Fog seals can be a valuable step as preventive maintenance on weathered asphalt paved surfaces. Fog seals can delay costly corrective measures caused by unravelling surface aggregate and harmful environmental conditions such as sunlight, rain, and snow. Fog seals are typically done on weathered roads that display the signs of oxidation, small cracks (less than 1/8” wide), pitting, and raveling.
The Asphalt Emulsion Manufactures Association (AEMA) defines a fog seal as “a light spray application of dilute asphalt emulsion used primarily to seal an existing asphalt surface to reduce raveling and enrich dry and weathered surfaces.” According to the Texas A&M Transportation Institute (TTI) the purpose of a fog seal is to “coat, protect, and/or rejuvenate the existing asphalt pavement.” In addition, a fog seal will decrease the permeability to rainwater (and air) into the pavement surface and decrease the rate of the oxidation of the asphalt pavement. Furthermore the Asphalt Institute states that a fog seal can also seal small cracks on the surface of the roadway.
To be effective, a fog seal must at least:
- Seal the surface of an asphalt pavement
- Be insoluble in rainwater
- Improve the pavements water resistance
- Decrease the oxidation of the pavement surface
- Penetrate into the voids of the pavement surface
In addition to treating weathered asphalt paved surfaces, fog seals can also be applied to chip seal surfaces. Here a fog seal can lock down marginally embedded chips therefore extending the life of the surface treatment. Furthermore, the fog seal will reduce the likelihood of chips breaking windshields when opened up to traffic. The life of a chip sealed surface can be extended (at least a year) when a fog seal is applied. In addition, a fog sealed road can have an aesthetically-pleasing dark paved look that will also extend the life of road stripping.
Fog seals are commonly slow-setting asphalt emulsions such as CSS-1, CSS-1h, SS-1, or SS-1h. These emulsions are normally diluted with water for application purposes. The dilution rate is contingent upon the condition of the asphalt pavement surface to be treated. In most cases the emulsion will be diluted with 1-part water to 1-part emulsion concentrate, but at times, it may be diluted with water up to a 2:1 dilution.
The application rate, the amount of diluted fog seal applied per square unit area, will be contingent upon the condition of the road…the surface texture, degree of cracking, and dryness of the road. The rougher the road surface, the more fog seal can and need to be applied due to the increased surface area. In contrast, smooth tight surfaces will require less fog seal. Typically the application rate ranges from 0.08 to 0.15-gallons per square yard.
The optimum application rate can be determined by doing a quick simple field test. Here you add a half-quart sample of the diluted fog seal to a plant watering can and sprinkle it evenly over a 1-square yard of the surface that you plan to treat. If the diluted emulsion is not absorbed in 10-minutes, then start the test over on a fresh surface by adjusting the quantity or dilution rate. Repeat until the proper application/dilution rate is determined. Keep in mind that a fog seal that is applied too thick will not properly penetrate the surface voids and the asphalt residual will be in excess which can cause a slippery surface. On the other hand, if the application is too light, it may underperform and compromise its effectiveness and longevity.
Fog seals are normally applied using computerized rate controlled distributor trucks. The diluted fog seals are typically applied at warm ambient temperatures but are sometime slightly heated (i.e., up to 122-degrees F). The rate of curing for slow set diluted fog seals is contingent upon the weather. The dryer and warmer it is, the faster it will cure and the sooner the road can be opened back to traffic.
Blue Line Road Products manufactures BL-Fog and MicroCoat® that are engineered to be exceptional alternatives to the traditional slow-set emulsion fog seal products. Both BL-Fog and MicroCoat® were engineered to promote better adhesion on a variety of asphalt surfaces. In addition, both BL-Fog and MicroCoat® are formulated for increased durability and faster return to traffic. For more information regarding BL-Fog and MicroCoat®, please contact a Blue Line Road products sales engineer at 503-279-2600.
Alan Yamada, “Asphalt Seal-Coat Treatments”. US Department of Agriculture/Forest Service, April 1999, https://www.fs.fed.us/eng/pubs/html/99771201/99771201.htm#APPA
Caltrans Division of Maintenance, MTAG Volume I Flexible Pavement Preservation 2nd Edition, October 2007, https://www.csuchico.edu/cp2c/_assets/documents/caltrans/fpmtag-chapter-6—fog-seals.pdf
Nikornpon Prapaitrakul, Tom Freeman, and Charles J. Glover, “ANALYZE EXISTING FOG SEAL ASPHALTS AND ADDITIVES: LITERATURE REVIEW”. Texas Transportation Institute The Texas A&M University System, December 2005, https://static.tti.tamu.edu/tti.tamu.edu/documents/0-5091-1.pdf
United States Department of Transportation, “Fog Seal Application Checklist”, https://www.fhwa.dot.gov/pavement/preservation/ppcl04.pdf