orous pavement is a special type of pavement that allows rain and snowmelt to pass through, thereby reducing the runoff from a site and surrounding areas. In addition, it filters certain pollutants from the runoff, adding to its ecological advantage.
There are two types of porous pavement:
- Porous asphalt
- Pervious concrete
Porous asphalt pavement consists of an open-graded coarse aggregate, bonded together by asphalt cement, with sufficient interconnected voids to make it highly permeable to water.
Pervious concrete consists of specially formulated mixtures of Portland cement, uniform, open-graded coarse aggregate and water. It is a special type of concrete with a high porosity, which, when it is used for concrete flatwork applications, allows water from precipitation and other sources to pass directly through. Consequently, runoff from the site is significantly reduced and groundwater is also able to recharge more rapidly. Typically pervious concrete has little or no fine aggregate and has just enough cementitious paste to coat the coarse aggregate particles while preserving the interconnectivity of the voids.
Show Me the Money
When compared to asphalt, concrete pavement has a significantly lower life-cycle cost. Though the initial cost of pervious concrete installation may be slightly higher, concrete saves money in the long run due to superior durability and strength. It requires fewer repairs and has a longer life span as well.
The proper utilization of pervious concrete is also a recognized Best Management Practice by the U.S. Environmental Protection Agency (EPA) for providing first flush pollution control and stormwater management. Reduction of the runoff from pervious concrete areas reduces the need (and cost) for separate storm water retention ponds and allows the use of smaller capacity storm sewers. Property owners can then develop larger portions of their properties at a lower cost. Natural water filtration through the soils also reduces pollutants that enter into our waterways.
In addition, because of its benefits in controlling stormwater runoff and pollution prevention, pervious concrete has the potential to help earn a credit point in the U.S. Green Building Council’s Leadership in Energy & Environmental Design (LEED) Green Building Rating System.
Finally, the color of concrete pavement absorbs less heat from solar radiation than darker pavements and the relatively open pore structure of pervious concrete stores less heat, helping to reduce heat island effects, another potential LEED credit. Trees and shrubs adjacent to pervious concrete pavement or sidewalks receive more air and water thereby growing better and further reducing the heat island effects. Landscapers and architects who wish to incorporate greenery in urban areas often consider pervious concrete as a solution.
Maintenance of pervious concrete consists mainly of prevention in the clogging of the void structure system. As part of pre-construction preparation, drainage of surrounding landscaping should be designed to prevent flow of materials onto pavement surfaces. In colder climates a thick layer of 8-24 inches of open-graded stone base will allow for entrained air flow that may improve the freeze-thaw durability.
Limited application of salt and sanding in cold climate regions is also suggested, as is annual vacuuming. Other recommended cleaning options include power blowing or pressure washing. Pressure washing of any clogged areas typically will restore infiltration into subgrade.
Pervious concrete as currently designed is not an appropriate product for our highways because the pressure of steady and constant vehicle traffic would most likely compact the concrete and cause it to lose its porosity. This is unfortunate, because the nature of its surface would tend to reduce road hazards such as surface ponding and hydroplaning.
Without doubt, pervious concrete offers many advantages to builders and society alike. Given these factors, it may well be improved, over time, for suitable use in more areas of construction.
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Case in Point
In 2007, UNH researchers watched the installation of a 21,000 square-foot parking lot adjacent to Williamson Hall using pervious concrete. As one of the first major previous concrete installations in New England, this parking area and has been closely studied by researchers.
When asked, a year later, about the pervious concrete's durability in the New England climate, Mr. Robert Roseen, Director of UNH Stormwater Center, stated, “It did very well. In fact, the hillside around the lot used to flood in heavy rains, and that has not happened since the new lot was installed.”
UNH Stormwater Center continues to monitor the durability of this test parking lot for long term sustainability.
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