Air entraining cement

Air entraining cement

Air is entrained in the concrete in the form of bubbles which modify the properties of fresh concrete i.e. workability, segregation, bleeding and finishing characteristics. It also modifies the properties of hardened concrete i.e. resistance to frost action and permeability. Air bubbles acts as flexible ball bearings so that friction between the various ingredients of concrete is reduced and workability is automatically improved without additional water. The desirable air content is 3-6%. The various air entraining agents generally used are:

(a)  Alkali salts of wood resin

(b)  Synthetic detergents

(c)  Calcium ligno-sulphate

(d)  Aluminum powder, animal fats etc.

These are also available under commercial names such as vinsol resin, airalon, darex, teepol etc. Just for clarity, entrained air is different from entrapped air. The difference between them is given in the Table

Difference between entrained and entrapped air

Entrained Air	Entrapped Air
1- Entrained air is intentional and it is distributed evenly. 2-The size of bubbles in entrained air is very small ranging from 5to 80µ. 3-The shape is nearly spherical. 4-It is a desirable process.	1-Entrapped air is due to insufficient compaction and unevenly distributed. 2-The size of entrapped air pockets may vary from 10µ to 1000µ. 3-Entrapped air may be of any shape as it follows the contours of aggregate surface. 4-It is an undesirable inclusion.

Air entraining cement is not very popular and it is not covered by Code.