What is the purpose of air-entraining agents in concrete, and how do they affect durability?

• A. They create microscopic air voids to improve freeze-thaw resistance; may reduce strength slightly.
• B. They accelerate curing.
• C. They increase slump significantly.
• D. They increase cement content.

Answer: (A)

Air-entraining agents are added to concrete to create a network of microscopic air voids throughout the hardened matrix. These tiny voids act as little cushions that absorb the pressure when water within the concrete freezes, so cracks don’t form or propagate as the ice expands. That gives the concrete much better durability in environments with freeze–thaw cycles and deicing salts. The trade-off is a small reduction in strength and density because some solid volume is replaced by air, but with proper dosage the durability benefits far outweigh the strength loss. They don’t speed up curing, they don’t significantly increase slump, and they don’t raise cement content; their main purpose is to introduce controlled air voids to enhance freeze-thaw durability.