The term "tar macadam" or sometimes "bitumi nous macadam" has been used to cover the use of bituminous materials for bonding together or preserv ing the surface of broken-stone roads in a variety of ways. In some instances, the tar is only used to coat the surface of a macadam road, with a view to prevent the formation of dust and the raveling of the road; in others, the tar is mixed with the broken stone used in building the road to form a bituminous concrete before laying it on the road.
Tar Macadam in England. Tar macadam pave ments have been used for quite a number of years in England, and have met with fairly good success. The usual method of construction, in England, is to first dry the stone by heating it upon a hot hearth. The tar is boiled for three or four hours and is then mixed with the hot stone, and turned with hot shovels. The stone is graded usually into two or three sizes; the larger stone, from I to 2 inches in diameter, is used for the bottom layer; the stone from one-half to I inch, for the upper layer; and sometimes the fine stone, from one quarter to one-half inch, is used on top to fill the inter stices in the surface layer. The quantity of tar varies from 8 to 12 gallons per cubic yard of stone. Each layer is separately rolled, and the top is covered by a coating of stone screenings. In some instances, the lower layer is not mixed with the tar, but is placed and rolled and then coated with tar before placing the upper layers. The tar concrete, formed, as above described, by mixing hot tar with broken stone, is sometimes stored in bins for about three weeks before using, to permit the tar to thoroughly saturate the stone before placing it in the road.
This method of construction seems to have given satisfactory results, and its use is rapidly extending. It forms, when well constructed, a smooth, practically noiseless pavement, which is easily cleaned and less to maintain than an ordinary macadam road.
Tarred Surfaces on Macadam Roads. The applica tion of tar to the surfaces of macadam roads, for the purpose of eliminating dust and preventing the ravel ing of the surface, has been found to be efficient and economical in many places. This method is largely
used in France, where it has been employed for several years. French engineers who have reported upon the matter express the opinion that the cost of main tenance is considerably reduced and the life of the road prolonged by such treatment.
This method is similar to that used in oiling mac adam surfaces and differs considerably in different localities. The road must be very dry when the tar is applied, and the work should be done during warm, dry weather in order to secure the best results. The tar will not penetrate properly if the road be damp.
When an old road is to be treated without resurfac ing, all of the dust should be removed and the surface swept clean. The tar is then applied at a temperature of about 200 degrees F. and allowed to stand for at least 8 or Io hours, in order to permit the tar to penetrate well into the surface of the road. The amount of tar used should be only sufficient to form a thin coating over the surface of the road, and any tar which may collect in low spots in the road surface should be brushed out evenly and uniformly distributed. After sufficient time has elapsed for the penetration of the tar, a light layer of stone screen ings or sand is placed over the surface to take up the surplus tar, and the road is rolled with a heavy roller.
When the surface of the macadam road requires repairing and reshaping, the dust is first removed and the surface loosened by means of spikes in the wheels of the roller. New material is added where necessary and the road reshaped and rolled to a hard surface. The tar is then applied as in the other case.
The amount of tar required varies with the character of the material composing the road surface; ordinarily about one-third gallon to one gallon per square yard of road-surface will be required. A second application is usually desirable after a year. This second application requires less tar. On some of the French roads an application of tar is made about once in two years. This, it is claimed, is cheaper than sprinkling and materially reduces the cost of maintenance.