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.
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.
matt...good discussions.
In regards to what voids to spec. Just wanted to share some data compiled in an article we put together. This is the rough draft of what eventually become a published article for Concrete International (March 2010).
http://docs.google.com/fileview?id=0Bw2WJrV11mhMM2Y4NTFlMjUtNGUwZC00Yjk1LTk1ZmItZTZkOGRkOTA0ODJm&hl=en
For this project we assumed we would gain 3% more than the standard 1688 results. So we initially chose a unit weight that would yield 14% per 1688 or 133.5 pcf. Of the five cores taken, the average void content was 17%. Pretty good results when the team chose 15-20% as an acceptable average range.
Since this was the first project that we used 1688 on, we just tryed to gather as much data as we possibly could in relationship to ASTM 1688. We also conducted ASTM 1701 just 4 days after its release on October 3rd, 2009.
If the link doesn't work...let me know and I can email you the drafted article.
Posted by: Jereme Montgomery | 02/21/2010 at 10:03 AM