Colorado Tries Porous

Story by Carol Carder | September 28, 2010

The Colorado Asphalt Pavement Association observed Earth Day in April with the grand opening of a porous asphalt parking lot at the Denver Wastewater Management Facility on West 3rd in Denver. In this partnership with Urban Drainage and Flood Control District and Denver Public Works, 11 CAPA members donated time and expertise to design and reconstruct a portion of this existing urban lot.

"We have a commitment to sustainable building to preserve our environment," Tom Peterson, CAPA director, told the 100 plus contractors, industry participants and media representatives at the opening.

"This asphalt is a new tool in our construction tool box, and we are proud to be a host site for this pilot application," said Terry Baus, P.E., program manager for Denver Public Works.

Ken MacKenzie, P.E., master planning program manager, UDFCD added, "We still have many questions such as, 'How long will it last? How well will porous asphalt remove pollutants? Will installations be properly maintained so they don't fail?' "

The following Denver-area companies provided products and services in the design, construction and performance monitoring of the parking lot constructed April 14 and 15, 2008: Aggregate Industries, Asphalt Paving Co., Brannan Sand and Gravel Co., Kiewit Western Co., Lafarge West, Premier Paving Inc., SEM Materials, CTL/Thompson Inc., Kleinfelder, Terracon, and Vance Brothers.

First developed in the 1970s at the Franklin Institute in Philadelphia, porous asphalt pavement consists of standard bituminous asphalt in which the aggregate fines (particles smaller than No. 30 sieve) have been screened and reduced, allowing water to pass through the asphalt. A bed of uniformly graded and clean-washed aggregate with a void space of 40 percent is placed underneath the pavement. Storm water drains through the asphalt and is held in the stone recharge bed as it slowly infiltrates into the underlying soil mantle. Or in the case of expansive soils, it can be captured and piped into the storm sewer system. A layer of geotextile filter fabric separates the stone bed from the underlying soil, preventing the movement of fines into the bed. In areas of expansive soils, a waterproof barrier can be placed at the bottom under the aggregate and drainage pipes can be installed on top of the barrier.

The layers at the CAPA pilot lot at the wastewater facility start with a layer of geotextile and perforated pipe for stormwater monitoring. Next is a 6-1/2-inch layer of C-33 sand, then 7 inches of 1-1/2-inch minus gravel (No. 3 aggregate), a 2-inch choker course of three-quarter-inch minus rock (No. 67 aggregate), and 3 inches of open graded friction course hot mix asphalt with half-inch aggregate.

Other Porous Asphalt Installations

Wal-Mart's experimental store: In 2004, Wal-Mart's concrete and asphalt contractors installed 19 different pavement mixes in 14 acres of paving at its experimental green store in Aurora. To set up these experiments, store designers met with the professional paving associations that will share in the information learned. These include the National Asphalt Pavement Association and the Colorado chapter CAPA; the National Ready Mixed Concrete Association, which coordinates with local Colorado partners Colorado Ready Mixed Concrete Association; the Rocky Mountain Cement Council; and the American Concrete Pavement Association Colorado Chapter.

Brannan Sand & Gravel Co. installed Colorado's first porous asphalt in three different sections totaling approximately 1.5 acres of this project lot. All three mixes are gap graded with a minimum of fines. One section has zero-percent Recycled Asphalt Pavement in the surface course while the other two sections have 5-percent RAP and 10-percent RAP in the surface courses. All three sections have 15-percent RAP in the first lift.

A stone infiltration bed underlies all the pavements and connects to two 400-foot-long bioswales planted with xeriscape grasses, trees and shrubs to help trap sediment and pollutants as they capture and filter storm water. Local firms Valley Crest Landscaping and Fiore and Sons built the bioswales designed by Land Resource Design, Steve Clark and Associates Inc. and Kimley-Horn and Associates. The National Renewable Energy Laboratory in Golden contracted with Wright Water Engineers to take flow measurements of water from the bioswales. However, no monitoring has been done of storm water quality.

Golden fire station: "While some municipalities are waiting until porous asphalt is tested, I think we need to experiment more with pilot projects and then monitor the performance," observed Jeremy Reichert, public works inspector for the city of Golden. In June 2008, Brannan Sand & Gravel Co. installed a porous asphalt lot of approximately 28,000 square feet for employee parking at the downtown fire station in Golden.

"While some people install porous asphalt so a detention pond is unnecessary, our goal was filtering the water," Reichert explained. "We are right next to the creek, and this system will pre-treat the first flush."

Since the perk rates on the soil were good, this system doesn't use piping. It's all filtration that will migrate into the soil. The sandwich layers for this installation include filter fabric at the bottom of the bed, 8 inches of 1-1/2-inch minus rock, then 4 inches of porous asphalt with a polymer additive. On the next installation of porous asphalt, Golden will install a choker course of smaller aggregate to help stop movement in the stone bed during the paving operation, according to Reichert.

One of Public Works' concerns was whether the porous asphalt would rut. Despite traffic loads up to 30,000 pounds for the trash truck and up to 75,000 pounds for a fully loaded fire truck, the structure of the stone bed so far has held up without rutting. Although the fire trucks exit from the building over concrete to 10th Avenue, the trucks have driven on the porous pavement section of the lot without consequence. According to Reichert, the city did allow the lot to cure for seven days before use, particularly as installation took place during a long stretch of hot weather.

Another concern is whether asphalt, being a tacky material, will collect fines that will affect the filtration performance. Maintenance will be high-power vacuuming to remove dust and sands. The city also plans to do some coring as it monitors the lot for fines, Reichert said.

Denver apartment parking lot: Mike Gerber of MGL Partners, developer and owner of the Paloma Villas at Morrison Road and South Stuart Street in Denver, said he is pleased with the new porous asphalt lot supplied by Lafarge and installed by Gilbert Contracting of Brighton this past July. Without the filtration provided by the porous asphalt, a detention pond eliminating one-third of the available rental units would have been required.

Bob Thayer of BC Builders, Centennial, said the work space was tight for trucks bringing in the aggregate for the stone reservoir and the paving for the three lots located between the existing buildings. Gilbert Contracting installed the porous asphalt from Lafarge on approximately 2,000 square yards in the three lots, providing parking for 60 cars. Since the soil at this site is expansive, a vapor barrier and underdrain system make up the bottom layer, then a 12-inch-deep stone reservoir course of 3-inch minus aggregate, a 2-inch choker course and a 4-inch layer of open graded hot mix asphalt.

Monitoring Porous Asphalt

While the Urban Drainage and Flood Control District expects porous asphalt to improve the quality of storm water, much sampling and analysis remains, according to UDFCD's Ken MacKenzie.

"In storm water monitoring you can take a number of samples and get wildly differing numbers from each one of them," he explained. "However, when you take enough samples, a trend will emerge."

How many samples are enough? Ideally eight to 10 storms per season for three seasons could reveal a trend, MacKenzie said. Denver has had a long, dry summer with only a few significant storms, but UDFCD is committed to monitoring for at least three years to measure the storm water quality of the water filtering through the new porous asphalt at the Denver Wastewater Management Division Facility.

Carol Carder is a Denver writer who covers Colorado construction for Rocky Mountain Construction.