Compaction is a process of expelling the entrapped air. If we don’t expel this air, it will result into honeycombing and reduced strength. It has been found from the experimental studies that 1% air in the concrete approximately reduces the strength by 6%.
DIFFERENT METHODS OF CONCRETE COMPACTION
There are two methods of compaction adopted in the field as given below.
- Hand compaction
- Mechanical Compaction
(A) HAND COMPACTION
Hand compaction is used for ordinary and unimportant structures. Workability should be decided in such a way that the chances of honeycombing should be minimum. The various methods of hand compaction are as given below.
It is a method of poking with 2m long, 16 mm dia rod at sharp corners and edges. The thickness of layers for rodding should be 15 to 20 cm.
It is generally used for compaction on ground in plain concrete. It is not used either in RCC or on upper floors.
It is a method in which the top surface is beaten by wooden cross beam of cross section 10 cm x 10 cm. both compaction and leveling are achieved simultaneously. It is mainly used for roof slabs and road pavements.
(B) MECHANICAL COMPACTION
Vibration is imparted to the concrete by mechanical means. It causes temporary liquefaction so that air bubbles come on to the top and expelled ultimately. Mechanical vibration can be of various types as given under.
1. INTERNAL VIBRATION
It is most commonly used technique of concrete vibration. Vibration is achieved due to eccentric weights attached to the shaft. The needle diameter varies from 20 mm to 75 mm and its length varies from 25 cm to 90 cm. the frequency range adopted is normally 3500 to 5000 rpm. The correct and incorrect methods of vibration using internal vibration needles are shown below.
2. EXTERNAL VIBRATION
This is adopted where internal vibration can’t be used due to either thin sections or heavy reinforcement. External vibration is less effective and it consumes more power as compared to the internal vibration. The form work also has to be made extra strong when external vibration is used.
3. TABLE VIBRATION
It is mainly used for laboratories where concrete is put on the table.
4. PLATFORM VIBRATION
It is similar to table vibrators but these are generally used on a very large scale.
5. SURFACE VIBRATION
These are also called screed board vibrators. The action is similar to that of tamping. The vibrator is placed on screed board and vibration is given on the surface. It is mainly used for roof slabs, road pavements etc., but it is not effective beyond 15 cm depth.