Placing Pervious Concrete
Pervious concrete mixtures are stiff, zero-slump mixtures with placement, finishing, and curing requirements falling outside of normal concrete flatwork processes. Due to the dry nature of the mixture (w/cm < 0.35) and high surface area, it is important to consider rapid concrete placement methods. As these mixtures are not appropriate for pumping, rapid placement methods may include chute placement directly from the truck mixers, wheel barrows or buggies, conveyors, or dump placement into an asphalt type paving machines. Regardless of the placement method, the quicker the placement is, the better.
Strike-off of the pavement may also be accomplished in a number of ways:
Finishing of pervious pavement requires a minimum of effort and is simple. After strike-off and compaction with a roller, a finned roller (sometimes referred to as a pizza cutter) is used to tool joints, and the edges are compacted with an edging tool. Additional decorative treatments can be added such as stamped impressions using braided rope over plastic and an additional pass with the roller to embed the shape of the rope. Other more traditional stamping tools may also be used provided the operation is done in rapid order.
- While strike-off with a simple straight edge is rare it is acceptable as long as the forms are elevated with a removable strip ½- to ¾-inch in thickness which can be removed to allow compaction of the pavement using a steel roller.
- Low frequency vibrating screeds and asphalt pavers may also be used for strike-off.
- Again it is recommended to strike-off slightly higher than the final elevation and compact with a roller to achieve the final height.
- Hydraulically powered roller screeds rotate against the direction of travel providing strike-off and compaction in a single operation.
The reason rapid placement and finishing techniques are stressed is due to curing requirements for pervious pavements. Since the moisture content of the mixture is initially so low it is particularly susceptible to moisture loss that would inhibit hydration of the cement. For this reason curing is typically specified as a minimum 6 mil plastic cover placed and securely anchored within 15 minutes of the pervious mixture being discharged from the mixer truck. Additionally, white plastic is used for hot weather to reduce the solar loading on the pavement.
Leming, M. L., Malcom, H. R., and Tennis, P. D., Hydrologic Design of Pervious Concrete, EB303, Portland Cement Association, Skokie, Illinois, and National Ready Mix Concrete Association, Silver Spring, Maryland, 2007, 25 pages.
Tennis, Paul, D.; Leming, Michael, L.; and Akers, David, J., Pervious Concrete Pavements, EB302, Portland Cement Association, Skokie, Illinois, and National Ready Mixed Concrete Association, Silver Spring, Maryland, USA, 2004, 36 pages.