Why is air entrainment important in concrete exposed to freeze-thaw cycles?

Air entrainment is crucial for concrete that will be exposed to freeze-thaw cycles because it introduces tiny air bubbles into the mix. These air bubbles create small voids within the concrete structure that serve as pressure relief zones. When water within the concrete freezes, it expands. If there are no air voids, this expansion can lead to cracking and spalling, as the internal pressure builds up. The presence of entrained air bubbles allows for this expansion to occur without causing damage to the concrete, significantly improving its durability and longevity under harsh conditions. Thus, incorporating air-entraining agents is essential for enhancing the freeze-thaw resistance of concrete.