Crushed stone and sand and gravel are the main types of naturalaggregate used in the United States. Aggregate is used in nearlyall residential, commercial, and industrial building construction,and in most public works projects including roads and highways,bridges, railroad and light rail beds, airports, water and sewersystems, and tunnels. Much of the infrastructure built during the1950s and 1960s has deteriorated to a point that requires extensiverepair or replacement. All this construction requires enormousamounts of aggregate. In Colorado, for example, nearly 45 milliontons of aggregate, or about 12 tons per person, were producedduring 1994.
