1. 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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2. Asphalt Concrete Aggregates
Granular Soil Group (Aggregates)
Physical weathering
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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3. 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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4. 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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5. 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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6. 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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7. 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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8. 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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9. Asphalt Concrete Aggregates
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10. Asphalt Concrete Aggregates
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11. Asphalt Concrete Aggregates
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12. Asphalt Concrete Aggregates
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13. Asphalt Concrete Aggregates
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14. 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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15. 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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16. 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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17. 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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18. Asphalt Concrete Aggregates
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19. Asphalt Concrete Aggregates Superpave consensus aggregate properties
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20. Asphalt Concrete Aggregates
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21. 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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22. Asphalt Concrete Aggregates Flat and Elongated Particles
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23. Asphalt Concrete Aggregates Flat and Elongated Particles
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24. 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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25. Asphalt Concrete Aggregates
Clay Content
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26. Asphalt Concrete Aggregates Clay Content
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27. 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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28. 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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29. 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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30. 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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31. 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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32. 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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33. 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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34. Asphalt Concrete Aggregates Gradation Chart
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35. Asphalt Concrete Aggregates Gradation Chart
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