Download Asphalt Concrete Aggregates

Asphalt Concrete Aggregates
• Soil Definition (Engineering)
– “refers to all unconsolidated material in the earth’s
crust, all material above the bedrock”
• mineral particles (gravel, sand, silt, clay)
• organic material (top soil, marshes)
– Aggregates
• mineral particles of a soil
– specifically, granular soil group
• gravel, sand, silt
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Asphalt Concrete Aggregates
• Granular Soil Group (Aggregates)
– Physical weathering
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action of frost, water, wind, glaciers, plant/animals ..
particles transported by wind, water, ice
soils formed are called granular soil type
“grains are similar to the original bedrock”
– Larger grain sizes than clays
– Particles tend to be more or less spheres/cubes
– Bound water is small compared to overall mass
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Asphalt Concrete Aggregates
• Granular Soil Group (Aggregates)
– ability to achieve greater densities
• well graded granular material
– increased soil strength
– lower permeability
– reduced future settlement
• These improvements dictate the use of aggregates
in pavement layers where wheel loads are greater
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Asphalt Concrete Aggregates
• Aggregates for asphalt concrete include:
– coarse aggregates
• aggregate particles larger than the 4.75 mm sieve
– fine aggregates
• aggregate particles smaller than the 4.75 mm sieve
– mineral filler
• aggregate particles smaller than the 75 um sieve
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Asphalt Concrete Aggregates
• Mineral Filler
– provides the fines that are important in
producing a dense-graded strong material
– however the amount of mineral filler must be
limited
• covering them would require excess asphalt cement
• strength of the concrete mix would be reduced as the
mixture would depend on friction between smaller
particles, which is less than between larger particles
– limestone dust is the most common material
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Asphalt Concrete Aggregates
• Aggregate Properties
– Well graded-dense including mineral filler
– Hard-resistance to wear and traffic polishing
– Sound-resistance to breakdown due to
freeze/thaw cycles
– Rough Surfaced-crushed rough surfaces
• higher friction strength
• better adhesion to asphalt cement
– Avoid cubical-thin elongated particles that
break easier
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Asphalt Concrete Aggregates
• Aggregate Properties (cont’d)
– Free from Deleterious Substances
• clay, dust, dirt, lightweight pieces
• lower quality of asphalt film on the particle
• breaking of some particles
– Hydrophobic “water hating”
• siliceous aggregates such as quartz are hydrophilic
• greater affinity for water than asphalt cement due to
surface charges
• stripping, asphalt coating comes away from the
particle in the presence of water
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Asphalt Concrete Aggregates
• Stripping Tests
– Visual tests (Saskatchewan Highways)
– samples saturated under vacuum / % stripped
– Strength Tests
• ASTM D1075, Effect of water on Compressive
Strength of Compacted Bituminous Mixtures
– samples are submerged for 4 days
• ASTM D4876, Effect of Moisture on Asphalt
Concrete Paving Mixtures
– samples are submerged under vacuum 80% saturation
• AASHTO T 283, Modified Lottman Test
– 80% saturation / freeze thaw
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Asphalt Concrete Aggregates
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Asphalt Concrete Aggregates
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Asphalt Concrete Aggregates
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Asphalt Concrete Aggregates
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Asphalt Concrete Aggregates
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Asphalt Concrete Aggregates
• Superpave Aggregate Properties
– three critical distress mechanisms
• rutting
• fatigue cracking
• low temperature cracking
– central role in overcoming permanent
deformation ie. Rutting
– lesser role in pavement fatigue and low
temperature cracking
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Asphalt Concrete Aggregates
• Consensus Aggregate Properties
– pavement experts agreed that these aggregate
properties were critical to well performing mixes
and wide agreement in their use and specified
values
– criteria are based on traffic levels and position
within pavement structure
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Asphalt Concrete Aggregates
• Source Aggregate Properties
– pavement experts agreed that there were other
critical aggregate properties that were dependant on
local sources and experience and were left to local
agencies to specify
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Asphalt Concrete Aggregates
• Superpave consensus aggregate properties
– Coarse aggregate angularity
• ensures a high degree of internal friction and rutting
resistance
• percent by weight of aggregates larger than 4.75 mm
with one or more fractured faces
– Fine aggregate angularity
• ensures a high degree of fine aggregate internal
friction and rutting resistance
• percent air voids in loosely compacted aggregates
smaller than 2.36 mm
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Asphalt Concrete Aggregates
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Asphalt Concrete Aggregates
Superpave consensus aggregate properties
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Asphalt Concrete Aggregates
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Asphalt Concrete Aggregates
• Superpave consensus aggregate properties
– Flat and Elongated Particles
• percentage by mass of coarse aggregates that have a
maximum to minimum dimension ratio greater than
five
• particles are undesirable because of their tendency
to break during construction and under traffic
• procedure uses a proportional caliper device to
measure the dimensional ratio of a representative
sample of aggregate particles
• percentage of flat + percentage of elongated
particles
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Asphalt Concrete Aggregates
Flat and Elongated Particles
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Asphalt Concrete Aggregates
Flat and Elongated Particles
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Asphalt Concrete Aggregates
• Superpave consensus aggregate properties
– Clay content
• percentage of clay material contained in the
aggregate fraction that is finer than the 4.75 mm
sieve
• sand equivalency test is used in which a sample is
mixes with a flocculating solution forcing clay
particles into suspension
• after a settling period the height of suspended clay
and sedimented sand is measured
• sand equivalent is the ratio of sand to clay readings
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Asphalt Concrete Aggregates
Clay Content
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Asphalt Concrete Aggregates
Clay Content
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Asphalt Concrete Aggregates
• Superpave source aggregate properties
– Toughness
• percent loss of material from an aggregate blend
larger than 2.36 mm using a Los Angeles
Abrasion Test
• estimates the resistance of coarse aggregate to
abrasion and mechanical degradation during
handling, construction and in service
• maximum loss values typically range from 35 to
45 percent
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Asphalt Concrete Aggregates
• Superpave source aggregate properties
– Soundness
• percent loss of material from from an aggregate
blend during the sodium or magnesium sulfate
soundness test
• can be used on both coarse and fine aggregates
• resembles freeze/thaw in the field
• test result is total percent loss over various sieve
intervals for a required number of cycles
• maximum loss values typically range from 10 to 20
percent for five cycles
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Asphalt Concrete Aggregates
• Superpave source aggregate properties
– Deleterious materials
• defined as mass percentage of contaminants such as
clay lumps, shale, wood, mica, and coal in a blended
aggregate
• performed on both coarse and fine aggregates
• mass percentage of material lost as a result of wet
sieving is reported as the percent of clay lumps and
friable particles
• values range as low as 0.2 percent to 10 percent
depending on the exact composition of the
contaminant
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Asphalt Concrete Aggregates
• Superpave aggregate properties
– Gradation Chart
• the 0.45 power gradation chart is used in which
sieve sizes are raised to the power 0.45 on the
horizontal axis (normal sieve gradation graph
already in use)
• maximum density gradation is a straight line from
the maximum aggregate size to the origin
• maximum size is defined as one sieve size larger
that the nominal maximum size
• nominal maximum size is defined as one sieve size
larger than the first sieve to retain more than 10 %
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Asphalt Concrete Aggregates
• Superpave source aggregate properties
– Gradation Chart (cont’d)
• specifies two new features to the gradation chart
• Control Points placed on the nominal maximum
sieve, an intermediate sieve (2.36 mm), and the
smallest sieve (75 um) through which the gradation
must pass
• Restricted Zone placed on the maximum density line
between an intermediate sieve and the 0.3 mm sieve
through which the gradation cannot pass. Gradations
that pass through the restricted zone are called
“humped gradations”
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Asphalt Concrete Aggregates
• Superpave source aggregate properties
– Gradation Chart (cont’d)
• Humped gradations indicate an:
• oversanded mixture and/or a mixture that
contains too much fine sand in relation to total
sand
• restriction zone discourages the use of fine
natural sand and encourages the use of a clean
manufactured sand
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Asphalt Concrete Aggregates
• Superpave source aggregate properties
– Gradation Chart (cont’d)
– Results :
• mixtures that poses compaction problems during
construction “tender mixes”
• offers reduced resistance to rutting
• gradations that follow to close to the maximum
density line for fine aggregates often have
inadequate VMA to allow enough asphalt for
adequate durability
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Asphalt Concrete Aggregates
Gradation Chart
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Asphalt Concrete Aggregates
Gradation Chart
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