Asphalt, << AS fawlt, >> is a black cementlike substance that is found in most crude petroleum. It has hundreds of uses. It is used to pave streets, highways, and airfields; to make floor tiles and roofing, waterproofing, and insulating materials; and to line reservoirs, waste storage ponds, dams, and irrigation canals. Asphalt is also used in varnish and inks. In addition, asphalt coatings protect underground pipelines from corrosion.
Asphalt is thermoplastic—that is, it softens and becomes a liquid when heated, and returns to a solid when cooled. Asphalt wears well, is highly waterproof, and is unharmed by most acids and salts.
Asphalt production.
Asphalt is separated from crude petroleum by refining methods that also produce gasoline, kerosene, and other products. Usually, a distillation (boiling) process removes gasoline and other products with low boiling points. The oil that remains is commonly called topped crude. Topped crude may be used as a fuel oil, or further refined to asphalt or other products. By varying the refining processes, refineries may obtain different kinds of asphalt. For example, they make blown or oxidized asphalts by blowing hot air through topped crude. These asphalts are widely used for roofing, enamels, and other industrial applications. Most topped crude is refined to produce asphalt cement, a semisolid asphalt used for paving.
Asphalt also occurs in natural deposits in pits, lakes, and rocks. But only a small part of the asphalt used in the world comes from natural deposits. Some natural deposits found in pits and lakes are pure, but most have become mixed with mineral matter, water, and other substances. One of the best-known deposits is Pitch Lake on the island of Trinidad in the Caribbean Sea. The English explorer Sir Walter Raleigh discovered this 114-acre (46-hectare) bed in 1595. One of the largest deposits of asphalt is in Lake Guanoco in Venezuela, near the Gulf of Paria. The deposit covers about 1,000 acres (400 hectares). Uintaite or gilsonite, a solid form of asphalt, is found in Utah and Colorado in the United States.
Paving with asphalt.
Asphalt is used mainly to pave streets, highways, and airports. Blacktop is the common name for many types of asphalt paving. Asphalt pavements are made in several ways. But usually, asphalt cement is mixed with mineral aggregates, such as crushed stone, gravel, and sand. These aggregates vary in size. The largest particles are usually about 3/4 inch (19 millimeters) in diameter.
The aggregates are blended, dried, and heated to about 300 °F (149 °C) in a paving plant. Hot mixes are prepared by adding hot asphalt cement. Paddles mix the asphalt with the aggregates in a pugmill mixer. The mix contains only about 5 to 10 percent asphalt by weight. Sometimes, ground rubber from recycled tires is added to the mix, because rubber improves a pavement’s heat resistance and flexibility. At the job site, a paving machine spreads the mixture evenly on the roadbed, and a roller flattens it into a smooth, hard pavement. Cold mixes are made with liquid asphalt. Liquid asphalt is a blend of asphalt cement and a light petroleum solvent (substance that can dissolve other substances). Cold mixes can also be made by blending asphalt cement with water. Cold mixes often can be prepared directly on the roadbed because little or no heating is needed in their preparation.
Surface treatment is used to resurface pavements or to pave lightly traveled roads. Hot asphalt cement or liquid asphalt is sprayed evenly over the roadway surface. Mineral aggregates are then spread over the surface and rolled into the asphalt.
