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Pervious concrete pavement is a unique and effective means to address important environmental issues and support green, sustainable growth. By capturing stormwater and allowing it to seep into the ground, porous concrete is instrumental in recharging groundwater, reducing stormwater runoff, and meeting U.S. Environmental Protection Agency (EPA) stormwater regulations. In fact, the use of pervious concrete is among the Best Management Practices (BMPs) recommended by the EPA-- and by other agencies and geotechnical engineers across the country-- for the management of stormwater runoff on a regional and local basis.

  This pavement technology creates more efficient land use by eliminating the need for retention ponds, swales, and other stormwater management devices. In doing so, pervious concrete has the ability to lower overall project costs on a first-cost basis. In pervious concrete, carefully controlled amounts of water and cementitious materials are used to create a paste that forms a thick coating around aggregate particles.

 A pervious concrete mixture contains little or no sand, creating a substantial void content. Using sufficient paste to coat and bind the aggregate particles together creates a system of highly permeable, interconnected voids that drains quickly. Typically, between 15% and 25% voids are achieved in the hardened concrete, and flow rates for water through pervious concrete are typically around 480 in./hr (0.34 cm/s, which is 5 gal/ft²/ min or 200 L/m²/min), although they can be much higher. Both the low mortar content and high porosity also reduce strength compared to conventional concrete mixtures, but sufficient strength for many applications is readily achieved.

 While pervious concrete can be used for a surprising number of applications, its primary use is in pavement. This site focuses on the pavement applications of the material, which also has been referred to as porous concrete, permeable concrete, no-fines concrete, gap-graded concrete, and enhanced-porosity concrete.

Pervious concrete pavement systems provide a valuable stormwater management tool under the requirements of the EPA Storm Water Phase II Final

Rule. Phase II regulations provide programs and practices to help control the amount of contaminants in our waterways. Impervious pavements-- particularly parking lots-- collect oil, anti-freeze, and other automobile fluids that can be washed into streams, lakes, and oceans when it rains.

As for oil that drips on and into pervious pavement, several studies have been conducted on the behavior of oil introduced into a pervious pavement system. According to PCA, findings show that the oil is contained within the pervious system as a coating on the large surface area of the void system and at the location of any geotextile fabric separating the pavement from the subbase or subgrade support. Hydrocarbons, such as oils, are a food source for many naturally occurring bacteria and fungi. The microorganisms feed on the oil and biodegrade it into simpler chemical components that are released into the atmosphere. Research has determined that 97.6 to over 99 percent of oils introduced into pervious pavements are trapped and biodegraded.

EPA Storm Water regulations set limits on the levels of pollution in our streams and lakes. To meet these regulations, local officials have considered two basic approaches: 1) reduce the overall run­off from an area, and 2) reduce the level of pollution contained in runoff. Efforts to reduce runoff include zoning ordinances and regulations that reduce the amount of impervious surfaces in new developments (including parking and roof areas), increased green space requirements, and implementation of “stormwater utility districts” that levy an impact fee on a property owner based on the amount of impervious area. Efforts to reduce the level of pollution from stormwater include requirements for developers to provide systems that collect the “first flush” of rainfall, usually about 1 inch (25 mm), and “treat” the pollution prior to release. Pervious concrete pavement reduces or eliminates runoff and permits “treatment” of pollution: two studies conducted on the long-term pollutant removal in porous pavements suggest high pollutant removal rates. The results of the studies are presented in Table 2 below

 Economic Benefits

Alternative to Costly Stormwater    

     Management Methods

 Parking areas paved with pervious concrete reduce the need for large detention ponds, because the pavement itself acts as a detention area. Parking lot owners that use pervious will spend fewer dollars on the labor, construction, and maintenance of detention ponds, skimmers, pumps, drainage pipes, and other stormwater management systems. Expensive irrigation systems can also be downsized or eliminated. In reducing runoff from paved areas, pervious concrete reduces the need for separate stormwater retention ponds and allows the use of smaller-capacity storm sewers. This allows property owners to develop a larger area of available property at a lower cost.

Stormwater Impact Fees

 Many government agencies are now implementing stormwater impact fees for all impervious areas. As regulations further limit stormwater runoff, it is becoming more expensive for property owners to develop real estate, due to the size and expense of the necessary drainage systems. Pervious concrete can reduce these fees for the property owner by helping to minimize demands upon sewer systems.   

 Developers are using pervious concrete for parking areas in order to increase utilization of commercial properties. The land ordinarily devoted to costly stormwater management practices or compliance with maximum impervious area ordinances can now be developed or preserved, enhancing the bottom line.

Strength and Durability

 Pervious concrete is a strong and highly durable material. Parking areas properly designed and constructed will last 20-40 years with little or no maintenance. Unlike asphalt, surface raveling (the loosening of surface aggregates) is common only in the first few weeks after the concrete is laid, and it can be reduced with proper compaction and curing techniques.

Pervious concrete mixes contain minimal amounts of water and therefore have very low slump (i.e. a stiff consistency). A much smaller amount of drying shrinkage occurs in the placement of pervious concrete than dense, and it develops sooner, as well. This allows many pervious pavements to be constructed without crack-preventing control joints. Random cracks that do form are not abundant, and they have no significant impact on the structural integrity of the pavement. They also generally do not detract aesthetically from the concrete's appearance.

 Pervious pavements can achieve strengths in excess of 3000 psi (strong enough to support a fire truck), and even more with special mix designs, structural designs, and placement techniques. The key to high-performance concrete is the use of supplementary cementitious materials such as silica fume, fly ash, and blast furnace slag, all which increase durability by decreasing permeability and cracking.

Pervious Concrete Parking Lots are Safer and More Attractive than Asphalt.

  Arizona's first use of pervious concrete on something other than a small   demonstration project was recently placed and is in service at Northern Arizona University's Applied Research & Development building in Flagstaff. The pervious concrete parking area is part of an overall project registered as LEED Platinum and presented as a special focus project at the recent US Green Building Council GREENBUILD Conference in Denver.

Comparison of Post-Snowstorm Pavement Surfaces

 These photos were taken within minutes of each other in two supermarket parking lots across the street from each other in Denver, CO. Both lots were plowed in the morning following the overnight snow storm. The air circulating beneath the pervious concrete accelerates the melting of remaining snow and allows the melt water to drain immediately. Besides providing a safe and tidy surface during the day, the totally dry pervious pavement greatly reduces the risks associated with refreezing of puddles as the temperature drops later.

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