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June 2, 2006

Bye-bye retention ponds, hello pervious concrete

  • Pervious concrete retains all water on site, doing away with expensive stormwater retention systems and allowing microorganisms to flourish and eat away pollutants.
    Pervious Concrete Inc.

    Photo courtesy Pervious Concrete Inc.  [enlarge]
    A coarse rock base is required to handle water after it passes through the pervious concrete.

    It's time to rethink concrete.

    You know the stuff: impervious to water, channels runoff. But what happens when — without sacrificing strength or durability — water drains right through it? Consider if roads and driveways, sidewalks and parking lots could let rain wash directly into the ground, where it's naturally filtered on its way to our aquifers. No runoff, no drains, no catch basins, detention vaults or piping systems. No kidding.

    It's called pervious concrete. And I think that it's about to change how we build things in Washington.

    The big idea

    Imagine concrete without the fine sand, and you have the coarse aggregate texture of a gray Rice Krispies Treat, with a void structure of 14-18 percent. Thick, rigid and good for 40-plus years. It does require a special blend, and there are some tricky installation issues, but the results have been proven to more than pay for themselves.

    That wasn't the case just seven or eight years ago when the product first emerged in the Northwest. Land was less expensive and stormwater issues less at the forefront. When my engineer, Noel Higa of Higa Burkholder Associates, brought pervious to my attention in 1999, only a few forward-thinking scientists, engineers, mixing plants and governmental planners were looking into its viability — and no jurisdiction in Washington state had approved its use. That was then.

    Pervious concrete is a two-part on-site stormwater management system consisting of the concrete pavement and a coarse gravel retention layer for stormwater storage. The system retains all water, including all pollutants, on site. And because the layers are porous, air is present and microorganisms flourish, eating away pollutants and other non-desirable elements in the water.

    This system has been used successfully throughout most of the country, but it seems that a major test case was needed to demonstrate its advantages in our region. I never intended to be a pioneer, but I'm excited about what we've learned.

    The critical test

    Stratford Place, a 20-lot plat in the city of Sultan, became our test. My company, CMI Homebuilders, bought it in 2002 and then ran into neighborhood stormwater issues. After considering the problem from all angles, we determined that creating an entire pervious stormwater management system would make the design financially viable by eliminating stormwater piping, catch basins and detention systems in favor of additional units. What's more, the system would be far superior environmentally.

    Without realizing how audacious our proposal was, we submitted it to city planner Rick Cisar and city engineer Jon Stack. After we did some learning together, these two men proved invaluable in helping us secure city approval to use pervious for the entire development, including roadways, parking areas, sidewalks and driveways, as well as footing and roof stormwater.

    There was just one catch: I had to give the city my personal guarantee that all 32,000 square feet of product would work for a certain number of years. As a result, I reluctantly became something of an expert — and then my own supplier — to ensure the project's success, and started a new company, Pervious Concrete Inc., to supply the pervious infrastructure.

    Through research, experimentation and testing, we figured out how to control and place the mix to accommodate differences in temperature, wind speed and other variables, load by load. We learned that proper curing is critical, and how to ensure it. We found that most ready-mix concrete plants in Western Washington can produce one or more pervious mixes, and that we can tint the mix for better way-finding.

    Most importantly, we developed a consistent and repeatable placement process (and are the only ACI-certified pervious flatwork technicians and finishers in the state, outside of Vancouver). And, we saved $260,000 over a traditional system.

    Within 21 days after pouring, the road held up to all heavy concrete and lumber trucks as well as general traffic, and it took water as fast as a garden hose could flow, with virtually no water spread.

    Despite the heavy rains last November and December, there has been a complete absence of ponding or overflow. In fact, not one drop of water left the site during that record seven-week rainfall, and my guess is that no traditional stormwater vault or pipe system can make that claim. But don't take my word for it, give Jon Stack a call.

    The unique challenges

    Yes, pervious is trickier than standard concrete. System design is site-specific, for example, and requires a soils survey and stormwater calculations that factor in the percability and characteristics of native soils.

    If the pervious is too wet or overworked during placement, the voids between the stone are reduced or eliminated, and it won't drain. If the concrete is too dry, it's impossible to get proper compaction for cross section strength. And if curing precautions aren't tightly maintained for the first 20 minutes of placement and seven to 14 days after that, pervious concrete can fail. Problems will begin to show after the initial seven-day cure and will be evident within a month, and the only solution is to remove and reinstall a corrected mix.

    When the site is properly prepared, the mix is right and your installers are knowledgeable and certified, you can significantly reduce the potential for costly errors.

    Care should be taken to keep the surface free from silty or clay-like material, and to avoid clogging it with sand, topsoil, beauty bark and other debris. Chemical cleansers are not recommended; plan to use plain water to flush the pervious pavement voids, and to sweep or vacuum once or twice a year to remove soil and debris.

    The major advantages

    Pervious concrete eliminates stormwater detention vaults, ponds and piping systems, which are not only the most time-consuming and costly elements in plat development, they can take up a couple of otherwise profitable lots. Getting those lots back can often pay for the entire system.

    Environmentally, it makes sense to let rainwater directly recharge our groundwater. By eliminating untreated stormwater and runoff, the system mitigates "first flush" pollution and protects our streams, watersheds and ecosystems in much the same way as bioswale and natural soil drainage and filtration, rather than concentrating pollutants by channeling stormwater.

    Pervious doesn't get as hot as standard concrete, which reduces heat island effects, and provides a higher albedo surface reflectivity index (0.35 or higher). In short, the earth is an excellent processing plant that we've been bypassing for years.

    In addition, pervious concrete has been designated a Low Impact Development (LID) tool for stormwater management and a Best Management Practice (BMP) by the EPA.

    Concerned about plugging? Don't be. Properly placed pervious takes water at more than 200 inches per hour. Seattle's record rainfall is about 5 inches in 24 hours, or about 0.2 inch per hour, so even if a pervious surface becomes 95 percent plugged, it can still pass a minimum of 50 times the maximum rain from Seattle skies.

    The future

    From every viewpoint — dollars, aesthetics, environment and performance — pervious concrete makes good sense. Costs are coming down, acceptance is rising, and my bet is that we're all going to see a lot more of this smart stormwater solution.

    Craig L. Morrison is the owner and president of both CMI Homebuilders and Pervious Concrete Inc. in Snohomish.

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