuitablefordirectuseasasubstituteforasphaltcementbecauseitstheologicalpropertiesdidnotmeettheperformancecriteria.Thebinderwasalsonotcompatiblewithasphaltcement.Apyrolysis-hydrogenationprocess,however,producedanoilwhichwassuitableforuseasanextenderofasphaltcementinpavingoperations.BasedonlimitedexperimentsthedurabilityofaHMAwearingcoursemixturecontaininghydrogenatedpyrolysisoilwasnotfoundsignificantlydifferentfromthatcontainingthereferenceasphaltcementonly.Extensivelaboratoryandfieldexperimentsareneededtoevaluatethecompatibilityandperformanceofthecellulosicoilswhenusedwithasphaltcementsfromvarioussources.
WOODLIGNINS
Woodligninisahighvolumewasteproducedinthemanufactureofpaperproducts.AttemptshavebeenmadetoconvertwoodlignintoahighwaybindermaterialanduseiteitheraloneasasubstituteforasphaltcementorasanextenderforasphaltcementinHMAmixes( 4).Suitablelignin-asphaltbinderformulationswerepreparedfromligninsfrombothmajormanufacturingprocesses,thatis,kraftandsulfiteprocesses.Noneofthebinderformulationswassuitablealoneasasubstitutepavingbinder.However,a30percentreplacementofasphaltcementappearedfeasiblewithnosignificanteffectonphysicalproperties.Kraftligninappearedinsolubleinasphaltcement.Lignin-asphaltcementbindersarestifferandrequireslightlyhigherbindercontentthanconventionalHMAmixtures.PrecoatingofthecoarseaggregatehasbeenrecommendedtoreducethebindercontentinHMAmixtures.Basedonlimitedlaboratory
experiments,Terreletal.(4)concludedthatHMAmixturescontaininglignin-asphaltbinderscanbedesignedtomatchthestructuralstrengthofHMAmixturescontainingconventionalmaterials.
BOTTOMASH
BottomAshisawastematerialfromcoalburningpowerplants.Itistheslagwhichbuildsupontheheat-absorbingsurfacesofthefurnace,andwhichsubsequentlyfallstothebottomofthefurnaceandcollectedinanashhopper.Thebottomashiscategorizedasdrybottomashorwetbottomashdependingupontheboilertypeused.Theashwhichisinsolidstateatthefurnacebottomiscalleddrybottomash.Theashwhichisinmoltenstatewhenitfallsinwateriscalledeitherwetbottomashormorecommonlyboilerslag(5).
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ExtensivelaboratorystudieswereperformedonrepresentativesamplesofIndianabottom
ashes,todeterminetheirphysical,chemicalandmechanicalproperties( 6).Dependinguponthesource,bottomashescanhavedifferentphysicalandchemicalproperties.Thesestudiesshowedthatthebottomashesarenon-hazardousinnatureandhaveminimaleffectsongroundwater
quality.However,thebottomashescanbehighlycorrosiveand,therefore,shouldnotbeplacedveryneartoanymetalstructure.
Laboratorystudies(7,8)havealsobeenconductedtoevaluatethefeasibilityofusingbottom
ashesasapartialorfullreplacementofnaturalaggregatesinHMAmixesanddevelopguidelinesfortheiruse.ThepropertiesofHMAcontainingbottomasharedependentonashcontent.Generally,astheashcontentisincreased,theoptimumasphaltcontentisincreased,themixturedensityisdecreased,andtheairvoidsandvoidsinthemineralaggregateareincreased.ThestabilityofHMAmixdecreasesuptoanashcontentof30percentandthenlevelsoff.Themixcontainingbottomashissusceptibletorutting.However,themixishighlyresistanttomoistureinduceddamage(stripping).Itwasconcludedfromthesestudiesthatthepropertiesofmostdryandwetbottomashescanmeetthespecificationsforconventionalaggregates.
Wetbottomash(orboilerslag)canimprovetheskidresistanceofHMAwearingcourses.WestVirginiaobtainedsatisfactoryperformancefromHMAwearingcoursemixescontaining50percentboilerslag(9).LigniteboilerslaghasalsobeenusedtoresurfaceresidentialstreetsinTexas.TheHMAmixcontained75percentboilerslag,25percentlimestonescreenings,and6to7percentasphaltcontent(10).
FLYASH
Flyashisthefineparticulatematterwhichisprecipitatedfromthestacksofpulverizedcoal-firedboilersatelectricalpowergeneratingplants.Itrepresentsnearly75percentofallashwastesgeneratedintheUnitedStates.Thequalityofflyashisaffectedbythetypeofcoalused,theashcontentofthecoal,thedegreetowhichthecdhasbeenpulverizedpriortocombustion,
andtypeofashcollectorsutilized.Thesevariables,therefore,impartawiderangeofphysicalandchemicalpropertiestotheflyash.Itiscomposedoffinelydividedpiecesofsiliceousglasswhichrangefrom1to50micronsindiameter(11).
FlyashhasprimarilybeenusedasamineralfillerinHMAmixturesinseveralstatessuchas
Illinois,Michigan,Montana,NorthDakota,andWestVirginia( 11).Becausetheflyashparticlesareextremelyfine,itisquitelikelytoactasanextenderofasphaltcementinHMAmixtures.Therefore,cautionshouldbeexercisedwhenusingflyashinrut-resistantHMAmixturesforheavydutypavements.
INCINERATORRESIDUE
AccordingtotheestimatesoftheU.S.EnvironmentalProtectionAgency(EPA),about23
millionmegagrams(Mg)ofmunicipalsolidwaste(MSW)wasincineratedintheUnitedStatesin1988(12).Itisprojectedthatabout41millionMgand50millionMgofMSWwillbeincineratedintheyears1995and2000,respectively.Twoashproducts:bottomashandflyash,resultfromincineratingMSW.Bottomashistheunburnedandincombustibleresidueleftontheboilergratesafterincineration,andconsistsoflargeparticles(0.1-100mm)ofslag,glass,rocks,metal,andothermaterials.Flyashconsistsofburnedorpartiallyburnedorganicmaterialparticleswhichareusually1-500microninsize( 5).Theheterogeneouscharacterofthesetwoincineratorresiduescausesawidevariationincompositionfromoneplanttoanother.
TheincineratorresidueisnotsuitableforuseinHMAmixturesunlessitisprocessedfurther.
TheFranklinInstituteResearchLaboratoryinPhiladelphiadevelopedaprocessofdensifying(fusing)theunfusedincineratorresidueandconvertingitintousefulconstructionmaterials.The
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incineratorresidueisgroundwithahammermill,preheatedat688C,andthenmeltedandfused
at1093Ctoformacolumnofsemi-moltenproductwhichislatercooledandcrushedtoaggregatesizes(11).PreliminarylaboratoryevaluationbythePennsylvaniaDepartmentofTransportation(13)indicatedthatthefusedincineratorresiduewasnotsuitableforHMA
mixturesbecauseitdidnotcomplywiththespecifiedgradation,particleswereflatandelongated,andtheHMAmixturedidnotmeetdesigncriteriaforstability,airvoids,orvoidsinthemineralaggregate.Subsequentimprovementsintheprocess( 14,15)producedahighqualityaggregatematerialwhichperformedwellinaHMAsurfacecourseinstallationinHarrisburg,Pennsylvania(16).
SeveralinstallationsusingunfusedincineratorresidueasanaggregateinHMAmixtureswere
madeduringthe1974-1979periodinTexas,Pennsylvania,Washington,DC,andMassachusetts(2).Detailsoftheseinstallationsaregivenintheliterature( 17,18,19,20,21,22).Thefollowingrecommendationshavebeenmade:(a)residuesshouldbewell-burnedout(lossonignitionshouldbelessthan10percent),(b)HMAmixturesforbasecoursescontaining50percentnaturalaggregateand50percentincineratorresidueholdthemostpromise,and(c)2percent
limeshouldbeaddedtominimizestrippingproblems.TIRESCRAPRUBBER
About285milliontiresarediscardedeveryyearintheUnitedStates.Ofthese,about55millionareretreadedorreused(resold),andabout42millionaredivertedtovariousalternativeusessuchascombustionforgeneratingpowerandadditivetoHMAmixes.Theremaining188milliontiresareaddedtostockpiles,landfillsorillegaldumps.AccordingtoEPAestimates,2to3billiondiscardedtiresareavailableatthepresenttime( 23).Severalstateshaveenactedlegislationtoregulatethescraptireproblem.Atthenationallevel,Section1038oftheIntermodalSurfaceTransportationEfficiencyAct(ISTEA)of1991specificallyaddressesthestudyanduseofthescraprubberbythehighwayindustry.
Crumbrubberobtainedfromtirescaneitherbeambientground(grindingatroomtemperatureorabove)orcryogenicallyground(grindingbelowembrittlementtemperature,liquidnitrogenisoftenused).Ambientgroundcrumbrubberhasasponge-likesurface.Duetoveryhighsurfaceareathisrubberreactswithasphaltcementreasonablyfast.Cryogenicallygroundrubberusuallyhasundesirableparticlemorphology(structure).Thisprocessproducescleanflatsurfaceswhich,inturn,reducesthereactionratewithhotasphaltcement.Cryogenicallygroundrubberalsogiveslowerelasticrecoverycomparedtotheambientgroundrubber( 24).
Theuseofcrumbrubbertomodifyasphaltcementhasbeendevelopedoverthepast25years.
CrumbrubberisprimarilyusedinHMAmixesbytwoprocesses:
WetProcess(Asphalt-Rubber)
Thewetprocessblendsthecrumbrubberwiththeasphaltcementpriortoincorporatingthebinderintotheproject.Themodifiedbinderiscommonlycalled"asphalt-rubber."Generally,18-26percentcrumbrubber(16mesh)byweightofasphaltcementisreactedwithasphaltcementat190to218/Cfor1to2hours.Theblendisformulatedatelevatedtemperaturestopromotepotentialchemicalandphysicalbondingofthetwoconstituents.Thefirsttechnologywhichappliedthe"wetprocess"iscalledthe"McDonaldProcess."Thisprocessisalsousedforconstructingstressabsorbingmembrane(SAM)andstressabsorbingmembraneinterlayer(SAMI),andmanufacturingcracksealers.SAMisasealcoatwhichusesasphalt-rubberasabinder.WhenSAMisplacedasaninterlayeritiscalledSAMI.
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DryProcess(Rubber-ModifiedMix)
Thisprocessmixesthecrumbrubberwithaggregatebeforeincorporatingtheasphaltcement.About3-5percentofcoarserubberparticles(1.6-6.4mm)byweightofaggregatearegenerally
used.Thenaturalaggregateisusuallygap-gradedtoaccommodatetherubberparticlesasaggregate.Theamountofcrumbrubberusedinthedryprocesscanbe2-4timesthatusedinthewetprocess.Thefirstapplicationofthe"dryprocess"intheUnitedStatesiscalledthe"PlusRideProcess."Ithasbeenclaimedthaticedebondseasilyfromthepavementsurfaceconsistingofrubber-modifiedmixturebecauseofhigherthanusualresiliencyofthemix.
DetailsofWetProcess
Thewetprocess(asphalt-rubber)willnowbediscussedinmoredetail.Toproduceanacceptableasphalt-rubberbinderitisnecessarytoestablishthedigestiontemperatureandtimeforaspecificcombinationofasphaltcementandcrumbrubber.Viscosityoftheblendischeckedatdifferenttimeintervalsduringtheblendinganddigestionprocess.Viscosityoftheblendincreaseswith
digestiontimeandthenlevelsoff.Achievingareasonablyconstantviscosityindicatesthattheinitialreactionisnearlycompleteandthebinderisreadytouse.Thisinitialreactionisnotwellunderstood,butappearstobeduetoachemicalandphysicalexchangebetweentheasphaltcementandrubberparticlesinwhichtherubberswellsinvolumecausinganincreaseinviscosity.Continuedmixingofasphaltcementandrubberafterinitialreactioncanbegintoreducetheviscosityoftheblendastherubberparticlesbreakdownduringmixing.However,breakdownofrubberparticlesisnotrapid,andmayrequireseveralhoursathightemperatures.
Iftheasphalt-rubberblendistooviscousanextenderoilisusuallyadded.However,iflessthan
10percentcrumbrubber(byweightofasphaltcement)isused,extenderoilisnotneeded.
Theasphalt-rubberblendshouldbeusedassoonaspossibleaftertheinitialreaction.Untilused
itshouldberecirculatedcontinuously.Blendshavebeenallowedtocoolinstoragetanksand
reheatedpriortousewithoutdifficulty.Whenasphalt-rubberbinderisusedinHMAmixesthefollowingmixingtemperatureranges
havebeenused:
Dense-gradedHMAmixes:163-190C
Open-gradedHMAmixes:135-163C
Stackemissionscanbehigherbecauseoftheelevatedmixingtemperatures.
HMAmixescontainingasphalt-rubberbindershouldnotbesubjectedtoprolongedstorageina
silo.Processingthemixthroughasurgesiloisacceptable.Dieselfuelshouldnotbeusedontruckbedsasareleaseagentbecauseitmakesthemixstickmoretothebed.Useoflimewater,
soapsolutionorsiliconeemulsionisrecommendedinstead.
Itisnecessarytomaintainthemixtemperaturewithinthedesiredrangeduringplacementofthe
mix.Staticsteelwheelrollersaregenerallyusedforcompaction.Rubber-tiredrollerstendtopick-upthemat.Vibratoryrollerstendtotearandshovethemat.Iftheroadwayhastobeopenedtothetrafficrightaftercompactionandthetraffichasatendencytopick-upthemat,itisrecommendedtoapply0.5-1kgofconcretesandpersquaremeterofthematsurface.
RecyclabilityoftheHMAcontainingcrumbrubberisaveryimportantissuewhichmustbe
addressedbeforeusingscraptirecrumbrubberonalargescale.Thereisnoexperienceatthepresenttimeinrecyclingsuchmixes.Itisquitelikelythatrecyclingthesemixesinthefuturecouldposeairpollutionproblemsduringproduction.Whatwillhappentotheasphalt-rubber
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binderduringrecycling?Howwilltherecycledmixbedesigned?Iftheseconcernsarenot
addresseditmaynotbepossibletorecycletheHMAmixescontainingcrumbrubber.InthatcasewewillhavetodisposetheRAP(reclaimedasphaltpavement)whichwillbemuchworsethantheproblemofdisposingtires.
In1988,underalegislativemandate,theFloridaDepartmentofTransportation(FDOT)beganaconcertedefforttoevaluatethepotentialuseoftirescraprubberinHMApavements.FDOTcommissionedtheNationalCenterforAsphaltTechnology(NCAT)toprepareastate-of-the-artreportontheuseofgroundtirerubber(GTR)inHMAandrecommendastrategytouseGTRinFloridaHMAmixes.Thisreport(24)waspublishedinAugust1989.Sincethen,FDOThasconstructedthreedemonstrationprojects.AtthepresenttimeFDOTisimplementingthefollowingrecommendations:
UseGTRinfrictioncoursesonly.TypicallyFDOTresortsto75mmcoldmilling(25mmfrictioncourse+50mmstructuralcourse)whichisrecycled.IfthefrictioncoursecontainsGTR,only1/3oftheRAPwillcontainGTR.
Dense-gradedfrictioncourses:Use3-5percentGTRbyweightofasphaltcement.In
thepast,18-20percentGTRhasbeenusedasmentionedearlier.UseNo.80meshGTR.Inthepast,16meshGTRhasgenerallybeenused.FinerGTRisappropriatefortheFloridafrictioncoursemixeswhichapproachsand-asphaltmixtures.Moreover,thefinerGTRsizereactswithasphaltcementmoreeasily,andthusacontinuousblendingofasphaltcementandGTRbecomespossible.
Open-gradedfrictioncourses:Use5-10percentGTRbyweightofasphaltcement.UseNo.40meshGTR.
FDOTdoesaconsiderableamountofhotrecyclingeveryyear.SincetheRAPwillcontainlow
amountsofGTR,noproblemsareanticipatedinrecyclingtheHMApavementsinthefuture.Itisestimatedthatstate-wideuseofGTRwillincreasethepriceofHMAmixesinFloridaby15percent.
NewConceptsThe"McDonaldprocess"(wetprocess)and"PlusRide"(dryprocess)asdiscussedearlierarepatentedprocesses.However,theoriginalpatentonthe"McDonaldprocess"hasexpiredin1992.Recentlysomenewerconceptswhichincludethegenericdryprocess,chunkrubberasphaltconcreteandcontinuouslyblendedasphaltrubber,havebeenintroduced( 23).
Thegenericdryprocessallowsincorporationofcrumbrubberinavailable"generic"aggregate
gradationratherthangap-gradedaggregaterequiredinthePlusRidesystem.ExperimentalfieldapplicationsofgenericdryprocesshavebeenmadeinNewYork,Florida,Iowa,Kansas,Oregon,andIllinois(23).
ChunkrubberasphaltconcreteconcepthasbeendevelopedbytheColdRegionsResearchand
EngineeringLaboratory(CRREL)oftheU.S.ArmyCorpsofEngineers.Thisconceptusesalargemaximumcrumbrubbersize(4.75-12.5mm),andanaggregategradationtoprovidespacefortherubberaggregate.However,thisconcepthasbeenconfinedtolaboratoryonlyandnofieldinstallationshavebeenmade.
ThecontinuousblendingasphaltrubberconcepthasbeenusedbyFloridaDOT(FDOT)as
describedearlier.Itisawetprocesswhichusesfinecrumbrubber(180:orNo.80mesh)toavoidreactiontanks,andtofacilitatecontinuousblending.TheperformanceofFDOTexperimentalsectionsisbeingmonitored.
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WASTEGLASS
WhencrushedwasteglassisincorporatedinHMAmixtheresultingmixtureissometimesreferredtoas"glasphalt."
Severallaboratoryandfieldevaluationsofglasphaltwereconductedintheearly1970sintheU.S.andCanada.Afternosignificantinterestforadecade,thepotentialuseofglasphaltisnowbeingreassessed.ThreestatesConnecticut,VirginiaandFloridahaverecentlyconductedfeasibilitystudiesandissuedresearchreports( 25,26,27).
TheConnecticutreportgivesanexcellentreviewofliteratureonbothlaboratoryandfield
evaluationsofglasphaltsince1969:Glasphaltwassuccessfullymixedandplacedinatleast45locationsintheU.S.and
Canadabetween1969and1988.However,mostglasphalthasbeenplacedoncitystreets,drivewaysandparkinglots,andnotonhigh-volume,high-speedhighways.
Potentialproblemswithglasphaltinclude:lossofadhesionbetweenasphaltand
glass;maintenanceofanadequatelevelofskidresistance,especiallywithcoarseparticles;breakageofglassandsubsequentraveningunderstuddedtires;lackofadequateandconsistentsupplyofglass;andincreasedproductioncosts(estimatedat$5/MgmorethantheconventionalHMAmixinConnecticut).
Glasphaltshouldbeusedonlyasabasecoursetoalleviatepotentialskidresistanceandsurfaceraveningproblems.
Nomorethanamaximumof9.5mmglassshouldbeusedinglasphak,withhydratedlimeaddedtopreventstripping.
TheVirginiareportisbasedonlaboratoryevaluationandeconomicanalysisofglasphalt.Two
glasscontents,5percentand15percent,andtwoasphaltcontentswereusedinthelaboratoryevaluationofVirginiaS-5surfaceHMAmix(12.5mmtopsize).Adenselygradedrecycledglasswithamaximumnominalsizeof9.5mmwasused.
Thefollowingaresignificantconclusions:
TheuseofglasstendstoreducetheVMAandairvoidsinMarshallspecimens;therefore,optimumasphaltcontentwillalsobereduced.
Neitherresilientmodulusnorindirecttensilestrengthsareadverselyaffectedbytheadditionofupto15percentglass.
Althoughbothwetstrengthandretainedtensilestrengthlaties(TSR)wereunaffectedbythepercentageofglass,someseparationattheasphalt/glassinterfacewasobserved.
Amaximumof15percentcrushedrecycledglassshouldbeallowed(100percentpassinga9.5mmsieveandamaximumof6percentpassinga75pmsieve)inHMAmixes.
Thereislittlemonetaryincentivetouserecycledglassatthepresenttimebecausethecostofglassvariesconsiderably.Anexperimentalsectionshouldbelaidpriortoextensiveuseofwasteglass.
TheFloridaDepartmentofTransportationtestedthreeHMAmixturestodeterminetheeffectsof
crushedglass:1.Controlmix(9.5mmnominalmaximumsize)2.15percentcoarseglassmixturesameasthecontrolmixtureexceptthat15percent
ofthescreeningswerereplacedwithcoarse(9.5mm-2.06mmnominalsize)crushedglass.
3.15percentfineglassmixturesameasthecontrolmixtureexceptthat15percentofthescreeningswerereplacedwithfine(2.06mm-75:mnominalsize)crushedglass.
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AC-30asphaltcementwithandwithoutantistrippingagentwasusedtoprepareMarshall
specimenswhichwerealsotestedfortensilestrength.Thefollowingconclusionsweredrawnbasedonthelimitedlaboratoryevaluation:
Marshallstabilityvaluesdecreasedby15-20percentanddryindirecttensilestrengthdecreasedby20percentwhen15percentofthescreeningswerereplacedwitheithercoarseorfineglass.
MoistureconditioningofMarshallspecimenscauseda15percentand50percentdecreaseintensilestrengthwhencoarseandfineglass,respectively,wereincorporatedintothemixture.Retainedtensilestrengthratio(TSR)valuesindicatedthattheantistrippingadditivewasineffectiveinreducingthemoisturedamage.
ItisunlikelythattheuseofcrushedglassinHMAmixtureswillbeeconomicallyfeasiblewhensuitablelocalmaterialsareavailableatorneartheHMAfacility.
TheCityofNewYorkplaces360,000MgofHMAcontaining10percentwasteglassevery
year.Thisamountstoanannualconsumptionof36,000Mgofglass.
ROOFINGSHINGLES
Approximately90millionroofingshinglesquaresareproducedperyearby77plantsintheUnitedStates.About1/3oftheshinglesareusedonnewhousesandtheremaining2/3oftheshinglesareusedforreroofinghouses.Whenahouseisreroofed,oftenanequivalentamountofoldshinglesisremovedanddiscarded.Moreover,eachroofingplantgeneratesscrapmaterialsandsecondsthatcanrangefrom5to10percentoftheproductioncapacity.Thedisposalofoldshinglesandthescrapmaterialhascreatedadifficultdisposalproblem( 28).
Itisestimatedthatroofingwastecontainsabout36percentasphaltcontent,22percenthardrock
granules(minusNo.10),8percentfillerandsmalleramountsofmiscellaneousmaterials( 29).
RoofingshingleshavebeenusedsuccessfullyintheHMApavingoftheparkinglotsatDisney
World,Florida.Shinglesneedtobeshreddedtoatleast12.5mmorsmallerpriortointroductioninthemixtoensuremeltdownanduniformdispersionintheHMAmixture.Accordingtocostestimates(28),theHMAcostcanbereducedby$3.08permegagram(Mg)byintroducingonly5percentorganicshingles.
COALMINEREFUSE
AnthraciteCoalRefuse
TheprincipalsourceofanthracitecoalisinnortheasternPennsylvania.Anthracitecoalminerefuseiscomposedofunprocessedminerefuseandprocessedcoalbreakerrefuse.Itisagrayslate-likematerialcontainingoxidesofsilicon,aluminum,iron,calcium,magnesium,sodiumandpotassium.Morethan50percentofaverageanthraciterefuseisgreaterthan25.4mminsize
(11).
ThePennsylvaniaDepartmentofTransportation(PennDOT)conductedalaboratoryevaluation
ofunburnt(raw)anthraciterefuseasanaggregateinHMAmixtures( 30).Theaggregateparticleswereveryabsorptive,particularlyinsizeslargerthanthe4.75mmsieve,andcouldnotbecoatedcompletelywithasphaltcement.Freeze-thawfailuresalsooccurredinlessthanhalfthecyclesofacontrolmix.AnthraciterefusemustbeincineratedbeforeuseintheHMAmix.FourHMApavingprojectsinLuzemeCountyofPennsylvaniawereconstructedwithcrushedincineratedanthraciterefuse(31).Theperformanceintermsofskidresistanceandwearwasreportedtobesatisfactory.
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BituminousCoalRefuse
TheprincipalsourcesofbituminouscoalarelocatedinKentucky,WestVirginiaandPennsylvania.Bituminouscoalrefuseissomewhatsimilartoanthraciterefuseinappearance.
TheUniversityofKentuckyevaluatedtheuseofbituminouscoalrefuseinHMAmixturesin1964(32).Themixturescontaining100percentcoalrefuseweresusceptibletomoistureinduceddamage.
SUMMARY
Recyclingofwastematerialsinhighwayconstructionshouldbeencouraged.However,itisnecessarytoaddresstheengineeringconcerns,environmentalconcernsandeconomicconcernsmentionedinthispaperbeforeanylargescaleuseofthesematerials.Theuseofwastematerialsshouldnotbemandated.HMAcontainingawastematerialshouldperformaswellorbetterthanconventionalHMA.Itshouldalsobeenvironmentallysafebothforthefirstconstructionandfuturerecyclability.
Thispaperdiscusseslaboratoryand/orfieldevaluationofseveralwastematerialswhichhavebeenusedinHMA.ItisrecommendedtoconstructdemonstrationprojectstoevaluatetheperformanceofHMAcontainingwastematerials.Wastematerialsshouldnotbeusedasastandardpracticeuntiltheperformancedataisavailablefromthedemonstrationprojects.REFERENCES1.
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Snyder,R.R.,"EvaluationofFusedIncineratorResidueasaPavingMaterial."FHWAReportNo.FHWA-TS-88-229,1980.Haynes,J.,andLedbetter,W.B.,"IncineratorResidueinBituminousBaseConstruction."ReportFHWA/RD-76/12,FederalHighwayAdministration,1976.Teague,D.J.,andLedbetter,W.B.,"ThreeYearResultsonthePerformanceofIncineratorResidueinBituminousBase."ReportFHWA/RD-78/144,FederalHighwayAdministration,1978.Erdeley,J.,andLedbetter,W.B.,"FieldPerformanceofLittercrete(IncineratorResidue)inBituminousBase."ReportFHWA/RD-88/022,FederalHighwayAdministration,September,1981.Pavlovich,R.D.,Lentz,H.J.,Ormsby,W.C.,"IncineratorResidueasAggregateforHot-MixAsphaltBaseCourse."TransportationResearchRecord734,1979,pp.38-44.Griffith,
J.L.,"SummaryUpdateofResearchProjectswithIncineratorBottomAshResidue."CommonwealthofMassachusetts,ExecutiveOfficeofEnvironmentalAffairs,DepartmentofEnvironmentalManagement,BureauofSolidWasteDisposal,Boston,MA,1982.Turo,M.D.,andLeonido,A.M.,"IncineratorResidueasaComponentofBituminousPavements."MassachusettsDepartmentofPublicWorksReportR-37-O,1984.Heitzman,M.S.,"StateofthePractice-DesignandConstructionofAsphaltPavingMaterialswithCrumbRubberModifier."ReportFHWA-SA-92-022,FederalHighwayAdministration,May1992.Roberts,F.L.,P.S.Kandhal,E.R.BrownandR.L.Dunning,"InvestigationandEvaluationofGroundTireRubberinHotMixAsphalt."NCATReport89-3,August1989.Larsen,D.A.,"FeasibilityofUtilizingWasteGlassinPavements."Connecticut
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Mellott,D.B.,"AnthraciteRefuseinID-2ABituminousConcrete."PennsylvaniaDepartmentofTransportation,ResearchProjectNo.70-8,July23,1970.Howard,G.G.,"ALaboratoryInvestigationofthePropertiesofCoal-BitumenPavingMixtures."UniversityofKentucky.EngineeringResearchBulletinNo.71.March1964.
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