Asphalt

Asphalt, black, cementlike material varying in consistency at room temperature from solid to semisolid. It can be poured when heated to the temperature of boiling water and is used in surfacing roads, in lining the walls of water-retaining structures such as reservoirs and swimming pools, and in manufacturing floor tiles and roofing materials. It is not to be confused with tar, a black substance derived from coal, wood, and other substances.

Asphalt is found in natural deposits, but almost all of the asphalt used commercially is now derived from petroleum. Straight-run asphalts, which are made up of the nonvolatile hydrocarbons left after petroleum has been refined into gasoline and other products, are used for paving. Air-blown asphalts, produced from petroleum residues at temperatures of from 204° to 316°C (400° to 600°F), are used to make roofing materials and similar products. A small amount of asphalt is “cracked” at temperatures of about 500°C (about 930°F) to make some insulation materials.

Natural asphalt was used extensively in ancient times. Ancient Babylonians used it as a building material, and it is referred to several times in the Old Testament books of Genesis and Exodus as a caulking material (see Bitumen). Natural deposits of asphalt occur in pits or lakes as residue from crude petroleum that has seeped up through fissures in the earth. Typical of these deposits are the La Brea tar pits in Los Angeles, in which the remains of prehistoric flora and fauna have been found. A natural asphalt pool is Pitch Lake, Trinidad. Deposits of asphalt-impregnated rock, called rock asphalt, are found throughout the world. An asphalt deposit of some commercial importance is Gilsonite, also called uintaite, found in the Uinta River Basin of Utah and used in the manufacture of paints and lacquers.

The use of asphalt for street paving in the United States began in 1870; by 1903 more than 35 million sq m (about 42 million sq yd) of U.S. streets were paved with asphalt. Today, asphalt derived from petroleum is used to surface about 90 percent of paved roads in the United States. About 75 percent of the more than 24 million metric tons of asphalt produced in the United States each year is used for this purpose.