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In “High strength concrete” in buildings, 56 or 91-day compression test results are sometimes adopted instead of 28-day compression test results. Why?

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 high-rise buildings involves the construction of lower levels firstly in which they are not loaded for a period of a year and more. Substantial material savings shall be resulted from using 56 or 91-day compression test results. Moreover, with later ages of test results used, other cementing materials can be incorporated into the concrete mixture which improves the durability of concrete in terms of heat generation in hydration and other aspects.

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 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.