A year later, “The Burnout Slab” was produced, with reduced cement content, silica fume, and pan finishing.  After only four days of curing, this slab was subjected to a pickup truck doing ‘burnouts’
Dave Frentress A year later, “The Burnout Slab” was produced, with reduced cement content, silica fume, and pan finishing. After only four days of curing, this slab was subjected to a pickup truck doing ‘burnouts’

The market for pervious concrete has seen significant growth, primarily in parking lots and walkways. Since pervious concrete provides distinct advantages over conventional impervious surfaces, it allows the ready-mix industry to compete for the parking lot market, especially where asphalt has historically been the pavement choice by default.

Pervious concrete low-cost stormwater management system provides a safe, durable traffic surface while often saving on land costs. For several years, I have described it as a “cement-based (storm)water treatment system,” and stormwater regulations across the country are now encouraging or requiring techniques that emphasize infiltration instead of detention.

Mix designs

Since it is not natural to intentionally make a mix with voids, ready-mix QC departments must think backwards about designing pervious concrete mixes. It is best to design around your aggregate gradations and void content, then proportion cement content leaving enough space to achieve 20% hardened voids. The ACI 522R-10, Report on Pervious Concrete, is a good guide to the mix design process. Alternate mix design resources, such as the NRMCA Mix Proportioning Software for Pervious Concrete Mixtures, have been developed to help producers design mixes that perform well.

Basic pervious mixes do not require exotic admixtures — regular water reducer, and a hydration stabilizer are two common admixes that are on hand at most batch plants and will usually provide durable, placeable pervious mixes. NRMCA’s Pervious Concrete Promotion Subcommittee just completed a “Guide to Specifying Pervious Concrete,” which is available for free download at www.perviouspavement.org.

Know your pervious and your contractor

As a producer, we do not supply pervious concrete mixes to contractors that have not successfully completed the NRMCA Pervious Concrete Installer certification. Pervious concrete requires a proper drainable sub-base and placing and curing operations differ significantly from regular concrete. To ensure proper field placement and a good outcome, the producer should also send key staff members to the certification classes so they understand the entire process to be able to oversee and work with the contractor.

If the QC staff is not familiar with pervious, get your state ready-mix association involved. They are usually the local sponsoring group that conducts the NRMCA certification classes and will be a good source of information about pervious concrete mix designs and proper practices. Another good idea is attending the annual Pervious Roast in Las Vegas. This event usually attracts 60 to 100 of the top pervious contractors, engineers, specifiers, state association directors, and various ready-mix producers. Sponsored by Bunyan Industries, CalPortland, Oldcastle Materials, and the NRMCA, this event is a combination of lectures and presentations, as well as placing and testing of experimental mixes, demonstrating finishing, decorative, and repair techniques.