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
alternate bays. Otherwise, cracking may result which is undesirable in
water retaining structure.
10. Why shouldn’t internal vibrators be forced down into fresh
concrete? (C1)
Internal vibrators operate by generating impulses which liquefy the fresh
paste so that the internal friction between aggregates is reduced. As a
result of vibration effect, the mix become unstable and trapped air would
rise to the top while aggregates would settle to the bottom.
Internal vibrators should be allowed to fall in fresh concrete under its own
weight and should not be forced to drag down into concrete. The reason
behind this is that the use of force to push down internal vibrators would
leave a mortar channel in fresh concrete and this result in the formation of
weak concrete along this channel.
11. What is the function of rebate in a typical construction joint? (FJ1)
Construction joints are created on sites to facilitate the construction
process. However, if improperly constructed, the completed construction
joints will leave an uneven scar on the concrete surface and affect
significantly its appearance. To avoid this, a rebate is formed during the
first pour of one side of construction joint. After the other pour is concreted,
it will hide the uneven joint inside the rebate.
Fig. A rebate.
12. Where are the desirable locations of construction joints? (FJ1)
Construction joints are normally required in construction works because
there is limited supply of fresh concrete in concrete batching plants in a
single day and the size of concrete pour may be too large to be concreted
in one go.
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