What is the function of rebate in a typical construction joint?

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.

rebate

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.

Why shouldn’t internal vibrators be forced down into fresh concrete?

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.

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 reasons in setting maximum and minimum time for concreting successive lift in water-retaining structures?

Maximum time for concreting successive lifts is required for concrete structures to reduce the potential differential strains. However, minimum time is needed to allow for possible shrinkage and thermal contraction to take place in the event of alternative bay construction before concreting to alternate bays. Otherwise, cracking may result which is undesirable in water retaining structure.

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 reasons for blockage in pumping concrete?

Concrete pumping is commonly adopted in highly elevated locations for which access for concrete trucks is difficult. Construction works can be speeded up by using concrete pumping because a larger volume of pours can be achieved within a specified duration when compared with normal concrete placing methods.

Blockage may occur during pumping operation for the following two common reasons:

(i) For saturated concrete mixes, the pump pressures may force water out of the concrete resulting in bleeding. The flow resistance is then increased and may contribute to the blockage of pipelines.

(ii) If the cement content (or other components of concrete mixes that increase the frictional forces) is high, a higher frictional resistance to pumping may develop and the concrete may not be pumpable.

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 problems if reinforcement is in contact with internal vibrators?

During concreting, if internal vibrators are placed accidentally in contact with some of the reinforcement bars, some undesirable effects may result. The most obvious one is that the reinforcement bars may become damaged or displaced if loosely tied.

Air bubbles tend to move towards the source of vibration. For poker vibrators touching the reinforcement bars, air pockets may be trapped in the vicinity of the reinforcement because the vibration generated by internal vibrators attracts these air bubbles. Consequently, the bond between the reinforcement and surrounding concrete would be impaired.

To produce good surface finish close to densely-packed reinforcement cage, workers may insert the poker vibrators in the gap between the reinforcement cage and formwork because the reinforcement cage tend to damp down the vibration effect when the vibrators are placed at a distance from the formwork. However, concentrated vibration within the cover region causes the migration of finer cement mortar to this region and results in changes in concrete colour. If the concrete cover is small, the chance of getting the poker vibrators jammed within the gap is high and the formwork is likely to be damaged by the vibrators.

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.

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