20 BULLETIN 691, U. S. DEPARTMENT OF AGRICULTURE. 
hot sand and followed by the asphalt. The mixture should con- 
tain from 9 per cent to 12 per cent bitumen and will vary within 
these limits with the exact grading of the aggregate. After 
thorough mixing the hot mixture should be deposited in a 
uniform layer on a binder course which has been previously 
laid over a concrete base. It should be laid to such depth that 
it will yield a uniform layer li or 2 inches in thickness as may 
be required after thorough rolling. 
As a seal coat for tar macadam or tar concrete roads it is 
applied during construction of the road at a temperature of 
from 300° F. to 350° F. It is used as a substitute for tar seal 
coat and may be applied at the same rate and covered in the 
same manner as described under Specifications TP-2 and TP-3. 
