Pervious Concrete: The Road Less Raveled

ACI 522 Pervious Concrete Fall 2012 Meeting Update

With the committee chair's trip canceled at the last minute, a surprise guest chaired the Fall 2012 ACI 522 meeting -- me!

Here's what happened:

1) While a little behind schedule, the committee continues to make progress on the third version of ACI 522R. Based on where we stand, I'd guess the committee is still a year away from submitting to TAC. Norb Delatte reported his task group will submit a draft that includes structural design tables.

2) The standard details are coming closer to reality. The original intent behind this was to create a stand-alone document to post online on the CKC, however, with the timing of 522R, it is more likely to be added as an appendix to the next version of 522R.

3) I'm happy to report the committee successfully balloted its response to TAC comments. The debate lasted over 2-1/2 hours covering exciting topics like internal curing, adoption of ASTM C 1754, basis of purchase, and responsibilities.

ASTM Pervious Concrete Winter Meeting 2011 Update

To say the body of knowledge in pervious concrete is growing is an understatement -- it is exploding!  This week the ASTM meeting added to that explosion with the first ASTM Symposium on Pervious Concrete.

The symposium featured eight speakers covering some of the latest research and updates on test methods for pervious concrete.  Some of the more interesting presentations included:

 

• "Validation of the Performance of Pervious Concrete in a Field Application with Finite Element Analysis (FEA)" Md. Ashraful Alam, Liv Haselbach, William F. Cofer, Washington State University, Pullman, WA USA 
• "Pervious Concretes Pore Structure Characterisation by 3D image analysis - Impact on their Permeability" Samuel Meulenyzer, Eric Stora, Fabien Perez Lafarge Centre de Recherche, Saint-Quentin Fallavier, Franc
• "More Information on ASTM C 1688 Variability" L. K. Crouch, Ph.D., P.E. Tennessee Technological University,  Cookeville, Tennessee, USA; John Hendrix, P.E. Tennessee Technological University,  Cookeville, Tennessee, USA; Alan Sparkman, CAE, CCPf, LEED AP Tennessee Concrete Association, Nashville, Tennessee, USA; Daniel Badoe, Ph.D. Tennessee Technological University,  Cookeville, Tennessee, USA 
• "The Development, Implementation and Use of ASTM C1701 Field Infiltration of In Place Pervious Concrete" Heather J. Brown, MTSU, Murfreesboro, TN, USA; Alan Sparkman, Tennessee Concrete Association, Nashville, TN, USA 
• ...and, of course:"Development of a Test Method to Determine the Raveling Potential of Pervious Concrete" Matthew Offenberg, W.R. Grace, Canton, GA, USA 

The papers and presentations are not available, yet.  But I'll post an update as soon as they are, or you can sign up to get an alert from ASTM here. 

ASTM's subcommittee on pervious concrete also met on Monday.  The raveling test has moved a step closer as it passed the main committee ballot, so is headed off for staff review.  The hardened density and voids test likewise is advancing, but had a few negative votes to resolve by the close of the meeting.  The compressive strength task group has a draft standard, but will also be testing out a new compaction technique using a hammer drill.  They expect to open their first ballot this coming spring.

BREAKING NEWS: New UFGS Pervious Concrete Specification

BREAKING NEWS:

The United States Federal Government's new pervious concrete specification has finally been published!

The new section, numbered 32 13 43, replaces the old section 32 13 13.06 which was the plain concrete pavement spec that had pervious language scattered throughout.  Likewise, an updated 32 13 13.06 has also been published to remove the old pervious concrete references.  A copy of the new pervious concrete specification can be downloaded for free from the National Institute of Building Sciences. 

Many thanks to the project team that made this happen.  Specifically, Chuck Weiss from the Army Corps of Engineers, Toy Poole, and the NRMCA staff who participated along the way.

 

The Future of Raveling Testing for Pervious Concrete

It's nice to come up with a new, repeatable test method, but before unleashing it on the world, a few questions must be asked:

1) Does the world need it?

2) Is the test meaningful?

3) Will it do more good than harm?  

At both the ASTM winter and summer meetings there was lively debate about the new test method, under development, for raveling resistance.  Trying to answer these questions was not an easy task for the subcommittee, and at more than one point I almost talked myself out of continuing on the path towards publication.  So, here's an attempt to summarize the discussion and answer these three critical questions as gathered from my thoughts, and those in the meeting room.

To provide some background, the test is being developed under ASTM Work Item #23367 (if you need a copy of the lastest draft email me).  In the current version, it is intended only to help a concrete producer or researcher select the best possible mixture proportions based on potential raveling resistance, or to compare the potential raveling resistance of different pervious concrete mixture proportions.  Samples may be cast from either laboratory or truck mixed concrete.  The round robin testing showed a very low coefficient of variation, but was based on controlled conditions – not cores or field cast specimens.  

1) Does the world need a test method to determine the raveling resistance potential of a pervious concrete mixture?  

When pervious concrete fails, it usually fails in raveling.  The goal of any producer, therefore, is to make a mix that is resistant to such distress.  Likewise, researchers need a tool to assess the impact of varying mixture proportions on the raveling resistance of pervious concrete.  Therefore, it is easy to say, there is a need for this test method.

2) Is the test meaningful?

Unfortunately, I can’t say it is with 100% confidence.  

Some of the properties of pervious concrete that we would expect to impact raveling resistance (void content, ash content), impact the performance of pervious concrete in this test.  For example high void content and high fly ash content reduce raveling resistance.  Until we have enough data to develop correlations (if those correlations truly exist) between test results and field performance, we can’t definitively state that the test is meaningful.  But, if we don’t publish the standard and incorporate it in specifications, producers won’t readily use the test to select mixture proportions.

3) Will a test method to determine the surface durability potential of a pervious concrete mixture do more good than harm?  

This is where I get nervous.  

Anybody in the concrete industry has experience with specifiers either misinterpreting or using test methods where they are explicitly stated not to be used.  So, as the subcommittee debated the fate of this test method, we discussed which applications specifiers could misuse the test, and the implications of such misapplication.  The list of improper applications of this test includes:

• Construction acceptance of delivered concrete.  At some point in the future, it would be nice to use this test as a means of quality control and acceptance. Comparing the mass loss of specimens cast every 100 yards might prove to be a good QC/QA test.  However, there has been no research to verify that it works as a QC/QA test – the test isn’t ready for this application, yet.  Of critical concern is the variability of the test as conducted in a field application.  As the standard is written today, it is intended for use in a controlled environment.  If a specifier or purchaser were to require such testing for a field application, the results would likely not agree with the laboratory results used for mixture selection.  The end result of such an inequitable comparison could unreasonably penalize the concrete producer.

• Construction Acceptance of in-place concrete.  The test method, as written and evaluated in the round robin testing, is for cylindrically cast specimens.  The applicability with respect to cores has not been broadly evaluated.  Some testing has shown that the test can differentiate between different levels of raveling resistance based on compaction.  However, neither the applicability nor the repeatability has been established. Again, if a specifier or purchaser were to require such testing for a field application, the results would likely not agree with the laboratory results used for mixture selection.  The end result of such an inequitable comparison could unreasonably penalize the concrete producer and contractor.

• Specification limits for mixture submittal and prequalification.  Plain concrete mixtures today are prequalified based on compressive strength of laboratory specimens or field history.  This is based on a history of what works.  With pervious concrete, we don’t know what specification limits for mass loss from this test might work, and for which applications.  Or, they might need to be correlated with some other test that we haven’t developed, yet. As suggested earlier, until we have enough data to develop correlations between test results and field performance, we can’t set specification limits for mixture prequalification.

The consensus is that this test is not a perfect test (as few are), but is the easiest, cheapest way to determine the potential raveling resistance of a pervious concrete mixture.  Hopefully, somebody smarter than me will develop a better test in the near future.

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.

Update to ACI 522.1 Pervious Concrete Specification

ACI specifications are updated regularly so they reflect the current state of the technology.  The ACI 522.1 pervious concrete specification is an example of how to do this in hyperdirve.  The original spec was released in 2008, and the committee is already working to update it.  At the fall ACI 522 Pervious Concrete meeting, proposed changes to the 522.1 specification were presented and debated by the committee. 

Committee members have until 12/15/10 to submit feedback in the form of proposed changes.  To ensure we have the best spec possible, beyond just the committee, I'm soliciting the pervious concrete community as a whole for any suggested updates/corrections/changes.  So, if you, my reader, have any, you may email them to me, or post them as comments to this blog.

Please, if you have technical experience with pervious concrete, please take some time and critically review the old specification, compare it to other specifications you might come across, and really take the time to provide good feedback to make this a solid spec.  You can download a copy of 522.1 from the ACI website.  After all feedback is incorporated, we'll open it up to a committee ballot in January 2011.  After TAC approval, ACI will open it up for a formal public comment period.

High Heels on Pervious Concrete

The two questions that always come up about pervious concrete:

1) Does it meet ADA requirements?

2) What about high heels?

And, we can answer both questions with 'Yes'.  But, let's focus on the high heels.  

We often say that smaller aggregate is better as the voids are smaller, so there's less of a chance of catching a heel.  Here is a link to a site where they actually documented some of this in a little field study.  It seems to be conclusive, that high heels will work on pervious concrete.  

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).

Specified Void Content of Pervious Concrete

This type of question keeps coming up, "What void content should be specified for field acceptance of pervious concrete by ASTM C 138 or ASTM C 1688?  For example, should we specify 15% +/- 3%?"

There is not an easy answer to this question, today, since test methods are still in their infancy, but C 1688 should be the preferred method over C 138. 

As C 1688 is still young, and broad testing of pervious concrete is only beginning, there is not a large body of data on field tests of pervious concrete using either C 1688 (or C 138).  Further, there is no long-term history as to whether field testing leads to durable pervious concrete pavements.  Thus, we don't know if specifying a tight range of voids (+/-3%), at the bottom end of the practical scale (15%), will lead to a successful project or not. 
 
The literature reports that for a pavement to drain, it needs at least 15% voids in the pavement (not a test pot), but doesn't say how to test for 15% voids.  So, we don't know if testing for 15% +/- 3% voids in the fresh concrete by C 1688 will lead to a well drained pavement.  Additionally, we don't yet have a reliable test method that will give the true void content of a hardened pervious concrete pavement.  To demonstrate this, we studied 9 different documented methods for testing voids in pervious concrete pavements on a set of field cores -- the results for void content on a single core ranged from 4.5% to 37.6%.  Therefore, 15% voids by one test method, might be 8% or 30% voids by other test methods.
 
Further, we know that 15% voids by 1688 may lead to 10% voids in the field, or it may lead to 30% voids in the field -- it all depends on how the mix compacts under the contractor's equipment.  Thus, specifying such a tight range of voids does not necessarily correlate to a similar tight range in the finished pavement. 
 
Based on all this, I'd suggest following the industry standard for field density (which accounts for voids) as published by ACI 522:
" Fresh density shall be within ±5 lb/ft3 (80 kg/m3) of the specified fresh density. "
 
The specified fresh density in this case would come from the concrete producer's C 1688 laboratory testing of the proposed mix design.

Change to ACI 522.1 Specification for Pervious Concrete Pavements

Breaking news!  If you remember back to the Fall 2009 ACI 522 committee meeting, the committee decided to move forward with a minor change to 522.1 to incorporate the latest technology of ASTM C 1688 in place of the older test methods.  The ACI website now shows the change to 522.1-08 here.  This will be corrected in paper in the next printing, and online in the coming weeks.