Pervious Concrete FAQs

The NRMCA conducted a field study of 10 pervious concrete pavements, ranging from 2- to 13-years-old, in various freeze-thaw climates. The most moderate climate in the study area experienced an average of 50 freeze-thaw cycles per year while the most severe averaged 210 freeze-thaw cycles, with an average below freezing for 60 days. The results of the study (available at www.nrmca.org) show pervious concrete that is partially saturated should have "sufficient voids for the movement of water and thus demonstrate good freeze-thaw resistance." The key point here is that total saturation should be avoided for optimum freeze-thaw durability.

What about clogging?
The majority of pervious concrete pavements will function very well with little or no maintenance. However, there may be instances where sand, dirt, leaves and other debris may infiltrate the void structure of the pervious concrete and inhibit its permeability. In most cases, the clogging is limited to the first 1 to 1½ in. of the pavement thickness. Routine cleaning can help avoid this situation and restore better than 90 percent of original permeability.

A recent study by the University of Central Florida supports this claim. The research, conducted at the Stormwater Management Academy and funded by the Ready Mixed Concrete Research and Education Foundation, looked at eight pervious parking lots ranging in age from 6- to 20-years-old. These lots had seen little or no maintenance since their construction. The cleaning techniques investigated were pressure washing, vacuum sweeping and a combination of these two methods.

Pressure washing dislodges the clogging particles, washing a portion offsite and flushing the remaining portion through the pavement surface. Vacuum sweeping dislodges the dirt and debris by means of the sweeping action and removes them via the vacuum. Results of the UCF study show that utilizing either pressure washing or vacuum sweeping can improve the infiltration rate of clogged pervious concrete by 90 percent. Cleaning with a combination of the two methods actually caused a 200 percent increase in infiltration rates over the benchmark infiltration rates of the clogged samples.

However, the best maintenance practice appears to be prevention. Proper design and construction of pervious concrete should include consideration of the drainage of surrounding areas to prevent the flow of potentially clogging materials onto the pavement surface. Whenever possible, drainage of all unpaved areas should be away from the pervious concrete pavement. Additionally, landscaping materials, such as mulch, should not be stored on the pavement even temporarily.

Your state's regulatory agency and pervious
In 1972, the National Pollutant Discharge Elimination System (NPDES) program was established under the authority of the Clean Water Act. This regulation requires communities and public entities that own and operate a municipal separate storm sewer system (MS4) to obtain an NPDES permit for stormwater discharges. As a part of the permitting process, the permit applicant must establish a best management practice (BMP) for stormwater management. The U.S. Environmental Protection Agency has approved several BMPs for stormwater management, and pervious concrete is among them.

Though pervious concrete is not new, its use in stormwater management is a relatively new concept to the engineering community. As such, projects utilizing pervious concrete are undergoing special scrutiny. Most states include porous pavements, in general, as an accepted BMP but are reluctant to give blanket approval for all projects utilizing pervious concrete. Instead, they provide approval on a case-by-case basis, usually through special provisions to existing regulations.

Tom Evans, promotion director for the Maryland Concrete Promotion Council, explains the process: "If you were going to build a road that crosses a stream, you can either build a bridge or place a culvert with backfill for an at-grade crossing. Either way, you must submit a completed design for approval. You can't just say, 'I'm going to put in a bridge.' The same applies for pervious concrete pavement. You can't just say, 'I'm using pervious concrete' and expect blanket approval. You need to submit a completed design with full details."

To facilitate a smoother approval process, it helps to do your homework. The common concerns of regulatory agencies deal with pervious concrete's freeze-thaw durability, infiltration capabilities and pollutant removal efficacy. Research has been conducted on all of these issues, and the results are readily available.