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February 1, 2013

The Economic and Environmental Benefits of Pervious Concrete

by the U.S. Green Building Council Long Island Chapter

Pervious concrete pavement is one of the leading materials used by the concrete industry in effecting significant “Green” industry practices and is recognized as a Best Management Practice by the U.S. Environmental Protection Agency (EPA) for providing pollution control, storm water management and sustainable development. The increased interest in pervious concrete is due to those benefits in storm water management and sustainable development. This paper will provide
technical information on the application, mixture design and construction methods of pervious concrete. It will also discuss many of the environmental and economic benefits of using pervious concrete.
WHAT IS PERVIOUS CONCRETE?
Pervious concrete is a zero-slump, open-graded material consisting of hydraulic cement, coarse aggregate, admixtures and water. Because pervious concrete contains little or no fine aggregates such as sand, it is sometimes referred to as “no-fines” concrete. When the cement and water are combined, it forms a paste that binds the coarse aggregate together in a hardened product with connected pores that allow water to pass through easily. The pores can range from 0.08 to 0.32 inches (2 to 8 mm), and the void content usually ranges from 15% to 25% with compressive strengths of 400 to 4000 psi (2.8 to 28 MPa). However, strengths of 600 psi to 1500 psa (2.8 to 10 MPa) are more common. Many of the void spaces are interconnected, forming channels that let water and air pass through the pavement. The draining rate of pervious concrete pavement will vary with aggregate size and density of the mixture, but will generally fall within the range of 2 to 18 gallons/minute/foot2 (81 to 730 liters/minute/m2). The density and flow rate depends on the properties and proportions of the materials used.  Therefore, mix designs must take into account the different aggregates used in each geographic locality.
PERVIOUS CONCRETE MIX DESIGN
As mentioned above, pervious concrete uses the same materials as conventional concrete, except that there is usually little to no fine aggregate. The quality, proportions and mixing techniques affect many of the properties of pervious concrete, in particular the void structure and the strength. Freshly mixed pervious should be plastic and capable of being shaped like modeling clay when squeezed by hand. It should hold its shape without slumping.
Aggregate
The size of the coarse aggregate used is kept fairly uniform in size (most common is size 89, or 3/8 inch) to minimize surface roughness and for a better aesthetic.  The use of the pervious concrete will dictate the size of the aggregate used, and sizes can vary from ¼-inch to ½-inch (6.35mm-12.7mm) in size. Aggregate can be rounded like gravel or angular like crushed stone and still make for a good mixture. The decision about the best aggregate for any project is usually a joint decision between the producer and the contractor. Part of that decision-making process has to do with the compaction equipment, the availability of materials, the production capabilities and economic concerns. It is good to remember that rounded aggregate requires less compactive effort than angular aggregate, and can produce higher strength pervious concrete.
Water
Water that is potable is generally fine for use in the mix. Coarse aggregate should be kept damp before batching, especially if the weather is very hot with low humidity in order to ensure consistency and uniformity from batch to batch of plastic pervious concrete.If the aggregate is too dry before being mixed, the mixture will not place or compact well. But excess free water on aggregates contributes to the overall mixing water and will create a wet, soupy mix in which
the paste flows off, and the voids are filled. Water to cement ratios should be between 0.27 and 0.30 including any chemical admixtures. Ratios as high as 0.34 to 0.40 have also been used successfully. Unlike the relation between strength and water-to-cement ratios in regular concrete, the relationship for pervious is not as well defined. In pervious, the total paste content is less than the voids content between the aggregates, so making the paste stronger may not reliably lead to
increased overall strength.  Needless to say, the water content should be tightly controlled so that the mixture has a sheen to it without it being so soupy that it flows off the aggregate. Optimum water content produces a fully wetted cement paste with a high viscosity that can be described as sticky.  Cementitious Materials call for pervious concrete in constructing large areas of pavement has encouraged technological advancements in placing and finishing pervious concrete. The quick application of pervious concrete is critical to a successful placement due to its rapid hardening.
References
National Ready Mixed Concrete Association (NRMCA), Text Reference for
“Pervious Concrete Contractor Certification,” NRMCA Publication #2PPCRT,
Silver Spring, Maryland, February 2007.
Pervious Concrete: “When it Rains it Drains,” http://www.perviouspavement.org

Full Article at http://usgbc-li.blogspot.com/2013/02/the-economic-and-environmental-benefits.html

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