September 2008

Pervious Pavements

New findings about their functionality and performance in cold climates

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Widespread misconception exists in the industry about pervious pavement systems, specifically about their functionality in cold-weather environments. The prevalent belief is that pervious pavements are not an effective stormwater management option for cold-weather climates because of concerns related to diminished permeability during freezing and that the material is not durable enough to withstand freeze-thaw conditions. Cold climates are typically very hard on constructed systems, and naturally, questions should arise about the effectiveness of pervious pavements in these environments—especially due to concerns about freezing of the filter media.

However, according to Dr. Robert Roseen, director of the University of New Hampshire Stormwater Center (UNHSC), stormwater management systems using infiltration and filtration mechanisms, if properly designed, can work well in cold-weather environments. He has been leading a four-year research effort focused on monitoring the year-round performance of a porous asphalt placement that was installed on the UNH campus. In addition, the UNHSC is hoping to shed light on the functionality of pervious concrete by testing a large placement that was also installed on the university campus in August 2007—the first major pervious concrete parking facility in New England. The purpose and function of the UNHSC is to evaluate the range of stormwater treatments systems available to designers, including proprietary and nonproprietary systems. The UNHSC is funded by the Cooperative Institute for Coastal and Estuarine Environmental Technology and the National Oceanic and Atmospheric Administration.

Findings from the porous asphalt study have demonstrated functionality that exceeds conventional practices by measures of both water quality and hydraulics.

Porous Asphalt Study
Design and Durability. The principal cause of parking lot pavement breakdown in northern climates is freeze-thaw cycling. Parking lots in these regions typically have a lifespan of about 15 years. By design, an open-graded, well-drained porous pavement system incorporating significant depth will have a longer life cycle from reduced freeze-thaw susceptibility and greater load-bearing capacity than conventional parking lot pavements. “Design guidelines for freeze-thaw consideration reflect frost depth ranges from 48 to 52 inches from coast to inland,” says Roseen. “For porous pavements, greater depth of frost is not the concern, but rather, the increase in the rate of cycling between freeze and thaw. This rate is highest near the coast.”

Photo: UNHSC

Steady-state infiltration with 30 gallons per minute from a 2-inch hose

This UNHSC system included porous asphalt at the surface, a stone choker course immediately below, and an underlying layer of finer filter material consisting of sand and gravel. “The mix design for porous asphalts has been in use for decades as an open-graded friction course—a pavement mix with a void content commonly in the 18 to 20% range,” says Roseen. In low-permeable soil, Roseen says, the filter course will need to be underlain by a drainage layer consisting of stone and drainage pipe. “For the UNHSC site, this drainage layer was constructed in order to monitor and sample the water that passes through the porous asphalt system,” he says.

Hydraulics. Monitoring infiltration rates for the porous asphalt placement, the UNHSC found excellent performance results year round. However, one of the most significant findings of the study was the winter hydraulic functionality of the placement. Indeed, the study determined that surface infiltration rates were not negatively impacted from frost penetration but were actually higher during winter months as compared to the summer. “The strong winter performance of the pervious asphalt placement was the opposite of what we expected to find,” says Roseen. “One of the most significant results from this study was that frozen filter media and freeze-thaw were not an issue. The well-drained nature of the parking lot sub-base ensures that the void space remains open, even during periods of prolonged freezing. While the filter may indeed freeze, it does not freeze solid, and infiltration capacity is preserved.”

The UNHSC collected monthly surface infiltration rates for three years and found a repeating trend of oscillating infiltration rates of about 2,000 centimeters per hour in the winter to 1,000 centimeters per hour in the summer. “Our suspicion is that the seasonal variation is likely caused by the asphalt binder, which is the weak link in the system,” says Roseen. “It is petroleum based and becomes sticky and tacky during the hot summer months, likely causing swelling and reduced pore size. Then, when the winter season arrives, the cycle is reversed. Our data over three winters and summers show that this is a reoccurring cycle.” More importantly, infiltration capacity remained high during the winter, even when there was significant frost penetration—sometimes in excess of 12 inches. “The porous asphalt does freeze; however, it generally freezes as a porous medium and not a solid block. Freezing rain and rain on snow can freeze the material at the surface, but minor salting and plowing at such times can return the surface to high infiltration,” says Roseen.

Photo: John Kuell

Construction of a "hybrid" parking lot in New London, NH

Throughout the research period, the study found that surface runoff did not occur from the parking lot, even though the Northeast region experienced an increase in extreme storm events. “We witnessed two 100-year storm events during the monitoring period and have never seen surface runoff, only runoff through the subdrainage system as designed,” says Roseen. The net water balance for the site was a 25% reduction in volume, with little or no runoff during the hottest months. “This was accomplished for a site with relatively poor infiltrating soils where infiltration as a stormwater management tool is often not considered,” he says.

The study also examined clogging of the surface and the resulting decline in infiltration rates. Roseen says clogging has two causes: the first is surface particulates and the second is a combination of liquid binder and surface particles, which can permanently clog an asphalt system. Both causes can be addressed—the first through routine cleaning approximately two to four times per year, depending on the frequency of use, and the second through the appropriate selection of a binder and admixtures to minimize binder draindown. Moreover, Roseen states that if 99% clogging were to occur, the infiltration rate would still be greater than 10 inches per hour, which is more efficient than most sand and soil mediums.

Water Quality. The UNHSC also monitored the quality of water draining at the base of the pervious asphalt system. Temperature, conductivity, dissolved oxygen, pH, and turbidity were monitored every five minutes in addition to automated sampler collections during storms. Water samples were sent to a certified lab for analysis of water quality. Results showed exceptional water-quality performance for the porous asphalt system with no seasonal variation. Typical performance efficiencies exceeded 95% for total suspended solids, total zinc, and total petroleum hydrocarbons in the diesel range, and approximately 42% for total phosphorous. “The porous asphalt design is distinctive in its use of a medium-grained sand for a reservoir base and filter course,” says Roseen. “This refinement enhances its effectiveness in treating water quality.”

As expected, nitrogen removal did not occur, as the placement was a nonvegetated system; however, if nitrogen were a concern, it could be addressed with a small vegetated system located at the subdrain outlet.

Winter Maintenance. From a winter maintenance perspective, the UNHSC also compared the porous asphalt placement with a nearby nonporous parking lot, evaluating salt application rates; the degree (percentage) of snow and ice cover; and the friction factor, which is measured by a standardized test method. “In northern climates, winter pavement maintenance requires a substantial effort and entails substantial cost,” says Roseen. “Routine plowing, anti-icing, and deicing is common practice for more than four months out of the year. Therefore, maintenance strategies for improving efficiency and effectiveness of winter practices are valuable.”

Research findings showed that salt application for porous asphalt could be reduced by 75%, based on snow and ice cover. “With only 25% of the salt, the snow and ice cover on the porous asphalt was the same as on the dense-mix asphalt parking lot,” says Roseen. “And even with no salt, porous asphalt has higher frictional resistance than dense-mix asphalt with 100% of the normal salt application,” he says. “Therefore, a sizable reduction in salt application rate is possible for porous asphalt without compromising braking distance or increasing the chance of slipping and falling.”

Porous Concrete Study
For the recently started porous concrete study, the UNHSC is working in conjunction with the Northern New England Concrete Promotion Association, the Northeast Cement Shippers Association, and others who are contributing materials and installation costs. The research team will examine pervious concrete’s performance in relation to treating water quality, reducing the volume of runoff, and minimizing the need for salting and sanding during winter months. The team will also test how pervious concrete holds up to freeze-thaw conditions.

Photo: John Kuell

The 26,000-square-foot lot has 11,000 square feet of pervious concrete.

“What is important to note is that the UNHSC project is not being run by the concrete industry,” says Jon Kuell, executive director of the Northern New England Concrete Promotion Association. “It is being conducted by an independent testing center, measuring system performance for water quality and quantity. The study will evaluate the effectiveness of the technology in cold environments for treating pollutants such as automotive fluids, phosphorous, and even what’s coming down in the rain itself.”

Kuell explains pervious concrete’s capacity to work and perform in cold environments by the “igloo effect,” where a shelter can be built out of snow yet contain a comfortable and warm interior. “In a cross section of a pervious concrete installation, you have an open-graded structure over a sub-base, which then sits atop the natural soil bed,” says Kuell. “The sub-base warms the pervious concrete above through the pure connection.”

The UNHSC project will give researchers a forum and the opportunity to show the industry that pervious concrete can function effectively in cold weather conditions. “Nobody has done this kind of experiment in the Northeast before,” says Kuell. “In a way, the Northeast is acting as a beneficiary for all the other parts of the country. It is important to shed light on pervious concrete’s effectiveness in cold climates.”

Design Criteria. At the UNHSC, the makeup of the sub-base in the newly installed pervious concrete placement is considerably different from what is typical of most pervious pavement systems. The system being tested consists of a media of large coarse stone with considerable void space and storage capacity, in addition to a sand filter layer that enhances the system’s overall performance and provides substantial and improved water-quality benefits. “A typical sub-base media consisting of 1- to 3-inch stones functions more like a detention pond,” says Roseen. “However, after adding a sand layer, the overall placement begins to function as a filtration system.” The sand filtration media consists of a chemical and a physical process. “There is chemical absorption to the sand particles in addition to mechanical filtering—small pore spaces that work to catch the sediment load,” says Roseen.

Kuell says there are three important criteria for constructing pervious concrete projects, especially in cold environments. First, he says, the concrete should consist of a quality product and contain a proper mix design. “You don’t want too wet of a mixture, because the cement paste could leach down through the sub-base into the soil and reduce infiltration rates. On the other hand, you want to make sure the mixture isn’t too dry, because raveling could occur,” he says.

The second criterion is related to the subbase. “When selecting the subbase material, it is important to look at the structural loading requirements and the water-quality benefits,” says Kuell. “It must equally be able to support cars and treat water.” He says the sub-base should be able to provide storage for an extreme storm event with enough permeability to effectively infiltrate water to the native soils. It should also be designed to drain well and effectively work with a total drawdown of no more than five days. “In freeze-thaw environments, you don’t want standing water resting for too long a time,” he says. A system with subdrains and well-drained materials will ensure that it doesn’t.

Photo: John Kuell

Travel lanes were constructed of conventional concrete.

The third criterion is making sure contractors are educated about the material and proper placement techniques so that the end result is a high-quality, durable pavement capable of lasting 20 to 40 years.

According to Kuell, the void content in pervious concrete installations is so high that it is possible to add fines into the mix without affecting infiltration rates. “If a pavement will be receiving heavier point loads, such as tractor-trailer traffic, we can put some of the sand back in the mix, which will help increase our compressive strengths,” he says. “With a void content of 20 to 35% in standard pervious concrete, there is considerable room to add fines and still maintain void contents that will accommodate up to 10 times the amount of runoff for a 100-year rain event for most areas.”

Kuell says the UNHSC project will be very important in terms of its perceptions among industry professionals. “With this study, we will be able to refer to a large-sized placement and show empirical evidence of its performance,” says Kuell. “However, it is still going to take plenty of convincing. Still, I’m confident that once the engineering and architectural community can actually see the evidence, they will begin to accept it.”

Differences Between Porous Asphalt and Pervious Concrete
There are some notable differences between porous concrete and porous asphalt. The mix production of porous asphalt is a bit tougher than that of pervious concrete; however, Roseen says, it is easy to install, and nearly any qualified installer can manage it. Pervious concrete, on the other hand, is fairly simple to produce but is tricky to install and generally requires certified installers for its placement. Other differences are related to the actual color of the substance. Because concrete is lighter, it absorbs less heat, contributing less to the urban heat island effect than traditional asphalt, which is responsible for adding heat to many cities. For the same reason, porous asphalt will perform better in the winter, as it will be warmer and promote greater deicing. Another advantage to concrete is that less nighttime lighting will be required because the lighter-colored surface reflects more light.

Incentives for Using Pervious Pavement Systems
In keeping with EPA requirements for Phase II of the National Pollutant Discharge Elimination System (NPDES) and the total maximum daily load (TMDL) program, most municipalities are requiring improved stormwater management. These practices are designed to limit additional contaminant loading, and in some instances, to reduce existing loadings, as well as to limit effective impervious cover. “This is a high standard that simply cannot be met with most conventional stormwater management practices using curbs and gutters and stormwater ponds and swales,” says Roseen.

Public heath concerns over mosquito-born illnesses such as West Nile virus and eastern equine encephalitis are also driving more interest in using stormwater management practices that do not include standing bodies of water like wetlands or ponds. “Pervious pavements and other low-impact development [LID] systems that do not have permanent pools of water are increasingly being considered as primary treatment options due to these disease concerns,” says Roseen.

However, beyond a regulation and health standpoint, pervious pavement is attractive in terms of its stormwater management benefits. For instance, in addition to removing contaminants, it is very effective at reducing the thermal impact of stormwater. “During the summertime, runoff can get very hot, sometimes as high as 120 degrees,” says Roseen. “When you have stormwater at that temperature flowing into a groundwater-fed stream that is at 65 degrees, severe impacts can occur in the aquatic ecosystem. Pervious pavements can significantly cool runoff.”

Lower winter maintenance is another distinct advantage of pervious pavement. Melting snow and ice can infiltrate directly into the porous material to facilitate faster melting.

“Along with less plowing and deicing, the need for constant sanding and salting in the winter months is greatly reduced,” says Roseen. This translates into greater safety. Standing water does not collect on the surface of a porous pavement system, which eliminates the formation of ice.

Stormwater management options such as pervious pavement that offer subsurface treatment are also becoming economically more attractive because of their efficient use of space. “Developers and business owners are willing to pay more for subsurface stormwater management because these areas can also be used for parking or other purposes,” says Roseen. And, with pervious pavement, the need for large detention ponds and other costly stormwater management systems is eliminated, allowing for even more parking space.

Additionally, many state regulations that require LID are also requiring some level of infiltration of stormwater runoff. In Rhode Island, for example, LID systems are required statewide for new and redevelopment beginning in July 2008. “They are really pushing the envelope,” says Roseen. “The Rhode Island Sound watershed is fairly developed with a high degree of imperviousness in some areas. It has almost reached build out, and as such, stormwater management in that area is being driven by water quality and water supply. Pervious pavement and other LID systems will be an essential tool in Rhode Island for improving the overall quality of the water.”

For redevelopment, pervious pavement systems are not as competitive or cost effective, because existing infrastructure is typically already in place, and pervious pavement requires that an appropriate sub-base be installed anew. “However, it can help reverse impairments commonly associated with urban waters, which are often conditions for redevelopment,” says Roseen. “In areas with impaired water where EPA enforcement is active, redevelopment cannot add additional contaminants, so innovative BMPs, such as pervious pavement, will need to be considered. That is exactly what is being required in Providence, RI, by the EPA.”

Costs
In addition to the misperceptions about its effectiveness in cold climates, another barrier to pervious pavement’s acceptance into the industry is the belief that it is much more expensive. However, a life-cycle-cost analysis shows that pervious pavement systems are more economical in the long run.

“Normal parking lots made from impervious pavement typically last 12 to 15 years in northern climates where freeze-thaw is prevalent, while pervious pavement lots can last more than 30 years,” Roseen says. “The recent reconstruction of the pervious concrete lot on the UNH campus is more costly than repaving, but over the long term, UNH will see a return on this investment. UNH Transportation Services understands this, and their commitment is crucial to making this technology demonstration happen.” UNH Transportation Services is also planning to use more pervious pavements in future parking lots.

For new development, Roseen says that although material costs are approximately 20 to 25% more than those for traditional pavement, the total project cost for these systems with reduced stormwater infrastructure is comparable to standard pavement applications with which stormwater infrastructure is required.

“The more these systems are used, the more affordable they will become,” says Roseen. “Pervious pavement systems also need to be considered as high-performance stormwater management. Given proper design and oversight, they can be affordable, effective solutions to stormwater management. They are important strategies for communities wanting development, while still providing protection for watershed health.”           

Author's Bio: Jeff Gunderson is a writer specializing in water-quality topics.