Air entrainment is provided in concrete with air entraining Portland cement or air entraining agents like admixture. The amount of air in such concrete generally remains among four to seven percent of the volume of concrete.
It is calculated with galvanometric method, volumetric method and pressure method. The air bubbles lessen internal pressure on the concrete by arranging chambers for water to spread when it freezes.
The objective of air entrainment is to enhance the strength of the hardened concrete, particularly in climates prone to freeze-thaw as well to raise workability of the concrete while in a plastic state.
Method - In the following ways, the air is entrained into the concrete:
1. By applying gas forming materials like aluminium powder, zinc powder and hydrogen peroxide.
2. By applying surface active agents that minimizes surface tension. They range from natural wood resins and their soaps, animal or vegetable fats or oils, alkali salts of sulfonated or sulphated organic compounds.
3. By applying cement dispersing agents.
Advantages - Given below, some of the advantages of the entrained concrete:
a. Workability of concrete is raised.
b. Impact of freezing and thawing is decreased significantly.
c. Bleeding, segregation and laitance in concrete are also minimized.
d. Entrained air makes the strength of concrete better against sulphate attack.
e. Minimizes the chance of shrinkage and crack development in the concrete surface.
Drawbacks - Given below, some of the drawbacks of air entrained concrete:
1. The strength of concrete is reduced.
2. The application of air entraining agent enhances the porosity of concrete and as a result the unit weight is decreased.
3. Air-entrainment in concrete should not be accomplished if the site control is not proper. It occurs as the air entrained in a concrete fluctuates with the alteration in sand grading, errors in proportioning and workability of the mix and temperatures.