What? A bonded wearing course (BWC) is a gap graded, ultra thin hot-mix asphalt (HMA) mixture applied over a thick polymer modified asphalt emulsion membrane. The high binder content seals the underlying road thereby protecting it from water infiltration and slows down the aging process. The gap-graded HMA layer above of the thick binder layer provides an open surface texture to allow water to flow through the surface. Because the top layer is permeable, it allows water to quickly flow away from the surface of the road reducing tire splash and safety hazards when it is raining. The characteristics of a gap graded HMA layer also provide good stone to stone contact.
Bonded wearing courses have been placed on numerous city streets and highways in California, as well as on very high to low traffic streets and highways throughout the U.S. The earliest projects are giving over 10 years of excellent performance, and most projects have exceeded expectations.

Why? The bonding wearing course preserves the underlying pavement structure and provides a good wearing surface for cars to travel on. It slows down the aging process and enhances safety.

When? Bonded wearing courses may be selected in place of tradition dense graded HMA overlays because of its ability to drain water and reduce hydroplaning of traveling cars.

Sources for more information:
http://www.dot.ca.gov/hq/maint/TAGBondedWearingPilotProjects.pdf
http://www.dot.ca.gov/hq/maint/MTAGChapter11-BondedWearingCourse.pdf
http://www.dot.ca.gov/hq/maint/FPMTAGChapter11-BondedWearingCourse.pdf
https://semmaterials.pbworks.com/w/page/15730353/Bonded-Wearing-Course