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If on-site slump test fails, should engineers allow the contractor to continue the concreting works?

This is a very classical question raised by many graduate engineers. In fact, there are two schools of thought regarding this issue.

The first school of thought is rather straightforward: the contractor fails to comply with contractual requirements and therefore as per G. C. C. Clause 54 (2)(c) the engineer could order suspension of the Works. Under the conditions of G. C. C. Clause 54(2)(a) – (d), the contractor is not entitled to any claims of cost which is the main concern for most engineers. This is the contractual power given to the Engineer in case of any failure in tests required by the contract, even though some engineers argue that slump tests are not as important as other tests like compression test.

The second school of thought is to let the contractor to continue their concreting works and later on request the contractor to prove that the finished works comply with other contractual requirements e.g. compression test. This is based upon the belief that workability is mainly required to achieve design concrete compression strength. In case the compression test also fails, the contractor should demolish and reconstruct the works accordingly. In fact, this is a rather passive way of treating construction works and is not recommended because of the following reasons:

(i) Workability of freshly placed concrete is related not only to strength but also to durability of concrete. Even if the future compression test passes, failing in slump test indicates that it may have adverse impact to durability of completed concrete structures.

(ii) In case the compression test fails, the contractor has to deploy extra time and resources to remove the work and reconstruct them once again and this slows down the progress of works significantly. Hence, in view of such likely probability of occurrence, why shouldn’t the Engineer exercise his power to stop the contractor and save these extra time and cost?

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.

What are the differences between shear slump and collapse slump in slump test?

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.

types of concrete 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 “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.