16 BULLETIN 691, U. S. DEPARTMENT OF AGRICULTURE. 
325° F.. and the mineral aggregate to a temperature of from 
250° F., to 300° F., before mixing. The mixture should contain 
from 69c to 8% bitumen, and will vary within these limits with 
the character of the rock and the exact grading of the aggregate. 
The hot mixture should be deposited 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 to 2* inches in 
thickness after thorough rolling. A seal coat of the hot oil 
asphalt should then be applied at the rate of 0.4 to 0.6 gallon per 
square yard and covered with a sufficient amount of stone 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 Specifications 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. 
