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Is slump test a good test for measuring workability?

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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In carrying out compression test for concrete, should test cubes or test cylinders be adopted?

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).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

If concrete compression test fails, should Schmidt hammer test be adopted as an alternative test to prove the concrete strength?

The Scmidt hammer test is based on the elastic rebound of hammer which presses on concrete surface and it measures the surface hardness of 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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In concrete compression test, normally 150mm x150mm x150mm concrete cube samples is used for testing. Why isn’t 100mm x100mm x100mm concrete cube samples used in the test instead of 150mmx150mmx150mm concrete cube samples?

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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Is compressive strength test required for standard mixes of concrete?

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.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.