30 BULLETIN 691, U. S. DEPARTMENT OF AGRICULTURE. 



exact grading of tlie aggregate. The hot mixture should be de- 

 posited in a uniform layer over a concrete or well compacted 

 broken stone or gravel foundation to such depth that it will yield 

 a uniform layer 2 inches in thickness after thorough rolling. 

 A seal coat of the same bituminous material should then be 

 applied at the rate of 0.4 to 0.6 gallon per square yard and covered 

 with a sufficient amount of chips to just absorb the excess bitumen. 

 Roads of this character may be maintained with oil surface 

 treatments, using the materials and methods given under Specifi- 

 cations OC-2 or OH-1. The latter is usually to be preferred. 

 Such maintenance treatments should be applied whenever the 

 wear comes directly upon the wearing course rock. 



