Asphalt Mix Design - FPA

Asphalt Materials, Mix Designs, and PlantsAsphalt Mix DesignDanny Gierhart, P.E.Asphalt Institute Regional EngineerTuttle, Oklahoma

HMA Mix DesignSmooth Quite ResistanceRavelingShovingFlushing

HMA Mix CharacteristicsLet’s start with the basics – A layer ofHMA pavement has 3 components:Typical %By Mass:94 - 961) Aggregate4-62) Asphalt Binder03) AirTypical %By Volume:81 - 8511 - 124-7

VolumetricsWe evaluate the quality of theHMA by setting parameters onthese three components, whichhave historically provided agood indication of a mixture’sprobable performance.

VolumetricsThe way that these three components fittogether is largely based on the way that theaggregate particles fit together in the mix Gradation- Particle size distribution Shape- Relative cubicity, angularity Strength- Resistance to destructive forces Surface Texture- Smooth or rough Type & Amount of Compactive Effort- Static pressure, impact, shearing

Basic Design ProcedureNo matter whether it’sSuperpave, Marshall, Hveem,Texas Gyratory, or somethingelse, the mix design processhas some common proceduresand goals.

Basic Design Procedure Choose binder type and test binder- Usually specified in contractBinder Specific Gravity

Basic Design Procedure Choose aggregate types, sources, and test- Sometimes specify polish-resistant aggregates insurface- Locate aggregate sources that can be combined tomeet specifications- Determine if RAP and/or RAS can be used

Basic Design Procedure Determine trial combination(s) and batch dry aggregates- Each aggregate or batch is sieved and carefully combined

Basic Design Procedure Heat aggregates and binder, then mix- Until all aggregate is sufficiently coated

Mixing / Compaction TemperaturesViscosity, Pa s1051.5.3.2.1Compaction RangeMixing Range100110120130140150 160Temperature, C* Works well with neat binders only170 180 190 200

Mixing / Compaction TemperaturesTex-205-FLaboratory Method of Mixing Bituminous MixTable 1 - Asphaltic Material Mixing Temperatures by Grade and TypeType-GradePG 70-28, PG 76-22PG 64-28, PG 70-22PG 64-22, PG 64-16AC-3, 5, 10; PG 58-28, PG 58-22Mixing Temp. F ( C)325 (163)300 (149)290 (143)275 (135)1. If using RAP or RAS and a substitute binder in lieu of originally-specifiedbinder, select mixing temperature for originally specified binder grade2. When using RAP or RAS, mixing temperature my be increased to 325 Fto achieve adequate coating

Temperature IssuesMaltenes: Oilyand resinous inappearance – thefirst to beabsorbed & thefirst to evaporateat high tempsAsphaltenes:Hard, brittle,insoluble,unaffected byoxidationNever heat over 350oF!

Basic Design Procedure Oven-age the mixture to account forabsorption, binder stiffening (TxDOT275oF, 2 hrs for HMA, 4 hrs for WMA)Think about how this relates to field production andplacement!

Binder ContentThe absorptiveness of the aggregateaffects the final binder contentSurface VoidsSolid Agg.ParticleAbsorption takes place at hightemperature and usually reachesa maximum in 1 - 6 hours,depending on the absorptivenessof the aggregate (Most aggregatesin 2 hours max)Vol. of water-perm. voidsnot filled with asphaltAbsorbed asphalt

Binder Content

Basic Design Procedure Determine the theoretical maximumspecific gravity, Gr (Gmm)

Basic Design Procedure Mold specimens to determine the bulkspecific gravity, Ga (Gmb)

VolumetricsVarious volumetric properties are thencalculated, such as: Percent binder Percent air voids Percent Density Voids in the Mineral Aggregate (VMA) Voids Filled with Asphalt (VFA)

Mechanical Tests - Moisture Sensitivity Tex-530-C Boil Test Prepare 1000 g of mix, cool at room temp for 24 2 hrs Bring oil bath to between 325 and 350oF Obtain a 200 g representative sample of the mix Fill 2000 mL beaker about halfway with distilled waterand heat to boiling Add mix sample to beaker, maintain boil for 10 m 30 s Remove beaker and skim asphalt from water surface Decant water and empty wet mix onto white towel Visually estimate degree of stripping Repeat after mix has dried for 24 2 hrs Report as estimated % stripping after drying period

Mechanical Tests - Rut Testing

Mechanical Tests - Indirect Tensile Strength Tex-226-F Indirect Tensile Strength Calibrate loading press to a deformation rate of 2”/min Ensure loading strips remain parallel during testing Determine the height and diameter of the test specimen Place test specimen in the constant temperatureapparatus long enough to ensure a consistenttemperature of 77oF 2oF throughout test specimen Carefully place specimen on lower loading strip Slowly lower top loading strip into light contact Apply load at 2” per minute and determine the totalvertical load at specimen failureCalculate ST using load, specimen height & diameter

VolumetricsLet’s take a closer look at: Binder Content Lab-Molded Density / Air Voids Voids in the Mineral Aggregate (VMA)

Binder ContentThe goal of establishing thecorrect binder content is to: Provide a sufficient film coatingaround the aggregates to bind andwaterproof Provide enough coating to make theHMA durable Not so much as to make the HMAsusceptible to rutting

Binder ContentThe correct amount of binderincreases as the nominal maximumaggregate size decreases- the finer itis, the more surface area for a givenvolumeMix TypeAverage BinderContentABCDF4.24.54.75.05.5

Lab-Molded Density / Air VoidsWe use lab-molded properties toestimate the aggregate structureand binder content needed towithstand the anticipated trafficat the designed pavementthickness.

Lab-Molded Density / Air Voids% Air Voids 100 - % Density% Density 100 - % Air Voids

Lab-Molded / Roadway Air VoidsWhy are the target values for lab-molded air voids androadway air voids different? Lab-molded air voidssimulate the in-place density of HMA after it hasendured several years of traffic in the roadway.Lab-MoldedDensityIn-placeDensityLabAir Voids 20-25% Before Rolling6 - 7 % After RollingFutureTrafficAir Voids3.5%

Lab-Molded / Roadway Air VoidsDon’t confuse roadway densitywith lab-molded density: Lab-molded density tells us aboutthe mix properties Roadway density tells us about thequality of compactive effort on theroadway

VMA (AASHTO definition)VaVMAAirVbeACAbsorbed ACVmbAggregateVMA is the volumetric voidspace created by theaggregate particles in anasphalt mixture. It is filled withthe volume of air voids plus thevolume of the binder notabsorbed into the aggregate.The mix needs aminimum VMA to haveenough volume to holdthe proper amount of airvoids and the properamount of binder.

VMA (TxDOT definition)VaVMAVbAirACAbsorbed ACVmbAggregateTxDOT includes the volume ofthe absorbed binder in thecalculation of VMA.Therefore, moreabsorptive aggregatesinherently producehigher VMA and viceversa.*TxDOT VMA minimumsare 1% higher thanAASHTO, which mitigatesthe issue to some degree.

VMAIf the VMA drops below thespecified minimums, the asphaltfilm thickness gets thinner and thepavement becomes less durableDesign VMA Requirements (341 Table 8)Mix TypeMinimum VMAABCDF12.013.014.015.016.0

VMAQuestion:Why can’t you add the percent binder(As) to the percent air voids (100-%Density) to get the VMA?Answer:Because As is a percentage by massand percent air voids is a percentageby volume.

Mix CompositionTxDOT’s automated Mix Design Report includesthe following worksheets: Combined Gradation Material Properties Aggregate Classification Blending Weigh-Up Sheet Aggregate Bulk Gravity Summary Power 0.45 Curve Asphalt Content versus Density, VMA, Ga, and Gr

Mix CompositionThe rule of thumb would be to never allow a differentmaterial or different source to be used than what is onthe mix design. You need to know if your local agencyallows: Switching binder grade on same design (maybe) Switching binder source within same grade (maybe) Aggregate Types (never) Aggregate Sources (never) Changing % of each aggregate used ( small tolerance) Individual and combined aggregate gradations (maybe) Changing design binder content (maybe)

HMA Mix Selection vs. Lift Thickness Optimum lift thickness 4x NominalMaximum Aggregate Size (NMAS) Acceptable lift thickness 3x to 5xNMAS Problems Compacting - 2x NMAS or less6x NMAS or more 2x guidelines especially critical if surfaceto be laid upon is uneven.

QUESTIONS?Good Reference Materials on the Topic:MS-2: Mix Design MethodsSP-2: Superpave Mix DesignMS-4: The Asphalt HandbookMS-22: HMA Constructionhttp://www.asphaltinstitute.org

Mechanical Tests - Moisture Sensitivity Tex-530-C Boil Test Prepare 1000 g of mix, cool at room temp for 24 2 hrs Bring oil bath to between 325 and 350oF Obtain a 200 g representative sample of the mix Fill 2000 mL beaker about halfway with distilled water and heat to boiling Add mix sample to beaker, maintain boil for 10 m 30 s