Pervious Concrete: The Road Less Raveled

Pervious Concrete - Asphalt Transition

In the NRMCA training for both stormwater designers and contractors, we teach the use of header curbs between the pervious concrete and adjacent asphalt to prevent water from migrating from under the pervious concrete to under the conventional pavement.  Such flows often cause subgrade failure under the plain pavement due to differential subgrade support as the area near the pervious concrete tends to be saturated while the soils farther away tend to be drier.  

Here are the slides from the contractor training class:

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.

Pervious Concrete and FOD

F.O.D. = Foreign Object Damage

It's what happens when something gets sucked into a jet engine, and usually is not a good thing.

The two major concerns with using pervious concrete on the "air side" of an airport, are:
1) Raveling caused by the heavy wheel loads, and
2) The FOD due to the above raveling.

There has been some debate that planes can land on dirt landing strips, so it shouldn't be a problem; and that the jet engines would be more likely to blow debris away rather than suck it in, so it shouldn't be a problem.  However, in speaking with knowledgeable sources on this (my father, a jet engine guy, but not a FOD expert) I been convinced that pervious concrete could lead to FOD.  FOD could occur during taxi and takeoff from the ground-engine vortex, or in landing from debris kicked up by the nose gear or thrust reversers.

A quick summary, planes that land on dirt strips are typically prop planes (not jets), so less likely to suck debris off the ground. Jets that do land on sodded or gravel landing strips are modified to protect them from debris and deflect it from the engines, however your average commercial aircraft has not been equipped as such.  Further, jets may land on unpaved strips without the assistance of the thrust reversers to avoid kicking up loose material.

So, we have two options:
1) Equip all jets with Unpaved Strip Kits
2) Not use pervious concrete on the "air side" of airports until we know better how to control raveling.

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).

Pervious Concrete Vermin Barrier

I'm a little tardy in reporting this, but the new "Imagination Playground in Burling Slip" located in Manhattan on John Street between South and Front Streets will be, to my knowledge,  the first installation of a pervious concrete vermin barrier.  The pervious will be placed below 24" of sand for the dual purpose of allowing stormwater to percolate down while preventing vermin from tunneling up. 

If you are in the city, please take a photo of either the construction or finished project, I'd love to see it!

Parking Islands and Pervious Concrete Paving

Why do landscape architects do this to us?  Maybe we need to let them know not to drain landscaping onto pervious concrete pavements.  From an LID perspective, the better design is to depress the parking islands to detain the stormwater.

Pervious Concrete in SFWMD!

My titles don't usually include exclamation points, but this is truly exciting!

This came from the South Florida Water Management District (SFWMD), the stormwater permitting agency for South Florida up into Orlando.  The draft memo has been sent to the review staff to be used for permit applications.

Download SFWMD Guidance Memo Pervious Pavements DRAFT 2009-11-02

Interesting Pervious Concrete Detail

This article describes a pervious concrete parking lot where the stalls are pervious concrete, and the drive lanes are grass.  Hmmmmm, I think I might have left out the grass and done it all in pervious concrete -- but I'm biased.

Pervious Concrete on Clayey Soils

Maybe you’ve heard this, “Due to the soil type here, drainage would be very poor and thus we can’t use pervious concrete on this project.”

 

This is a common misconception of pervious concrete. Leming (2007) sums it up best:

 

"It is important to recall that runoff is relatively high in areas with clayey soils or clayey-silts, even with natural ground cover, and properly designed and constructed pervious concrete pavement systems can provide a positive benefit in many situations...

 

Pervious concrete pavement systems may be used for active mitigation even with very tight, non draining soils when designing the system as a detention rather than retention device, although additional structural details must be provided. In these situations, since the soil will take in very little runoff anyway, regardless of the cover, the intent is to simply reduce the peak flow by holding the runoff for some period of time. Infiltration is not considered a critical feature of the design since virtually all of the captured runoff will be released directly into natural channels or the storm [water system]."

 

Reference: Leming, M. L., Malcom, H. R., and Tennis, P. D., Hydrologic Design of Pervious Concrete, EB303, Portland Cement Association, Skokie, Illinois, and National Ready Mixed Concrete Association, Silver Spring, Maryland, USA, 2007, 72 pages.