What is the principal reason for air-entrainment in concrete?

• A. Improve surface finish
• B. Prevent freeze-thaw cracking the concrete
• C. Increase strength
• D. Reduce set time

Answer: (B)

Air-entrainment introduces a controlled network of microscopic air voids throughout the concrete. These tiny, well-distributed pockets provide space for water to expand as it freezes, so internal pressures don’t build up enough to crack the concrete during freeze–thaw cycles. In effect, the air voids act as pressure relief zones that keep the paste from cracking as ice forms inside the pore structure.

This is why air-entrained concrete is much more durable in climates where freezing and thawing occur repeatedly. While air-entrainment can influence workability and surface finish to some degree, its main purpose is to prevent freeze-thaw cracking. It doesn’t meaningfully accelerate or retard setting, nor is it primarily used to increase strength.