Pervious Concrete: The Road Less Raveled

UNH Pollutant Removal Efficiency of Pervious Concrete

Somehow I missed this the first time by, but it's definitely worth mention, even if it is 2 years old.

When it came to the pollutant removal efficiency of pervious concrete, all we really had to cite was this table:

The Stormwater Center at the University of New Hampshire, however, has published their data in a new table, and it shows just how good pervious concrete is in pollutant removal efficiency.  Page 11 is the highlight.

The report also discusses the practice of sealcoating asphalt parking lots and the impact on downstream water quality, "Although it is intended to remain on the pavement surface, UNHSC researchers found that much of the sealcoat flakes and then washes off during storm events, ending up in nearby streams and stormwater treatment devices. ...UNHSC field data shows that just nine months after sealing only 4 percent of the entire parking lot area, PAH levels increased in the swale receiving runoff from the entire lot to 95.7 mg/kg, double the regulatory limit of 44.7 mg/kg."  That's a 60-fold increase in Total PAH from pre sealant levels!

Also of interest, are their practical notes on how well pervious concrete performed through the tough New Hampshire winter, "While the pavement froze sooner, deeper, and for longer periods than the reference condition, the pores remained open and well-drained year round."  The authors also included some new recommendations for winter maintenance on page 17.

Utah Museum of Natural History's Pervious Concrete

Recently, I had a chance to visit the construction site of the new Utah Museum of Natural History.  The museum sits on a beautiful, hillside location, but more impressive is the 60,000 sq ft pervious concrete parking lot.  

Having a site on a hill with a 30% slope and limited space for conventional stormwater retention, the landscape architect, Design Workshop, had to incorporate LID stormwater management techniques to fit everything on the site, while still preserving half of the real estate as open space.  

On my tour of the site, I'd heard rumor that the subbase to the pervious was up to 18 feet thick in some areas.  However, in speaking with Project Manager Troy Cook, with Design Workshop, he set the story straight -- the subbase for the pervious is only 2-2.5 feet thick.  There is, however, a large underground infiltration system on site that handles the stormwater for the rest of the site.  The pervious concrete acts as a passive system, draining only itself, but overflows in heavy storm events to the underground system.

Maybe one of the best uses of technology, the website for the project shows a time lapse video of the construction on the site. If you look in the lower right hand corner, you can see the footage of the construction of some of the pervious concrete -- from the earthwork, to subbase, to construction and curing and then watch it weather its first winter. I don't want to understate how cool this video is, but would love to see it with frames from every day.

Feds to Pay Stormwater Utility Fees

If you remember the discussion of the debate about whether the feds had to pay stormwater fees (because they might really be taxes), here's an update:

The lame duck session of congress passed a bill that would require the federal government to pay stormwater fees for its facilities.  So it doesn't resolve the debate as to whether they are fees or taxes, but settles the issue between the local and federal governments.

EPA Pervious Concrete Test Section

The US EPA has a unique pervious pavement test site at it's facility in Edison, NJ. Unique in how rigorously is was constructed, instrumented, and will be tested. The slides in this presentation, from the ACI spring meeting, describe all of the testing being conducted on a range of pervious pavements, including pervious concrete. The tests on water quality coming through the systems include ISE, COD, Solids, Nutrients, Organic Carbon, SVOC, and Microbial Indicators.

They are looking to partner with municipalities on new parking lots, in other climates, to perform similar testing. If you know of any such opportunities, please pass on that information.  You can see their website on the project here.

The Fallacy of Clay Soils and Pervious Concrete

It's good to see so much press on pervious concrete. If you look at Google news with the search term "pervious concrete" you'll be amazed at how many articles are published every week. This article, however, seemed to stir some controversy.

First, I'll applaud the Atlanta Journal Constitution for interviewing Dr. Wanielista for the article. If you want a good source on pervious concrete, Marty is the man -- you won't find many people with more credibility.

However, I'm disappointed that they'd paint pervious concrete in such a negative light with this quote, "Georgia's hard-packed clay soil makes porous pavement or driveways pointless, unless layers of rocks are installed below the pavement to help water drain..." To say 'duh' here would be less than professional, so instead, I'll just say that nobody in Georgia is promoting pervious concrete on clayey soils without an open-graded aggregate base. Maybe a good writer would have reported this as, "Even Georgia's hard-packed clay soils are perfect for placing pervious concrete, with an aggregate base, to reduce the impact of development, and recharge groundwater supplies."

There was a companion article in this series discussing stormwater utilities and the "user fees" they collect. It seems the state and federal government are claiming they don't have to pay these "taxes" to the local government. It will be interesting to see how this one plays out in court, and the impact it will have on sustainable development.

ACI 522R-10

Thanks to Jereme Montgomery for the tip.  You may now download ACI 522R-10 here.  This has all of the latest info on pervious concrete (that's not on this blog).

Pollutant Removal for Pervious Concrete Pavements

The Center for Watershed Protection has a report here that details new pollutant removal efficiencies for permeable pavements.

Now, I'm not a stormwater engineer, but in my mind, if all of the stormwater has been infiltrated into the subgrade soil, then all of the pollutants have been removed from the runoff.  I'd say the upper end of the scale should be 100% to account for this.

ERIK for Pervious Concrete

If you're familiar with the pervious concrete research out of the University of Central Florida (discussed previously here), then you know I'm a fan of the ERIK (Embedded Ring Infiltrometer Kit).  If you want to measure the permeability of the entire pervious concrete system, you need some apparatus that penetrates through each layer of the pavement as the ERIK does.  As another option, I've advocated the 10X10 cell - that's half of a parking spot curbed flush to grade, extending 6" into the subgrade.  Such a cell would require a lot of water to test, but would give the most realistic results of infiltration rates through the system. 

Recently, I had the opportunity to visit a site with ERIK's built in for monitoring.  The first image shows why we need to educate the stripers about how to handle such devices.  The second image shows the difference in compaction between the concrete inside the ring and outside the ring.  In general, the ERIKs on this site showed more compaction inside the ring than outside the ring.  A quick image analysis of the areas (inside vs outside) showed that inside had about 25% less voids than the rest of the pavement.  Higher density inside the ring is not a bad thing - it will just yield a conservative estimate of the infiltration rate of the system.

A Painted ERIK for testing infiltration rate of a pervious concrete pavement system.

Notice the difference in porosity between the pervious concrete inside and outside the ERIK.

Pervious Concrete Pavements in Coastal Georgia

Last week, I learned of the Coastal Stormwater Supplement to the Georgia Stormwater Management Manual that was released this year. It is very progressive in including good new guidance on incorporating LID in stormwater management.  I'll admit, I didn't read all 542 pages, but I will quote my favorite parts discussing pervious concrete pavements:

"The alternatives to impervious surfaces recommended for use in coastal Georgia include:

- Green Roofs

- Permeable Pavement"

"...site planning and design teams may also want to consider the use of alternative paving surfaces, such as pervious concrete...for roadway construction"

"If parking lot footprints cannot be minimized using any of the techniques described above, site planning and design teams should consider the use of alternative paving surfaces, such as pervious concrete..."

"Porous asphalt is thought to have a limited ability to maintain its structure and permeability during hot summer months and, consequently, is currently not recommended for use in coastal Georgia."

You can download the full document here.

Pervious Concrete Article on Permitting

You can find my article from Land Development Today on permitting pervious concrete pavements here.