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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 precast concrete is compacted by means of external vibrators bolted to the formwork that fluidise the concrete and permit the entrapped air to rise to the surface. In addition, internal poker vibrators (see Plant & Equipment) are used for thick sections or special products. The moulds for small precast units may be placed on vibrating tables to achieve the required compaction.
Acknowledgement: The Concrete Society