what is a fog seal?
A fog seal is a surface application that involves the spraying of a thin coating of asphalt emulsion over an existing asphalt surface as a fine mist, a procedure better known as “fogging.” This inexpensive maintenance treatment lengthens pavement life and postpones the need for major repairs or rehabilitation.
Fog seals serve multiple functions, including rejuvenating aged and oxidized pavement, sealing minor cracks, and slowing down surface deterioration such as raveling. By creating an insulating layer, fog seals reduce the rate of moisture infiltration, which would otherwise accelerate pavement wear and deterioration. They provide temporary protection for deteriorated road segments, keeping them serviceable while more extensive repairs are taking place.
Asphalt emulsions Fog seal asphalt emulsions consist of tiny globules of paving asphalt suspended in water with an emulsifying agent-or surfactant-and sometimes a rejuvenator. The surfactant keeps the asphalt globules uniformly dispersed until the emulsion is applied to the pavement, where the water starts to evaporate. A rejuvenator is an additive that slightly softens the existing pavement upon application, enhancing adhesion and improving pavement condition.
Overall, fog seals are a means of prolonging pavement life and improving the pavement surface. Most importantly, fog seals serve to temporarily stabilize a deteriorating roadway until proper maintenance or rehabilitation work can be completed.
During application, the emulsion wets the surface of the aggregate and the existing binder film. Cationic (positively charged) emulsions can displace water from the surface of an aggregate or aged asphalt film. The emulsion then breaks by loss of water and chemical action, forming a film of new binder on the aggregate and existing binder film. The rate at which the emulsion breaks is dependent on several factors with weather conditions (e.g., wind, rain, temperature, etc.) being dominant factors. For anionic (negatively charged) emulsions, there is no surface-specific interaction with most aggregates. The emulsion breaks due to water loss by evaporation and absorption of water by the aggregates and surface voids of the pavement.
The most common cationic emulsion bitumen used for fog seals are CSS-1 and CSS-1h.
Fog Seals Process
Surface Preparation
Sealing of minor cracks and cleaning of the surface prior to treatment.
Spray Application
Light spray application of approximately 0.10 gallons per square yard of a diluted asphalt emulsion.
Fine Aggregate Application
Light spreader application of approximately 0.5 pounds per square yard of a fine cover aggregate to aid in surface texture.
Curing
Typical curing in 30 to 60 minutes (sometimes shorter or longer depending on road and atmospheric conditions) prior to returning to traffic.
Road Requirements for Fog Seals
Fog seals can be used on a variety of road types, with a full range of traffic types, and applied in any climate type. Fog seals are applicable for pavement surfaces where the penetration of the diluted emulsion can be expected. This typically includes roads with aged and raveled asphalt surfaces, as well as chip and seal and open-graded surfaces. In addition, the roads need to have adequate structural capacity and are not applied on any roads that have signs of structural distress such as excessive cracking and rutting or shoving.
Benefits
- A low-cost way to keep good roads in good condition
- Protects pavement from moisture and oxidation
- Addresses minor surface cracks and raveling
- Increases road safety by improving the contrast between pavement and road markings
