The New Student's Reference Work/Asphalt

Asphalt (ǎs' fǎlt) is bitumen of purer form, being a mineral and solid or semi-solid. Its name comes from Locus Asphaltites, Latin for the Dead Sea, where asphalt once abounded. It is black or brown in color, brittle in consistency (though this varies from a bright pitchy condition to thick masses of mineral tar) and compact. It melts easily about the boiling point of water, burns without making ashes, and emits a thick smoke of pitchy odor. It is widely distributed, especially in tropical and subtropical regions, but deposits of sufficient quantity for commerce and the industries occur only in a few localities. It is a product of the decomposition of vegetable and animal substances, the three preceding products being naphtha, petroleum and mineral tar. Asphaltic stone is limestone impregnated with bituminous matter. Asphalt cement is refined cement tempered with petroleum. Mastic is asphalt cement mixed with powdered limestone and sand. Asphalt concrete is crushed stone cemented with mastic and compressed. Simple asphalt is found at Auvergne (France), Caxatambo (Peru), Cuba, southern California, Switzerland, Trinidad (an island off Orinoco River) and Venezuela. Colorado, Italy, Kentucky, Oklahoma and Utah also contribute. Asphaltic limestone occurs largely in Europe. Auvergne once supplied most asphalt, Caxatambo exports a very pure variety of high luster, Cuba yields considerable asphalt of fine quality, but the supreme sources of supply in quality as in quantity are Trinidad, California and Bermudez in Venezuela. Trinidad contains a lake of asphalt about one mile in diameter, and containing perhaps 6,000,000 tons. An American company exports 100,000 tons a year from this lake to the United States, the supply partly renewing itself by a constant flow of soft pitch from subterranean sources. Bermudez, where an asphalt lake covers 1,000 acres, exports very pure and hard asphalt. In California asphalt was discovered in 1879, and about 1894 beds of very pure, high grade, liquid asphalt were found, which even in its natural state already has the proper consistency for paving. The value of our domestic product almost equals that of the imported product.

Bituminous rock, which is principally used for paving streets, is usually shipped unrefined, and mixed at the place of use with other ingredients. Raw asphalt generally is impure, and must be refined before it can be used. It is manufactured into a cement by mixing it with other forms of bitumen, this cement being used to bind particles of limestone and sand in an asphalt pavement. First the asphalt is melted, the residuum from petroleum being added and this mixture being the paving cement. Clean, sharp sand, heated to 300° is added, and carbonate of lime last. The three substances are mixed thoroughly, and the product is the mixture used, in paving streets. Asphalt is most used for street paying, but it is employed also for the distillation of lubricating and illuminating oils, for cements, for making black Japan varnish, drainpipes of compressed asphalted paper and roofing felts, or paper waterproofed with asphalt, and for waterproofing foundations in bridges and buildings.

Paving streets with asphalt is a simple process. The street is graded solidly to within eight inches of the proposed surface, and rolled. Then a bed of hydraulic concrete is laid, rammed and rolled. On the thoroughness of this preliminary work largely depends the life of the paving. The asphalt is laid in two courses—a cushion one half to one inch deep, and a coat one and one half or two inches thick. The asphalt is applied at a temperature of 250°, dumped and spread evenly with hot rakes. Then hot tampers smooth the surface. It is rolled with three rollers of increasing weight, and sprinkled with cement before the heaviest roller is run.