The quality of tar varies greatly for this purpose. A rather light material which flows readily at the temperature of application is required, and it should not contain an appreciable quantity of water. On account of the difficulty of securing a uniform quality of tar, the best results have been obtained with cer tain materials specially prepared for the purpose. A preparation known as "tarvia" has been quite exten sively used, most of the successful tarred roads being constructed by its use.
The tar is commonly heated by steam coils in the tank cars in which it is shipped, and is distributed upon the street in tank wagons which have a small fire-box underneath the tank, to keep the tar hot while trans porting it to the street. Various forms of sprinklers have been employed for the purpose of spreading the tar upon the road, but the best results seem to have been obtained by the use of a hose attached to the rear of the tank wagon and fitted with a valve for controlling the flow, the tar being spread to a width of 12 to 15 feet as the wagon progresses. Laborers with brooms follow the tank and spread the tar evenly over the surface. The tar should be spread as evenly and quickly as possible, in order that it may be accomplished before cooling takes place. Some attempts have been made to use tar for this purpose without heating. This requires tar light enough to flow readily at ordinary air temperatures. There is as yet no experience on which to form a judgment concerning the success of these lighter materials.
The coating of screenings upon the surface of the road should not be placed until time has been given for the tar to penetrate into the road and should only be sufficient to cover the surface lightly, so as to dry the surface and take up any surplus tar, without leaving any residue to be reduced to powder .and be blown or washed away.
The cost of treating a road in this manner is compara tively slight, having been in a number of instances from 5 to 8 cents per square yard. Experience has shown that the surface of a macadam road treated in this manner with good materials will not grind up into deep dust, and therefore the cost of maintaining is reduced and the life of the road prolonged. Such roads are likely to come into more general use in the future, and to prove economical and desirable for country roads and for village and suburban streets of light traffic.
Bituminous Concrete. Tar concrete has, as already noted, been quite extensively used in England. This construction has been employed to a limited extent in America, but has not been applied to ordinary road work. Bituminous concrete of better grade, using carefully graded stone and specially prepared bitumi nous cement, is extensively used under the name of "bitulithic pavement." This is discussed in Art. 62.
Macadam With Asphalt Filler. In several instances, roadways have been constructed of macadam the surface of which is filled with raw rock asphalt. The method of construction employed in these roads is to form the macadam surface in the usual manner and then, in place of the usual binding material to apply a top dressing of ground rock asphalt and roll to a smooth surface.
The asphalt which has been applied to this use is Kentucky rock asphalt, containing at least 8 per cent bitumen. It is applied in fine condition, without heat, and is found to pack firmly under the rolling and traffic, the asphalt being forced into the interstices in the stone and serving as a binder for the macadam surface.
It is claimed that the surface of a road formed in this manner will not form dust and will not ravel, on account of the oily nature of the binder. Some roads con structed in this manner are reported to have given satisfactory results for the short time they have been in use. These roads are sometimes called by the name of the company introducing them, " Wadsworth Macadam." Oiled Macadam. Gravel and macadam roads are constructed in California, using asphaltic oil in much the same manner that the tar macadam is used in the Eastern states. The following extract from the specifications of City Engineer Homer Hamlin for graveled streets in Los Angeles shows the method of construction employed.
Surfacing Roadway. "Upon the surface prepared and brought to subgrade in the manner above specified, shall be spread one layer of good gravel to have a thick ness of 4 inches, unless otherwise shown on the plans, profiles or cross sections, after having been rolled. The surface of this layer of gravel to a depth of one inch shall be raked free from all stones larger than i inch in the greatest dimension. If no gutters are provided, these larger stones shall be raked to the curb and distributed over a strip two feet in width next to the curb.