Porous Paving Smackdown:  Vendors Take It Into the Alley

Can the right pavement improve water quality and cut down on the amount of storm water running to sewage treatment plants? The Board of Public Service of the City of St. Louis is trying to find out.

The Board of Public Service (BPS) has initiated a process to test three paving systems for how well they reduce storm water runoff and improve the quality of runoff, and for how costly or difficult they are to install. The project involves paving three alleys in south St. Louis, one with porous asphalt, one with pervious concrete, and one with pervious paving blocks. The first alley up was Cardinal Alley in a neighborhood near the St. Louis University Medical School.

Byrne and Jones Construction Company recently installed a porous asphalt mix at Cardinal Alley on top of a roadbed prepared by Gateway Contractors Inc. Pace Construction Company supplied the mix. In a recent test, a tanker truck sprayed 4400 gallons of water onto the asphalt surface and there was no runoff – all the water disappeared through the asphalt surface.

Andy LaPlante, quality control manager for Pace Construction, said the pavement on Cardinal Alley is an “open graded friction course,” or OGFC. It has 42 percent voids to allow water to run through to the crushed rock base, compared to four percent voids in a traditional asphalt mix, he said. “The difference is in the oil,” he said. “We use a polymer-modified oil to make the mix durable,” he said. Pace Construction’s mix includes three percent recycled shingles. “The fiberglass in the shingles increases the strength of the bonds holding the rock together,” LaPlante said.

Byrne and Jones Construction installed two sections of pavement. One section got a six-inch thick layer of asphalt, the other section got a four-inch layer, “so we can see how each holds up,” LaPlante said.

Michael Woodling, president of Gateway Contractors, noted that the road base for this experimental alley is thicker than the typical road base. “Normally a base is 12 inches maximum,” he said, while the base of Cardinal Alley is 24 inches deep. The reason is that Cardinal Alley’s base serves a purpose that a typical base does not. “The smaller base is just for stabilization,” Woodling said. “This base is for stabilization and water retention,” he said.

Gateway Contractors began its work by tearing up the old alley. They sent the old bricks to a recycler to be crushed and used as warning track material on baseball fields. After excavating the roadway, they installed a geotechnical fabric to keep dirt from filtering up into the rock and filling voids that are supposed to catch water. The base consists of about 40 percent voids, Woodling said. Two-inch thick clean rock forms the bulk of the base. The laying bed, however, consists of two-inch diameter crushed concrete. From the base to the asphalt, the builders of Cardinal Alley looked for every opportunity to use recycled materials.

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