Our Bay: Green concrete can help slow increase in bay's nitrogen load

Pervious concrete - the technology Queen Anne's County will use in Bloomfield Farm's parking lots - allows water to drain straight through into a reservoir below the pavement where it can subsequently soak into the ground. Most concrete and asphalt don't allow rainwater to penetrate the ground, instead the water runs off and pollutes waterways.

Urban and suburban runoff is one of the top contributors to the Chesapeake Bay's nitrogen load, and is also the fastest-growing nutrient pollutant source in the bay.

"Urban and suburban runoff account for 16 percent of Maryland's nitrogen load to the bay," said Jay Apperson, deputy communications director at the Maryland Department of the Environment.

Pervious concrete, along with other stormwater runoff prevention measures, is becoming more important in Maryland in light of the Stormwater Management Act of 2007, which Stu Schwartz, a senior research scientist at the University of Maryland, Baltimore County, said provides very aggressive criteria for new construction.

The key feature of the law is that it requires Environmental Site Design, also known as ESD, Apperson said.

"What ESD is, is the use of planning techniques and design techniques that are designed to replicate runoff conditions, like what you would get from a forest," Apperson said. "Before that was encouraged, but now it's required. It's required to be done, basically as much as possible. The legal term is, 'to the maximum extent practicable.'"

Excess nitrogen causes algae blooms, which consume the water's oxygen and lead to fish kills.

In Queen Anne's County, ESD requirements have translated to the use of more than one innovative technique. The large permeable concrete installation is just the newest, said Lee Edgar, project engineer with Queen Anne's County public works.

"One of the big things that the county commissioners have been focused on the past few years is leading the push for sustainability in stormwater," Edgar said. "Some of these ESD techniques that we've started in Queen Anne's County include bioswales, (a design element that allows runoff to flow through rock or other particulate matter before it flows into waterways), an award-winning rain garden and now we're looking at pervious concrete."

Concrete is an aggregate material, said Schwartz. It is composed of rocks or gravel, well-mixed cement and water. Where most concrete has fine particles - sand-sized particles - to fill gaps between larger pieces, permeable concrete has none.

"It is sometimes called no-fines concrete," Schwartz said. "The aggregate used to make pervious concrete doesn't have sand- or silt-sized particles."

Or, as Edgar put it: "It kind of looks like a Rice Krispie treat. It's a little different, but that's where all those voids are. You have a void where water can run through it."

The project costs $375,000, of which $200,000 is being paid for with a federal stimulus grant, Edgar said. The county government is picking up the remaining cost.

Pervious pavement requires more maintenance than regular pavement, usually vacuuming or power washing, but it is quite effective at stopping stormwater runoff.

Schwartz cautioned that pervious paving materials are not always the most-effective way to control runoff.

"It's probably not a universal substitute for paved surface and like all of the innovative stormwater surfaces, pervious pavement is not a magic bullet," Schwartz said. "That's simply because there are no magic bullets."

By the end of the 2009, Queen Anne's Bloomfield Farm - once a working farm near Centreville that now encompasses a 19th-century homestead along with soccer, baseball and lacrosse fields, a fishing pond and a driving range - will sport 30,720-square-feet of pervious cement, along with 16,000-square-feet of permeable pavers, Edgar said.

"We're trying to make the development as green as possible," Edgar said. "We're very enthusiastic about this … the Chesapeake Bay and her tributaries are an invaluable resource on the national scale, while her protection remains on the local level."

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More infomation - 2009-11-23 17:19:23

I've been around the product quite a bit. I've see more grass grow on my asphalt driveway than any of the pervious concrete parking areas I've seen. If anything, there would be less growth because there is less surface area for a plant to take root. Vacuuming isnt nearly as effective as a power wash from what Ive seen, so no need for RoundUp. You can go years between cleanings, but it really depends on what is happening on that piece of property.

In regards to freezing, there have been a lot of advances in engineering, mixing and placing pervious concrete over the past few years. First, the drainage should be designed so the concrete does not hold the water, and second, the void ratio of pervious concrete is about 20% while water only expands 9%. There are hundreds of projects in far colder climates that Maryland and several in Maryland over the past 7 or 8 years.

Pervious concrete also helps out with the urban heat island effect as does traditional concrete. The brighter pavements reflects the heat and light which lowers the electric bill.

unhide Comment hidden due to low ranking. Why is this comment hidden?

Steven Tripp - Waldorf, MD - Karma: Neutral

Other issues - 2009-11-21 11:29:36

I would have liked to have heard discussion of how one deals with plants growing from this material. I could see many homeowners offsetting the potential benefits of this type of material by using tons of Roundup or other herbicides on their driveway.
Also, and I'm sure this relates to the vacuuming, I wonder how quickly these materials might become none porous due to sediment filling in the spaces in the top layer or even down into the material.
I like the idea a lot and think it's a hugely important concern and potentially a great approach to remediating one of the problems that paving creates. I'd also like to hear any ideas that address the heat island effect of pavements.

Mark McCabe - Eldersburg, MD - Karma: Neutral

Missing Critical Info - 2009-11-21 08:04:15

Too bad the reporter failed to talk about some of the reasons permeable concrete is typically not used around here. Most importantly, in the past, permeable concrete has not been used in climates where there is a risk of below freezing temperatures. If there are wet conditions on the permeable concrete and the water freezes then the permeable concrete gets broken up. I would be interested to know if someone has come up with a solution to this problem.

Spencer Leech - Annapolis, MD - Karma: Neutral

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