15 May 2001
Void content of base course.
Curing time of prime material.
(2) The recommended priming materials are emulsified asphalts and cutback (liquid) asphalt.
The recommended types and grades are shown in table 3-1.
Prime Coat Materials
(3) A prime coat can only work if it penetrates into the base course. Open-textured (high-void
content) bases can be primed easily, but a tight surface (low voids) cannot be readily penetrated. In
cases of low voids the less viscous cutbacks such as RC-70, MC-30, MC-70, and SC-70 should be
considered. If penetration does not occur, an asphalt film will be left on the surface of the base causing
slippage of the bituminous surface during and after construction. Caution should also be urged in using
RC-70 or RC-250 because the solvent in the cutback may evaporate rapidly or be absorbed by the base-
course fines and leave an asphalt film deposited on the surface. Undiluted emulsions can also cause
asphalt film problems if the base-course surface is tight.
(4) Weather can influence the choice of the correct priming materials. Since emulsions are
dependent on the evaporation of water for curing, low temperature or high humidity can slow or stop the
curing process. Cutbacks are not as dependent on weather conditions as emulsions. In cold weather,
however, the rapid curing cutbacks (RC's) may perform better than the slower curing cutbacks (MC's
(5) Environmental restrictions have begun to limit the types of prime coat materials that are
available in some areas of the United States. As a result, some cutback asphalts are not available for
priming. Therefore, asphalt emulsion primes are becoming more numerous. Asphalt emulsions must be
diluted with water before being applied as a prime, and special handling and storage considerations to
prevent freezing, settling, and breaking must be exercised.