A water management alternative

Depending on the site and the use, the porous asphalt pavement typically uses an aggregate layer 12 to 24 inches thick, installed in lifts of 8 to 12 inches. He says the number of lifts of aggregate varies depending on how thick the lifts are.

Because the base is constructed of larger aggregate and because the fines have been removed, the angular base has air voids of around 40%. This allows water to be stored within the aggregate until it gradually seeps into the natural soils.

Potts says that in between the soil and the aggregate construction often calls for a geotextile fabric that provides separation between the open-graded stone and the soil. "By using the geotextile it makes sure the stone doesn't settle into the soil and the soil doesn't infiltrate up into the stone," Potts says.

He says placing the stone is similar to placing stone on a traditional paving job, but compacting the stone is different. Each lift is compacted just enough to set it up. "Unlike a conventional subbase it doesn't require 95% compaction," Potts says.

Hot mix asphalt surface
In most porous asphalt pavement constructions hot mix asphalt is placed in a single lift of 3 ½ inches and then compacted to 2 ½ to 3 inches. Potts says the mix can be placed by a standard paver, but track pavers are required.

"You can't use a tired paver because you're working on a base that's a little more unstable," Potts says. "The base is not compacted like standard subbase, so you need a track paver so you don't rut the stone as much."

He says planning of the paving is important for two reasons. First, the HMA itself is a different mix with a higher asphalt cement content and a stiffer binder that is sticky and difficult to work by hand. So paving should be planned in as many straight runs as possible. "Try to use the paver as much as possible so you can minimize handwork," he says.

The second reason planning is important is scheduling of mix pickup from the plant and staging of haul trucks. "Because it's a stiffer binder the mix needs to be hotter so you can work it easier," Potts says. "So you don't want trucks sitting there lined up and waiting too long. The mix will cool down, and it can be very difficult to place."

He says that while compaction of a porous asphalt mix is similar to compaction of a tranditional HMA pavement, there are some differences contractors need to be aware of. First, and probably most important, let the mat cool slightly before starting compaction.

"You don't want to overcompact it because you can cause a breakdown of the aggregate itself or a reduction in the permeability, so try to let the porous material cool off a little before you hit it with a roller," Potts says.

He says an 8 to 10 ton static roller is best because a vibratory roller might overcompact it or damage the aggregate, and he recommends only "a couple of passes" before it will be properly compacted. "Then the seams can be cleaned up with smaller rollers," he says.

Why it's the future
"Porous asphalt pavement turns a problem into a solution in terms of stormwater," Potts says. "It turns an enemy of stormwater - impervious surfaces - into a way to capture stormwater."

Potts says that once the porous asphalt pavement is constructed it's important to keep it really clean. "You need to keep it a porous system," he says. "It sounds obvious but often we see corners cut and things can get messed up if you're not careful." After construction, Cahill Associates recommends sweeping the pavement with a vacuum sweeper twice a year to maintain the permeability of the system long-term.