Air voids created by air-entraining agents have which trade-off?

• A. They improve freeze-thaw resistance but may reduce strength slightly.
• B. They have no effect on durability.
• C. They always increase strength significantly.
• D. They eliminate permeability.

Answer: (C)

Air voids from air-entraining agents act as tiny, well-dispersed pockets that give water somewhere to expand during freezing, so the concrete’s internal pressures don’t crack as easily. That boosts freeze-thaw durability. But those voids also replace some solid material in the matrix, so the concrete’s compressive strength can drop a bit. The aim is to gain durability with only a small loss in strength, and the magnitude depends on how much air is introduced and how the mix is designed. The other statements aren’t accurate: the durability is not unchanged, strength isn’t always increased, and permeability isn’t eliminated by these voids (it’s altered, not erased).