Pervious Parking Surfaces
Parking lots and driveways are considered to be primary point sources of pollution. When rain falls on a conventional parking lot or driveway, it usually runs off carrying pollutants with it. Polluted stormwater runoff eventually finds its way into our streams, rivers and lakes.
Parking lots paved with pervious surfaces, however, assist the process by capturing rainwater and allowing it to pass through and percolate into the underlying soil. Because water is allowed to absorb into a special sub-base under the parking surface that can break down some of these pollutants, nearby vegetation is watered, groundwater is recharged and stormwater run-off and pollution is reduced. Also, the special sub-base underneath these surfaces contains air voids that promote habitat of aerobic bacteria that can break down hydrocarbon pollutants found in automotive fluids and fuels.
Types of Parking Surfaces
Pervious concrete is a mixture of hydraulic cement, coarse aggregates (stone), water and admixtures. Pervious concrete contains little or no sand, and is sometimes referred to as a "no-fines" concrete. The cement and water forms a paste that binds the coarse aggregates together. Only enough paste is added to the mix to glue the aggregate together where they touch each other, but not enough to fill all the space between the aggregates.
A typical pervious concrete mixture will contain about 17-25% void space within the concrete. Many of the void spaces within the pervious concrete will be interconnected, forming channels that allow water and air to pass freely through the pavement structure.
A variation of Pervious Concrete is a product called Filterpave® which is a mixture of granite chips and crushed recycled glass which is bound with a 60% natural plant based resin. The pervious pavement is placed over a crushed rock bed which functions much the same as pervious concrete. This paving is suitable for parking areas, pathways or driveways. This paving offers the advantage of an environmentally sound use for recycled glass and is available in different colors depending on the color of glass used.
Pervious Concrete Pavers
Pervious concrete pavers systems typically consist of strong materials (bricks or blocks) that have gaps along the abutting sides. These gaps are typically filled with a pervious material (such as crushed rock) which allows surface water to infiltrate to a special rock base below. Pervious concrete pavers may be installed as pre-cast modular systems that may be made up of lattice grids. These include modular units (look like bricks or blocks) and interlocking systems that may be connected by cables. Depending on the depth of the "sub-base" rock layer underneath these systems, a large enough quantity of stormwater could be captured such that a pervious paver application may factor in as a percentage of stormwater detention for a building site.
Pervious Grass Pavers
Pervious grass pavers systems typically consist of modular grids that keep vehicular traffic from compacting soils in the top layer and distributing weight to a crushed rock base. By not allowing soil compaction on the top, the grass can thrive and water can infiltrate to the rock layer below. Pervious grass pavers may be installed as modular systems that may be made up of lattice grids that interlock to form an overall vehicle pad. The "sub-base" rock layer underneath these systems should have some soil mixed in to allow for maximum root growth. This application has been shown effective in reducing the "heat island" effect associated with typical concrete parking surfaces. It is suggested that the use of "drought tolerant" grasses should be incorporated for this application.