What characteristic of a prime coat helps in the adhesion of overlying asphalt courses?

The correct answer highlights the importance of low viscosity in a prime coat's ability to promote adhesion with overlying asphalt courses. A prime coat is typically a liquid tack coat applied to the surface of a base material to enhance the bonding between the base and the asphalt layer that will be placed on top.

When the viscosity of the prime coat is low, it allows the liquid to penetrate into the surface of the base material more effectively. This penetration helps to fill voids and create a textured surface that provides a stronger mechanical bond for the subsequent asphalt layer. The low viscosity ensures that the prime coat can spread easily over the surface, maximizing contact with the underlying material and facilitating a strong adhesive bond.

In contrast, while thickness can influence the performance of the prime coat, it is not as crucial as viscosity in enabling good adhesion. A prime coat that is too thick may not spread evenly or may dry too quickly, hindering proper bonding. Color and temperature may affect visual monitoring and application conditions but do not directly impact the adhesive qualities of the prime coat in the same way that viscosity does.