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Gases can be deliberately or accidentally incorporated into concrete leaving voids in the hardened concrete. The compressive strength of concrete tends to decrease with increasing voids content. Concrete should be well compacted to remove entrapped air. All concretes will contain some entrapped air voids, even if very well compacted.
Entrained Air: Large numbers of very small discrete bubbles are introduced deliberately into the concrete by an admixture. Entrained air is added because it considerably increases the resistance of concrete to the damaging effects of freeze-thaw cycles. Entrained air is also beneficial in reducing the bleeding of concrete.
Foamed Concrete: Lightweight concretes can be made by producing concrete containing large quantities of air voids by use of an admixture or by mixing in a preformed foam. Foamed concrete is often used as a backfill material for trench reinstatement or a lightweight fill material for voids such as old cellars, tanks, etc.
Aerated Concrete: This is made by introducing into the concrete a material which causes the generation of gas producing a foam like material, eg. some types of lightweight concrete building blocks. The gas produced in the foaming reaction may not be air.
Acknowledgement: The Concrete Society