Ginger Dvoracek, age 15, of Casper, Wyo., for her question:
CAN YOU TELL US HOW ROADS ARE BLACKTOPPED?
Blacktop is the common name for many types of asphalt paving. Asphalt is used mainly to pave streets, highways and airports. Over 90 percent of all paved roads in the United States have blacktop asphalt surfaces.
Asphalt is a black cement like mineral substance that is found in most crude petroleum. Asphalt is separated from crude petroleum by refining methods that also produce gasoline, kerosene and other products. Usually, gasoline and other products with low boiling points are removed by a distillation process. The oil that remains is commonly called topped crude.
Most topped crude is refined to produce asphalt cement, a semisolid asphalt used for paving. Production of petroleum asphalt has increased steadily during recent years.
Asphalt or blacktop 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 three fourths of an inch in diameter.
The aggregates are blended, dried and heated to about 300 degrees Fahrenheit 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 paving mix contains only about five to 10 percent asphalt.
Trucks then carry the hot mixture to 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. This is a blend of asphalt cement and a light petroleum solvent. Cold mixes can also be made by carefully blending asphalt cement with water. Cold mixes often can be prepared directly on the roadbed becuse little heating is needed.
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.
Asphalt is also used to make roofing, waterproofing and insulating materials and floor tiles.
In addition, asphalt is used to line reservoirs, waste storage ponds, dams, irrigation canals, and, as an ingredient in varnish and inks.
Asphalt coatings protect underground pipelines from corrosion.
Asphalt is themoplastic. 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.
Although most asphalt comes from the separation of crude petroleum, asphalt also occurs in natural deposits in pits, lakes and rocks. But only a small part of the asphalt used in the United States comes from natural deposits.
Some natural deposits found in pits and lakes are pure, but most deposits 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. One of the largest deposits is found in Lake Guanoco in Venezuela, near the Gulf of Paria.