Transcript PPGS - Asphalt Academy

HMA Design:
Component Selection
TRH3/HMA Workshop
12-14 June 2001
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
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
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
Physical properties
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Hardness & toughness
Durability & soundness
Particle shape & surface texture
Cleanliness
Variability
Hardness & Toughness
• Important for rut resistance and lowspeed skid resistance
• Tests:
– 10% FACT: >160kN (general) ; >210kN (stone
skeleton structure - SKS)
– ACV: <25% (general) ; <21% (SKS)
– Los Angeles Abrasion:
• 10% for very hard aggregate (typical)
• 60% for very soft aggregate (typical)
Durability & Soundness
• Ability to resist breakdown &
disintegration in operational
environment
• Test:
– Sulphate soundness test: 12-20% usually
acceptable
– Ethylene glycol soundness test
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 < 14 < crushed
– PSV: >50 (general) ; >45 (gap-graded)
– Fractured faces: >95% (surfacing) ; >50% (base)
Cleanliness
• Presence of foreign and deleterious material:
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–
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Weak or weathered particles
Friable particles
Clay lumps
Organic matter
resulting in ravelling, stripping, pop-ups
• 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 shapes
• Some mixes (SMA) require narrow gradation
envelopes:
– Provision of sufficient bins for the different
aggregate fractions?
Binder Adhesion
• Test for assessing susceptibility to stripping:
– Modified Lottman test (AASHTO 283)
• Ageing of loose mix (4hr @ 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
Recommended TSR criteria
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
Properties for Design
Calculations
• Relative density:
– Bulk RD < Effective RD < Apparent RD
• Absorption:
– Coarse: <1% by mass ; Fine: <1,5% by mass
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
Types of Filler
• Hydrated lime (active filler):
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–
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:
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Improves mix compactability
Low bulk density
Relatively high cost
Variable characteristics require greater control
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
Filler-Binder Ratio
• TRH8: for gap- and semi-gap-graded mixes:
– 1:1 < filler-binder ratio < 3:2
• 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 mixes
Viscosity Ratio (Visc at 60 deg.)
Effects at 60ºC
14.0
filler : binder = 2:1
12.0
10.0
filler : binder = 1:1
8.0
6.0
4.0
filler : binder = 1:2
2.0
0.0
0
0.5
1
1.5
Filler / Binder Ratio by mass
2
2.5
Viscosity Ratio (Visc at 135 deg.)
Effects at 135ºC
250.0
filler : binder = 2:1
200.0
150.0
100.0
filler : binder = 1:1
50.0
filler : binder = 1:2
0.0
0
0.5
1
1.5
Filler / Binder Ratio by mass
2
2.5
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 < 5: good quality fillers
• Values > 5: perform hydrometer analysis &
determine Atterberg limits
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)
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
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
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
Bituminous binders
• Modified binders & special binders:
– Used for heavy traffic applications or where
special mix requirements exist, such as:
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Flexibility (rubber crumbs, SBS, SBR)
Stability (SBS, EVA, APP, natural hydrocarbons)
Durability (rubber crumbs, SBS, SBR)
Stiffness (APP, natural hydrocarbons)
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