After placement of fresh plastic concrete, it needs to be consolidated so that it can achieve its desired strength. The process of consolidating the plastic concrete is called Compaction of concrete. In this process, efforts are made to reduce the voids manually or mechanically from the plastic concrete and obtain a homogeneous concrete. When compaction is done manually, it is called Hand compaction and when done mechanically it is called Machine compaction. Here we will discuss both types of compaction methods, as both are used during the construction of house.
1) Hand compaction:
Hand compaction is done with the help of steel tamping rods or timber screeds. Narrow and deep members like columns, are compacted with tamping rods. Thin slabs like roofs and floors are compacted with the help of timbers screeds. In this method, compaction is done in layers of 30cm for mass concrete and 15cm for reinforced concrete. Compaction is carried out for such a time that a layer of mortar starts appearing at the compacted surface. Excessive compaction and under compaction both are harmful for concrete. Due to excessive concrete, coarse aggregate particles i.e. stone chips sink to the bottom and cement and sand mortar appears at the top, which makes concrete structure heterogeneous and hence affects strength.
2) Machine compaction:
Machine or mechanical compaction of concrete is done with the help of vibrators. Vibrators produce vibrations which when transmitted to plastic concrete make it to flow and affect compaction. The air bubbles are forced out of the concrete due to vibrations. Over vibration should not be done otherwise coarse aggregates i.e. stone chips will accumulate at the lower layers and mortar will come to the surface.
There are three types of vibrators which are commonly used:
a) Internal vibrator: This vibrator is also known as immersion, poker or
needle vibrator. It consists of a power unit and a long flexible tube at the end of which a vibrating head is attached. This vibrator develops about 7000 vibrations per minute. Wherever compaction is to be done, the vibrating head is inserted in the concrete. This vibrator is used for depths above 20cm such as beams.
b) Form vibrator: It is also called external vibrator. This vibrator is clamped to the formwork and imparts vibrations to the concrete through formwork. The vibrator is used only when internal vibrator cannot be used as in case of thin and deep structures.
c) Surface vibrator: It is also named as screed or pan vibrator. It is clamped to the screed. It imparts vibration to the concrete from the surface when screeding operation of concrete is carried out. It is suitable for structures having depth less than 20cm like roofs and floors.