Compaction of Concrete

Compaction of concrete has to be done as early as possible after placing the concrete in place. The purpose of compaction is to expel air and bring the particles closer so as to reduce the void and make the concrete denser. This increased density will give higher strength and make the concrete impermeable. Over-compaction leads to segregation while under-compaction makes the concrete lean. To check for correct compaction, the compaction should be stopped as soon as the cement paste starts appearing on the upper surface of the concrete.

Compaction by hand may be performed by rodding, tamping, ramming or hammering. Wooden or steel hammers are used for ramming massive concrete works. Tampering is adopted for compacting slabs or other horizontal surfaces. Rodding is followed for compacting vertical sections. In all cases of hand compaction high water–cement ratio should be adopted.

Compaction by machines is performed using mechanical vibrators. This method of compaction has several advantages as detailed below:

1. The concrete produced is dense and impermeable.
2. A lesser water–cement ratio results in about 15% reduction in the use of cement.
3. A better bond exists between steel and concrete.
4. The surface of the concrete is uniform because of machine compaction.
5. Because of a high aggregate–cement ratio, there is a possibility for the reduction in creep and shrinkage.
6. Filling small openings is feasible because of good consistency in concrete.
7. It is relatively fast in placing concrete.
8. It consumes comparatively less time, materials and labour and is hence economical.