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Typically used for off-road pavements that handle a low volume of traffic flow - parking lots, industrial parks, and driveways - porous asphalt pavement has been around for more than 30 years. But only recently, as land values have climbed and stormwater regulations have evolved, have so many developers begun to embrace this paving technology.

"Ten years ago it was almost unheard of, and five years ago people had become much more aware of it but it still wasn't being used very much," says Andrew Potts, water resource engineer for Cahill Associates, a consulting firm specializing in porous asphalt pavement. "But in the last three years use of porous asphalt pavement has grown exponentially. It's now on an upward swing of the curve, and we're seeing a lot more going into the ground."

Simply put, porous asphalt pavement is constructed of hot mix asphalt that has the fine material screened out. Screening out the fines creates an asphalt mix that has an air void matrix that allows water to move through it. Potts says that a traditional HMA pavement has voids ranging from about 4% to 7% but porous asphalt pavement has voids ranging from 15% to 20%. These voids enable rainwater to move from the pavement surface into the aggregate beneath it, where it can be stored until it eventually seeps into the natural soil beneath the aggregate.

"Stormwater management has come a long way in the last 30 years or so," Potts says. "Before we were only concerned with the peak rates of runoff from a project, and we handled that with detention ponds. But we've learned over the years that we were not doing a good enough job and that detention ponds were not the most effective way to manage stormwater runoff."

He says the current approach to stormwater regulation has evolved, so instead of looking only at moving the water off the pavement surface current stormwater management considers ground water recharge, water quality, and streambank protection, among other things.