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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.
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.
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.
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.
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.
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.
When a building blocks the wind blowing across it, part of the wind will escape over the top of the building. Some will pass around the edges of the building while a majority of the wind will get down to the ground. The channeling effect of wind for an escaping path, together with the high wind speeds associated with higher elevations, generates high wind speeds in the region at the base of the building. At the base level of the building, there are three locations of strong pedestrian level winds:
When railway tunnels are built close to buildings, ground-borne vibration is transmitted to the building by means of compression and shear waves.
It is desirable that the position of anchors be located symmetrical to the centre of gravity of the precast concrete units. Otherwise, some anchors would be subject to higher tensile forces when compared the other anchors depending on their distance from the centre of gravity of the precast concrete units. As such, special checks have to be made to verify if the anchor bolts are capable of resisting the increased tensile forces.
