Hot Mix Asphalt consists of a mixture of aggregates (about 95 percent) and an asphalt binder (about 5 percent).
The aggregate is typically crushed to certain size or gradation specifications. In Hot Mix Asphalt, gradation helps determine almost every important property, including stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance, and resistance to moisture damage.
The fundamental requirements for aggregates to be used in asphalt manufacture are defined by a combination of their engineering and physical properties.
The engineering properties required of the aggregate are that it must be durable, strong, and resistant to the polishing effects of traffic (obviously not important in the lower pavement layers) and exhibit good adhesion to the asphalt cement binder.
The main physical properties required for an aggregate to be used in asphalt manufacture are proper gradation and shape, absorption and water content, and cleanliness (the absence of any deleterious material and/or contaminants).
Asphalt or bitumen is a dark brown to black, highly viscous hydrocarbon produced from petroleum distillation residue. This distillation can occur naturally, resulting in asphalt lakes, or take place in a petroleum refinery. Asphalt cement refers to asphalt that has been prepared for use in Hot Mix Asphalt.
Asphalt can be classified by its chemical composition and physical properties and is a measure of rheology and durability (hardening). In North America, almost all asphalt cement is characterized by the Superpave performance grading (PG) system. Superpave performance gradings are reported using two numbers: the first is the average seven-day maximum pavement temperature (in degrees Celsius), and the second indicates the minimum pavement design temperature likely to be experienced (also in degrees Celsius).
Asphalt cement can be further modified to meet certain additional performance characteristics; namely, higher stiffness at high temperatures to reduce rutting, and lower stiffness at low temperatures to reduce thermal cracking. Asphalt cement can also be modified to increase the adhesion between the asphalt binder and the aggregate in the presence of moisture.