Bituminous Coatings on Concrete Surfaces.—The concrete road surface is sometimes coated with a layer of bituminous material and stone chips or gravel pebbles. This is particularly advisable where no really satisfactory aggregates are available and the concrete surface would not possess sufficient durability. The bituminous material is applied hot to the surface and is then covered with stone chips or gravel pebbles, ranging in size from ¾ inch down to ¼ inch, the resulting coating being about ¾ inch thick. Many failures of this type of surface have been recorded due to the difficulty of securing adhesion to the concrete. This seems to be due in part to inability to get the proper bituminous materials and in part to climatic effects. Considerable progress has been made in developing this type of surface and it may eventually become a satisfactory maintenance method.
Characteristics.—The concrete road is of a granular texture and is not slippery. It is of course rigid and noisy for steel tired vehicles. It is an excellent automobile road and its low tractive resistance makes it a desirable surface for horse drawn vehicles. It possesses a high degree of durability if properly constructed. It is likely to crack indiscriminately but as a general rule the cracks are not a serious defect.
Maintenance.—The cracks that appear in the concrete surface are filled once or twice a year, tar or asphalt being employed. The dust and detritus is cleaned out of the cracks and the hot filler poured in, with enough excess overflowing to protect the edges.
Chapter IX
VITRIFIED BRICK ROADS
Vitrified brick roads consist of a foundation course of Portland cement concrete, broken stone or slag macadam, or of brick laid flat, the first named being by far the most generally used, and a wearing course of vitrified brick.
Vitrified Brick.—Vitrified brick are made from clay of such a character that when heated to the required temperature they will fuse into a glassy texture. Brick roads are constructed on roads carrying the severest of traffic and the brick must therefore be tough and of high resistance to wear.
Not all of the clays from which brick may be manufactured will produce a product suitable for road construction, and paving brick, even though truly vitrified, are of different degrees of durability, depending upon the nature of the clay and the care exercised in the manufacture.