ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 9

Asphalt Concrete Aggregates

ENCI 579 5 10

Asphalt Concrete Aggregates

ENCI 579 5 11

Asphalt Concrete Aggregates

ENCI 579 5 12

Asphalt Concrete Aggregates

ENCI 579 5 13

Asphalt Concrete Aggregates

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 18

Asphalt Concrete Aggregates

ENCI 579 5 19

Asphalt Concrete Aggregates Superpave consensus aggregate properties

ENCI 579 5 20

Asphalt Concrete Aggregates

ENCI 579 5 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

ENCI 579 5 22

Asphalt Concrete Aggregates Flat and Elongated Particles

ENCI 579 5 23

Asphalt Concrete Aggregates Flat and Elongated Particles

ENCI 579 5 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

ENCI 579 5 25

Asphalt Concrete AggregatesClay Content

ENCI 579 5 26

Asphalt Concrete AggregatesClay Content

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 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 %

ENCI 579 5 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”

ENCI 579 5 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

ENCI 579 5 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

ENCI 579 5 34

Asphalt Concrete Aggregates Gradation Chart

ENCI 579 5 35

Asphalt Concrete Aggregates Gradation Chart