Thursday, July 16, 2009

Pervious Concrete Pavement, It Rains, It Pours, and Runs Right Through

Water, water, everywhere and not a drop to drink. Drive past Brush Creek during a rainstorm and notice one minute it is dry and the next full of raging dirty water. Water probably dirtier than you would like to because the rainfall often overwhelms the sewer system causing a discharge of extra stuff into aptly nicknamed Flush Creek. The extra water comes from our urban environment where impervious roofs, parking lots, and roads prevent moisture from infiltrating in the ground and is rapidly conveyed to our storm sewer system. Well everything would be much better if that water just went through the pavement, right? Luckily that technology has reached Kansas City and is called pervious concrete pavement.

Pervious concrete has the same basic ingredients as your standard sidewalk with the exception of a single-sized coarse aggregate to create pores for the water to flow, imagine a harder less-tasty rice krispy treat. Typical permeability values for pervious concrete range from 500 in./hr to over 2,000 in./hr, values that well exceed any possible storm. The stormwater runs through the pavement and is stored in an aggregate layer beneath until it soaks into the ground or evaporates. Even sites with low permeability clay around Kansas City can experience significant infiltration. Other benefits of pervious concrete include cleaning stormwater through filtration.

The most common concerns for the public are freeze-thaw durability, clogging, and excessive cost. Much of the research in the last couple years has focused on freeze-thaw durability. We now have pervious concrete mixtures more durable than traditional concrete. Although it turns out that no pervious pavement has ever experienced freeze-thaw deterioration. Pervious concrete is a filter and can become clogged if not maintained. Normal routine street sweeping keeps the pores open and if clogging occurs (usually from nearby open soil), permeability can easily be restored by pressure washing and vacuuming. While the pervious concrete itself costs more from additional chemicals in the mixture, overall project can save money by reducing the size of detention ponds and the amount of stormwater pipes required.

On-going research at UMKC's Department of Civil and Mechanical Engineering is investigating the mitigation of the urban heat island effect (heat build-up around cities from dark and impervious surfaces) with pervious concrete and improved winter safety from reduced potential for black ice.

The largest pervious concrete installation in the Kansas City area can be seen at Oregon Trail Park in Olathe, KS. For more information on pervious concrete in Kansas City download a podcast discussing pervious concrete here.

-Written by John T. Kevern, Ph.D., LEED AP, Assistant Professor of Civil Engineering, UMKC

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