Compaction of concrete is the process of removal of entrapped air from the concrete. During the process of concrete mixing and placement, the air gets entrapped in it. Workable mix has a lower amount of entrapped air. In other words, stiff concrete has a high amount of entrapped air therefore would require more compacting efforts to remove the air bubbles.
This entrapped air reduces the strength of the concrete. Approx. 5% of entrapped air decreases the strength of concrete up to 30% and 10% entrapped air can reduce the strength by as much as 60%.
Thus it is very necessary to remove the air from the concrete as it not only reduces the strength of the concrete also due to the presence of air, the porosity of the concrete increases and foreign material finds its way and attack concrete and steel.
Equipment used for Compaction of Concrete
Hand compaction of concrete is done in the case of small concrete works. It consists of rodding, ramming, and tamping. For hand compaction higher w/c ratio is required. It is limited up to the thickness of 15-20 cm.
In this method of hand compaction, concrete is compacted by using the rod of 16 mm dia. and 2 m length between the reinforcement and edges. Rodding is done continuously to effectively pack the concrete and remove entrapped air. Bamboos can also be used in place of the rod.
Ramming by steel or wooden rammer is essentially done in the case of unreinforced concrete on the ground floor only. Ramming can’t be done on reinforced concrete because it will displace the reinforcement from its position. Also, it cant be done on the upper floors because its impact can displace the formwork or strut.
You might have seen that during the construction of slabs of buildings or road pavements the surface is compacted by wooden crossbeams or steel beam or section 10 x 10 cm. This method of hand compaction is called tamping. Tamping is adopted at a place where the thickness is concrete is comparatively less and the surface is required to be finished smooth and level.
Compaction by vibration
The strength of the concrete compacted by hand compaction will always be low because of the higher w/c ratio required for the compaction of concrete. If high strength concrete is required, the w/c ratio would have to be low thus in this case hand compaction can not be used. Thus we have to compact the concrete using mechanical vibrators and the method is called compaction by vibration.
Vibrators set the concrete in motion which reduces the friction between the particles of the concrete. Vibration does not directly affect the strength of the concrete but it permits the use of concrete with a lower w/c ratio which ultimately increases the strength of concrete.
It is the most commonly used vibrator also known as needle vibrator, poker vibrator, and immersion vibrator. It consists of a power unit, a shaft and a needle can be powered by electricity, or petrol or by an air compressor. An eccentric weight is attached either to the shaft or motor which creates the vibration.
Dia. of the needle varies from 20 – 70 mm length is 30-90 cm. The bigger the size of the construction bigger the needle will be used. This Is portable and can be shifted very easily. This method of compaction by vibration can also be used in difficult positions and situations.
External Concrete Form Vibrator
This method of compaction by vibration is used in the case of columns, thin walls and ni precast construction works. The vibrator is attached to the external wall surface of the formwork and vibration is given to the formwork. This is particularly suitable in case of very heavy reinforcement works where a needle vibrator will not enter.
It gives a very good finish to the concrete surface. The efficiency of this type of vibrator is lower than the needle type because vibration is given to the formwork thus consuming more power.
As the name suggest it is a special vibrator where vibrator is attached to the table. It is used mostly used in laboratories to compact the concrete cubes and in small precast unit fabrication.
If size of the table vibrator is increases we will call it platform vibrator. This method of compaction by vibration is used for manufacturing of large prefabricated concrete units like railway sleepers, prefabricated electrical poles.
Also known as screed board vibrator. In this type of vibrator a small vibrator is attached to the screed board. It is used for the compaction of thin concrete member like slab, floor, pavement and can be employed for a maximum thickness of 15 cm.
Compaction by pressure and jolting
Very dry concrete is vibrated pressed and jolted to compact hollow blocks, cavity blocks, and solid concrete blocks. By the combined action of vibration, jolts, and pressure concrete gets compacted to a dense form.
Compaction by spinning
This method of compaction is used for the fabrication of concrete pipes/hume pipes. Plastic concrete when spun at high speed gets compacted by the effect of centrifugal force.
In this method of compaction, a heavy roller that vibrates while rolling is employed for the compaction of concrete dams, concrete and earthen roads.
Proper Use of Vibrators
Depth of Concrete layer
Although concrete up to 1m depth can be compacted using heavy vibrators the depth of the concrete layer should not generally be more than 60 cm.
Depth of immersion of vibrator
The active part of the vibrator should be completely immersed for good compaction of concrete. The vibrator shall be immersed 10-20 cm in the already compacted concrete to create a good bond between the compacted and new layer of concrete.
Spacing of vibrator insertion
Spacing should be such that the range of action overlaps to some extent and concrete shall be sufficiently compacted. For example for concrete of workability 0.78 to 0.85, a vibrator shall be inserted at a distance of 35 to 90 cm apart.
Speed of insertion and withdrawal
Speed of 3 cm/s is sufficient for the proper compaction by vibration.
Duration of vibration
Duration of vibration depends on the depth of the concrete layer, workability, and type of vibrator. The vibrator should not be kept in one place for a longer duration. Instead, it is better to insert a vibrator at a number of places for a shorter duration for good compaction of concrete.
Vibrating concrete with hardened concrete
In the case where concrete has to be joined with hardened concrete, for proper bonding hardened concrete should be roughened and cleaned first. Cement sand grout of 1:1 shall be applied then only fresh concrete shall be placed over it and vibrated.