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.

THE BIG IDEA

Imagine concrete without the fine sand, and you have the coarse aggregate texture of a grey Rice Krispies Treat, with a void structure of 14 to 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 to eight years ago when the product first emerged in the Northwest. Land was less expensive then, and storm water issues were less at the forefront. When my engineer, Noel Higa, 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--no jurisdiction in Washington State had approved its use. That was then ...

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

Today, this system has been used successfully throughout most of the rest 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 storm water issues. After considering the problem from all angles, we determined that creating an entire pervious storm water management system would make the design financially viable by eliminating storm water 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 all did some learning together, these two men proved invaluable in helping us secure the City of Sultan approval to use pervious for the entire development, including roadways, parking areas, sidewalks and driveways, as well as footing and roof storm water.

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 at Stratford Place.

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-mixed 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, at present, the only ACI-certified pervious flatwork technicians and finishers in Washington State, outside of Vancouver). And, while we were at it, we saved some $260K 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 heavy rains, there has been a complete absence of ponding or overflow In fact, not one drop of water left the site during a record seven-week rainfall.

THE UNIQUE CHALLENGES

Yes, pervious is trickier than standard concrete. System design is site-specific, for example, and requires a soils survey and storm water 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 pervious concrete can fail. On the other hand, when the site is properly prepared, you can significantly reduce your 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 one to two times a year to remove soil and debris.

THE MAJOR ADVANTAGES

Pervious concrete eliminates storm water 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 just makes good sense to let rainwater directly recharge our groundwater. By eliminating untreated storm water 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 storm water.

Pervious doesn't get as hot as standard concrete, which reduces heat island effects, and it provides a higher albedo surface reflectivity index (0.35 or higher).

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

Concerned about plugging? Don't be. Properly placed pervious takes water at more than 200 inches per hour.

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 storm water solution.

Craig L. Morrison is the owner and president of both CMI Homebuilders and Pervious Concrete in Snohomish. He thanks the Smokey Point Concrete team and GM Scott Mickels for their dedication and hard work, Bruce Chattin from WACA, and especially the City of Sultan, Planner Rick Cisar, Engineer Jon Stack, Public Works Director Connie Dunn, Building Official Craig Bruner, the mayor and the city council for their support of Stratford Place.



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