Prime coat is not required when asphalt is placed directly on what?

• A. Subbase.
• B. Base course.
• C. Subsurface.
• D. Subgrade.

Answer: (D)

Prime coat is used to improve adhesion between asphalt and a granular base, sealing and priming a base surface so the asphalt binder bonds well. When asphalt is placed directly on the subgrade—the natural soil—there isn’t a base layer for the prime coat to prime, so applying a prime coat isn’t needed. In that situation, you focus on proper subgrade preparation and may use a tack coat between lifts for bonding rather than a primer on the surface. That’s why the correct scenario is placing asphalt directly on the subgrade.