What: The application of a chip seal followed by a slurry seal. The chip seal acts as an interlayer to address surface distresses, while the slurry seal ties down the chip seal and provides a smooth, protective road surface. A laboratory design is performed to determine the right combined amount of binder for both chip and slurry seals. After surface preparation, the chip seal is applied using standard application methods. After the chip seal thoroughly cures (typically 48 hours to a week), the slurry layer is placed and allowed to cure. Micro surfacing may also be used in place of the slurry seal. Some suppliers use polymer modified asphalt emulsions for earlier and stronger chip retention to speed faster traffic return and construction. Some cape seal processes also incorporate crumb rubber in the slurry seal.
Where: Aging asphalt pavements in good structural condition with low to moderate severity surface distresses. Cape seals can be placed on pavements with higher levels of distress than normally recommended for a single slurry seal. They are an ideal pavement preservation treatment for pavements where surface smoothness and/or black surfaces are wanted and damage from loose chips is a concern. The slurry surface is smoother than a chip seal, and therefore friendlier for residential neighborhoods.
Why (advantages): The cape seal increases chip seal durability by enhancing binding of the chips and by protecting the surface. The chip seal serves to seal and bridge cracks, and can be designed to address dry or bleeding surfaces. With no loose aggregate, the cape-seal surface is a dense, smooth mat. Cape seals are less susceptible to damage from snow plows. The total treatment seals and waterproofs the surface, fills cracks, restores skid resistance and provides a black surface. Cape seals extend the life of existing pavements by correcting surface distresses such as cracking, raveling and friction loss and by protecting the underlying pavement structure. Cape seals provide a smooth finish that is similar to an asphalt overlay, but at a lower cost.
Why not (disadvantages): Since cape seals are two-step processes, road closures may be longer than other treatments. There should be strict traffic control until the treatments have cured. Micro surfacing will cure much more quickly than slurry seals. Cape seals are more expensive that a single chip seal or a single slurry seal.
When: Cape seals should be applied when the pavement structure is in good condition, but there are minor to moderate surface distresses.
Estimated life extension: 6-10 years over pavements in good condition; 5-6 years on pavements in fair condition; 2-5 years for pavements in poor condition.
For more information on successful cape seals, consult the Chip Seal and Slurry Seals Checklist put together by the Foundation and FHWA