Pervious Concrete: The Road Less Raveled

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 522 Pervious Concrete Spring 2010 Update

The American Concrete Institute's committee on pervious concrete (ACI 522) met last week in Chicago.  A few highlights from the meeting include:

1) George Seegebrecht's presentation on a steel reinforced pervious concrete wall in the Chicago area that's almost 100 tears old and showing no signs of corrosion. 
2) An in-depth report by Dr. Neithalath on the current research happening on pervious concrete.
3) A discussion/critique on the draft set of pervious concrete details the committee is developing.

Task groups were formed to start working on the next versions of 522R and 522.1, so in 2-3 years, we should have updated documents, again.

Durability of Pervious Concrete Made with Gravel

If you remember the previous discussions of Gravel vs. Crushed Rock for coarse aggregate for pervious concrete, you know I'm not a fan of rounded aggregates for traffic applications.

Looking at the unpublished data from the ASTM round robin testing of the raveling potential standard, some interesting trends are present:

1) Crushed aggregates have a lower potential mass loss than rounded aggregates, but rounded aggregates seem to perform better at higher void contents.  So if I was designing a mix to be as durable as possible, I'd still use a crushed aggregate to push the mass loss as low as possible. 
2) In general, rounded aggregates showed a very narrow range of mass loss over a range of void contents.  So whether designed at 15 or 25 percent voids, the performace will not change significantly.
3) There seems to be no correlation between mass loss and mass loss COV.  This is a opposite of expected -- you'd expect as the material is weaker, the more variable it would be.

A few words of caution about the data: 
1) These mixes are all old meatball style mixes, so I'm not sure how the more modern mixes would perform. 
2) This was the first time for the many of labs and operators to be running the tests, so I'm not sure if we'd see lower variability on another round of testing.

3) The proposed test method is undergoing some changes as we speak, so this data should not be considered absolute scietific truth.  For example the revolutions used in the test have changed from 50 to 500 to reduce the COV.  Specimens are tumbled in triplets now instead of singles.  However, the trends shown here are expected to have some reflection in reality.

Relationship between Voids and Mass Loss in Pervious Concrete with Crushed and Rounded aggregates

Relationship between Mass Loss and Mass Loss COV in Pervious Concrete with Crushed and Rounded aggregates

Happy Birthday to Me!!!

Celebrating one year of writing this blog.  Thanks to all of you for reading it!

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.

You Could Call It Porous Asphalt

You could call it porous asphalt, or maybe just the mother of all rutted pavements.  Either way, you now it will move some water.

ACI 522R-10 Updated Content on Pervious Concrete - Already!

If you remember all the way back to the discussion of ACI 522R-10, you know it came out this week.  For those of you who have read it, chapter 12 was dated by the time it was released.  So, in a first for ACI, we've updated chapter 12, and you can find the new version at the ACI Concrete Knowledge Center.

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).

Batching Pervious Concrete with Hot Water

For the past few years, the industry has been debating whether or not to use hot water when batching pervious concrete. A few projects have had issues, so we'd been leaning away from it.  However, Bill Lenz in Canada offers the following advice:

"In Canada, we use hot water to batch concrete for approximately 9 months in the calendar year.  The key to using hot water is to add the majority (80%) of it with the aggregates in the mixer (the mixer can be either the truck or a central mixer) ... the purpose of this is to warm up the aggregates in order to achieve a suitable batching temperature e.g. 65 deg F.  The last remaining amount of water is added in the tail end of the load, however, it is not enough to cause any adverse effect with the admixture system.  Also, if you are noticing problems, and you suspect the hot water is causing the problem, you can increase the hot water going into the aggregates to 100%.

Hot water, when used properly, can be used successfully."

Thanks, Bill!