HMA Design: Component Selection

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HMA DesignComponent Selection

• TRH3/HMA Workshop
• 12-14 June 2001

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Mix components affecting performance

• Aggregate
• Type and properties of aggregate
• Aggregate grading
• Filler
• Physiochemical properties of filler
• Amount of filler
• Bituminous binders
• Physiochemical properties of binder
• Amount of binder

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Aggregate

• Constitute 90-95 by mass of asphalt
• Provide a substantial proportion of load carrying
  capacity
• Their 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
  subjected

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Physical properties

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

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Physical properties

• Hardness toughness
• Durability soundness
• Particle shape surface texture
• Cleanliness
• Variability

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Hardness Toughness

• Important for rut resistance and low-speed skid
  resistance
• Tests
• 10 FACT gt160kN (general) gt210kN (stone
  skeleton structure - SKS)
• ACV lt25 (general) lt21 (SKS)
• Los Angeles Abrasion
• 10 for very hard aggregate (typical)
• 60 for very soft aggregate (typical)

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Durability Soundness

• Ability to resist breakdown disintegration in
  operational environment
• Test
• Sulphate soundness test 12-20 usually
  acceptable
• Ethylene glycol soundness test

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Particle Shape Texture

• Greatly affects strength, stability, workability
• Rough textured and angular, near cubical shape
  preferred for stability, stronger mechanical bond
  with binder low-speed skid resistance
• Tests
• Flakiness index criteria dependent on mix type
  maximum stone size
• Particle index test natural lt 14 lt crushed
• PSV gt50 (general) gt45 (gap-graded)
• Fractured faces gt95 (surfacing) gt50 (base)

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Cleanliness

• Presence of foreign and deleterious material
• Weak or weathered particles
• Friable particles
• Clay lumps
• Organic matter
• resulting in ravelling, stripping, pop-ups
• Tests
• Sand equivalent test gt50 (tot. fines) gt30
  (natural)
• Clay lumps friable particles lt1

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Variability

• Changes 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
• Provision of sufficient bins for the different
  aggregate fractions?

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Binder Adhesion

• Test for assessing susceptibility to stripping
• Modified Lottman test (AASHTO 283)
• Ageing of loose mix (4hr _at_ 60C)
• 6 samples compacted voids 6-8
• 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)
  calculated

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Recommended TSR criteria
Climate Permeability Permeability Permeability
Climate Low Medium High
Dry 0.60 0.65 0.70
Medium 0.65 0.70 0.75
Wet 0.70 0.75 0.80
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Properties for Design Calculations

• Relative density
• Bulk RD lt Effective RD lt Apparent RD
• Absorption
• Coarse lt1 by mass Fine lt1,5 by mass

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Filler

• Material passing 0,075 mm sieve
• 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

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Types of 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
• Improves mix compactability
• Low bulk density
• Relatively high cost
• Variable characteristics require greater control

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Types of Filler

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

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Filler-Binder Ratio

• TRH8 for gap- and semi-gap-graded mixes
• 11 lt filler-binder ratio lt 32
• Study has shown that viscosity ratios increase
  significantly when
• Filler-binder ratio gt 12 at 60ºC
• Filler-binder ratio gt 11 at 135ºC
• Limiting filler-binder ratios important for the
  compactability of sand-skeleton mixes

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Effects at 60ºC
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Effects at 135ºC
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Other Issues

• Safeguarding durability
• Natural fillers with excess clay or adsorption
  potential may cause
• Early hardening
• Stripping
• Can be assessed by the Methylene Blue test
• Values lt 5 good quality fillers
• Values gt 5 perform hydrometer analysis
  determine Atterberg limits

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Bituminous binders

• Type and properties of bituminous binder will
  affect
• Visco-elasto-plastic behaviour and performance of
  the asphalt mix
• Resistance to cracking
• Resistance to permanent deformation
• Temperature- and shear-susceptibility of the mix
• Durability of the mix (adhesiveness
  cohesiveness)

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Bituminous binders

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

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Bituminous binders

• 60/70 penetration grade bitumen
• Typically used for
• Asphalt surfacings subjected to light to heavy
  traffic
• Used for typical asphalt applications in most
  climatic zones

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Bituminous binders

• 80/100 penetration grade bitumen
• Typically used for
• Mixes 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/modified

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Bituminous binders

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

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What has changed?

• Additional aggregate tests
• Modified Lottman test for moisture susceptibility
• Recognise binder-filler interaction
• Increases in viscosity
• During compaction
• In service
• Methylene Blue Test for assessing the quality of
  fillers