In recent years, the development community, permitting agencies, engineers, and owners have been seeking out new and innovative ways to reduce stormwater runoff and build low-impact, sustainable communities. One of the “new and innovative” ways that assist in these efforts just might be a product that has actually been around for some time–pervious concrete.
History and Uses
Pervious concrete, in its earliest form, has been in use for more than 150 years, although it was not until a little over 20 years ago that it was successfully employed in various applications in the United States (American Concrete Institute 2006). The development community has now found many ways to incorporate pervious concrete into practice; some of these include tree protection, stormwater runoff reduction, and increased pervious area on a site (Offenberg 2005a).
With the rising focus on stormwater treatment, pervious concrete is becoming a viable option for owners and designers to consider. Typical pervious concrete consists of cement, gravel, and water with generally no fine aggregates in the mixture. Typically, the absence of fine aggregate permits properly placed pervious concrete to be composed of 15% to 20% void space, which allows stormwater to pass through the pavement. This percolation allows for a recharge of groundwater tables, and the system can be a key tool in the effort to reduce stormwater runoff.
Pervious concrete pavement has also been permitted within the drip-lines of trees, as the pavement allows for the passage of air to the tree roots. Because the roots are obtaining sufficient air, they will tend to grow out and not up, thus preventing the surface of the pavement from popping up.
Many permitting agencies also permit pervious pavements to be used to contribute to the impervious surface ratio. This pavement can often be permitted at various levels of perviousness, but typical values range from 50% to 100%, depending on the local regulations.
Design Considerations
Proper design is one of the first steps to making the system function properly, with a key element being the thickness of the concrete. Here, the thickness requirements may be based on traffic conditions, stormwater storage, or even subgrade conditions. Higher traffic volumes may require thicker sections, but sections typically vary from 6 to 8 inches. The higher stress levels that exist around the free edge of pavements may necessitate a thickened edge or curbing.
A project at the Veterans Administration hospital in Riviera Beach, FL, used a curbed pervious concrete pavement parking lot in an environmentally sensitive area in 2004. This parking lot expansion consisted of 80,000 square feet of 6-inch pervious concrete placed on a 6-inch bed of clean #57 aggregate with perimeter curbing. In this case, the system was designed with rock and curb to contain higher levels of volume and to protect nearby environmentally sensitive areas.
When the pavement, as well as the subgrade and volume above the pavement, are used to retain a volume of water, it will likely be necessary to install curbing around the perimeter of the pavement to contain the entire volume. Credits that can be achieved vary by local jurisdiction, so it is important to verify local regulations to make sure the credits are optimized for each application.
Pervious concrete pavement is typically used in light- to medium-duty traffic applications, such as parking lots, subdivision streets, driveways, or sidewalks. Today’s technology is not well suited to heavier-duty applications, such as sites with constant heavy truck traffic, but advances in the future may permit such applications.
In addition to pavement thickness, history has shown that the subgrade is a crucial element of the system. Sections of pavement by themselves have shown astronomically high (2,000 inches per hour) percolation rates, so it was observed that overall infiltration rates of the system are generally controlled by the subgrade. The minimum percolation rate of a subgrade should be incorporated into the overall site drainage design. An overcompacted subgrade will lead to lower percolation rates of the system. In some cases, it may be necessary to install a layer of clean rock on top of the subgrade to provide some additional infiltration capacity.
Construction Practices
A successful placement depends not only on correct design practices but also on correct installation procedures (Offenberg 2005b). It is therefore up to the project team to ensure that the contractor selected to perform the work follows the proper placement methods from start to finish. As a part of the National Ready Mixed Concrete Association’s Pervious Concrete Contractor Certification Program, contractors can achieve either technician or craftsman levels of certification.
Proper placement methods typically include the following steps:
- Placement. Prior to concrete placement, the properties of the subgrade should be verified for compliance with the project plans and specifications. The concrete should be placed as close to its final location as possible. It can be conveyed, buggied, or bucketed, but not pumped, to its final location.
- Strike off. After placement, the concrete is struck off with some type of vibratory screed. It is typically struck off about three-eighths to 0.5 inch high to allow for the compaction operation.
- Compaction. Next, the pavement is rolled with a steel pipe roller to provide some compaction. It is important not to overcompact in this step, as the void space may be reduced if overcompaction occurs.
- Jointing. Joints are important to any concrete pavement, and this is no exception with pervious concrete. Joints are typically cut at the time of placement with a hand tool. This tool is usually a roller with a cutting disk in the center to cut to a depth of 0.25 inch.
- Curing. Aside from edging the pavement, the final step is the curing process. It is vital that a pervious concrete pavement receive proper curing. For this step, the pavement should be covered within 20 minutes of the strike-off step and should remain covered for a minimum of seven days. During this time, no traffic should be permitted on the pavement.
The American Concrete Institute Committee 522 has just published the 522R-06 document, which is a guide to the use of pervious concrete. The committee is also diligently working to complete the 522.1 specification, which will provide guidance for specifiers who may be incorporating the material into their projects.
Maintenance
Maintenance of pervious concrete pavements is a subject of longstanding debate. Proper maintenance generally consists simply of vacuum sweeping or power washing. Ongoing research shows that systems that are not maintained still perform very well over time (Wanielista et al. 2005), but not at their original astronomical infiltration rates. However, a good cleaning generally will improve the infiltration rate of the system. Some permitting agencies require maintenance agreements prior to putting a pervious concrete pavement into service, or include language in their codes requiring periodic inspections and maintenance. Similar to nearly all other stormwater treatment tools, proper maintenance will keep the system running at higher performance levels.
What’s to Come
The future of pervious concrete may give an idea of what is to come with the technology. Many improvements have been made in a relatively short time, in the areas of both design and construction. Currently, there is extensive research under way to review various aspects of pervious concrete pavements, including freeze/thaw performance, pollutant treatment, and long-term performance. The research list continues, but as we endeavor to further develop our precious resource more carefully, pervious concrete pavements will likely be an important element of the push to build low-impact, sustainable communities for the future.
