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Benefits
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Hydrologic Design of Pervious Concrete Part 1 w Part 2 w Part 3 w Part 4 w Part 5
Introduction
Pervious concrete pavement systems can be an important part of context-sensitive construction, and Low-Impact Development (LID), designed to meet a number of goals related to the function of the site and structure. Goals include the owner‘s objectives and society‘s requirements, both of which are site specific. The needs of society are often at least nominally addressed through permitting requirements, including land use restrictions, zoning limitations, and fees, and reflect increasing concerns related to control of surface runoff associated with development. These needs can vary by community, location, and application, and include both flood control and water quality.
Although pervious concrete* has been used in some areas for decades, recent interest in sustainable development or “green” building and recognition of pervious pavements by the US Environmental Protection Agency (EPA) as a best management practice (BMP) for storm water management has heightened interest in its use throughout North America. Its use supports national initiatives such as EPA’s Heat Island Reduction Initiative (USEPA 2007a) and Low Impact Development (USEPA 2007b) and provides a potential for credit in the LEED® (Leadership in Energy and Environmental Design) rating system for sustainable building construction (US Green Building Council 2005). LEED® includes provisions for control of both water quantity and water quality in storm water design. [*Note: A pervious concrete pavement system is a combination of elements including pervious concrete, usually a base course of clean stone, and may include filter fabric or geotextile, or curbs. Pervious concrete is a material typically produced with a conventional quantity of cementitious material, low water content, little or no sand, a relatively small, uniformly-sized coarse aggregate , and commonly used admixtures and air entraining agents. Pervious concrete generally has a relatively high permeability and high porosity. See Tennis, Leming, and Akers (2004) for more details.]
The hydrological performance of the pervious concrete pavement system is usually the characteristic of most interest to agencies with permit granting authority. In some cases, limits are placed on the percentage of land which may be developed for a given site without the use of specified remedies including structural Best Management Practices (BMPs) such as detention or retention ponds. Alternately, limits on the quantity of runoff after development may be specified. Pervious concrete pavement systems can be an important part of a sustainable site while simultaneously providing access and parking space.
Unlike many other structural BMPs, pervious concrete paving systems can be used effectively and economically to retrofit existing built-up sites to attain desired hydrologic performance. The EPA’s Preliminary Data Summary of Urban Storm Water Best Management Practices (1999) notes that retrofitting to correct or attain specific runoff limits for an existing, built-up site can be extremely expensive. Pervious concrete pavement systems can be the exception to this rule since they can re-use existing parking areas for impoundment purposes. For example, analysis of one potential parking lot the size of a football field indicated that the runoff from a reasonably urbanized, 9-acre (3.6 ha) area over a sandy silt subgrade would be the same as from grassy pasturage in that area (Malcom, 2002).
Pervious concrete can be a BMP used to mitigate problems associated with surface runoff through several mechanisms. Hydrologically, pervious concrete paving systems can:
1. Capture the “first flush” of runoff from the surface so the pollutant load including trash, “floatables,” and other debris in overland surface runoff and, ultimately, streams and rivers, is reduced, and
2. Create short term storage detention of rainfall which: a) Reduces the volume of surface runoff, b) Provides for additional infiltration, thereby recharging groundwater and increasing base flow, c) Thereby also reducing the velocity of water in both natural and constructed drainage channels, and reduces surface runoff, which can reduce the sediment load carried into receiving waters since both the erosion of channels and the quantity of materials carried into those channels are reduced.
Pervious concrete paving systems also benefit the environment in other ways. The surface temperature of the pervious concrete is lower than, for example, an asphalt pavement, which reduces the “heat island” effect common in built-up areas. The initial runoff from conventional pavements can be much warmer than the receiving water temperature, raising the overall temperature of the receiving water and causing environmental distress. Since runoff is held in the pervious concrete paving system rather than running directly into receiving waters, potential temperature increases in the receiving waters can be significantly reduced.
Pervious concrete pavement systems not only positively impact water quality and water quantity, but can provide other benefits. Pervious concrete can reduce “black ice” formation, reducing potential slipping hazards, as melting snow drains into the pavement rather than ponding on the surface.
The material characteristics of the pervious concrete and other elements of the system significantly affect the final design. The porosity of pervious concrete affects both hydrologically important properties (permeability and storage capacity) and mechanical properties (strength and stiffness). Pervious concrete used in pavement systems must be designed to support the intended traffic load (axle loads and repetitions) and contribute positively to local storm water management strategies. The designer must specify the appropriate material properties, the appropriate pavement thickness, and other needed characteristics, including the absence or presence of features such as base course, filter fabric, or geotextile reinforcement, to meet the hydrological requirements and anticipated traffic loads simultaneously.
This publication provides an overview and discussion of design techniques which can be used for hydrological design needs, considering the inter-relationship between runoff characteristics, material characteristics, and site geometry. It also provides guidelines for the preliminary selection of appropriate pervious concrete characteristics for specific applications and environments. This publication is intended to assist:
1. Civil engineers, landscape architects, and other designprofessionals-of-record for guidance in the selection and design of an appropriate pervious concrete pavement system, and includes a discussion of design methods;
2. Permit granting agencies in the review and acceptance of proposed pervious concrete pavement systems; and
3. Developers and owners interested in a more complete technical review of pervious concrete pavement systems. A companion publication, Pervious Concrete Pavements (Tennis, Leming, and Akers 2004) provides an introduction to the technology of pervious concrete, including applications, engineering properties, and construction techniques.
Reference: Leming, M.L., Malcom, H.R., and Tennis, P.D., Hydrologic Design of Pervious Concrete, EB303, Portland Cement Association, Skokie, Illinois, and National Ready Mixed Concrete Association, Silver Spring, Maryland, USA, 2007, 72 pages. |
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