“Aggregate” is a collective term for sand, gravel and crushed stone mineral materials in their natural or processed state (NSSGA 1991).  In 2009, the U.S. produced nearly 2 billion tons of aggregate at a value of about $17.2 billion.  Roads and highways constitute31 percent of the total sand, stone and gravel market (NSSGA 2010).  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 25 percent of the cost of an HMA pavement structure.  Aggregate is also used by itself or with a stabilizer for base and subbase courses.

“Asphalt” is a dark brown to black, highly viscous, hydrocarbon produced from petroleum distillation residue.  This distillation can occur naturally, resulting in asphalt lakes, or occur in a petroleum refinery.  In 2008, the U.S. produced about 30 million tons of asphalt at a rough value of around $12 billion.  Roads and highways constitute the largest single use of asphalt at 90 percent of the total.  In HMA, asphalt functions as a waterproof, thermoplastic, viscoelastic adhesive.  By weight, asphalt generally accounts for between 4 and 8 percent of HMA and makes up about 25 – 30 percent of the cost of an HMA pavement structure depending upon the type and quantity.    The paving industry also uses asphalt emulsions, asphalt cutbacks and foamed asphalt.