PCI Systems, LLC




Contact Us 877.839.2727

Frequently Asked Questions

Q: Is pervious concrete effective in clay soil?
A: There are some areas of the US that truly have impervious soils, but they are not common. Generally if you commonly have septic tanks in your region, your soils have a sufficient perk rate for pervious concrete pavement. A layer of clean washed #57 stone (25-35% voids) under the pervious concrete paving (15-20% voids) can store the design volume of water for percolation over several hours.

Q: What happens when the subgrade gets saturated?
A: Pervious concrete pavements are recommended for auto parking only. Occasional trucks can be accommodated, but heavy semi traffic is not recommended. (Although there have been some subdivision streets in Florida with pervious pavement installed.) Pervious concrete pavement should not be placed directly on fine grained subgrade soils (placement directly on sandy soils have been done successfully in Florida for many years). For fine grained soils (clays or silts) a layer of engineering fabric should be placed to separate the subgrade soil from a porous granular base at least 3 inches thick. This layer of porous granular acts to strengthen the pavement system and add stormwater storage capacity.

Q: Is freeze-thaw a problem?
A: This should not be an issue in the Sunbelt, but freeze-thaw could be an issue if you have winter weather that includes snow/ice that stays on the ground for an extended period of time. If the right combination of conditions occur where the entire void structure of the pervious concrete is filled and freezes, damage to the structure can occur. Water drains through pervious concrete very rapidly so completely filling the void structure completely may be unusual, particularly in the Sunbelt region. Installations of pervious concrete have been in place in Raleigh, NC, Boone, NC and Chattanooga, TN for as long as 12 years without freeze-thaw problems.

Q: What about clogging?
A: Clogging is mainly a matter of design. If the site plan allows stormwater from outside of the parking lot to flow across introducing fines into the system, some clogging can occur in the vicinity of the flow. Areas of the pavement not exposed to this flow will remain unclogged, resulting in the "average" performance of the entire parking lot to remain good. If the parking lot is designed to take care of the rain that falls directly on its surface, the only source of fines will be wind blown or tracked in on vehicle tires. Coring studies in Florida indicated that on well-designed pervious concrete parking lots less than 10% loss of internal volume has occurred due to infiltration of fines after 12 years.

Q: What maintenance is associated with pervious?
A: Sweeping, blowing, vacuuming and other normal methods of maintaining parking lots are important in minimizing the materials available to clog pervious concrete pavements. Landscape maintenance personnel should be cautioned to avoid introducing fine materials onto the surface of pervious concrete pavements. Tests in Florida indicate that the pervious concrete pavements can be vacuumed or pressure washed to re-establish permeability.

Q: What about cost?
A: It is not unusual for the pervious concrete paving option to be less expensive than other alternatives. When the parking lots doubles as the stormwater management system, the cost of the land for detention ponds, the cost of detention pond construction, and the cost of "first flush" mitigation facilities are saved. The material cost of pervious concrete can be 20% or more higher then regular concrete due to additional material costs to the supplier. The in-place cost can be double the cost of conventional concrete paving.