Pervious Concrete:
The Smart Stormwater Solution

By: Craig Morrison, Founder, PCI

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.

 

 

Imagine concrete without the fine sand, and you have the coarse aggregate texture of a grey Rice Krispies Treat, with a void structure of 14-18%. Thick, rigid and good for 20 to 40+ 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 7-8 years ago when the product first emerged in the Northwest. Land was less expensive then, 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 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 to our region. I never intended to be a pioneer, but I'm excited about what we've learned.

 

 

Yes, pervious is trickier than standard cement. 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 it will ravel or chip off. If curing procedures aren't tightly followed immediately after placement and for up to 7-21 days thereafter, pervious concrete can fail. Problems will begin to show after the initial 7-day cure and will be evident within a month, and the only solution is to remove and re-install a corrected mix.
 
On the other hand, when the site is properly prepared, the mix is right and your installers are knowledgeable and certified, your potential for costly errors is significantly reduced.
 
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.  While the cleaning interval will depend upon the specifics of your location you can expect to sweep or vacuum 1-2 times a year to remove soil and debris.

 

 

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 reduce the number or lots or size of buildable area. Getting those lots and/or buildable area back can often pay for the entire pervious system.

 

 

A simple, two-layer pervious concrete stormwater system eliminates:

 

Environmentally, it just makes good 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, instead of concentrating pollutants by channeling stormwater. Pervious concrete doesn't get as hot as standard concrete, reducing heat island effects, and it provides a higher albedo surface reflectivity index (0.35 or higher). In short, the earth is an excellent processing plant we've been bypassing for years.  In addition, pervious concrete has been designated an LID (Low Impact Development) tool for stormwater 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. 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% plugged, it can still process the rain from Seattle skies.

 

 

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 it.

 

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