There are three types of slump that may occur in a slumps test, namely, true slump, shear slump and collapse slump.

True slump refers to general drop of the concrete mass evenly all around without disintegration.

Shear slump indicates that the concrete lacks cohesion. It may undergo segregation and bleeding and thus is undesirable for the durability of concrete.

Collapse slump indicates that concrete mix is too wet and the mix is regarded as harsh and lean.

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 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.

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.

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.

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.