ROADS 209 



a properly proportioned mixture of stone dust, sand, pebbles 

 or broken stone, and bituminous cement, all of which are 

 heated, mixed, and spread while hot on a foundation of 

 Portland cement concrete, macadam, or gravel. Sheet 

 asphalt pavement has been used for many years in the 

 cities and is made of natural asphalt mixed with sand 

 and spread on a Portland cement foundation 4 to 6 inches 

 thick. The asphalt is put down in two layers, an elastic 

 layer 1 to 1}^ inches thick and a wearing surface 2 inches 

 thick. Both coats are mixed, spread, and rolled while 

 hot. 



Portland cement concrete. A base for various forms of 

 pavement has been constructed of burned rock cement for 

 many years. The fabrication of a wearing surface using 

 Portland cement as a binder is a comparatively recent 

 development which is coming rapidly into use in all parts 

 of the country. It is placed on the road in either one or 

 two courses. The single course is 6 to 8 inches in thickness, 

 the center being slightly thicker than the edges. The sand 

 and pebbles or broken stone are selected for their wearing 

 qualities and proportioned according to their grading as to 

 fineness. The materials are mixed on the site, placed, fin- 

 ished to the proper surface, thoroughly tamped or rolled, 

 and properly cured. 



Local roads. In certain localities various materials dif- 

 ferent from those specified are used for road construction. 

 Such materials usually have only local application, but may 

 be very effective and satisfactory where used, while they 

 would not be practical in localities differently situated. 



Corduroy. Corduroy roads are used in timbered coun- 

 tries and consist of brush, poles, or logs placed across the 

 line of the road. In roads crossing swamps the timber 

 material is packed closely together. Occasionally it is 

 used without any covering, but it will last longer and be 

 much smoother if the timber is entirely covered from one 

 end to the other with a coating of sand, clay, or gravel. 



