Compaction of concrete

Concrete compaction is the process of compacting fresh concrete in forms such that it surrounds reinforcing elements and other embedded items, like tubes, in the mold. If concrete is not correctly compacted, a number of issues could occur, including stuck in concrete paste and honeycomb. Furthermore, inadequate concrete compaction may result in permeability issues, steel corrosion, and a reduction in the final capacity of hardened concrete.It is done to remove entrapped air and set particles of fresh concrete in motion reducing friction between them. Insufficient compaction increases the permeability of concrete resulting in easy entry for aggressive chemicals which attack concrete and reinforcement hence reducing the durability of the concrete. In order to achieve full compaction and maximum density with reasonable compacting effort available at site, it is necessary to use a mix with adequate workability.

The mix should not be too wet for easy compaction which also reduces the strength of concrete. There are two main methods of compacting concrete they include:-

1. Hand compaction.
2. Machine compaction.  

Compaction of concrete using vibrator

It is adopted in case of an important concrete work of small magnitude. Also applied where a large quantity of reinforcement is, which cannot ne normally compacted by mechanical means.

It consist of rodding (poking the concrete to pack it between the reinforcement and sharp corners and edges)

Tamping (adopted where thickness of concrete is comparatively less and surface to be finished is required to be smooth and leveled). It consists of beating the top surface by wooded cross beam.

Ramming adopted where the reinforcement may be disturbed or formwork may fail.

Its advantages include.

1. Permit use of less water hence concrete of high strength
2. It makes it possible to place economically concrete which is impracticable to place with hands.
3. Improves durability of structures

There are various types of vibrators

1. Internal vibrators
2. External vibrators
3. Table vibrators
4. Surface vibrators

Internal vibrators (needle vibrator)

It essentially consist of power unit that may be electrically driven or operated by petrol engine, a flexible shaft which is attached to eccentric weight that cause vibration and a needle which can be preplaced by a blade to facilitate vibration of members.

It has the following advantages:-

1. Can be shifted from one place to another
2. Can be used in difficult position and situations
3. Speed is relatively constant.

External vibrator/ from work vibrators

The machine is damped on the external surface of formwork. Vibrator is attached to the formwork so that concrete near the shutters gets vibrated.

Adopted where reinforcement and spacers interfere much with the internal vibrators and it provides a good finished to the concrete surface. Though its efficiency is lower than that of internal vibrator.

Table vibrator.

The table is clamped to the table or table mounted on springs which are vibrated transforming the vibration to the table.

It is commonly used for vibrating concrete cubes.

Surface vibrator

It’s a vibrator placed on a screed board and mostly used in floor and slabs whose thickness is small.

Platform vibrator

It is just like the table vibrator but larger in size.

Compaction by pressure

Mostly used for compacting hollow blocks or cavity blocks where stiff concrete is used by employing great pressure, where the ratio of water to cement is low.