What are asphalt seal coats and why are they used?
An asphalt seal coat as defined by the Asphalt Institute is “a thin surface treatment used to improve the surface texture and protect an asphalt surface.” Moreover, asphalt seal coats are preventative maintenance procedures that are applied to the surface of pavements to prevent or delay corrective measures (Yamada, 1999).
Fundamentally, asphalt seal coats are topical treatments designed to seal and protect the surface of an asphalt pavement from harmful environmental conditions such as sunlight and precipitation. Additionally, surface treatments are also applied to improve the surface wearing properties and improve traction for traffic.
Asphalt seal coats are made of asphalt material such as asphalt cutbacks, asphalt emulsions, or paving-grade asphalt cement. In addition, modifiers such as rubber, latex, polymers, and rejuvenators are often added to the asphaltic liquid mixture.
Asphalt seal coat treatments specifically address the road surface and do not address issues with the road base or any other material beneath the asphalt pavement.
In summary, the benefits of asphalt seal coats include:
- Protects the pavement surface from moisture intrusion
- Shields the pavement structure from oxidation
- Improves pavement life
- Slows reflective cracking
- Delays the need for major maintenance or rehabilitation
- Seals small to moderate cracks
- Increases skid resistance
- Improves color contrast between pavement and striping
- Restores surface characteristics
Presently in the United States, there are eight different types of seals coats that are most often used including:
- fog seals
- rejuvenating fog seal
- sand seals
- slurry seals
- cape seals
- sandwich seals
- chip seals
Following is a brief overview of each type of seal coats as defined by the Asphalt Institute:
A fog seal is a light application of diluted asphalt emulsion that is used to renew old asphalt surfaces, seal small cracks and surface voids, and inhibit raveling.
A rejuvenating fog seal is a light application of a diluted asphalt emulsion that contains additional additives such as petroleum oils, maltenes, or resins. The benefit of a rejuvenating fog seal is by restoring oxidized components in the existing road asphalt binder and minimize the rate of aging and oxidization.
A sand seal is a sprayed application of asphalt emulsion followed by a covering of clean sand or fine aggregate. A pneumatic-tire roller is often used after applying the sand. Excess sand is removed from the road surface after rolling. Sand seals enrich weathered pavements and fill fine cracks in the pavement surface. The sand can provide additional skid resistance to the pavement while also inhibiting raveling.
A slurry seal is a mixture of emulsified asphalt, well-graded fine aggregate, mineral filler or other additives, and water. A slurry seal will fill minor cracks, restore a uniform surface texture, and restore friction values.
Micro-surfacing is a mixture of polymer modified asphalt emulsion, crushed dense graded aggregate, mineral filler, additives and water. It provides a thin resurfacing of 3/8 to 3/4 inches (10 to 20 mm) to the pavement.
A cape seal is a surface treatment where a chip seal is followed by the application of either slurry seal or micro-surfacing.
A sandwich seal is a process that consists of the uniform application of one course of aggregate to a prepared surface, followed by the application of an asphalt emulsion, which is then followed by the uniform application of a second course of smaller aggregate, which is then rolled. A sandwich seal is similar to a double chip seal but the first layer is without a binder.
A chip seal is a road surface treatment that consists of a layer of asphalt binder (hot or emulsion) with a layer of embedded aggregate. There can be more than one layer of binder and aggregate. Chip seals add skid resistance to an existing road surface, stops raveling, seals minor cracks, and delays further deterioration of the existing roadway.
Blue Line Road Products manufactures specially engineered asphalt emulsion seal coats. For more information regarding our seal coats, please contact a Blue Line Road products sales engineer at 503-279-2600.
Asphalt Seal-Coat Treatments
Alan Yamada; Civil Engineer; Project Leader
San Dimas Technology & Development Center
United States Department of Agricultural April 1999
Asphalt Institute: Glossary of Terms
Pavement Preservation and Recycling Alliance
Asphalt Seal Coats
Washington Department of Transportation, Technology Transfer (T2), 2003