pavement: Aggregate

“Aggregate” is a collective term for the mineral materials such as sand, gravel and crushed stone that are used with a binding medium (such as water, bitumen, portland cement, lime, etc.) to form compound materials (such asasphalt concrete and portland cement concrete). By volume, aggregate generally accounts for 92 to 96 percent of HMA and about 70 to 80 percent of portland cement concrete. Aggregate is also used for base and subbase courses for both flexible and rigid pavements.

Aggregates can either be natural or manufactured. Natural aggregates are generally extracted from larger rock formations through an open excavation (quarry). Extracted rock is typically reduced to usable sizes by mechanical crushing. Manufactured aggregate is often the byproduct of other manufacturing industries.

This section will briefly discuss aggregate sources and quarrying operations then describe the basic aggregate mineral, chemical and physical properties most important to pavements and the typical tests used to determine these properties. The following source contains more detailed information on aggregate:

Aggregate Sources

Aggregates can come from either natural or manufactured sources. Natural aggregates come from rock, of which there are three broad geological classifications (Roberts, et al., 1996):

Igneous rock: These rocks are primarily crystalline and are formed by the cooling of molten rock material beneath the earth’s crust (magma).
Sedimentary rocks: These rocks are formed from deposited insoluble material (e.g., the remains of existing rock deposited on the bottom of an ocean or lake). This material is transformed to rock by heat and pressure. Sedimentary rocks are layered in appearance and are further classified based on their predominant mineral as calcareous (limestone, chalk, etc.), siliceous (chert, sandstone, etc.) or argillaceous (shale, etc.).
Metamorphic rock: These are igneous or sedimentary rocks that have been subjected to heat and/or pressure great enough to change their mineral structure so as to be different from the original rock.

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).

Aggregate Production

Aggregates are produced in a quarry or mine (Figure 1) whose basic function is to convert in situ rock into aggregate with specified characteristics. 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.