The old Astrodome parking lot (all concrete and asphalt) couldn't hold events on 100-plus-degree summer days. Now, with a turf-based lot, Reliant Park has an additional revenue stream between the football season and the National Rodeo.

Different types of materials and continuing research offer more options.

By Tara Hun-Dorris

Pavements are an intrinsic, seldom-thought-about part of life, particularly in urban areas. However, for developers, industrial facilities, and municipalities addressing stormwater and associated water-quality guidelines and regulations, pavement stays very much at the forefront of planning issues. “Pavements are the most ubiquitous structures built by man. They occupy twice the area of buildings. Two-thirds of all the rain that falls on potentially impervious surfaces in urban watersheds is falling on pavement,” says Bruce Ferguson, professor and director of the School of Environmental Design at the University of Georgia in Athens.

Porous pavements, designed to allow air and water to pass through, are today just a small fraction of all pavement installations. However, their popularity is steadily increasing on a percentage basis, and they have been installed in all regions of the United States, Ferguson says. “This is potentially the most important development in urban watersheds since the invention of the automobile. The automobile is causing us to build all these pavements and have all these oils that we spill. If we can transfer the environmental function of the pavement, we’ve done two-thirds of the work.”

If used properly, porous pavements can facilitate biodegradation of the oils from cars and trucks, help rainwater infiltrate soil, decrease urban heating, replenish groundwater, allow tree roots to breathe, and reduce total runoff, including the magnitude and frequency of flash flooding. Stormwater, particularly urban runoff and snowmelt, is the wastewater of the 21st century, according to John Sansalone, associate professor in the Department of Civil and Environmental Engineering at Louisiana State University (LSU) in Baton Rouge. As reuse becomes more necessary, runoff will eventually be seen as a valuable commodity, he explains. This makes porous pavements, with their potential to revolutionize stormwater management, an important technology for the future.

Ferguson has been studying porous pavements for more than a decade. In his book, Porous Pavements (2005), Ferguson identifies nine categories of porous pavement: decks, open-celled paving grids, open-graded aggregate, open-jointed paving blocks, plastic geocells, porous asphalt, pervious concrete, porous turf, and soft paving.

Categories of Porous Pavements
Decks are level or elevated wooden structures that serve as porous pavements. They bear foot traffic and allow freedom for tree roots to grow, Ferguson explains. They are beneficial in situations where they can be built around the existing environment, such as in wetlands or on coastal dunes. For example, in the case of dunes, the sand can drift between the planks and vegetation can grow right up through the slats. This sort of technology can “preserve the ecosystem in every regard,” he notes.

Open-celled paving grids are open spaces with ribbing in between. A potential disadvantage is that they can be difficult to walk on, Ferguson observes. 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 is “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. Today 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% to 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, Ferguson notes, a driveway into 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, according to Presto Products Co. of Appleton, WI, manufacturer of the Geoblock. Ferguson says Geoblock is a durable paver. However, he notes that some models are lighter in weight and research needs to be done to justify them as an addition to porous pavement technologies. In many cases, aggregate alone may be suitable for a project.

Porous asphalt was developed around 1970. Some early installations failed because the original tarry asphalt binder that holds the aggregate together never really hardened; it migrated down due to gravity and created a clogging layer, explains Ferguson. Today, polymers are added to the asphalt binder to prevent migration, and polymer-reinforcing fibers further hold it together. In addition, large enough particles are used so if a little migration occurs, openings remain that are large enough to allow infiltration. The most widespread use of porous asphalt today is as an overlay on interstate highways. All interstate highways in Georgia and Oregon are repaved with porous asphalt. This is done for safety—better drainage, traction, and visibility, Ferguson explains. “It increases the capacity of the highways without the expense of widening.”

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 Pervious Concrete Pavements, published in 2004 by the Portland Cement Association in Skokie, IL. This mixture contains little or no sand and forms a system of “highly permeable, interconnected voids that drain quickly.” According to the association, 15% to 25% voids are achieved in the hardened concrete, and flow rates average around 480 in/hr. Pervious concrete is advocated as a best management practice (BMP) by the EPA. It has been used in residential streets and in a solid waste transfer station. The Portland Cement Association maintains a list of certified contractors.

Porous turf is used by itself as well as with modern reinforcements. It is well suited for infrequent or serial uses that allow the grass time to regenerate between events, notes Dustin Glist of Invisible Structures Inc., a Golden, CO–based porous pavement manufacturer. Examples include church, school, and stadium parking lots. The cost of bringing in a sand rooting zone needs to be considered when selecting this technology, Ferguson says, citing research from the US Golf Association. “Sand does not get compacted like clay,” he explains. “It maintains its permeability and penetrability by the grass roots and the aeration to grass roots.” Turf-based systems actively evapotranspire and cool off the immediate area by several degrees. “The cooling is so prominent you can feel it when you move from an asphalt street into a grass parking area.”

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. According to Ferguson, there are technical ways to measure the safety of these materials.

Problems and Myths
With so many choices and the EPA recommending some of these technologies as stormwater BMPs, use of porous pavements is increasing. New information is being collected about the durability and effectiveness of these technologies.

Still, concerns about them remain. “This is a technically challenging area, and there’s so much ignorance and speculation,” Ferguson says. “There are many rumors going around that you get challenged about this from all angles.”

In its 1999 stormwater technology fact sheet on porous pavement, the EPA says that traditionally porous pavements have had failure rates as high as 75%, mostly attributable to “poor design, inadequate construction techniques, soils with low permeability, heavy vehicular traffic, and resurfacing with nonporous pavement materials.” There are three things that absolutely must be done with each porous pavement installation, Ferguson stresses: (1) Select for location, (2) make sure the design is correct, and (3) build properly. “If you do all these things right, there is no reason to expect failure,” he adds. “If you do any one of them wrong, it probably will fail.”

There are several disadvantages commonly but not always accurately associated with porous pavements.

There is some risk that use of porous pavement could result in groundwater contamination. This concern results from the idea that surface water would infiltrate groundwater too quickly, not allowing time for pollution abatement. “The most common potential pavement-related pollutants are either rapidly sequestered during infiltration (e.g., heavy metals) or unaffected by any level of runoff treatment (notably chloride from road salt)” according to an article by Derek Booth and Jennifer Leavitt in the summer 1999 issue of the APA Journal.

However, the concern over groundwater pollution becomes reality in two site-specific conditions, Ferguson maintains. The first is a brownfield, where toxic residue remains from a previous use. “Those are limited, finite, identifiable places,” he says. The other is where the soil is so “grossly sandy and gravelly” that it becomes a conduit for untreated water into the groundwater. Almost any other soil has enough clay, silt, or fine sand to filter out and biodegrade the oils and fine particles carrying metals. The porous pavement structure, often up to 10 inches deep, also filters out pollutants and biodegrades them because of the huge surface area encountered by water as it trickles through. There is a lot of residence time, and the environment within the pavement is aerated and moistened from time to time. The result is a diverse ecosystem similar to what occurs in natural soils. As a result, oils are broken down into carbon dioxide and water, he explains.

Porous pavement has the tendency to become clogged. “Whether it gets clogged depends on how you design it,” Ferguson observes. The ones that get clogged tend to be at the low point in the site’s drainage. Those that don’t get clogged have only rainwater on them. Installations should be designed to drain away from the porous pavement in every possible direction, he advises. Clogging is a reasonable fear, given the history of clogging associated with porous pavement, says Sansalone. It is important that end users understand how the material functions, how it can best be maintained, and how its capacity can be restored after a certain period, he says. Because porous pavements are BMPs, maintenance information should be readily available to all users. “Wastewater treatment plants wouldn’t produce clean discharge without regularly scheduled maintenance. BMPs are no different,” says Sansalone.

Porous pavements need to be vacuumed or pressure-washed every six months. Ferguson notes that the clogged pavement he has seen could not have been prevented by washing. Designing properly can prevent the need for frequent vacuuming or washing.

Porous pavement is too expensive. Cost is a very site-specific question, dependent upon location, land-use context, and stormwater requirements and alternatives, Ferguson says. For example, unbound, open-graded aggregate can be used in residential driveways or parking stalls and is very inexpensive. “In return for saving money, you get the most porous and permeable material you could possibly make a pavement out of.”

Materials, such as metals, may accumulate in certain pavement types. Metals occur in generic pavements because they get ground out of brake pads and other components. They can get captured in the porous pavement or the soil underneath, Ferguson says. Theoretically, after an extended period, metals could accumulate to toxic levels, he continues. However, metals are either going to accumulate within pavements or at a site downstream. “That’s the only choice we have as long as we keep using automobiles and making them the way we do.”

Porous pavements cannot handle freez/-thaw cycles or temperature extremes. Turf-based systems have been successfully used as far north as Alaska, Ferguson says, noting the most important thing is to select the proper technology for the climate and site. Aggregate systems can take any amount of heating, he adds. Many technologies can also handle freezing temperatures well. Porous pavers, for example, have been installed in Ontario, Canada, and are doing fine.

Porous Pavements in Use
As porous pavement installations have become more numerous, information is becoming available about how this technology works in practice. Below are examples of turf, porous pavement, crushed stone, and porous asphalt applications.

The Orange Bowl
The turf-based system, Grasspave², has been installed at two major US football stadiums: the Orange Bowl in Miami, FL, and Reliant Stadium in Houston, TX.

The Grasspave² system consists of a sandy gravel base course, a Hydrogrow polymer-fertilizer mixture, the Grasspave² ring and grid structure, sharp concrete sand, and grass seed or sod, according to Invisible Structures, the manufacturer. Grasspave² can provide load-bearing strength and shield vegetation root systems from compaction due to vehicle weight. It also provides heat-island mitigation, Glist says. Void spaces allow roots to develop and also provide storage capacity for rainfall. As it moves through and across the Grasspave² surfaces, runoff slows, allowing suspended sediments to drop out and increasing the time to discharge. Table 1 shows expected storage volumes for the system in clay soils.

The system was first installed at the Orange Bowl in three phases beginning in 1995, for a total of 24,260 square meters of porous turf. Around 2,000 parking spaces are covered with Grasspave², while the drive lanes are made of asphalt, says Dale Sandlin, grounds and turf manager for the Orange Bowl. The idea was that cars could park on the turf and rainwater would percolate down through the soil. “We’ve had a lot of games where we’ve had a lot of rain,” he notes. “No one ends up getting stuck. We’ve had no ruts or anything like that.”

According to Ferguson, the system efficiently removes surface water in an area that has old sewer systems not equipped to handle additional drainage.

Overall, Sandlin says he has been happy with the system. However, he notes it is very important to follow the manufacturer’s recommendations regarding installation of turf-based porous pavement systems. In the case of the Orange Bowl, he says, the contractor combined two operations by purchasing a thicker-cut sod, laying it on top of the cells, and then rolling it out so extra soil would fill in the void. This looked OK the first year, he continues, but not all the cells filled up and additional top dressing is occasionally required.

Despite this, the Orange Bowl project went so well and water drained so quickly following flooding in 2002 that stadium architects began looking to it as an example to be applied elsewhere, Glist says. Most notably, in 2002 HOK Architects of Houston, TX, installed the world’s largest porous pavement parking area at Reliant Stadium. The 317,000-square-foot parking area was part of an effort to “green” the area around Reliant Park. The project serves to mitigate stormwater and reduce the urban heat island effect. The area—which, when paved, was not used during the hottest summer months—is now routinely used for festivals and rodeos.

The system is also regarded as a stormwater management BMP. The lot can store and clean up to 60,000 cubic feet of stormwater, preventing flooding and non-point source pollution. According to its manufacturer, the system cleans the water of toxic hydrocarbon drips, allowing clean water to recharge groundwater. This contrasts with asphalt or concrete lots, where hydrocarbons run off and flow into surface water.

Jordan Cove
At the Jordan Cove project in Waterford, CT, the EPA is actively monitoring a neighborhood divided between a 10.6-acre traditionally built segment and a 6.9-acre area built using BMPs intended to reduce stormwater runoff and pollution. BMPs include permeable interlocking concrete pavers (Eco-Stone from Uni-Group USA in Palm Beach Gardens, FL) on road surfaces and driveways, crushed-stone driveways, grass swales, and rain gardens. Monitoring has shown “night and day” differences between the traditionally built and BMP-designed segments, according to Mel Cote, manager of the Oceans and Coastal Protection Unit of EPA Region 1 in Boston. (For more information, see “BMP Research in a Low-Impact Development Environment: The Jordan Cove Project” in the January/February 2003 issue of Stormwater.)

Researchers at the EPA and the University of Connecticut conducted a sub-study during 2002 and 2003 to monitor runoff quantities and pollutants from driveways paved with asphalt, crushed stone, and Eco-Stone permeable pavement. The permeable pavement had the best infiltration rate (Table 2). The permeable pavement and the crushed stone greatly outperformed asphalt in terms of decreasing runoff (asphalt, 1.8 millimiters; permeable pavement, 0.5 millimiter; crushed stone, 0.04 millimiter) and reducing pollutants in runoff (Table 3).

The concrete pavers are desirable from an aesthetic standpoint, Cote says, noting they’ve helped with the marketability of homes in the development. Throughout the neighborhood, the permeable pavement is infiltrating 13% to 15% of rainwater. Donna DeNinno of Uni-Group USA notes that Eco-Stone is more expensive than asphalt or even non-porous pavers. However, if costs not spent on other stormwater controls are factored in (such as the cost of land used for a detention pond), total cost per project tends to equal out or be lower for permeable pavers.

Georgia DOT
Porous asphalts have been used for safety as well as water-pollution control. “There is definitely better traction on a porous pavement in wet weather than anything that has a sheet of water on it,” Ferguson says. The Georgia Department of Transportation (DOT) requires all roads in the state with traffic loads greater than 50,000 vehicles a day to be paved with open-graded friction course (OGFC), containing aggregate, polymer-modified asphalt cement, stabilizing fibers, hydrated lime, and mineral fiber.

OGFC reduces highway noise and increases surface drainage, says Peter Wu, assistant state materials engineer for the Georgia DOT. “Drive behind a truck and you do not see much splash.” The porosity of the pavement provides little reservoirs for the water to escape, greatly reducing the incidence of hydroplaning, though Wu says he has not seen any safety statistics. The primary downside to OGFC is that it costs significantly more ($65 a ton) than asphalt ($40 a ton), and both technologies have the same performance life (about 10 years), he says.

The use of porous pavement technology is increasing throughout the United States. “There’s a huge potential to restore places that are bearing traffic and acting just like cities, with all the economic and traffic implications,” Ferguson says. On that same “square inch of land,” stormwater infiltration resulting in better groundwater quality and quantity, stream preservation, and tree-root growth could be occurring, he explains. The integration of ecology with urban living “is really a revolutionary way to be building cities.” As an example, he notes the downtown Brooklyn, NY, MetroTech business improvement area. MetroTech features porous pavement in the pedestrian plaza. Under the pavement is structural soil where tree roots grow and take advantage of air and water that come through pavement. Above the plaza is a tree canopy, which enhances outdoor comfort, air quality, and ecosystem diversity.

Future Research
Much work remains to be done. LSU’s Sansalone has been researching porous pavements extensively from an engineering standpoint. There is still a need to learn more about how porous pavement functions and why, he says. He also wants to further define pore characteristics—not just total porosity, but a quantifiable distribution of the effective porosity that transmits water flow and filters pollutants. If design can be understood, better porous materials can be built.

Next on the research horizon is development of reactive porous pavements, Sansalone says, which will work reactively through a chemical process to treat the stormwater flow filtering through them. Solute material, such as phosphorous and metals, would have the potential to react with the porous pavement and become immobilized. He says a pilot site for this technology should be running within a year. “If you can take a pavement, which is part of the problem, and modify it to make it part of the solution, that really what’s driving our work in porous pavements.”

References
Booth, Derek B., and Jennifer Leavitt. 1999. Field evaluation of permeable pavement systems for improved stormwater management.” APA Journal 65, no. 3.

Ferguson, Bruce K. 2005. Porous Pavements. Boca Raton, FL: CRC Press.

Writer Tara Hun-Dorris specializes in water-quality issues.

SW March/April 2005

Home + About + Subscribe + News + Calendar + Glossary
Talk + Images + Advertise + Contact Us + Search + Register + Services

Distributed Energy | Erosion Control Magazine | MSW Management Magazine
Grading & Excavation Contractor | StormCon | ForesterPress | Forester Media

© 2000 - 2005 FORESTER MEDIA, INC.