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
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.
4. What are the reasons for blockage in pumping concrete? (PC1)
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 [18]:
(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.
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