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



to a temperature of from 275° F. to 325° F. and the mineral 

 aggregate to a temperature of from 250° F. to 300° F. before 

 mixing. The mixture should contain from 7 per cent to 11 per 

 cent bitumen and will vary within these limits with the character 

 of the rock and the exact grading of the aggregate. The hot mix- 

 ture 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 inches in thickness after 

 thorough rolling. A seal coat should not be necessary on this 

 type of pavement, which is usually maintained by patching with 

 the same mixture used in construction. 



