Every ASTM standard test is supposed to include statements on its precision and bias, which are a lot like the fine print on a legal document. These two sections describe how accurately one can expect the test results to reflect the true condition of what's being tested, and in that way, also are a lot like construction tolerances. As with the fine print on a contract, not paying attention to a test's precision and bias can trip you up.
A recent ASTM symposium presentation by Eric Koehler of W.R. Grace & Co. brought this point home in regard to the slump flow and J-ring tests for self-consolidating concrete (SCC). Koehler opened the symposium on SCC testing with a report on the results of studying 286 concrete mixes to determine how well current tests measure three key parameters of SCC: flowability, passing ability, and segregation. Although most of his findings affirm current practices, he noted some difficulties with J-ring test results.
The J-ring test (ASTM C 1621) is performed along with a second running of the slump flow test (ASTM C 1611). It consists of placing a small Stonehenge-looking device around the slump cone, such that when the SCC leaves the cone and spreads out onto the test surface it has to go past 16 vertical rods in its path. The idea is that the reduction in slump flow due to the vertical rods—simulating one layer of rebar—gives an indication of one mix's passing ability that can be compared to that of a different mix.
Both the unobstructed slump flow and the slump flow with the J-ring in place are reported. The difference between the two is the basis for the “blocking assessment.” Up to 1 inch difference is considered “no visible blocking.” Between 1 and 2 inches is labeled “minimal to noticeable blocking,” and more than 2 inches indicates “noticeable to extreme” blocking.
Now comes the time to consider precision and bias. In terms of testing, bias is a built-in difference between measured values and the values known to be true. A bias can be the result of a systematic error, for which corrections can be applied. Because both slump flow and passing ability are defined only in terms of these tests, neither has a bias.
Precision, according to ASTM's Form & Style Manual, “is the closeness of agreement between test results obtained under prescribed conditions.” Like construction tolerances in the field, it can tell you how close you can count on any one measurement being to the real thing.
Slump flow, it turns out, is a difficult to pin down. The precision statement in C 1611 for the slump flow test says, “results of two properly conducted tests by the same operator on the same batch of concrete should not differ by more than 3.0 inches.” Even under the best conditions, assessing C 1621 blockage based on the difference in slump flow due to the J-ring would require better precision than the C 1611 slump flow test can offer.
Faced with this, Koehler began measuring the difference in height of the concrete at four points inside and outside the J-ring and found that to be a better indicator of SCC's passing ability.
It will be some time before the J-ring test procedure in C 1621 is revised; those things always take time. But at least there's a meaningful alternative in the interim.