Suddenly pervious, or no-fines, concrete is the hottest thing in the concrete industry. Motivated by the need to control the runoff of rainwater, developers and building owners are considering using pervious for parking lots, driveways, and even residential subdivision streets. Pervious is fairly simple, but it's far from foolproof; when designed and built without knowledge of the proper techniques, it can and has failed.
To move the technology a step ahead, the National Ready Mixed Concrete Association sponsored a Concrete Technology Forum focused on pervious concrete. Held in Nashville on May 24 and 25, 2006, the Forum drew an audience of 250 people from across the country. CONCRETE CONSTRUCTION was a major conference sponsor (along with our sister magazines THE CONCRETE PRODUCER and PUBLIC WORKS). Speakers looked at the benefits, design methods, mix designs, and construction techniques.
Leading off the forum, Bruce Ferguson from the University of Georgia, highlighted why porous pavements in general have recently gained popularity, and focused on pervious concrete, noting that success is far from a given. “At the current state of technology, porous concrete's installed quality depends vitally on a qualified installer.” Ferguson stated that “Porous pavements' potential application is vast...primarily because of public concern about and legal requirements for stormwater management. Properly applied porous pavements can also enlarge urban tree rooting space, reduce the urban heat-island effect, reduce traffic noise, increase driving safety, and improve appearance.”
Turning to the design of pervious concrete, Michael Leming, North Carolina State University, described “active” mitigation techniques, meaning that the pavement is designed not only to allow rainwater to drain but also to actually store the water to prevent runoff. Depending on how much water is expected, and the soil characteristics, the base layer below the pervious concrete needs to be designed to store water as well, or even to drain. Sloping pavement surfaces can have a significant impact on the storage capacity.
A valuable presentation on design was also given by Manoj Chopra from the University of Central Florida. In summary, he presented a model specification for pervious that included the following portion on construction:
Subgrade Material—The top 6 inches shall be composed of granular or gravely, predominantly sandy soil. It is desirable for the soil to contain no more than a moderate amount of silt or clay. Granular or gravel sub-base may be placed over subgrade. Subgrade shall have a permeability of no less than 1 inch per hour.
Site Preparation—Subgrade shall be leveled to provide a uniform construction surface with a consistent slope not more than 5%. It is recommended that the slope be as flat as possible. After leveling, soils shall be compacted to a minimum density of 92% of a maximum dry density as determined by ASTM D 1557 or AASHTO T 180.
Forms— Forms may be either wood or steel and shall be the depth of the pavement. Forms shall have sufficient strength and stability to support pavement and mechanical equipment without deformation. The edge of existing pavement may be used as a form.
Placing and Finishing—Mixers shall be operated at the speed designated as mixing speed by the manufacturer. The portland cement aggregate mixture may be transported or mixed onsite and shall be used within 45 minutes of the introduction of mix water. Concrete shall be deposited as close to its final position as practicable and such that fresh concrete enters the mass of previously placed concrete. An internal vibrator should not be used to consolidate concrete. Following strike-off, the concrete shall be compacted to form level, utilizing a steel roller made from nominal 10-inch diameter steel pipe of ¼-inch thickness. The roller shall have enough weight to provide a minimum of 10 psi vertical force.