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.
