Prime Coat Mix Design

A traditional mix design is not done for prime coats.  Many user agencies have decision trees or design methodologies that they use in the design process. This subsection provides items that should be included in the thought process when a pavement designer or contractor is considering the application of a prime coat. Prime coat design is a very simplistic but important process that should embrace several factors, including:

• Location of the construction project (e.g., non-attainment area)
• Material to be primed
• Base construction process (e.g., slush roll or blade and roll)
• Thickness and composition of next pavement layer to be applied
• Experience, expertise, and equipment of available contractors
• Probability of inclement weather while the prime coat is exposed
• Projected need to carry significant traffic or carry traffic for a prolonged period

According to Senadheera and Vignarajah (2007), penetration of the prime coat into a base is very important to obtain the maximum benefit from the prime coat. The amount of penetration depends on a number of factors including the prime coating method, prime coat binder, base material, base finishing technique, and the permeability of the base course.Typical penetration depth of a sprayed cutback prime might range from 1/8 inch to 3/8 inch. Occasionally, penetration can exceed 1/2 inch. The photograph illustrates actual penetration of an MC-30 prime for a specimen prepared in a laboratory.