Q1:What is Pervious Concrete?
A1:Won’t the pores in the pervious concrete eventually fill up with sand, silt and debris and essentially become impervious? Prvious concrete at 15% porosity will handle approximately 300 inches of rain an hour. While it is true that in the coastal areas the pervious will collect sand, sand itself is pervious. Pervious concrete was installed on a street near the beach. No maintenance has been done at that location and the pervious is still performing as well as when first installed even though it has been repeatedly covered in sand.
Because the surface of the pervious concrete is compacted when it is installed, the pores at the surface of the concrete are smaller than those at the bottom the profile for a given pour. Thus, particulate accumulate at the surface where they can be easily removed by an industrial sweeping machine. Researchers at the University of South Carolina have been unable to make pervious concrete impervious by clogging it with sand.
Q2:Can pervious concrete be used even if the underlying soil is clay or “gumbo”?
A2: Yes. This becomes a balance between budget and function. In situations where the natural soils have limited porosity, some of the underlying soil is excavated and replaced with a gravel sub-base over which the pervious is poured. This gravel sub-base provides storage so that the water has more time to slowly infiltrate into the soils below.
Q3:Can compaction of the underlying soil during the construction phase cause the soil to become impervious and affect the function of the pervious concrete?
A3: While installers should be wary of the impacts of heavy machinery during the
construction phase, the Carolinas Ready Mixed Concrete Association recommends up to 90% soil compaction to create a stable base.
Q4:Even when there is highly porous sandy soil under the pervious concrete, won’t it compact and become less porous over time?
A4: Think of beach sand; sand is made of a solid that does not compress like silly putty or other more ductile substances do.
Q5:Isn’t pervious concrete is a lot more expensive than regular concrete?
A5: Pervious concrete may cost up to 25% more to install than conventional concrete, but when lifecycle costs and the savings associated with reduced stormwater management infrastructure are considered, pervious is much cheaper. It important to consider the cost savings attained when use of pervious concrete negates the need for constructing expensive off site stormwater retention (and devoting valuable real estate to do so). Further, when we consider the non-market values associated with using an unstable petroleum based product such as asphalt vs. a stable product such as pervious concrete that promotes pollutant removal, we find that the ecological cost of pervious is much lower.
Q6:Doesn’t pervious concrete require a lot of maintenance?
A6: Pervious concrete, when properly installed and maintained can last thirty to forty years, which is as long or longer than conventional concrete and much longer than asphalt, which much be seal coated or resurfaced about every 4 years.
Q7:Can the voids in the pavement become clogged, and thus ineffective?
A7:It’s unlikely that a pervious surface will become totally clogged. Reducing erosion and sediment runoff onto the pavement through good design can eliminate most problems. If any voids do become clogged, vacuuming or pressure washing of the pavement can restore most of the permeability.
Q8:Can decorative treatments be applied to pervious concrete pavements?
A8:Many landscape architects like the organic, rough-textured look of plain pervious concrete. But for installers who want to enhance the aesthetics, there are several options. Integral coloring agents can be added to the concrete before its placed. Or water-based concrete stains can be sprayed onto the surface after installation. (Dont use solvent-based coatings or sealers because they can clog the voids in the pavement.) Because pervious concrete is much stiffer than standard concrete, patterns can’t easily be stamped into it. But it is possible to score decorative lines into the surface.
Q9:How strong is a typical pervious concrete pavement?
A9:It’s possible to achieve pervious concrete compressive strengths of 3000 to 4000 psi and flexural strengths of 500 to 600 psi, which are on par with conventional concrete. With the use of admixtures, today’s pervious concrete mixes can be made with less water, which improves strength and durability. Also, better placement methods have resulted in tighter pavements that maintain point-to-point contact of all the aggregates while providing the necessary void content.
Q10:Can pervious concrete be used in climates exposed to freezing and thawing?
A10:In most cases, the strength and durability of pervious concrete make it resistant to potential cracking from freezing and thawing. Pervious concrete that is partially saturated should have sufficient voids to accommodate the expansion caused by freezing of water. Structural damage could occur, however, if the pavement becomes fully saturated or the ability of the concrete to drain water is compromised due to clogging of the void structure. We offers recommendations for preventing these problems in its publication Freeze-Thaw Resistance of Pervious Concrete.
Q11:Can pervious pavements be used over impervious, clayey soils?
A11:Yes, but the pervious concrete should be installed over a permeable subgrade layer of rock several inches thick. This additional layer will allow the pavement to hold more water until its absorbed into the ground. Consult a geotechnical engineer for design and construction options specific to the soil in your area.
Q12:What about maintenance?
A12:Generally the only maintenance needed is to occasionally sweep, pressure wash, or vacuum the pavement to remove any debris that might clog the voids and inhibit water penetration.