FREQUENTLY ASKED QUESTIONS
What is pervious concrete paving?
Pervious concrete pavement is a mixture of Portland cement, pea gravel, and water. It is ordinary concrete without the sand. Because of the absence of sand, the void space is between 15% and 30%, which makes the material look a bit like a rice cake. This allows water to percolate through the pavement to the subsoil beneath. Pervious concrete is not a proprietary product; it is a “recipe” for concrete which can be made to order by any concrete batch plant. Pervious concrete is installed without rebar; the thickness is minimum 6 inches for most applications. Color can be added to the mix if desired.
What is the history of pervious concrete?
It was originally used 100 years ago in Europe as structural insulation in buildings. It has been used as a paving material in Europe for the past 80 years and in the American Southeast for the past 20+ years. It is relatively new to the West Coast.
What is it used for?
Pervious concrete is used for parking lots, driveways, service roads, walkways, sidewalks, curbs and gutters, and secondary roads. It is suitable for light to medium vehicle traffic at low to moderate speeds. It is not suitable for highways or other high speed pavement.
What are the advantages of pervious concrete?
Because of its absorptive qualities, pervious concrete eliminates the runoff of rainfall. This results in zero discharge of polluted runoff into waterways, lessening of urban flooding, increased groundwater recharge, improved health of adjacent trees, on-site bioremediation of pollutants, and a reduction or elimination of the need for storm drain infrastructure. Because of its light color, it helps to reduce the urban heat island effect caused by large areas of dark-colored pavement. It can also significantly reduce the cost of paving. Pervious concrete meets NPDES regulations.
Are there situations where it shouldn’t be used?
Slopes over 5 percent present special problems for pervious pavement. Any site that is at risk of siltation from adjacent areas would be a poor choice for pervious concrete unless special measures are taken to protect the pavement.
How is pervious concrete installed?
Pervious concrete is prepared at a batch plant and delivered in a standard ready-mix truck. It must be placed within an hour of mixing. It must be poured from the truck directly into the forms; it cannot be pumped due to its low water content and coarse texture. Once in place, it is leveled with a vibrating screed and then compacted with a special heavy steel roller. Although control joints are not necessary in many cases, they can easily be made with a flanged roller. The slab is covered with plastic sheeting as soon as it is completed, and it is allowed to cure under the sheeting for one week. Because of its texture, there is no finishing required or possible. If a smoother finish is desired, the material can be ground off with a standard pavement grinder.
Won’t it plug up and stop working?
Because the material will absorb up to 450 inches of rainfall per hour, the vast majority of its absorptiveness could be lost to sedimentation and the pavement would still easily absorb even abnormally heavy rainfall of 3-4 inches per hour.
What about maintenance?
There is little or no maintenance required. An occasional cleaning with a vacuum sweeper truck can help restore void space if desired.
What is its lifespan?
The lifespan is the same as conventional concrete. Virtually all the pervious concrete installed in the Southeast is still in use.
How much does it cost?
The cost per square foot, installed, is usually comparable to conventional concrete. As installers become more familiar with the installation process, prices should drop because there is less to do on most installations. The elimination or reduction in drainage infrastructure often reduces the final cost of the project, making pervious concrete the least costly approach.
Where can I find more information?
- National Ready Mix Concrete Association
- Portland Cement Association
- United States Environmental Protection Agency