Air- Entraining admixtures

Air- Entraining Admixtures

Air-entraining admixtures are used to purposely introduce and stabilize microscopic air bubbles (of predominately between 0.25—1 mm diameter) in concrete.

·         Air-entrainment will dramatically improve the durability of concrete exposed to cycles of freezing and thawing.

·         Entrained air greatly improves concrete’s resistance to surface scaling caused by chemical de-icers.

·         Furthermore, the workability of fresh concrete is improved significantly, and segregation and bleeding are reduced or eliminated.

Air entraining agent materials and mechanism in concrete

The main chemicals used in air-entraining admixtures are:

• – Natural wood resins

• – Animal and vegetable fats and oils such as tallow, olive oil and their fatty acids such as stearic and oleic acids

• – Various wetting agents such as alkali salts or sulphonated organic compounds

• – Water soluble soaps of resins acid

• – Miscellaneous materials such as sodium salts of petroleum sulphonic acids, hydrogen peroxide and aluminium powder, etc.

A typical dose of admixture is 0.0005 to 0.05% of active ingredient by weight of cement.

Dosing: The dosage rate for air—entraining admixtures may typically be as low as 0.6 ml per 1 kg cement and accurate dosing is therefore essential.

Temperature: An increase in temperature will reduce air content. A rise in temperature from 10 to 32°C may halve the amount of air entrained but normal day-to-day temperature fluctuations are much smaller and do not cause significant problems.

The air voids are entrained and stabilized when a surfactant is added to the cement-water system:

·         At the air-water interface the agents are oriented toward the water phase lowering the surface tension, promoting bubble formation.

·         At the solid-water interface the agents making the cement surface hydrophobic so that air can displace water and remain attached to the solid particles as bubbles.