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
Level One (Core FAQs)
Part III: Construction of Concrete Structure
1. Should large-diameter or small-diameter vibrators be used in
compacting concrete? (C1)
There is a general rule regarding the size of vibrators in compacting
concrete. The diameter of the vibrator should be a quarter of the wall
thickness of the concrete being cast. In general, large-diameter internal
vibrators have higher amplitude with lower frequency while small-diameter
internal vibrators have lower amplitude with higher frequency. In particular,
small-diameter internal vibrators with high frequency are normally used in
compacting high slump concrete.
2. What is the problem of over vibration of fresh concrete? (C2)
For proper compaction of concrete by immersion vibrators, the vibrating
part of the vibrators should be completely inserted into the concrete. The
action of compaction is enhanced by providing a sufficient head of concrete
above the vibrating part of the vibrators. This serves to push down and
subject the fresh concrete to confinement within the zone of vibrating
action.
Over-vibration should normally be avoided during the compaction of
concrete. If the concrete mix is designed with low workability, over-vibration
simply consumes extra power of the vibration, resulting in the wastage of
energy. For most of concrete mixes, over-vibration creates the problem of
segregation in which the denser aggregates settle to the bottom while the
lighter cement paste tends to move upwards [40]. If the concrete structure
is cast by successive lifts of concrete pour, the upper weaker layer (or
laitance) caused by segregation forms the potential plane of weakness
leading to possible failure of the concrete structure during operation. If
concrete is placed in a single lift for road works, the resistance to abrasion
is poor for the laitance surface of the carriageway. This becomes a critical
problem to concrete carriageway where its surface is constantly subject to
tearing and traction forces exerted by vehicular traffic.
3. What are the problems if reinforcement is in contact with internal
vibrators? (C1)
During concreting, if internal vibrators are placed accidentally in contact
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