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.

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.

The Schmidt hammer test involves hitting the in-situ concrete with a spring-driven pin at a defined energy, and then the rebound is measured.

The rebound depends on the surface hardness of the concrete and is measured by test equipment. By referring to the some conversion tables, the rebound result of the test can be used to determine the compressive strength of the concrete. Although past investigations showed that there is a general relationship between compressive strength of concrete and the rebound number, there is a wide range of disagreement among various research workers regarding the accuracy of estimation of strength from Schmidt hammer. In fact, there is about a variation of 15-20% in concrete strength measured by the method.

Schmidt hammer is in not a standard test for acceptance testing of concrete strength. It is only a test used for estimating the strength of concrete in structure and it can hardly be considered as a substitute for compressive strength 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 pumping operation, the force exerted by pumps must overcome the friction between concrete and the pumping pipes, the weight of concrete and the pressure head when placing concrete above the pumps. In fact, as only water is pumpable, it is the water in the concrete that transfers the pressure.

The main problems associated with pumping are the effect of segregation and bleeding. To rectify these adverse effects, the proportion of cement is increased to enhance the cohesion in order to reduce segregation and bleeding. On the other hand, a proper selection of aggregate grading helps to improve the pumpability of concrete.

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.

Retardation of fresh concrete has several advantages as follows:

(i) A rapid hydration process results in loss in concrete strength because the concrete will have a poorer structure with a higher gel/space ratio compared with the concrete with a lower hydration rate.

(ii) During the hydration process, a substantial heat of hydration will be generated. If the hydration process is carried out too swiftly, it will cause a rapid rise in temperature and results in considerable early thermal movement in concrete.

(iii) In hot weather concreting, the loss of workability is substantial. In order to ensure sufficient compaction of fresh concrete, it is necessary to extend the time for fresh concrete to remain plastic.

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.