COAL-TAR PAVEMENTS. Numerous attempts have been made to construct a pavement by the use of coal tar as a cementing material in place of asphalt or in conjunction with it. This form of pavement was laid somewhat extensively in Washington, D. C., from 1872 to 1887. The earlier pavements were laid under many different patents, of many different mixtures, receiving their name generally from that of the patentee. Most of them were signal failures, although some were fairly durable. In 1886 Congress in making an appropriation for pavements in Washington stipulated that no contract for an asphalt pavement should be made at a higher price than $2.00 a square yard; and no bids having been received for asphalt pavements for less than $2.25, a considerable quantity of coal-tar pavements were laid during the years 1886 and 1887.
Specifications. The specifications employed at Wash ington, D. C., in 1886-87 for coal-tar distillate pavements are, as follows : * "Road-bed. The space over which the pavement is to be laid shall be excavated to the depth of six inches below the top of the surface of the pavement when completed. Any objectionable or unsuitable material below the bed shall be removed and the space be filled with clean gravel or sand well rammed. The bed shall then be trimmed so as to be exactly par allel to the surface of the new pavement when completed, and the entire road-bed shall be thoroughly rolled with a heavy steam-roller.
"Base. The base shall be composed of clean broken stone which will pass through a three-inch ring and which shall be thoroughly coated with No. 41 coal-tar paving cement in the proportion of about one gallon to the square yard of the base. This layer shall be well rammed and shall then be rolled with a steam-roller. The depth after rolling shall be 4 inches.
"Binder. The second or binder course shall be composed of clean broken stone, thoroughly screened, not exceeding II inches in the largest dimension, and No. 4 coal-tar paving cement. The stone shall be heated to a temperature between 230° and 250° F. by passing it through revolving heaters, and shall be thoroughly mixed by machinery with the paving cement in about the proportion of one gallon of tar to one cubic foot of stone. The mixture shall be hauled upon the work, be spread upon the base course at least 2 inches thick, and be immediately rammed and rolled with hand and steam-rollers. The base and the binder shall together measure at least 41
inches when compacted.
"Wearing Surface. The wearing surface shall be composed of the following materials, in the following proportions: "The sand shall be clean sharp river sand, free from clay, and of such a size that not more than twenty per cent shall be retained upon a sieve of twenty meshes to the inch, and not more than five per cent shall pass a sieve of seventy meshes to the inch, about sixty per cent to be coarser than forty meshes to the inch. The powdered stone or stone dust shall be the residue from crushing the stone for the base and the binder; and shall be of such a degree of fineness that 16 per cent by weight shall be an impalpable powder, and that the whole of it shall pass a No. 26 screen. The paving cement shall be composed of refined Trinidad asphalt, 25 to 30 parts, and No. 4 coal-tar paving cement, 75 to 70 parts. The refined asphalt shall con tain at least sixty per cent of bituminous 'matter soluble in carbon bisul phide. The No. 4 coal-tar paving cement shall correspond to a standard to be furnished by the Engineer Commissioner, and shall be free from cess of sooty matter, napthaline, and creosote oils. The hydraulic cement, lime, and sulphur shall be of the best commercial quantity. The materials for the wearing surface shall be heated to not over 260° F.,—the paving cement in kettles, the sand and stone dust in revolving heaters. The hy draulic cement, lime, and sulphur shall be added cold to the sand and stone dust in the sand box before going to the mixer. The ingredients shall be thoroughly mixed by approved machinery.
"Laying. The wearing coat shall be hauled upon the street, and be spread upon the binder course in a layer 2 inches thick with hot iron rakes and other suitable appliances. It shall be immediately compacted, while hot and plastic, with hot tamping irons and hand rollers. It must be thoroughly rolled and cross rolled until it has become hard and solid. In spreading the material the joints between the different loads shall be diagonal to the line of the street. The surface shall be finished with a dusting of dry hydraulic cement rolled in. In cool weather, or when ordered, the carts carrying the mixture shall be protected with canvas covers. The wearing coat shall be at least 1I inches thick after rolling."