Q: Is freeze-thaw a problem?
A: This should not be an issue if the recharge bed underneath the pervious concrete is designed properly (minimum 12″ of stone). 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, however, water drains through pervious concrete very rapidly so completely filling the void structure completely may be unusual, particularly in southern regions. Installations of pervious concrete have been in place in harsh winter environments in Oregon, Minnesota, Vermont, Maine, Ohio and New Hampshire without issues due to freeze-thaw.
Q: What, if any, maintenance is associated with pervious concrete?
A: There will always be some maintenance with pervious concrete patios, sidewalks, parking lots and streets. Each jobsite is different and your contractor should work with you and perform a flow test prior to the pervious concrete being put into service and inform you, in writing, as to how much water your pervious system can take in one minute and then perform the same test in six, twelve, eighteen and twenty four months and again, report to you in writing so that you can adjust your cleaning schedule if needed.
Typically 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 and Oregon indicate that pervious concrete pavements can be vacuumed or pressure washed to re-establish permeability.
Download our Owner’s Maintenance Manual
Q: What about cost?
A: If you look at pervious concrete as a stormwater management system and not just a type of concrete, it is common for the pervious concrete paving option to be less expensive than a typical asphalt paving design. By eliminating the need for expensive catch basins and perimeter drains as well as excavation costs even though pervious concrete can cost as much as twice as much as regular concrete, in the big scheme of a project, the pervious concrete for parking lanes and portland cement concrete for the drives lanes is a very affordable altternative.
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. When you consider the cost and the impact on the environment of toxic asphalt sealers that are needed with any asphalt paving project you can see how pervious concrete is a win-win for all involved. (See USGS Study on our links page)
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, this is in addition to the water that will evaporate into the atmosphere. A recharge bed of clean washed #57 stone (35-40% voids) under the pervious concrete paving (15-20% voids) can store the design volume of water for percolation over several hours and provide better drainage than pre-construction conditions. If extra drainage is needed, a small overflow drain pipe can always be installed within the recharge bed. (See the Estes Design, Inc study on our industry links page.)
Q: Is there anything that can be done to make pervious concrete more architecturally pleasing?
A. Yes! The easiest way to change the look of pervious concrete is to change the size of the stone used. While 3/8″ round gravel has been the preferred aggregate of choice in the past, mixes using a #9 crushed stone can produce a pervious concrete that appears to the untrained eye to be regular concrete or asphalt.
In addition several techniques to make it appear more architecturally pleasing such as using an integral coloring or stain can be used just like with conventional concrete. It can be also be stamped using special metal forms, if the aggregate is small enough. Another interesting and unique technique is to grind the surface and expose the aggregate used to highlight the many colors in the stone.