Testing - Concrete Mix Design, Quality Control and Specification - page 188

An important consideration is that it is not only the accuracy of a test
that matters but also its relevance and the accuracy of the assumptions
made in evaluating it. For example, a test cylinder left on an in situ slab
may give a very accurate strength but may have a very different maturity
and therefore a very different strength to the slab itself. A pullout test on
the same slab may be much more variable but at least it is measuring the
actual strength. A standard test cylinder combined with a maturity (e.g.,
equivalent age) measurement of both the cylinder and the slab might be
more accurate than the in-situ-cured cylinder and as relevant as the pullout
test, but it does depend on the accuracy of the maturity-strength correla-
tion and, for example, the compaction of the slab. An ultrasonic test would
also be very relevant and may be quite repeatable and accurate but would
be totally dependent on the strength-velocity relationship assumed, which
would be affected by such factors as moisture content.
The reader is referred elsewhere to Bungey (1993) for further details
of various NDT tests, but the authors certainly see a place for such tests
in the overall control operation. Particular examples are pullout tests on
suspended floor slabs prior to early stripping or stressing, and Schmidt
hammer tests on freshly stripped columns. The latter is not a very accurate
test (especially if used informally rather than according to the manufac-
turer's routine), but it is an extremely quick and cheap test that could be
used on every column as it is stripped and would give early warning of any
severe problems. It has even been suggested that the test could be worth
performing even if the strength scale is not read. The implication is that
the depth of indentation or even the sound of the impact would alert a
daily user to any drastic problem. Day found this to be the case with spun
concrete pipes, where sound was a good indication, and the process could
be compared to tapping the wheels of railway carriages to detect cracks.
However, a thorough examination by a U.S. university team (Telisak et al.,
1991) concluded that in situ maturity determination was the most accurate
criterion of early-age strength.
When regular NDT tests are carried out it is very desirable to enter the
results in the control system for graphing and analysis alongside the other
test data. Such action will soon establish the extent to which the variation
of strength in the structure is a consequence of basic concrete variation.
A development pioneered by Dr A.M. Leshchinsky is that of using
multiple techniques of NDT concurrently. The idea is that although the
correlation of any one such set of test results with compressive strength may
be upset by some influence (e.g., ultrasonic pulse velocity is greatly affected
by moisture content), it is less likely that two or more different tests will
be similarly affected. Therefore the use of two or more techniques will give
more certainty of a correct assessment than any number of repetitions of
the same type of test. This is a further illustration of a point previously