This is the only reference to prime coat in the book. The technical information agrees on the function of the prime coat as a protector for base, but gives no firm recommendation for its use.
The political side
I have personal knowledge of the political side of the "use of prime debate." I have served on the Specification Committee for the Asphalt Contractors Association of Florida for 19 years and was chairman for nine of those years. During that period, this committee reviewed all of the asphalt specifications for Florida.
Approximately 14 years ago, the FDOT came to the Asphalt Contractors Association to request that prime be eliminated as a requirement for the rock bases and that its use becomes optional.
At an approximate cost of $.25 per square yard, the FDOT felt they could save substantial amounts of money that they were paying purely to provide convenience to the contractor.
The association was against making prime optional, even though it acknowledged that prime has no structural value and is strictly a rain barrier for the rock base. However, it argued that in Florida the paving contractor and the grading contractor are often two different entities.
If priming were made optional, the grading contractors would want the rock bases paved each time they had a small area ready, resulting in paving work being performed in much smaller sections, similar to paving a checkerboard.
Since asphalt pavement is laid smoother and more productively in large, long, straight sections, we argued that allowing the grading contractors to pave the jobs in smaller pieces would result in ridability problems.
The association felt the FDOT should continue to require and pay for the prime for that purpose. If not for the actions of the Asphalt Contractors Association, prime would not be required for rock base by Florida specifications today.
Another common issue dealing with construction of prime coats is the application of sand. Correct construction involves spraying the prime, allowing it to cure for 24 hours, and applying sand only to those areas that have not dried after that period of time.
If sand is applied to a prime coat immediately after it is sprayed, the prime, taking the path of least resistance, will coat the loose sand, rather than serving as a moisture retarder for the underling surface. Also, since the prime coats the loose sand, it is broomed off prior to paving.
In summary, if the rock is prepared and paving is imminent, application of prime coat becomes optional. If the rock base is prepared and some time will elapse or inclement weather is anticipated prior to the area being paved, prime is useful to protect the base for that period of time.
Sanding should only be done in those areas that stay wet after the cure period. Sanding of prime in general is not necessary, except in those instances when traffic is going to be using the base for a period of time.
Jon Chellgren is a professional engineer with more than 30 years of experience managing paving and grading companies. He currently works as a consultant in Boca Raton, FL. You can contact him at JDChellgren@aol.com