Traditionally, the goal of a stormwater management system has been to collect, transport and dispose of stormwater as efficiently and quickly as possible. This decreases the interval between precipitation and runoff and increases peak runoffs and peak velocities, all of which contribute to erosion, pollution, flooding and a reduction in groundwater recharge. Overall, runoff stormwater systems increase flooding during storms and decrease stream levels between storms. Porous concrete pavement offers one way to eliminate or reduce stormwater runoff. Water simply filters through the slab and recharges the groundwater supply. Runoff volume and runoff velocity are reduced, and lag time between rainfall and runoff is increased. The result is a developed building site with runoff levels which closely approximate predevelopment runoff levels.
Porous concrete pavements can be made with a porous concrete mix which contains voids that allow water to penetrate through the hardened concrete pavement. Another way to make porous pavements is to cast cavities into a slab made with normal, relatively non-porous, nonpermeable concrete. A similar result can be obtained with lattice-type precast pavers. Porous concrete mixes generally contain no fine aggregate and a narrow gradation range of coarse aggregate. The water-cement ratio is low, and slump is extremely low, often zero.
Obviously, porous concrete is not a new material, but its use today as a paving material is expanding. In the past, pavement design has centered around keeping the moisture out of the subbase. With a porous concrete pavement, water is allowed to penetrate the pavement and recharge groundwater. In California, porous concrete has been used in longitudinal edge drains and as a base course under conventional concrete pavement. It has also been used for greenhouse floors, parking areas, and tennis courts. Other possible applications include roads and highways, airport runways, sidewalks, patios, swimming pool decks, nature trails, floors in animal barns, and sewage sludge beds.