Compaction of concrete.
After concrete has been mixed, transported and placed, it contains entrapped air in the form of large voids. If this air is not removed by proper compaction, the presence of these voids will:
- reduce the strength of the concrete
- increase the permeability and hence reduce the durability
- lead to blemishes on the surface, such as blowholes and honeycombing.
Most concrete is compacted by means of internal poker vibrators (see Plant & Equipment) that fluidise the concrete and permit the entrapped air to rise to the surface. External vibrators bolted to the formwork are occasionally used for in situ concrete though their main use is for precast concrete.
Slabs are best consolidated by vibrating beam compactors. These combine the action of a screed and a vibrator, but are only effective for limited depths. Deeper slabs should be compacted with poker vibrators and finished with a vibrating beam.