Aggregates for asphalt mixture are generally classified according to their occurrence. The three major sources of aggregates for INDOT use are sand and gravel operations, stone quarries, and industrial processing (synthetic aggregates). Crushed stone, sand, and gravel are classified as natural aggregates. Blast furnace slag, steel slag, granulated blast furnace slag, and wet bottom boiler slag are by-products of industrial processing and are classified as synthetic aggregates.
Crushed stone is produced from quarries where bedrock is blasted from a quarry face and then further processed by crushing and screening. Crushing is done by processing the shot rock through a series of crushers until the desired top size is produced. Materials are then graded in a screening operation similar to the sand and gravel operations.
Physical quality requirements for aggregates used in asphalt mixture are all specification provisions other than those dealing with gradation or usage requirements. These quality requirements may be divided into five distinct groups as follows:
1) Absorption/Specific Gravity
2) Abrasion Resistance
4) Deleterious Materials
5) Special Requirements