The removal of entrapped air during production of concrete and the uniform, dense arrangement of the constituents of concrete are effected during the compacting of corners. The density and the strength and durability of concrete depend upon this operation. Concrete should be thoroughly compacted during the operation of placing and thoroughly worked around the reinforcement, around embedded fixtures and into corners of the form work. Concrete is compacted by vibration, during which the vibrator communicates rapid vibrations of low amplitude to the particles, as a result of which the concrete becomes fluid, that is to say, its mobility is increased and the particles, in movement, under the force of gravity occupy a more stable position, with which volume of concrete is least.
Vibrators are of three general types:
Internal or immersion vibrator consists of a vibrating element enclosed in a casing which is immersed in fresh concrete and transmit vibrations through the vibrator body. External or form vibrators are fastened to the form work by a clamping device and transmit vibrations to the concrete through the form. In precast members of concrete, the vibrating tables are very helpful. These tables vibrate the entire mass of concrete uniformly. Surface vibrators, set up on the concrete surface after placing, transmit vibrations to it through a working platform. They are generally employed in concrete road construction.
Mechanical method of compacting the concrete is used only when the mix is stiff. Over-vibration or vibration of very wet mixes is harmful and should be avoided. Alternatively, concrete may be compacted manually by rodding, temping or hammering. Rodding is generally done to compact thin vertical members while temping is done for compacting concrete for slabs etc.
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