Highway Materials – Purpose of a Prime Coat In bituminous road construction, what is the principal purpose of applying a prime coat on a prepared granular base (WBM/WMM) before laying the first bituminous layer?

Introduction / Context:
In flexible pavements, interface treatments control how well successive layers act together. A prime coat is used when a bituminous layer is to be placed over a prepared granular base. It seals surface voids, hardens the top, and promotes adhesion of the forthcoming bituminous layer, ensuring composite action and durability under traffic and weather.

Given Data / Assumptions:

• Base: granular (e.g., WBM/WMM) prepared to the required density and surface finish.
• Next layer: bituminous binder or wearing course.
• Application: low-viscosity cutback/emulsion to penetrate and bind surface fines.

Concept / Approach:

The prime coat serves two linked functions: sealing and bonding. Sealing reduces dusting and water infiltration; bonding ensures adequate shear transfer at the interface so the bituminous layer does not slip or debond under wheel loads. This is distinct from a tack coat, which specifically promotes adhesion between two bituminous surfaces.

Step-by-Step Solution:

Verification / Alternative check:

Field practice shows that bases receiving a proper prime coat exhibit better adhesion and reduced stripping at the base–bitumen interface compared to unprimed surfaces.

Why Other Options Are Wrong:

• Option b describes a tack coat (bitumen over bitumen), not a prime coat.
• Options c and d concern deeper layers (sub-base–sub-grade), unrelated to prime coat application.

Common Pitfalls:

Using prime where a tack coat is required; over-application causing bleeding; inadequate curing before overlay, which can trap volatiles and weaken the bond.

Final Answer:

Promote the bond between the base and the wearing courses