Pervious Concrete: The Road Less Raveled

Changes to NRMCA Pervious Concrete Contractor Certification Policy

It would be cool to say that the committee that oversees the NRMCA Pervious Concrete Contractor Certification is a super secretive society that flies in for monthly meetings, in their cloaked rocket planes, to meet in a remote, hidden underground fortress. But, in truth, it's a much more pedestrian group of designers, contractors, and concrete producers that have irregular conference calls to provide guidance to the NRMCA on changes in the market and technology that might impact how they run their certification program.

In our committee meeting last month, we made some minor changes to the policies that govern the program for both installers and craftsmen:

For installers, the policy has been clarified with respect to participation in demonstrations and hands-on training events. "Applicant’s participation in a pervious concrete demonstration or hands-on training may only count for one (1) of the three (3) required projects." To draw the line between 'demo' and real project, if the pavement goes into service, then it is considered a live project, even if it was cast as part of a demonstration. This was done to encourage contractors to get hands-on training while ensuring they had suffiecient live project experience.

For craftsman, nothing has changed, yet. However, the committee is crafting policy to limit the submittal of applications through local LSG's only. This would give more local control to the certification process and prevent national entities from signing off on a craftsman application for a contractor whose work might be hundreds of miles away. The paperwork requirements for craftsman applications will continue to be cumbersome, but with 38 certified craftsmen and 145 installers, this is not an excessive hurdle for such a critical specification requirement. Further, the craftsman application still requires the approval of a 3-member certification committee of the LSG. These individuals are responsible for ensuring the applications is truthful, and that the contractor has actually done the work detailed in the submittal.

Pervious Concrete at the Fitzgerald, GA Wal-Mart

If you ever find yourself in south Georgia, at the crossroads of cotton and peanut country, near the home of the "Wild Chicken Festival" you'll be in Fitzgerald.  Recently, I had a chance to visit the Wal-Mart there.  If you don't know where Fitzgerald is, it's right here:

Hard to believe they have a Wal-Mart there, but you can find it at 120 Benjamin H Hill Drive West, Fitzgerald, GA.  The store and associated pervious concrete parking lot, were built in 2003.  The pervious section is the rarely used overflow lot on the north side of the property.  It's roughly 300' by 200' and in generally good condition for a pavement that's approaching 9 years old.  

Here are a few photos to give you a feel for how it's holding up.

The Store

Condition of the pavement and joints.  Not bad for its age.

 

Some joints are performing well...

...and some aren't.

The ribbon curb is doing a good job of protecting the pavement edge.  It's interesting to see that the asphalt next to the curb is shoved up.  I'm not sure if this is from the concrete expanding or the asphalt expanding, but I've seen this on other, older pervious concrete projects.

 

As we've seen with other pervious concrete installations where asphalt drains onto the pervious, there is a bit of clogging near the inlet to which the asphalt run-off is directed.  I didn't run the 1701, but water did still drain through the clogged panels.  In the future, this may become a maintenance issue for the owner.

This shot will give you a feel for how much asphalt area drains onto the pervious concrete.


For me, this was the highlight of the whole trip -- a beautifully patched core hole!  These were scattered all over the lot.  I've been looking for years for a photo that shows how to patch the core holes, and this is a great example.

Pervious Concrete Contractor Training for Union Workers

In a unique partnership with the National Ready Mix Concrete Association (NRMCA), the Operative Plasterers' and Cement Masons' International Association (OPCMIA) has begun training contractors in the art of pervious concrete.  Under this agreement, the OPCMIA will offer an intensive, hands-on training program to its members and provide the NRMCA pervious concrete contractor certification exam.  The exam will be administered through the NRMCA Local Sponsoring Group in coordination with the local union hall.

Individuals completing the class and passing the exam will be certified technicians. When they complete three projects with at least 10,000 square feet of pervious concrete pavement they will be able to upgrade their certificaiton to the installer level.  

Overall, this partnership will provide union-heavy markets with a pool of trained individuals, while still mainting a uniform level of quality, across the country, in the certification process.  

Interested individuals can find information on their local union hall through http://www.opcmia.org/.   

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).

Surface Texture Variability in Pervious Concrete

Two guarantees with pervious concrete pavements:

1) There will always be some raveling.

2) There will always be some variation in surface texture, especially on larger projects.

The combination of the contractor's skill at finishing and the concrete producer's ability to produce a consistent mix will determine the variability in the pavement's surface texture.  As the rare exception, I've seen craftsmen take some rough looking concrete, and work their magic to make it look beautiful.  However, more than likely, a consistent concrete mix will help the installer make a good looking pavement, especially for the unskilled contractor.  I don't often show my bias in this blog, but a good pervious concrete admixture, like Grace's V-MAR VSC500, can do a lot to make the concrete more consistent.

How much variation in surface appearance is acceptable?  Within the ACI 522.1 specification, there is no way to quantify variations in surface texture, thus, there is no limit on variability in surface appearance.  

What is the Architect/Engineer/Owner to do about accepting the surface appearance of pervious concrete pavements?  For reference, below are photos of projects demonstrating the range of acceptable variation in serviceable pavements.  Some have suggested using an image analysis technique as described in this previous post; however that has not yet been evaluated for standardization, so the validity of the method is not known. The testing that is in the current specifications includes density tests by ASTM C140 -- this should cover wide variations in surface texture that would also show up as wide variations in core density.  Wide variations, beyond the specification limit make the acceptance/rejection decision easier for the specifier.

Pervious Concrete Projects at World of Concrete

In a move to accelerate contractors' certification promotion, the World of Concrete has scheduled a revolutionary program to provide hands-on training at the world of concrete.  Contractors who sign up will receive individualized training in small groups to provide them the latest, most advanced training in pervious concrete placement techniques.  With a low student to instructor ratio (3:1), each piece of each students' work will be carefully coached to sustain future project success.  Additionally, these projects may count towards advancing the individual's certification level for both Installer and Craftsman in the NRMCA Pervious Concrete Contractor certification system.

For more information, or to register, visit the World of Concrete's website 

What Is A Certified Pervious Concrete Contractor?

When ACI 522.1 was written, there was a lot of debate about what should be considered a certified pervious concrete contractor.  Within the final document, formal reference was made to the National Ready Mix Concrete Associations' pervious concrete contractor certification program, and it's three levels of qualification.  The committee decided a contractor could be considered certified if he had 1 craftsman, 3 installers, or 5 technicians on the jobsite.  

The highest level of qualification, Craftsman, requires extensive work experience, an accompanying certification as an ACI Flatwork Technician, and passing a written test.  In a Utopian world, all pervious concrete would be perfectly placed by those with a Craftsman certification.  It truly indicates that somebody has spent the time learning the art of placing pervious concrete.  If you look at healthy pervious concrete markets like Seattle and Florida, you will find a healthy number of contractors employing at least one Craftsman.

The next level down is the Installer.  Installers have completed 3 projects totaling more than 10,000 sq. ft., maintain an ACI Flatwork Technician certification, and have passed a written exam.  This level was not in the original certification program, but was added to allow less experienced contractors to serve the newer markets.  The idea was that these contractors would quickly upgrade to Craftsman as the market matured.

Technician is the lowest level of certification.  To become a Technician one only needs to pass a written test.  One can actually become a techician without ever seeing real pervious concrete -- fresh or hardened.  There has been some debate in the industry -- was it a mistake to write 522.1 to allow five Technicians to place pervious concrete?  

Not speaking on behalf of the committee, only sharing my own opinion -- it was the right decision.  Five years ago, when pervious concrete was a rare sight, it made sense to allow crews with five Technicians to place pervious concrete -- otherwise there would be no contractors in the market.  As the market grows and matures, those Technicians are becoming Installers, and Installers are becoming Craftsmen.  As the ACI Pervious Concrete committee prepares to rewrite 522.1, we'll see how the opinions of those in the industry shape the definition of a "Certified Pervious Concrete Contractor".

Basic Equipment for Placing Pervious Concrete

Question: Is there a standard specification for the roller used in final compaction of pervious concrete?  Are there any specifications on any of the finishing tools used in pervious?


Answer: There is not any good spec language on this as it's up to the contractor to choose means and methods.  However, on their website, the NRMCA lists the minimum basic equipment for placing pervious concrete pavements.  The Schedule 40 steel pipe is about 40 pounds per lineal foot, which asserts a pressure of roughly 10 psi.  In the end, any equipment that gets the job done should be acceptable.

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).

Check Dams for Pervious Concrete

Originally Posted 7/17/09

The city of Olympia, Washington is a leader in the municipal implementation of pervious concrete technology.  On their website they show a good detail for check dams under a pervious concrete pavement.  There is also a presentation with a few good photos of check dam installation here.