Troubleshooting - Concrete Mix Design, Quality Control and Specification - page 274

Another useful distinguishing test is to place a straightedge at right angles
across a crack. If the straightedge will rock, this indicates that the slab has
deflected and therefore that the crack was probably caused by subgrade or
formwork movement, or structural inadequacy in the case of suspended
slabs.
Where cracks are three pointed, they are usually caused by a swelling or
settlement resisting rock immediately below the junction of the cracks, for
example, a “floater” in a soft subgrade subject to moisture movement.
In the case of suspected thermal cracks, it is useful also to check whether
the concrete had a high cement content, making it likely to generate more
heat, whether it was poured on a hot afternoon followed by a cold morn-
ing, and whether there was a delay in pouring, which could have allowed
the concrete to heat up while kept waiting in the truck.
Surface crazing occurs when the surface layer shrinks relative to the body
of concrete below it. This can be caused by allowing the surface to dry or
cool quickly and is more likely when a high shrinkage surface layer, rich in
cement paste and fine sand and of high w/c ratio, is present.
There is an almost universal tendency to use quality control personnel
for trouble shooting of the above nature. This may be a reasonable use of
any spare time, but it is important to ensure first that it does not disrupt the
QC routine and second that such work is separately costed from QC. This
is because the economic justification of QC should be clearly established as
it otherwise tends to be regarded as a luxury item, first in line for cutting
in hard times. Troubleshooting in general is not QC, indeed it may be the
result of inadequate QC, and it is rarely cost saving or revenue generating.
Many QC departments (not only in the concrete industry) have been axed
or decimated through a failure to recognise this.