Gary Agajanian, age 15, of Erie, Pa., for his question:
HOW IS ASPHALT PRODUCED?
Asphalt is a black cement like substance that is found in most crude petroleum. It is separated from crude petroleum by refining methods that also produce kerosene, gasoline and other products.
Usually, gasoline and other products with low boiling points are removed by a distillation or boiling process. The oil that remains is commonly called topped crude.
Topped crude may be used as a fuel oil, or further refined. By varying the refining process, different kinds of asphalt may be obtained.
Blown or oxidized asphalts are made 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.
Production of petroleum asphalt has increased steadily. In the United States, the amount of asphalt and road oil refined from crude petroleum rose from about 2 million tons in 1924 to about 50 million tons in the 1980s.
The use of asphalt for street paving in the United States started in 1870. Today, asphalt derived from petroleum is used to surface 90 percent of the paved roads in the U.S. Of U.S. production of petroleum asphalt, about 75 percent is used for paving, 15 percent for roofing and 10 percent for the more than 200 other known uses of asphalt, including waterproofing, pipe protection and the manufacture of paint and electrical insulation.
Asphalt also occurs in natural deposits in pits, lakes and rocks. But only a small part of the asphalt used in the U.S. 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 Trinadad in the Caribbean Sea. Sir Waiter Raleigh discovered this 114 acre bed in 1595.
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 three quarters 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 aggregates in a pugmiil mixer. The paving mix contains only about five to 10 percent asphalt in weight.
Trucks then carry the hot mixture to the job. 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. A petroleum solvent is a substance that can dissolve other substances. Cold mixes can also be made by carefully 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.