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Researchers at Iowa State University recently completed a study to develop an optimum cold weather mix design for pervious concrete. Numerous concrete mixes, with varying chemical and mineral admixtures, were subjected to freeze-thaw testing per ASTM C 666, Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing. The samples were fully saturated and experienced four to five freeze-thaw cycles per day for a full 300 cycles. Results of the testing showed that the inclusion of a small amount of fine aggregate, as well as air entraining admixture, provided the best freeze-thaw durability with less than 2 percent loss of mass after 300 freeze-thaw cycles.

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.