Pervious Concrete A Solution to Stormwater Runoff

Lionel Lemay, Sr. Vice President, Sustainable Development, National Ready Mixed Concrete Association. and
Colin Lobo, Sr. Vice President, Engineering, National Ready Mixed Concrete Association

Introduction

Pervious concrete is a mixture of hydraulic cement, coarse aggregates (stone), water and admixtures. American Concrete Institute (ACI) Committee 522 describes pervious concrete as a "near zero-slump, open-graded material consisting of hydraulic cement, coarse aggregate, little or no fine aggregate, admixtures, and water.”1 The cement and water in pervious concrete forms a paste that uniformly coats and bonds the aggregates together when properly compacted. An optimized quantity and consistency of paste is used in the mixture to allow the fresh pervious concrete mixture to flow adequately when placed and to obtain point-to-point contact of the coarse aggregates, creating an interconnected void system to allow stormwater to infiltrate through the pavement system when installed. A typical pervious concrete mixture will contain between 15 and 25% void content within the concrete. The drainage rate of pervious concrete pavement will vary with aggregate size and density of the mixture, but will generally fall into the range of 0.2 cm/s (288 in./hr) to .54 cm/s (770 in./hr)².

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).