A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
Gypsum is added to control the "setting of cement". If not added, the
cement will set immediately after mixing of water leaving no time for
concrete placing.
8. Can rapid-hardening cement be used in water-retaining structures?
(CM2)
Normal Portland cement is usually adopted in water retaining structures.
Where sulphates or chemical agents are anticipated in groundwater,
sulphate-resisting cement may be used to guard against sulphate and
chemical attack. However, it is normally not advisable to use
rapid-hardening cement in water-retaining structures because it involves
greater evolution of heat during hydration process, leading to increased
shrinkage cracks which form the location of potential leakage in the
structure. It is only applicable in cold weather condition where the rate of
hydration is low.
9. Is it desirable to use concrete of very high strength i.e. exceeding
60MPa? What are the potential problems associated with such high
strength concrete?
To increase the strength of concrete, say from 40MPa to 80MPa, it
definitely helps in improving the structural performance of the structure by
producing a denser, more durable and higher load capacity concrete. The
size of concrete members can be significantly reduced resulting in
substantial cost savings. However, an increase of concrete strength is also
accompanied by the occurrence of thermal cracking. With an increase in
concrete strength, the cement content is increased and this leads to higher
thermal strains. Consequently, additional reinforcement has to be
introduced to control these additional cracks caused by the increase in
concrete strength. Moreover, the ductility of concrete decreases with an
increase in concrete strength. Attention should be paid during the design of
high strength concrete to increase the ductility of concrete. In addition, fire
resistance of high strength concrete is found to be less than normal
strength concrete as suggested by Odd E. Gjorv (1994).
Though the tensile strength of high strength concrete is higher than that of
normal concrete, the rate of increase of tensile strength is not proportional
to the increase of compressive strength. For normal concrete, tensile
strength is about one-tenth of compressive strength. However, for high
strength concrete, it may only drop to 5% of compressive strength.
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