Colin Lobo, Sr. Vice President, Engineering, National Ready Mixed Concrete Association
Figure 1: The cement and water in pervious concrete forms a paste that uniformly coats and bonds the aggregates together when properly compacted but creates an interconnected void system that allows water to pass through it.
Pervious concrete, in some form, has been used in construction for more than 100 years. Today, in the United States, it is being used primarily as a pavement material for parking areas, drives, walkways and some lightly trafficked residential streets. Parking lots and roadways are major sources of stormwater pollution. When precipitation runs off a conventional, impervious parking lot or roadway, it carries with it a wide range of pollutants, such as debris, metals (brake linings) and hydrocarbons (oils and grease). Polluted stormwater runoff eventually finds its way into streams, rivers and lakes.
|Figure 2: Pervious concrete is primarily used as a pavement material for parking areas, drives, walkways and some lightly trafficked residential streets|
A major reason for the interest in pervious concrete is being driven by the benefits it provides for Low Impact Development (LID). In the United States, federal regulations require states, counties and municipalities to adopt procedures that address stormwater runoff and associated pollutants. These regulations generally require that property owners collect and treat stormwater on site as opposed to allowing it to runoff to adjacent sites or into municipal stormwater management systems. Infiltration systems, such as pervious concrete pavements, help satisfy these requirements. The US Environmental Protection Agency (EPA) recognizes pervious concrete as one of the best management practices for stormwater management.
A typical pervious concrete pavement system consists of a layer of pervious concrete to support traffic loading over a layer of aggregate base. The pervious concrete allows water to percolate quickly into the aggregate base layer, often called the storage layer, before slowly infiltrating into the subgrade soil. The aggregate base layer is typically a clean, single-sized aggregate between 18 mm and 40 mm (3/4 in. and 1-1/2 in.) nominal size. In some cases, perforated pipes are incorporated into the aggregate base layer to increase storage capacity or collect water from other surfaces on the site including roofs, walks and conventional impervious pavements. This supplemental drainage may also be necessary to increase capacity of systems on fine-grained soils that reduce the rate of percolation of stormwater. Perforated pipes in the base layer collect excessive stormwater and diverts it to supplemental retention areas. Pervious concrete pavement systems are often combined with other stormwater infiltration systems such as rain gardens and bio-swales (see figure 3).