Vibration of concrete is carried out for the sake of consolidation. The main objective of vibration is to compact the concrete and to achieve the maximum possible density of concrete. Almost 5 to 8% by volume of freshly placed concrete in the form is occupied by air bubbles. Air bubbles occupy this space in a high workable concrete mix.
Air bubbles can be removed by decreasing in the amount of required water, but it affects workability of concrete. Therefore, vibration of concrete is the only suitable method that helps without affecting much the properties of fresh concrete. Vibration is only affected in a fresh mix concrete or one can say up to that time, when concrete has the ability to flow. This stage exists before the starts of initial setting.
Vibration reduces the internal friction by randomizing concrete. This able the concrete to rearrange the particles (these particles include coarse aggregates and fine aggregates). When rearrangement of particles occur than a close configuration of coarse aggregate is achieved. Close packing expels most of the entrapped air in between, but still some air bubbles left. 100% air bubbles are difficult to remove from concrete.
During the vibration of concrete, site engineer should ensure that vibration is applied to the entire mass of concrete uniformly. This is so, because if not applied uniformly than some volume of concrete get fully compact and some parts remain un-compacted. Site engineer should also strictly follow the vibration time period, because over vibration of concrete cause segregation and bleeding of concrete. Therefore, proper consistency is required during the vibration.
- Internal vibrators
- External vibrators
- Vibrating tables
They are also called immersion, poker or needle vibrators. They essentially consist of a power unit and a long flexible tube at the end of which a vibrating head is attached. Power is provided by electric motor, compressed air or petrol engine. The long tube houses a flexible shaft which rotates an eccentric weight inside the vibrating head. The frequency of the vibrator is about 700 cycles per minute.
The vibrating head is inserted in the concrete. They are most effective as the vibrating head comes into intimate contact with concrete.
These vibrators are clamped rigidly to the form work at the pre-determined points so that the form and concrete are vibrated. They consume more power for a given compaction effect than internal vibrators.
These vibrators can compact up to 450mm from the face but have to be moved from one place to another as concrete progresses. These vibrators operate at a frequency of 3000 to 9000 rpm at an acceleration of 4g.
The external vibrators are more often used for pre-casting of thin in-situ sections of such shape and thickness as cannot be compacted by internal vibrators.
These are rigidly built steel platforms mounted on steel springs and driven by electric motors. The concrete is placed in moulds mounted on the platforms and securely clamped to the vibrating table, so that mould and concrete vibrate in union.
Vibrating tables are used for compacting stiff and harsh mixes used in precast structural members in factories and laboratory specimens.