What: A polymer modified asphaltic binder is spray applied to a prepared pavement surface followed immediately by an application of aggregate. The seal is then rolled to seat the aggregate in the binder. The binder may be applied as a water-based asphalt emulsion or hot applied polymer asphalt. A fog seal may be applied to aid in chip retention and to blacken the surface.
Where: Polymer asphalt chip seals have been successfully applied on all types of asphalt pavements, from low volume rural roads over a prepared base to heavily trafficked superhighways over hot mix asphalt. The road should be structurally sound with only minor surface defects.
Why (advantages): Polymer chip seals are more effective than conventional chip seals because they have a faster cure (and earlier chip retention), are more elastic and therefore more durable, are more effective at sealing cracks, and are more adhesive to chips resulting in lower chip loss, less snowplow damage and less damage to vehicles from flying chips. They are also more able to withstand expansion and contraction during temperature cycling, delaying reflective and thermal cracking as well as shelling of aggregate in cold weather. As with conventional chip seals, they seal and protect asphalt pavements from sunlight, rain and snow; improve skid resistance; seal minor cracks and surface imperfections; inhibit raveling; address bleeding and flushing when properly designed; protect pavement structure and extend pavement life. While the polymer adds to the cost, the overall cost increase is small when all material, construction, traffic control and user delay costs are considered. A recent Federal Lands Highway/ Federal Highway Administration study found that the added cost was justified by the improved performance and recommended that all chip seals be done with polymer modified asphalt.
Why not (disadvantages): Chip seals are not appropriate over rutted, potholed, rough, moderately or severely distressed surfaces. Chip seals may be noisy under traffic, but use of single-sized aggregate can greatly reduce the noise. Chip seals should be done in weather dry and warm enough to allow thorough curing, and with very strict traffic control to prevent damage before the seal has thoroughly cured. Polymer chip seal emulsions break very quickly; the chips should be applied immediately before the emulsion is completely broken, or the chips may not embed and stick as well as desired. Polymer chip seals should not be applied over pavements with entrapped moisture; they may seal in the moisture, allowing it to damage the pavement.
When: Ideally, all chip seals should be applied when the road is still in good condition, with only minor surface problems, such as loss of friction, drying surface or very minor cracking. They may also be applied as a wearing surface shortly after cold or hot in-place recycling or other treatment or over a prepared base on rural roads. Polymer asphalt chip seals are an excellent choice for keeping good pavements good.
Estimated life extension: 7-12 years over pavements in good condition; 5-7 years on pavements in fair condition; 3-5 years for pavements in poor condition.
For more information on successful chip seals, consult the Chip Seal Checklist put together by the Foundation and FHWA.