Compaction of concrete is the process adopted for expelling the entrapped air from the concrete. In the process of mixing, transporting and placing of concrete air is likely to get entrapped in the concrete. The lower the workability, higher is the amount of air entrapped. In other words, stiff concrete mix has high percentage of entrapped air and, therefore , would need higher compacting efforts than high workable mixes. If this air is not removed fully, the concrete loses strength considerably.
Methods of Compaction of Concrete
The following methods are adopted for compaction of concrete:
- Hand Compaction
- Rodding
- Ramming
- Tamping
- Compaction by Vibration
- Internal vibrator (Needle vibrator)
- Formwork vibrator (External vibrator)
- Table vibrator
- Platform vibrator
- Vibratory Roller.
- Compaction by Pressure and Jolting
- Compaction by Spinning.
Hand Compaction of Concrete
Hand compaction of concrete is adopted in case of unimportant concrete work of small magnitude or where a large quantity of reinforcement is used, which cannot be normally compacted by mechanical means. Hand compaction consists of rodding, ramming or tamping.
Rodding is nothing but poking the concrete with about 2 meter long, 16 mm diameter rod to pack the concrete between the reinforcement and sharp corners and edges.
Ramming should be done with care. Light ramming can be permitted in unreinforced foundation concrete or in ground floor construction. Ramming should not be permitted in case of reinforced concrete or in the upper floor construction, where concrete is placed in the formwork supported on struts.
Tamping consists of beating the top surface by wooden cross beam of section about 10 x 10 cm. Since the tamping bar is sufficiently long it not only compacts, but also levels the top surface across the entire width.
Compaction by Vibration
It is pointed out that the compaction by hand, if properly carried out on concrete with sufficient workability, gives satisfactory results. But where high strength is required, it is necessary that stiff concrete, with low water/cement ratio be used. To compact such concrete, mechanically operated vibratory equipment, must be used.
Internal Vibrator (Needle Vibrator)
Of all the vibrators, the internal vibrator is most commonly used. This is also called, “Needle Vibrator”, “Immersion Vibrator”, or “Poker Vibrator”. This essentially consists of a power unit, a flexible shaft and a needle. The vibrations are caused by eccentric weights attached to the shaft or the motor or to the rotor of a vibrating element. The frequency of vibration varies up to 12,000 cycles of vibration per minute. The needle diameter varies from 20 mm to 75 mm and its length varies from 25 cm to 90 cm.
Formwork Vibrator (External Vibrator)
Formwork vibrators are used for concreting columns, thin walls or in the casting of precast units. The machine is clamped on to the external wall surface of the formwork. The vibration is given to the formwork so that the concrete in the vicinity of the shutter gets vibrated. This method of compaction of concrete is particularly useful and adopted where reinforcement, lateral ties and spacers interfere too much with the internal vibrator.
Table Vibrator
This is the special case of formwork vibrator, where the vibrator is clamped to the table. or table is mounted on springs which are vibrated transferring the vibration to the table. They are commonly used for vibrating concrete cubes. Any article kept on the table gets vibrated. This is adopted mostly in the laboratories and in making small but precise prefabricated R.C.C. members.
Platform Vibrator
Platform vibrator is nothing but a table vibrator, but it is larger in size. This is used in the manufacture of large prefabricated concrete elements such as electric poles, railway sleepers, prefabricated roofing elements etc.
Vibratory Roller
One of the recent developments of compacting very dry and lean concrete is the use of Vibratory Roller. Such concrete is known as Roller Compacted Concrete. This method of compaction of concrete originated from Japan and spread to USA and other countries mainly for the construction of dams and pavements. Heavy roller which vibrates while rolling is used for the compaction of dry lean concrete.
Compaction of Concrete by Pressure and Jolting
This is one of the effective methods of compacting very dry concrete. This method is often used for compacting hollow blocks, cavity blocks and solid concrete blocks. The stiff concrete is vibrated, pressed and also given jolts. With the combined action of the jolts vibrations and pressure, the stiff concrete gets compacted to a dense form to give good strength and volume stability. By employing great pressure, a concrete of very low water cement ratio can be compacted to yield very high strength.
Compaction by Spinning
Spinning is one of the recent methods of compaction of concrete. This method of compaction is adopted for the fabrication of concrete pipes. The plastic concrete when spun at a very high speed, gets well compacted by centrifugal force. Patented products such a “Hume Pipes”, “spun pipes” are compacted by spinning process.
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