The next time you walk across a street or through a parking lot, take a closer look around. You’ll probably see some solid waste like paper, food, bird droppings or animal feces, or possibly some fertilizer from a neighboring lawn. More than likely, there will be some stains from cars that have leaked oil, brake fluid, transmission fluid or antifreeze. What do you think will happen to these materials during the next rain?
In all probability, they will end up in a storm drain that is discharged directly into a local lake, river or ocean. Recent studies have found that urban rainwater and snowmelt rivals (and in some cases exceeds) sewage plants and large factories as a source of damaging pollutants.1Storm water washing over impervious surfaces can pick up toxic chemicals, disease-causing organisms, dirt and trash. Conventional hard-surface pavements concentrate these pollutants at the storm drains, thereby creating what the U.S. Environmental Protection Agency (EPA) considers a point source emission of pollutants. In most cases, stormwater is not treated because it is too expensive or impractical.
A Filterpave® porous pavement system can reduce or eliminate stormwater runoff from many areas and provides some unique benefits compared to impervious pavement or even other types of porous pavement. It is appropriate for parking areas, driveways, walkways, patios, golf cart paths and other light- to medium-duty traffic areas.
The NeedTwo hundred years of unmanaged urban stormwater have contributed to public health problems and expensive natural resource losses in the U.S. Left unregulated and uncontrolled, urban stormwater may:
- Pollute drinking water sources, clogging reservoirs with silt and oxygen-depleting nutrients
- Fill in navigable waterways with contaminated sediment, increasing dredging and disposal costs
- Close or reduce lucrative rockfish, shad, flounder, crab, oyster, and other commercial fisheries due to chemical contamination and/or oxygen starvation resulting in loss of habitat
- Foul beaches and other recreational waters, causing losses in revenues from declines in boating, fishing, water fowl hunting, and coastal tourism
- Scour smaller stream channels and dump excessive gravel and silt loads that may affect fish and amphibian habitats
- Change the hydrology of streams, springs and wetlands, affecting sources of clean surface and ground water
- Damage homes and businesses during flash floods, which are more common where stormwater is left uncontrolled
The most common ways of treating stormwater, either with a combined sanitary sewer system or the use of a detention pond, are expensive or can be problematic. In the case of a combined sanitary sewer system, an overloaded system during a heavy storm may send raw sewage directly into a local waterway. In other cases, it may cause untreated waste water to flood homeowners’ basements. Each year in the U.S., thousands of homes are flooded in this manner, causing exposure to potentially deadly pathogens and creating conditions that could promote the growth of toxic black mold. Detention ponds use up valuable land resources and-if not managed properly-can become an eyesore, create nuisance odors, or become a breeding ground for insects.
In response to these storm water management issues, local and state governments are creating new regulations to limit the amount of impervious area in new developments or preventing impervious expansion of buildings or hardscape in existing developments.
System FeaturesFilterpave porous pavement system uses post-consumer recycled glass as the open-graded aggregate. According to the Container Recycling Institute (CRI), only 27.8% of glass bottles were recycled in the U.S. in 2006.2Considering the number of municipal recycling programs around the country, and the fact that industry experts claim up to 70% recycled glass can be used to manufacture new containers (thus saving energy and improving efficiency), this is a surprising statistic. The problem lies with sorting glass from other solid waste materials and sorting the crushed glass cullet based on color. Glass manufacturing can only tolerate a small percentage of foreign matter or glass of the wrong color. Right now, almost all the glass people think they are recycling ends up in a landfill.
Filterpave requires some additional processing of the glass cullet to make it suitable for use as the aggregate in the porous pavement. The cullet is tumbled or vibrated over screens to reduce or eliminate sharp edges. The material is sieved to obtain the correct particle size (approximately 0.12-0.25 in. in diameter). Then the glass is treated with a dilute solution of hydrolyzed aminosilane in water to develop a reactive surface on the glass for the polyurethane binder. The final steps are to dry and package the open-graded aggregate in waterproof super sacks to be used on the construction site. Figure 1 shows the improvement in compressive strength (approximately 40%) achieved by pretreating the glass with the aminosilane solution. In addition, it makes the composite pavement much more stable to environmental forces, such as heat and humidity, which could potentially reduce its strength and eventually cause failure of the pavement. Thermal cycling in high-humidity conditions shows little or no loss in compressive strength after 75 cycles between -10°C and 25°C.
The polyurethane contains over 50% renewable resource content in the form of natural oil-based components, which imparts the desired hydrophobicity that is often necessary for field applications. The chemical components of Filterpave PUR are similar to those of Elastocoast PUR, which has a proven track record for field use.3The PUR provides consistent physical properties for the composite over a wide range of temperatures, as shown by the dynamic mechanical analysis (DMA) of the cured elastomer provided in Figure 2. The Filterpave PUR can be pigmented with a variety of colors, but five base colors were chosen (sapphire blue, jade green, sedona red, amber brown and topaz brown) to limit complexity within the supply chain (see Figure 3).
Filterpave pavement is open pore, which allows air and water to flow directly through the composite. Even though the pavement uses much smaller aggregate than more common porous pavements (i.e., pervious concrete and porous asphalt), the composite maintains over 35% void volume. This allows the material to accept very large volumes of water in a short period of time. Testing at the University of Central Florida indicates that the pavement can accept 1,600 in. of water per hour. Also, the void volume makes the maintenance of the open structure easier, because sediment deposited in the pores can be removed with little effort. Air flow through the material allows it to release heat through convection much more quickly than conventional pavements, so the pavement cools down in a much shorter period of time (compared to its alternatives) after a heat source is removed.
The Solar Reflective Index (SRI) for the five colors of Filterpave is provided in Table 2 (ASTM E 1980 test method), along with some generally accepted values for concrete and asphalt. SRI is a measure of a surface’s ability to reject solar heat, as shown by a small temperature rise. It is defined so that a standard black (reflectance 0.05, emittance 0.90) is zero, and a standard white (reflectance 0.80, emittance 0.90) is 100. For example, the standard black has a temperature rise of 90°F (50°C) in full sun, and the standard white has a temperature rise of 14.6°F (8.1°C). Once the maximum temperature rise of a given material has been determined, the SRI can be computed by interpolating between the values for white and black.
System BenefitsThe Filterpave porous pavement system mimics the natural filtration process that occurs when water flows through the ground. Organic contaminants that are deposited on the pavement can be trapped in the storage layer under the wearing course and decomposed by bacteria. This allows the local aquifer to be recharged with clean, filtered water. It also reduces the amount of impervious surface on a given site so that the land use can be more efficient in new developments. It may reduce the size of a detention pond or eliminate it all together. For developed real estate, porous pavement will reduce the amount of storm water that is sent to the local treatment facility, thereby alleviating stress on the system, or it may eliminate a potential point source emission for the untreated discharge to a local waterway.
As long as the porosity is maintained in the pavement, it is very stable to freeze-thaw cycles. Ice will freeze through the composite to the point of least resistance. Consider the following analogy: if a capped glass-bottled beverage is placed in a freezer, the bottle will burst when it freezes; if the bottle is uncapped, the liquid will freeze and will move out of the opening as it expands-leaving the bottle intact. Thermal cycling from -10°C-25°C, both with the sample totally submerged and with only a small amount of water present in the composite, indicates no significant loss in compressive strength after 75 cycles. Another potential benefit of the Filterpave system in a cold weather climate is that it should not require salt for ice removal. In general, ice on pavement develops from snow melting and then refreezing. In the case of Filterpave and other porous pavement, if the porosity is maintained, the water from snow melt moves through the composite and does not refreeze on the surface. Snow removal from a Filterpave surface is the same as that for conventional pavement, except that a flexible blade tip should be used for plows or snow blowers to prevent gouging of the surface. Power brushes may be used without modification.
When properly installed, Filterpave pavement will remain remarkably stable. The thermoset polyurethane binder provides stability over a wide temperature range that will not soften significantly at high temperatures to allow rutting. A properly installed base for the pavement should be very stable, but if there is settling or partial washout of the base, Filterpave pavement can bridge substantial gaps. If it does begin to fail, the failure may not be catastrophic, because the material maintains a resistance to stress even after the peak stress level is reached (see Figure 4). Eventually, the material fails catastrophically, but not until a substantial deformation of the sample occurs. Another reason the pavement remains stable is that plant roots will have less of a tendency to crack or break up the pavement, because the porous composite allows water and nutrients through to any roots that infiltrate the base.
Filterpave pavement can also help reduce the urban heat island effect. All five color variants of the pavement have an SRI substantially greater than 29, which is the minimum requirement to achieve a Leadership in Energy and Environmental Design (LEED) credit for the reduction of heat island effect for non-roof structures. In addition, the porosity of the pavement allows it to cool more quickly than many alternatives because of heat loss due to convection and possibly evaporative cooling from any water within the structure.
Filterpave pavement is easy to install using conventional concrete equipment. It is formed and finished similar to low-slump concrete. The material can be mixed in a batch process on a small scale using a concrete or mortar mixer. The PUR is mixed and added to the aggregate in the mixer, blended for a few minutes, and then placed into the forms. The mixed PUR and glass aggregate has a working time of 30-45 minutes. A vibratory screed is often used to provide a slight compaction/settling of the material in the forms. A smooth finish is achieved by working the surface with a bull float (Fresno blade) or a power trowel, and the edges around the forms are finished with a ¼-in. radius edge trowel.
Depending on weather conditions, the material is tack-free within four to six hours, can be walked on in 24 hours and has achieved 95% of its final hardness within 72 hours. If the surface is to be driven on, it can typically withstand the force of vehicle traffic after four days. An aliphatic PUR surface coating is sprayed or rolled on after the material is tack-free to ensure a wearing surface that is very stable to torsional forces from tires turning upon it. Also, sand can be broadcast over the curing topcoat to provide an anti-skid surface on slopes or areas with high foot traffic.
For large-scale operations, the material can be mixed continuously or on demand with a slightly modified volumetric concrete mixer (see Figure 5). A two-component polyurethane machine with a static mixer is attached to the auger mixer and calibrated to the flow of aggregate. This configuration is capable of mixing 10-50 yards of material in an hour.
Colored Filterpave pavement can provide an aesthetic quality to pavement not achievable by other means. The glass sparkles in the sunlight, and different colors can be used for various effects and patterns. It can provide architects with additional options and unique site design alternatives. In addition, Filterpave porous pavement can contribute to credits and points through LEED and/or the National Association of Home Builders (NAHB), the two leading “green” certification systems for commercial and residential construction, respectively (see sidebar for additional details).
ApplicationsFilterpave porous pavement system is appropriate for light- to medium-duty traffic areas. Figure 6 shows a cross-section diagram of a typical porous pavement installation using the Filterpave system. The native soil is excavated to a depth appropriate for the regional weather conditions and how the soil drains. In some northern climates, this could be as much as 24 in. The area is then lined with a geotextile, and the storage layer, consisting of 0.38-0.75 in. diameter open-graded aggregate, is installed and compacted. A 2-in.-thick open graded choker course (0.25-0.5 in. diameter) is installed and compacted. Finally, the wearing course of Filterpave is installed: 3½ in. thick (2 x 4 in. forms) for driving surfaces and 2½ in. for other areas.
In the last two years, the pavement has been used in a variety of applications. Over 40 different installations have been completed, including residential driveways, walkways, patios, commercial parking areas, and golf cart paths. Figure 7 provides some examples of these installations.
ConclusionThe next time you walk across a dirty-looking parking lot, imagine how much mores aesthetically and environmentally pleasing it would be if it were made with a porous pavement. Filterpave can help manage stormwater by reducing or eliminating runoff, and it allows rain water to flow through the structure and naturally filters out many pollutants before the water replenishes the local aquifer.
Filterpave pavement is stable to freeze-thaw cycles, reduces urban heat island effect and is easy to install. It can be used for light- and medium-duty traffic areas such as driveways, parking areas, cart paths, walkways, and patios. Using post-consumer recycled glass aggregate and a pigmented thermoset polyurethane, the pavement provides an aesthetic quality that cannot be achieved with other pavements, and it contributes to LEED credits or NAHB Green Points.
For more information, contact BASF Corp. at 1609 Biddle Ave., Wyandotte, MI 48192; phone (734) 324-6660; fax (734) 324-5549; or visit www.basf.com.
Editor’s note: This article is based on a paper given at the Center for the Polyurethanes Industry’s (CPI) Polyurethanes 2010 Technical Conference, September 2010.
References1. National Resources Defense Council,The Problem of Urban Stormwater Pollution, November 2000,www.nrdc.org/water/pollution/fstorm.asp.
2. Container Recycling Institute,Recycling Rates by Material and Class, 2006, updated 2007,www.container-recycling.org/facts/all/data/recrates-depnon-3mats.htm.
4. Hicks, S., Bower, D.K., and Leberfinger, M., “Elastomeric Revetments-An Innovative Solution for Coastal Protection and Erosion Control,” presented at the Polyurethanes 2008 Technical Conference, September 29-October 1, 2008.