Pervious Concrete: The Road Less Raveled

Grouted Edges for Pervious Concrete

The goal of any pervious concrete installation is to make a strong, durable pavement that will drain water.  Seems easy enough, but 'strong and durable' are the opposite of 'drain water'.  So, the skilled pervious concrete contractor has to maintain a delicate balance between these two opposite targets to buld a pavement that will last for years while maintaining permeability.

Unlike conventional concrete pavements, when pervious concrete fails, it fails in raveling (the wearing away of the pavement surface caused by the dislodging of aggregate particles).  Raveling is usually most pronounced at the edges and joints where compaction didn't happen in a smooth operation like with the rest of the pavement.  Some strategies to prevent this distress include over-compacting the edges, and paying special attention to tightening up the joints.

Raveled edge along a construction joint

Our friend Dave "The PervMaster" Mitchell has come up with a unique way to prevent raveling along the construction joints -- he calls them 'grouted edges'.  He uses a bagged grout mix that, when mixed to a stiff consistency, the finisher works into the edge of the pavement adjacent to the form.  The result is a strong, decorative edge to the pervious concrete that shouldn't ravel.

 

Grouted edge along a formed joint

 

The advantages of this technique are obvious, the disadvantages might include: extra labor needed to mix and place the grout, reduced freeze thaw durability of the grout (if not selected properly), and difficulty in matching color for decorative work.  I like the way Dave's edges are wavy and irregular; however, every building owner might not appreciate the look which would require more labor to keep the lines straight.

Decorative grouted edge border

 

This previous post shows a driveway with grouted edges along the outside border (and The Pervmaster himself showing off his work).

ACI 522 Pervious Concrete Fall 2011 Meeting Update

Usually there are lively debates, and heated exchanges; however, in a relatively boring fashion, the American Concrete Institute's Pervious Concrete committee (522) met this morning.  For those who didn't make the meeting, here is what you missed:

1) Dr. Neithalath shared a report on the latest research on pervious concrete.  Jereme Montgomery presented his study on using density for field quality control.
2) Our task group developing standard details for pervious concrete is nearing completion on a ballot-able document.  We expect to see something this month.
3) The next technical convention session on pervious concrete will likely be at the Phoenix convention.  Dr. Kevern and Dr. Neithalath asked that committee members keep it in mind and submit when the call for papers comes out.
4) The next version of 522R is expected to be released in 2014.  Task groups focused on the individual chapters have been formed and are expected to wrap up revisions by the summer of 2012.
5) ACI 522.1's next version is coming close to being a finished product.  The committee has passed the response to TAC's comments, and will be ready to address the public comments in the spring.

Pervious Concrete Live 2012 Preview

Plans are coming along for Pervious Concrete Live at the 2012 World of Concrete.  So far everything looks very exciting!  

If you missed last year, you can see the highlight video here.

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.