8. Compaction

8.2 Definitions

It is important to understand the terminology associated with asphalt pavements. The following definitions are offered.

Compaction—the process that reduces the volume of an asphalt mixture, shrinking its air voids and reorienting its aggregate closer and closer together, inducing aggregate interlock. This forms what is known as the aggregate skeleton of a mixture, which is its main source of strength.

Density—the mass of a material per unit volume. In the asphalt pavement community, the term density is usually inferred to mean relative density.

Laboratory density—a calculated value that multiplies the measured bulk specific gravity (G_mb) by the density of water (62.4 lb/ft³ or 1,000 g/L) at the design air voids, most commonly following AASHTO T 166 or ASTM D 2726.

Theoretical maximum density (TMD)—a calculated value based on the laboratory testing procedure that follows AASHTO T 209. In this test, a sample of an asphalt mixture’s specific gravity with zero air voids is determined; the symbol G_mm is used for this value. To get its density, this value is multiplied by the density of water (62.4 lb/ft³ or 1,000 g/L). This value is often referred to as Rice specific gravity or Rice density if multiplied by the density of water. The test is named after Jim Rice, who was the developer of the testing procedure.

Relative density—the percentage of a reference density. Almost universally, the reference density for asphalt pavements is the TMD.