HMA Design: Component Selection TRH3/HMA Workshop June 2001

Mix components affecting performance AggregateAggregate –Type and properties of aggregate –Aggregate grading FillerFiller –Physiochemical properties of filler –Amount of filler Bituminous bindersBituminous binders –Physiochemical properties of binder –Amount of binder

Aggregate Constitute 90-95% by mass of asphaltConstitute 90-95% by mass of asphalt Provide a substantial proportion of load carrying capacityProvide a substantial proportion of load carrying capacity Their selection is critical to mix performanceTheir selection is critical to mix performance Requires careful evaluation of the quality and costs of available materials related to the service conditions to which the mix is to be subjectedRequires careful evaluation of the quality and costs of available materials related to the service conditions to which the mix is to be subjected

Physical properties Affected by:Affected by: –Mineralogy of the parent rock –Processes required to produce graded & blended aggregate Physical properties regarded as most important aspect of selectionPhysical properties regarded as most important aspect of selection

Physical properties Hardness & toughnessHardness & toughness Durability & soundnessDurability & soundness Particle shape & surface textureParticle shape & surface texture CleanlinessCleanliness VariabilityVariability

Hardness & Toughness Important for rut resistance and low- speed skid resistanceImportant for rut resistance and low- speed skid resistance Tests:Tests: –10% FACT: >160kN (general) ; >210kN (stone skeleton structure - SKS) –ACV: <25% (general) ; <21% (SKS) –Los Angeles Abrasion: 10% for very hard aggregate (typical)10% for very hard aggregate (typical) 60% for very soft aggregate (typical)60% for very soft aggregate (typical)

Durability & Soundness Ability to resist breakdown & disintegration in operational environmentAbility to resist breakdown & disintegration in operational environment Test:Test: –Sulphate soundness test: 12-20% usually acceptable –Ethylene glycol soundness test

Particle Shape & Texture Greatly affects strength, stability, workabilityGreatly affects strength, stability, workability Rough textured and angular, near cubical shape preferred for stability, stronger mechanical bond with binder & low-speed skid resistanceRough textured and angular, near cubical shape preferred for stability, stronger mechanical bond with binder & low-speed skid resistance Tests:Tests: –Flakiness index: criteria dependent on mix type & maximum stone size –Particle index test: natural < 14 < crushed –PSV: >50 (general) ; >45 (gap-graded) –Fractured faces: >95% (surfacing) ; >50% (base)

Cleanliness Presence of foreign and deleterious material:Presence of foreign and deleterious material: –Weak or weathered particles –Friable particles –Clay lumps –Organic matter resulting in ravelling, stripping, pop-ups Tests:Tests: –Sand equivalent test: >50 (tot. fines) ; >30 (natural) –Clay lumps & friable particles: <1%

Variability Changes in the quality of the rock soundness causes crushing processes to produce material with different fractions and particle shapesChanges in the quality of the rock soundness causes crushing processes to produce material with different fractions and particle shapes Some mixes (SMA) require narrow gradation envelopes:Some mixes (SMA) require narrow gradation envelopes: –Provision of sufficient bins for the different aggregate fractions?

Binder Adhesion Test for assessing susceptibility to stripping:Test for assessing susceptibility to stripping: –Modified Lottman test (AASHTO 283) Ageing of loose mix 60C)Ageing of loose mix 60C) 6 samples compacted: voids 6-8%6 samples compacted: voids 6-8% 3 samples vacuum saturated (saturation limit 55-80%),3 samples vacuum saturated (saturation limit 55-80%), frozen & placed in hot bath ITS determined of all samples and Tensile Strength Ratio [ITS(after) / ITS(before)] calculatedITS determined of all samples and Tensile Strength Ratio [ITS(after) / ITS(before)] calculated

Recommended TSR criteria ClimatePermeabilityLowMediumHigh Dry Medium Wet

Properties for Design Calculations Relative density:Relative density: –Bulk RD < Effective RD < Apparent RD Absorption:Absorption: –Coarse: <1% by mass ; Fine: <1,5% by mass

Filler Material passing 0,075 mm sieveMaterial passing 0,075 mm sieve three purposes:three purposes: –Acts as extender for binder (mastic stiffener  improved stability) –Acts as void-filling material – used to adjust gradings and volumetric properties –Some fillers: improve bond between aggregate & binder

Types of Filler Hydrated lime (active filler):Hydrated lime (active filler): –Improves adhesion & mix durability –Low bulk density and high surface area –Relatively high cost –Designer should monitor effect on stiffness to ensure compactability Fly ash:Fly ash: –Improves mix compactability –Low bulk density –Relatively high cost –Variable characteristics require greater control

Types of Filler Portland cement (active filler):Portland cement (active filler): –Relatively high cost –Designer should monitor effect on stiffness to ensure compactability Baghouse fines:Baghouse fines: –Variable characteristics require greater control –Some source types increase moisture susceptibility –Some types render mixes sensitive to small changes in binder content

Filler-Binder Ratio TRH8: for gap- and semi-gap-graded mixes:TRH8: for gap- and semi-gap-graded mixes: –1:1 < filler-binder ratio < 3:2 Study has shown that viscosity ratios increase significantly when:Study has shown that viscosity ratios increase significantly when: –Filler-binder ratio > 1:2 at 60ºC –Filler-binder ratio > 1:1 at 135ºC Limiting filler-binder ratios important for the compactability of sand-skeleton mixesLimiting filler-binder ratios important for the compactability of sand-skeleton mixes

Effects at 60ºC

Effects at 135ºC

Other Issues Safeguarding durability:Safeguarding durability: –Natural fillers with excess clay or adsorption potential may cause: Early hardeningEarly hardening StrippingStripping –Can be assessed by the Methylene Blue test: Values < 5: good quality fillersValues < 5: good quality fillers Values > 5: perform hydrometer analysis & determine Atterberg limitsValues > 5: perform hydrometer analysis & determine Atterberg limits

Bituminous binders Type and properties of bituminous binder will affect:Type and properties of bituminous binder will affect: –Visco-elasto-plastic behaviour and performance of the asphalt mix Resistance to crackingResistance to cracking Resistance to permanent deformationResistance to permanent deformation –Temperature- and shear-susceptibility of the mix –Durability of the mix (adhesiveness & cohesiveness)

Bituminous binders 40/50 penetration grade bitumen:40/50 penetration grade bitumen: –Typically used for: Mixes for high traffic application, where stiffness is requiredMixes for high traffic application, where stiffness is required Thick asphalt layers & base coursesThick asphalt layers & base courses –Typically less suitable for: Areas where support conditions are not of a high standard, or cold regionsAreas where support conditions are not of a high standard, or cold regions

Bituminous binders 60/70 penetration grade bitumen:60/70 penetration grade bitumen: –Typically used for: Asphalt surfacings subjected to light to heavy trafficAsphalt surfacings subjected to light to heavy traffic Used for typical asphalt applications in most climatic zonesUsed for typical asphalt applications in most climatic zones

Bituminous binders 80/100 penetration grade bitumen:80/100 penetration grade bitumen: –Typically used for: Mixes for low traffic applications, where lower stiffness is requiredMixes for low traffic applications, where lower stiffness is required –Typically less suitable for: Thick layers on a stiff support or mixes placed in hot regions, unless stabilised/modifiedThick layers on a stiff support or mixes placed in hot regions, unless stabilised/modified

Bituminous binders Modified binders & special binders:Modified binders & special binders: –Used for heavy traffic applications or where special mix requirements exist, such as: Flexibility (rubber crumbs, SBS, SBR)Flexibility (rubber crumbs, SBS, SBR) Stability (SBS, EVA, APP, natural hydrocarbons)Stability (SBS, EVA, APP, natural hydrocarbons) Durability (rubber crumbs, SBS, SBR)Durability (rubber crumbs, SBS, SBR) Stiffness (APP, natural hydrocarbons)Stiffness (APP, natural hydrocarbons)

What has changed? Additional aggregate testsAdditional aggregate tests Modified Lottman test for moisture susceptibilityModified Lottman test for moisture susceptibility Recognise binder-filler interactionRecognise binder-filler interaction –Increases in viscosity During compactionDuring compaction In serviceIn service –Methylene Blue Test for assessing the quality of fillers