400 ROADS AND PAVEMENTS 



produced by the cars carry the binding dust off, thus exposing 

 the surface stones to the action of rain and frost. To prevent 

 the rapid destruction resulting from such traffic, the method 

 of sprinkling with oil has been extensively practiced. Oiling 

 prevents the binding dust from flying off the surface, and 

 under the rolling action of the traffic this dust binds again with 

 the surface stones. This remedy is, however, of only tem- 

 porary nature; repeated applications are required, and besides 

 it is not always effective. The more recent practice of dealing 

 with macadam roads is to protect them by covering them with 

 bituminous materials. A macadam road so treated is called 

 a bituminous macadam road. There are many methods of 

 constructing this form of road, chief among them being the 

 surface method, the penetration method, and the mixing method. 



The surface method consists in applying the bituminous 

 material to the surface of a macadam-finished road; it is 

 especially adapted for roads that have already been built. 

 Before applying the bituminous material, all dust and dirt 

 must be removed from the surface. The material is then 

 applied either cold or hot, at a temperature of from 100 to 

 250 F., and in quantities from $ to | gal. per sq. yd. Means 

 must be provided also for an even distribution of the bitumi- 

 nous material. After this has been done, a thin layer of sand 

 or stone chips is spread on the surface and rolled with a heavy 

 roller. 



In the penetration method, the bituminous material takes the 

 place of the stone-dust binder used in the ordinary macadam 

 road. The macadam is built in the manner previously 

 described, but, instead of the stone binder, hot bitumen is 

 poured in quantities of about 1 to 1} gal. per sq. yd. Before 

 rolling, stone chips about $ in. in size are spread over the 

 surface. After rolling, another coat of bitumen, at the rate of 

 about 1 J gal. per sq. yd. is applied. Stone chips are then spread 

 again and rolled until a firm and smooth surface is obtained. 



When the mixing method is employed, the bitumen is mixed 

 with the upper layer of broken stone before placing the latter 

 on the road. This method is similar to the one known as 

 bitulithic pavement and described under the heading City 

 Pavements. The difference lies chiefly in the manner of 





