The process which involves the introduction of tiny air bubbles into concrete is called air entrainment. And the concrete formed through this process is called air entrained concrete. Using air entraining Portland cement or air entraining agents such as admixture, air entrainment is done in concrete. The amount of air in such concrete is usually between four to seven percent of the volume of concrete. It is measured by galvanometric method, volumetric method and pressure method. The air bubbles relieve internal pressure on the concrete by providing chambers for water to expand when it freezes.
Here are the ways of incorporating air in concrete:
- Using gas forming materials as aluminium powder, zinc powder and hydrogen peroxide.
- Using surface active agents that reduces surface tension. They may be natural wood resins and their soaps, animal or vegetable fats or oils, alkali salts of sulfonated or sulphated organic compounds.
- Using cement dispersing agents.
Some of the advantages of air entrained concrete are given below:
- Workability of concrete increases.
- Use of air entraining agent reduces the effect of freezing and thawing.
- Bleeding, segregation and laitance in concrete reduces.
- Entrained air improves the sulphate resisting capacity of concrete.
- Reduces the possibility of shrinkage and crack formation in the concrete surface.
Some of the disadvantages of air entrained concrete are given below:
- The strength of concrete decreases.
- The use of air entraining agent increases the porosity of concrete thereby reducing the unit weight.
- Air-entrainment in concrete must not be done if the site control is not good. This is due to the fact that the air entrained in a concrete varies with the change in sand grading, errors in proportioning and workability of the mix and temperatures.