A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
concrete. Since the test is very sensitive to the presence of aggregates and
voids at the concrete surface, it is necessary to take more than 10 readings
over the area of test. However, it should be noted that Schmidt hammer
test measures surface hardness only but not the strength of concrete.
Therefore, it may not be considered a good substitute for concrete
compression test.
6. In carrying out compression test for concrete, should test cubes or
test cylinders be adopted? (T1)
Basically, the results of compression test carried out by using cubes are
higher than that by cylinders. In compression test, the failure mode is in the
form of tensile splitting induced by uniaxial compression. However, since
the concrete samples tend to expand laterally under compression, the
friction developed at the concrete-machine interface generates forces
which apparently increase the compressive strength of concrete. However,
when the ratio of height to width of sample increases, the effect of shear on
compressive strength becomes smaller. This explains why the results of
compression test by cylinders are lower than that of cubes. Reference is
made to Longman Scientific and Technical (1987).
7. Is slump test a good test for measuring workability? (T3)
Though slump test is originally designed as a measure of workability, it
turns out to be an indicator of excessive water content in concrete only.
Slump test is not considered as a measure of workability because:
(i) There is no connection between the test results of slump test and
workability;
(ii) The test results exhibit large random variations which is greater than
that due to observed differences in workability;
(iii) Concrete of different workability may have the same slump.
8. In “High strength concrete” in buildings, 56 or 91-day compression
test results are sometimes adopted instead of 28-day compression
test results. Why? (T1)
In normal concrete structures, 28-day test results are often adopted.
However, in the construction of high-rise buildings using “High strength
concrete”, compressive strengths based on 56 or 91-day compression test
results are commonly used instead. Since the process of construction of
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