Entrapped and entrained air; air entraining agent

Entrapped air

Air voids that are unintentionally trapped in the concrete during mixing, handling and placing. The voids are large (above 1 mm), irregular in shape, not uniformly spread, and amount to 1-3 per cent of the volume (more in poorly compacted concrete). They reduce strength and should be removed by proper compaction.

Entrained air

Microscopic air bubbles (10-1000 µm, usually below 0.3 mm) that are intentionally introduced by an air-entraining agent. They are spherical, well-spaced and not connected to each other. The usual entrained air content is 3-6 per cent (about 4.5% for 20 mm aggregate) of the volume of the concrete.

Why an air-entraining agent (AEA) is used

• Improves resistance to freezing and thawing (frost damage) and de-icing salt scaling.
• Improves workability (acts like ball bearings), reduces bleeding and segregation.
• Reduces permeability somewhat and allows water reduction.
• Examples: Vinsol resin, wood rosin, fatty acid salts, synthetic detergents (IS 9103).

How it works

1. AEA is a surface-active chemical with a hydrophilic and a hydrophobic end. It lowers the surface tension of water.
2. During mixing, air is drawn in and forms stable, tiny bubbles, whose surface is coated by the agent so they do not coalesce.
3. These bubbles remain in the hardened paste as tiny voids. When water in the capillary pores freezes it expands by 9 per cent; the water is pushed into the nearby air bubbles, which act as pressure-relief chambers, so the paste does not crack.

Side effect: every 1% of air reduces compressive strength by about 4-6%, so the water-cement ratio is lowered to compensate.