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 in structure and it can hardly be considered as a substitute for
compressive strength test.
3. Is compressive strength test required for standard mixes of
concrete? (T1)
In some countries like Britain, specification for concrete does not normally
require cube tests for standard mixes of concrete. The quality control of
standard mixes in Britain is achieved by checking if the appropriate mix
proportions are adopted during the mixing of concrete. However, in Hong
Kong the requirement of testing for compressive strength is still required for
standard mixes in the specification because it is impractical to inspect and
check all constituent materials (e.g. cement, aggregates etc.) for concrete
for compliance. As there is high variability in mixing materials owing to
variance in the origin of production of constituent materials in Hong Kong,
there is a risk that the end-product concrete does not comply with the
design requirements even though the mix proportions of standard mixes
are followed closely by engineers.
4. In concrete compression test, normally 150mmx150mmx150mm
concrete cube samples is used for testing. Why isn’t
100mmx100mmx100mm concrete cube samples used in the test
instead of 150mmx150mmx150mm concrete cube samples? (T1)
Basically, the force supplied by a concrete compression machine is a
definite value. For normal concrete strength application, say below 50MPa,
the stress produced by a 150mmx150mmx150mm cube is sufficient for the
machine to crush the concrete sample. However, if the designed concrete
strength is 100MPa, under the same force (about 2,000kN) supplied by the
machine, the stress under a 150mmx150mmx150mm cube is not sufficient
to crush the concrete cube. Therefore, 100mmx100mmx100mm concrete
cubes are used instead to increase the applied stress to crush the concrete
cubes.
For normal concrete strength, the cube size of 150mmx150mmx150mm is
already sufficient for the crushing strength of the machine.
5. If concrete compression test fails, should Schmidt hammer test be
adopted as an alternative test to prove the concrete strength? (T1)
The Scmidt hammer test is based on the elastic rebound of hammer which
presses on concrete surface and it measures the surface hardness of
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