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"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.

Pile of aggregate Pile of aggregate
Figure 1: Aggregate Stockpiles Figure 2: HMA Plant Aggregate Feed Bins


Aggregate Origins and Production

Aggregates can either be natural (Figures 3, 4 and 5) 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).

Kapa'a quarry in Windward O'ahu

Figure 3: Kapa'a Quarry

Yamada quarry on the Big Island
Yamada quarry on the Big Island
Figures 4 and 5: Yamada & Sons Quarry on the Big Island Near Hilo


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

Maximum aggregate size can affect HMA and base/subbase courses in several ways. In HMA, instability may result from excessively small maximum sizes; and poor workability and/or segregation may result from excessively large maximum sizes (Roberts et al., 1996). ASTM C 125 defines the maximum aggregate size in one of two ways:

It is important to specify whether "maximum size" or "nominal maximum size" is being referenced.


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.

Gradation is usually measured by a sieve analysis (see Figures 6 and 7). 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 nth power (n = 0.45 is typically used) as the x-axis units. The maximum possible density appears on a graph like this 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).

Sieve washing Sieve analysis test
Figure 6: Washing the Aggregate Before a Sieve Analysis Figure 7: Weighing Aggregate During a
Sieve Analysis

Typical Gradations (see Figure 8)

Figure 8: Typical Aggregate Gradations
(Examples are shown for 3/4-inch maximum aggregate size)

Other Gradation Terms

Other Properties

Other important aggregate physical properties are:

In general, Hawai'i is blessed with fairly high quality aggregate.  Certain sources do, however, produce aggregate with high absorption (this can lead to the aggregate absorbing excess asphalt binder, which may lead to stripping or raveling).  Aggregate from a'a deposits is generally avoided because it tends to be high in voids and low in durability and toughness.


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