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How it works

Porous asphalt pavement is not new, but it's new to many asphalt contractors. Asphalt has a successful track record of porous pavements dating back to the late 1970s. Cahill Associates, an engineering firm, has been involved in the design and construction of more than 200 porous asphalt pavements since the 1980s.

Cahill has reported no failures on pavements for which proper design and construction procedures were followed, according to Kent Hansen, P.E., Director of Engineering, National Asphalt Pavement Association (NAPA).

Porous asphalt pavements are designed for dual duty. They provide pavements for parking and roads and serve as stormwater storage and infiltration systems.

What's more, the demand for porous asphalt pavements is growing because they offer site planners and public works officials the opportunity to manage stormwater in an environmentally friendly way, says Hansen.

From the bottom up, the standard porous pavement structure consists of:

• An uncompacted subgrade to maximize the infiltration rate of the soil.
• A geotextile fabric that allows water to pass through, but prevents the migration of fine material from the subgrade into the stone recharge bed.
• A stone recharge bed consisting of clean single-sized crushed large stone with about 40 percent voids. The stone chamber serves as a structural layer and temporarily stores stormwater as it infiltrates into the soil below.
• An optional stabilizing course or "choker course" consisting of clean single-sized crushed stone smaller than the bed's larger stone. That stabilizes the surface for the paving equipment.
• An open-graded asphalt surface with interconnected voids that allow stormwater to flow through the pavement into the stone recharge bed.