ASPHALT PAVEMENTS.
Asphaltic Paving Materials. All asphaltic or bituminous pave ments are composed of two essential parts—namely, the cementing material (matrix) and the resisting material (aggregate). Each has a distinct function to perform; the first furnishes and preserves the co herency of the mass; the second resists the wear of traffic.
Two classes of asphaltic paving compounds are in use,—namely, natural and artificial. The "natural" variety is composed of either limestone or sandstone naturally cemented with bitumen. To this class belong the bituminous limestones of Europe, Texas, Utah, etc., and the bituminous sandstones of California, Kentucky, Texas, Indian Territory, etc. The "artificial" consists of mixtures of asphaltic cement with sand and stone dust. To this class belong the pavements made from Trinidad, Bermudez, Cuban, and similar asphaltums. For the artificial variety, most hard bitumens are, when properly prepared, equally suitable. For the aggregate, the most suitable mate rials are stone-dust from the harder rocks, such as granite, trap, etc., and sharp angular sand. These materials should be entirely free from • loam and vegetable impurities. The strength and enduring qualities of the mixture will depend upon the quality, strength, and proportion of each ingredient, as well as upon the cohesion of the matrix and its adhesion to the aggregate.
Bituminous limestone consists of carbonate of lime naturally cemented with bitumen in proportions varying from SO to 93 per cent of carbonate of lime and from 7 to 20 per cent of bitumen. Its color, when freshly broken, is a dark (almost chocolate brown, the darker color being due to a large percentage of bitumen. At a tem perature of from 55° to 70° F., the material is hard and sonorous, and breaks easily with an irregular fracture; at temperatures between 70° and 140° F. it softens, passing with the rise in temperature through various degrees of plasticity, until, at between 140° and 160° F., it begins to crumble; at 212° it commences to melt; and at 280° F. it is completely disintegrated. Its specific gravity is about 2.235.
Bituminous limestone is the material employed for paving pur poses throughout Europe. It is obtained principally from deposits at Val-de-Travers, canton of Neufchatel,, Switzerland; at Seysell, in the Department of Ain, France; at Ragusa, Sicily; at Limmer, near Hanover; and at Worwohle, Germany.
Bituminous limestone is found in several parts of the United States. Two of these deposits are at present being worked—one in Texas, the material from which is called "lithocarbon"; and one on the Wasatch Indian Reservation. These deposits contain from 10 to 30 per cent of bitumen.
The bituminous limestones which contain about 10 per cent of bitumen are used for paving in their natural condition, being simply reduced to powder, heated until thoroughly softened, then spread while hot upon the foundation, and tamped and rammed until compacted.
Bituminous sandstones are composed of sandstone rock impreg nated with bitumen in amounts varying from a trace to 70 per cent. They are found in both Europe and America. In Europe, they are chiefly used for the production of pure bitumen, which is extracted by boiling or macerating them with water. In the United States, exten sive deposits are found in the Western States; and since 1880 they have been gradually coming into use as a paving material, so that now up wards of 150 miles of streets in Western cities are paved with them. They are prepared for use as paving material by crushing to powder, which is heated to about 250° F. or until it becomes plastic, then spread upon the street and compressed by rolling; sometimes sand or gravel is added, and it is stated that a mixture of about 80 per cent of gravel makes a durable pavement.
Trinidad Asphaltum. The deposits of asphaltum in the island of Trinidad, W. I., have been the main source of supply for the asphal tum used in street paving in the United States. Three kinds are found there, which have been named, according to the source, lake-pitch, land or overflow pitch, and iron pitch. The first and most valuable kind is obtained from the so-called Pitch Lake.