Aggregate

"Aggregate" is a collective term for sand, gravel and crushed stone mineral materials in their natural or processed state (NSSGA, 1991).  In 2000, the U.S. produced over 3 billion tons of aggregate at a value of about $14.2 billion.  Roads and highways constitute the largest single use of aggregate at 40 percent of the total (NSSGA, 2002).  In HMA, aggregates are combined with a asphalt binding medium to form a compound material.  By weight, aggregate generally accounts for between 92 and 96 percent of HMA and makes up about 30 percent of the cost of an HMA pavement structure.  Aggregate is also used by itself or with a stabilizer for base and subbase courses.

Figure 1: Sand and Gravel Pit	Figure 2: Aggregate Stockpile

Aggregate Origins and Production

Aggregates can either be natural or manufactured.  Natural aggregates are generally extracted from larger rock formations through an open excavation (quarry).  Usually the rock is blasted or dug from the quarry walls then reduced in size using a series of screens and crushers.  Some quarries are also capable of washing the finished aggregate.  Manufactured rock typically consists of industrial byproducts such as slag (byproduct of the metallurgical processing – typically produced from processing steel, tin and copper) or specialty rock that is produced to have a particular physical characteristic not found in natural rock (such as the low density of lightweight aggregate). 

Figure 3: Quarry Overview

Aggregate Physical Properties

Aggregates can be classified by their mineral, chemical and physical properties.  The pavement industry typically relies on physical properties for performance characterization.  An aggregate's physical properties are a direct result of its mineral and chemical properties.

Maximum Size

Gradation

An aggregate's particle size distribution, or gradation, is one of its most influential characteristics.  In HMA, gradation helps determine almost every important property including stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance and resistance to moisture damage (Roberts et al., 1996). Because of this, gradation is a primary concern in HMA mix design and thus most agencies specify allowable aggregate gradations.

Measurement
Gradation is usually measured by a sieve analysis.  In a sieve analysis, a sample of dry aggregate of known weight is separated through a series of sieves with progressively smaller openings. Once separated, the weight of particles retained on each sieve is measured and compared to the total sample weight.  Particle size distribution is then expressed as a percent retained by weight on each sieve size.  Results are usually expressed in tabular or graphical format.  The typical graph uses the percentage of aggregate by weight passing a certain sieve size on the y-axis and the sieve size raised to the n^th power (n = 0.45 is typically used) as the x-axis units.  The maximum density appears as a straight line from zero to the maximum aggregate size (the exact location of this line is somewhat debatable, but the location shown in Figure 4 is generally accepted).

Typical Gradations (see Figure 4)

Figure 4: Typical Aggregate Gradations

Other Gradation Terms

• Fine aggregate (sometimes just referred to as "fines").  Defined by AASHTO M 147 as natural or crushed sand passing the No. 10 sieve and mineral particles passing the No. 200 sieve. 
• Coarse aggregate.  Defined by AASHTO M 147 as hard, durable particles or fragments of stone, gravel or slag retained on the No. 10 sieve.  Usually coarse aggregate has a toughness and abrasion resistance requirement.
• Fine gradation.  A gradation that, when plotted on the 0.45 power gradation graph, falls mostly above the 0.45 power maximum density line.  The term generally applies to dense graded aggregate.
• Coarse gradation.  A gradation that, when plotted on the 0.45 power gradation graph, falls mostly below the 0.45 power maximum density line.  The term generally applies to dense graded aggregate.
• Mineral filler.  Defined by the Asphalt Institute as a finely divided mineral product at least 65 percent of which will pass through a No. 200 sieve.  Pulverized limestone is the most commonly manufactured mineral filler, although other stone dust, silica, hydrated lime, portland cement and certain natural deposits of finely divided mineral matter are also used (Asphalt Institute, 1962).

Other Properties

Other important aggregate physical properties are:

• Toughness and abrasion resistance.  Aggregates should be hard and tough enough to resist crushing, degradation and disintegration from activities such as  manufacturing, stockpiling, production, placing and compaction. 
• Durability and soundness.  Aggregates must be resistant to breakdown and disintegration from weathering (wetting/drying) or else they may break apart and cause premature pavement distress. 
• Particle shape and surface texture.  Particle shape and surface texture are important for proper compaction, load resistance and workability.  Generally, cubic angular-shaped particles with a rough surface texture are best. 
• Specific gravity.  Aggregate specific gravity is useful in making weight-volume conversions and in calculating the void content in compacted HMA (Roberts et al., 1996).
• Cleanliness and deleterious materials.  Aggregates must be relatively clean when used in HMA.  Vegetation, soft particles, clay lumps, excess dust and vegetable matter may affect performance by quickly degrading, which causes a loss of structural support and/or prevents binder-aggregate bonding.

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