Pervious Concrete Pavements Reduce Stormwater Runoff and Cut Costs
Poured-in-place pervious concrete pavements can offer an elegant, cost-effective solution to stormwater runoff management and reduce or eliminate the need for other methods of stormwater management. However, in order to take maximum advantage of these benefits, care must be taken in pavement design, concrete mix design, installation, and maintenance.
Why Pervious Pavements?
Normal concrete pavements are designed to keep water out. But that water has to go somewhere. When rain falls on traditional pavements, stormwater runoff must be managed through the use of swales, retention ponds and other engineering solutions. All of these can add significantly construction costs. Pervious concrete pavements are designed to let water pass through the pavement into the soil below, returning naturally into the groundwater.
According to the U.S. Environmental Protection Agency (EPA), “alternatives to traditional pavement on our paved surfaces can help reduce runoff by infiltrating rainwater and melting snow. In addition to reducing the runoff from the rain that falls on them, permeable pavements can help filter out pollutants that contribute to water pollution. Permeable pavements can also reduce the need for road salt and reduce construction costs for residential and commercial development by reducing the need for some conventional drainage features.”
EPA’s Phase II Final Rule implementing the Clean Water Act required municipalities in urban areas to implement programs to reduce pollutants in post-construction runoff in projects that disturb areas of one acre or more in order to receive a National Pollutant Discharge Elimination System (NPDES) permit. Municipalities are required to develop and implement strategies that include best management practices (BMPs). Pervious concrete pavement is recognized as a Structural Infiltration BMP by EPA for providing first flush pollution control and stormwater management.
Beyond these federal rules, there has been a strong movement in this country and others toward sustainable development. The US Green Building Council (USGBC) through its Leadership in Energy and Environmental Design (LEED) Green Building Rating System encourages sustainable construction. Projects are awarded Silver, Gold, or Platinum certification depending on the number of credits they achieve. Pervious concrete pavement qualifies for LEED credits, and can help bring about a high LEED certification.
What is Pervious Concrete?
Unlike traditional concrete, pervious concrete is highly porous. Its air-void content ranges from 18 to 35 percent versus standard concrete, which ranges from less than 2 to 8 percent. Compressive strengths can range from 400 to 4000 psi. It’s infiltration rate — the amount of water that can pass through the concrete — ranges from 2 to 18 gallons per minute per square foot, depending on the concrete mix design, the thickness of the pavement, and other factors.
Generally, pervious concrete has little or no fine aggregate and has just enough cementitious material to coat the coarse aggregate. This helps preserve the interconnected pours of the concrete. Pervious concrete has to be strong enough and stiff enough to support intended loads, while retaining the ability for water to pass through its pores as intended. Both of these factors contribute to determining the thickness of the pavement. Pervious concrete pavement systems should only be used where the underlying soil percolates well or when there is an adequate sub-base drainage system.
It All Starts With Good Design
For pervious concrete to function properly, care must be taken in both the overall pavement design and in the design of concrete mix.
The American Concrete Institute’s (ACI) publication, ACI 522.1-13: Specification for Pervious Concrete Pavement covers materials, preparation, forming, placing, finishing, jointing, curing and quality control of pervious concrete pavement. This and other publications related to pervious concrete are available from ACI at www.concrete.org.
Selection of a concrete supplier that can deliver the right mix design is critical. Many of the best concrete suppliers have engineers and other technical staff available to help pavement designers specify the right mix. Mark Orler of ConcreteSouth’s Walker Concrete Co. LLC says, “When specifying pervious concrete, designers need to really consider the capabilities of the company supplying the concrete. Not only does the mix have to be designed correctly, but both the batch-plant operator and the concrete truck driver have to understand that pervious concrete is installed differently than regular concrete. This is a case where you really need to pick your supplier carefully.”
Proper Installation is Key
When regular concrete is placed, installers are working for a densely finished surface that can either be polished, or broom-finished for skid resistance. When pervious concrete is placed, installers should be working to ensure proper compaction, aggregate interlock, and a porous surface. With regular concrete, the slab is placed, allowed to begin curing and then finished a few hours later to achieve the proper surface.
With pervious concrete, the placement and finishing occur simultaneously eliminating the need of the contractor to return some time later and then allowed to cure on its own. Special care has to be taken not to over compact the concrete.
The National Ready-Mix Concrete Association (NRMCA), offers three levels of certification related to pervious concrete: Pervious Concrete Certified Technician, Pervious Concrete Certified Installer, and Pervious Concrete Certified Craftsman The latter, according to NRMCA, is a person who has “demonstrated the ability to place, compact, finish, edge, joint, cure and protect pervious concrete pavements and has documented a higher level of field time-based experience in placing pervious concrete.”
A list of certified pervious concrete technicians, installers, and craftsmen is available from NRMCA.
Maintenance of Pervious Concrete
For pervious pavements to function as intended, they must be cleaned periodically. According to NRMCA’s publication, Pervious Concrete Maintenance and Operations Guide there are three levels of maintenance: routine maintenance, periodic maintenance, and deep cleaning and unclogging.
The necessity for the latter is greatly reduced by regular routine and periodic maintenance. Winter maintenance of pervious concrete pavement is critical, particularly in colder climates. If the pavement becomes clogged and thus saturated with water, freezing can cause pervious pavements to crack and fail.
Great Solution for Stormwater Management
“Pervious concrete can be a great solution for managing stormwater runoff, complying with federal, state, and local regulations, and sustainable development, says Orler of ConcreteSouth. “Design the pavement properly, pick a knowledgeable concrete supplier, select an installer who understands pervious pavements, and perform some simple, basic routine maintenance. Then you’ll have a big winner, both in terms of construction costs and protecting the environment.”
Scott Ramminger has written extensively about construction and has held executive management positions with a several building products trade associations.
Online Resources for Pervious Concrete:
Related Video: Pervious vs Impervious Comparison
- Published: 02-17-17