30 BULLETIN 691, U. S. DEPARTMENT OF AGRICULTURE. 
exact grading of the 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. 
