Porous Pavement: Slipping Through The Cracks
Pavements occupy twice the surface area of buildings in the U.S. In urban watersheds they produce two-thirds of excess runoff and are responsible for essentially all the runoff pollution and temperature gain. By utilizing porous pavements, developers, facility managers, and municipal officials can meet environmental safety requirements and turn the runoff pollution problem around.
Porous pavement is designed to allow air and water to pass through spaces between large pieces of gravel in the surface layer, rather than run off an impervious top. It helps rainwater and snowmelt infiltrate soil, significantly improves the water's quality through its filtering action, replenishes groundwater, and reduces total runoff. Providing, of course, that the pavement is installed properly—which is not always the case, as we shall see.
Porous pavements have been around since the 1950s, but the technology is only now poised for more mainstream use as environmental regulations have made it an attractive, inexpensive alternative to other methods of storm water management.
Federal guidelines supporting the Clean Water Act have stimulated owners and developers to consider porous asphalt and pervious concrete for commercial and light industrial construction. U.S. Environmental Protection Agency Phase II regulations require owners of newly developed or redeveloped sites of one acre or more to have an on-site management system for treating storm water. Owners often find themselves dedicating 10–20% of the overall site to retention/detention ponds, swales, underground piped systems, or other treatment devices. Pervious concrete and porous asphalt can be used for both parking and runoff management. Pervious concrete's use is an EPA-recommended "best management practice" for first-flush pollution mitigation in storm water management.
"It is a significant first-cost savings," said Glenn Ochsenreiter, vice president of the National Ready-Mixed Concrete Association (NRCA), Silver Spring, Md. "One, it's cheaper than traditional runoff solutions, such as retention ponds and swales; and two, owners are then able to utilize more of their land. So, it's a double benefit."
That savings for storm water management is gaining attention from big-box retailers and large industrial companies, says Bruce Ferguson, professor and director of the School of Environmental Design at the
University of Georgia and author of Porous Pavements (Taylor & Francis, 2005).
Pervious concrete is created by specially mixing and installing a mixture of water, Portland cement, and little to no sand or aggregate. Once it has set, the resulting surface will have a system of highly permeable, interconnected voids that drain quickly. 15% to 25% voids are achieved in the hardened concrete, and flow rates average around 480 inches per hour. Pervious concrete can support heavy traffic yet still allow stormwater and snowmelt to filter into the soil below.Photo: Jeff Yoders
Early last year, for example, Shelter Systems, a building component manufacturer in Westminster, Md., was planning to build a light industrial facility on a 12-acre site, with an eight-acre impervious parking lot. This would have required a 1.5-acre retention pond and underground drainage system. By specifying pervious concrete for the parking lot, the company was able to eliminate the retention pond and drainage system from the original plans, thus recovering 13% of the site and $400,000 in underground drainage costs alone.
Concrete or asphalt?
The most common porous pavements are porous asphalt and pervious concrete. Both have been used for the last 30 years, but their use has been stunted by a less-than-stellar track record for some of the early porous pavement installations.
In a 1999 fact sheet on storm water technology and porous pavement, the EPA stated that traditional porous pavements had a failure rate as high as 75% because of "poor design, inadequate construction techniques, soils with low permeability, heavy vehicular traffic, and resurfacing with nonporous pavement material."
Experts agree that proper installation is the key. "If it's installed improperly you will have a real problem," said Dan Huffman, the NRCA's senior national resource director. "We usually recommend starting over with a qualified contractor in those situations."
For porous asphalt, clogging can also be a problem. According to the University of Georgia's Ferguson, the binding agent in some specific porous asphalt installations over the last 30 years has become weak and gradually drains from the pavement surface through its pores, accumulating into a clogging layer. Highway departments in the United States and Europe have developed technology to stabilize the binder and assure a long life for porous asphalt installations. Still, past problems continue to haunt porous asphalt.
"You really need to do three things with every installation of a porous pavement," said Ferguson:
Select for location. Changes in the asphalt or concrete mix can be made for heavy snow, hot environments, clay soil, and other adverse conditions.
Make sure the design is correct.
"If you do all these things right, there is no reason to expect failure," says Ferguson, who consulted 170 experts and studied 800 technical articles in researching his book.
Pervious concrete, such as this parking lot at a Denver area Safeway store, is advocated as a best management practice by the U.S. Environmental Protection Agency.
Photos: National Ready Mixed Concrete Association
To follow those steps, however, Building Teams need qualified installers, which are still unavailable in many major markets in the U.S. As a result, the costs for installing porous asphalt and pervious concrete are escalating.
"We are very concerned about the cost issue," says the NRCA's Huffman. The NRCA and the American Concrete Institute recently unveiled a certification program, called ACI-522 Pervious Concrete, to train, test, and certify installers. For larger projects, owners are able to draw from a group of national contractors that travel the country and do this work.
Huffman says the cost of materials for pervious concrete is in line with that of conventional concrete, and the labor costs involved in installing is actually less—when you can get qualified installers.
Nonetheless, corporations that annually pave hundreds of large lots a year are testing the technology. Wal-Mart has used pervious concrete in two experimental stores in Texas and Colorado, and Safeway has a pervious lot at a store in Colorado.
Huffman, who has worked in the concrete industry for 30 years, said the cautious approach being applied to pervious concrete and other porous pavements will prove beneficial in the long run. He helped introduce the use of fly-ash in concrete, which took years for the construction industry to accept but is now popular in many projects.
More optimistic is Ferguson, who foresees a shift to porous pavement just around the corner. "I know of suppliers that are worried they won't be able to meet the demand for this technology," he said.BD&C
Types of porous pavement
Decks are level or elevated wooden structures that serve as porous pavements. They bear foot traffic and allow freedom for tree roots to grow. They are beneficial in situations where they can be built around the existing environment, such as in wetlands or on coastal dunes.
Open-celled paving grids are open spaces with ribbing in between. One disadvantage is that they can be difficult to walk on. Turf needs time to grow over the grids and open spaces, and then they can work well. Open-celled paving grids can be used in low-traffic areas, such as loading areas or emergency-access lanes.
Open-graded aggregate has been called “the most permeable material and the lowest cost material you can get anywhere, including conventional dense asphalt.” Aggregate used to have the disadvantage of creating dust. It is made out of single-sized angular particles and washed before application. It is available in all geographic areas of the country. About 30-40% of that material is void space, and its permeability is measured in thousands of inches per hour.
Open-jointed paving blocks are segmental pavers that bear enormous traffic. These pavements can handle high weights and perform in a variety of climates; for example, the driveway of a fire station in Ontario, Canada, is composed of these blocks.
Plastic geocells are plastic cells held together with ribs and filled with aggregate or turf. Plastic geocells can be used for emergency access lanes, auxiliary parking areas, trails, pedestrian and wheelchair access ways, and golf cart path shoulders and aprons.
Porous asphalt was developed around 1970. Some early installations failed because the original tarry asphalt binder that holds the aggregate together never really hardened or eventually broke down; it migrated due to gravity and created a clogging layer. Today, polymers are added to the asphalt binder to prevent migration, and polymer-reinforcing fibers further hold it together. Large enough particles are used so that, if a migration does occur, openings remain that are large enough to allow infiltration. The most widespread use of porous asphalt today is as an overlay on interstate highways.
Pervious concrete was also developed around 1970. It is created by mixing water and cement-like materials into a paste that forms a thick coating around the aggregate particles, according to the Portland Cement Association, Skokie, Ill. This mixture contains little or no sand and forms a system of highly permeable, interconnected voids that drain quickly. 15% to 25% voids are achieved in the hardened concrete, and flow rates average around 480 inches per hour. Pervious concrete is advocated as a best management practice by the U.S. Environmental Protection Agency. The National Ready-Mix Concrete Association and the American Concrete Institute sponsor a national certification program for porous concrete installers. For more information: http://nrmca.org/certifications/pervious/index.asp.
Soft paving materials include wood mulch, crushed shell, and other organic materials. These are used for areas of pedestrian traffic such as wood-mulching gardens and playgrounds.
Definitions courtesy of Porous Pavements, by Bruce K. Ferguson (Boca Raton, Fla.: Taylor & Francis, 2005).