Use of Prime Coat
The Role of Prime Coats in Asphalt
At one time, it was thought that a prime coat was an essential element of good asphalt pavement construction. However, asphalt prime materials and aggregate base gradation have changed over the years, and the use of prime is no longer necessary for most asphalt pavements. In fact, their use can be detrimental to pavement performance. This has led the Virginia Department of Transportation (VDOT), many state and municipal DOTs, and other engineers to remove the requirement from their specifications.
According to studies performed by The Asphalt Institute over the last 20+ years, few, if any, pavement failures can be attributed to the lack of a prime coat. The following is a brief explanation of what prime coats are, which materials are typically used, and why the Virginia Department of Transportation and other specifiers have discontinued their use.
What is the purpose of a prime coat?
There are four primary purposes for the application of a prime coat on an aggregate base course:
- Coat and bond loose material particles on the surface of the base.
- Harden or toughen the base surface to provide a work platform for construction equipment.
- Plug capillary voids in the base course surface to prevent moisture migration.
- Provide adhesion between the base course and the succeeding asphalt course.
What asphalt materials should be used for prime coats?
For a prime coat to be effective, it must penetrate the base course. Typically, a light-grade medium-curing cutback, such as MC-30 or MC-70, will work well. However, these are cutback asphalts (asphalt cement combined with light petroleum oil) that “cure” or harden when the light oils evaporate into the atmosphere. As the light oils evaporate they release hydrocarbons into the air, causing air pollution.
As a result, the EPA has severely restricted or eliminated the use of cutbacks for most areas of the U.S. Virginia Department of Transportation (VDOT) discontinued the use of cutbacks more than 30 years ago when Virginia agreed to severely restrict the use of cutbacks in the late 1970’s as an offset to the emissions from a proposed oil refinery on the Chesapeake Bay. The refinery was never built, but VDOT has used emulsified asphalts (asphalt cement liquefied by suspending it in water) for surface treatment (chip seal), slurry seal, tack, and prime applications ever since.
Virginia Department Of Transportation Discontinues Priming Aggregate Base or Subbase
In the 1990s, VDOT recognized issues with the use of prime and changed its specifications, requiring prime coats only when the total asphalt pavement layer thickness was less than 4 inches. In the early 2000s, VDOT revisited the use of prime coats, evaluating numerous projects where it had been used and where it had not. It was determined that there was no difference in the performance of pavements with or without a prime coat, and constructability/application issues had been experienced with its use. With no identified benefit for the cost, VDOT made the decision to discontinue the requirement to use prime coat entirely, issuing the following language in a Special Provision to its 2007 Road and Bridges Specifications:
“Priming aggregate base or subbase: Unless otherwise specified in the contract documents, priming with asphalt material will not be required on an aggregate subbase or base material prior to the placement of asphalt base, intermediate or surface layers.”
Other states’ DOTs, county and municipal DOTs, state agencies, and engineers have reached the same conclusions as VDOT: no longer specifying a prime coat on unbound aggregate base and subbase materials.
Where do I find VDOT’s Language Eliminating Prime?
The language quoted above can be found in Section 315.05(b)1(a)