Fog Sealing Older Pavements

In its “A Basic Asphalt Emulsion Manual”, the Asphalt Emulsion Manufacturers Association and the Asphalt Institute describe a fog seal as a “light application of slow-setting emulsion sprayed on an existing asphalt surface.” Properly applied and applied at the right time, a fog seal can be a valuable aid in the maintenance of an asphalt pavement. The question is, what is the proper application and when is the right time to use fog seals in a pavement maintenance program?

Again quoting from the above-referenced manual, “It is used to renew old asphalt surfaces that have become dry and brittle with age and to seal very small cracks and surface voids. It also coats  aggregate particles at the surface.” Too many times, a pavement owner or property manager is sold on a fog seal application when another type of treatment is more appropriate. The results from an incorrect application can be disastrous. If an asphalt pavement is very smooth and nonporous (i.e. a new fine density asphalt mix or a pavement that has been previously treated with an asphalt sealer or slurry seal), the fog seal application cannot penetrate the pavement and properly cure. When traffic, both  vehicular and foot, are allowed access to the pavement (especially in warm weather), the fog seal typically will track on the tires and people will track the sticky fog seal on their shoes, possibly damaging flooring, sidewalks, etc. Because a fog seal is a light application (typically a 50%-50% mix of an SS-1/SS-1h asphalt emulsion and water applied at 0.10/gallon to the square yard), the tendency of the owner or inexperienced person is to order the distributor truck operator to increase the application so as to make the pavement have a heavier sealed look. This heavier application compounds the  racking problem ten-fold.

A proper maintenance system for a smooth asphalt pavement (driveways and parking lots) would be either an asphalt sealer application (a mixture of asphalt emulsion, water, and various fillers) or a Caltrans Standard Specifications Section 37 slurry seal (a mixture of asphalt emulsion, water and fine aggregates).