Questions and Answers on practical civil engineering

Questions & Answers


1. 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

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?


2 In designing concrete structures, normally maximum aggregate sizes are adopted
with ranges from 10mm to 20mm. Does an increase of maximum aggregate size
benefit the structures?
To answer this question, let’s consider an example of a cube. The surface area to volume
ratio of a cube is 6/b where b is the length of the cube. This implies that the surface area to
volume ratio decreases with an increase in volume. Therefore, when the size of maximum

aggregate is increased, the surface area to be wetted by water per unit volume is reduced.
Consequently, the water requirement of the concrete mixes is reduced accordingly so that
the water/cement ratio can be lowered, resulting in a rise in concrete strength.
However, an increase of aggregate size is also accompanied by the effect of reduced
contact areas and discontinuities created by these larger sized particles. In general, for
maximum aggregate sizes below 40mm, the effect of lower water requirement can offset
the disadvantages brought about by discontinuities as suggested by Longman Scientific and
Technical (1987).



3. In concrete compression test, normally 150mmx150mmx150mm concrete cube
samples is used for testing. Why isn’t 100mmx100mmx100mm 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