ForConstructionPros.com

There is a substantial amount of misinformation concerning the function and proper use of prime. Prime is not glue ' causing the base to adhere to the pavement is not its purpose. Prime's purpose is to protect the base from rain and light traffic when the paving will be delayed.

A prime coat is an application of an asphalt cutback or emulsified asphalt to a prepared base. The base can be either a subgrade or a base rock. Prime functions as a protector from rain for both types of bases. A prime coat should not be confused with a tack coat. Tack is an adhesive for gluing two asphalt layers together.

A subgrade or rock base is not a single component. It's made up of millions of varying sized particles. In the process of finishing a rock base, the surface is graded and rolled repeatedly to obtain required compaction.

This process will grind the surface into a smooth texture. This results in millions of tiny particles being present at the surface. But, unlike concrete, which also floats small particles to the surface during finishing, there is no significant adhesion between the particles of rock. You can literally pinch up the loose material by hand.

Optimum moisture content of the base is important for density. Restricting a completed base from absorbing additional moisture retains the base's structural value. Prime is useful for moisture control of a completed base. It slows down the evaporation of the water applied for compaction and retards penetration of rainwater. Prime will also control dust if the base will be used for temporary traffic.