Essentially, pervious concrete is a structural concrete pavement with a large volume (15 to 35 percent) of interconnected voids. Like conventional concrete, its made from a mixture of cement, coarse aggregates, and water. However, it contains little or no sand, which results in a porous open-cell structure that water passes through readily.
When pervious concrete is used for paving, it can take in stormwater at a rapid rate of 3 to 5 gallons per minute per square foot of surface area, which exceeds the flow rate needed to prevent runoff in most rain events. The rainwater may be stored in a coarse gravel layer underneath the pavement or allowed to percolate into the underlying soil. Because the pavement itself acts as a retention area, it helps to prevent much of the polluted runoff that normally occurs with impervious pavements. The filtration process also helps to purify the water. As the water percolates through the open cells of the pavement, aerobic bacteria in the voids help to break down harmful pollutants and chemicals.