Final answer:
Air entrainment significantly improves concrete's resistance to freeze-thaw cycles by incorporating air bubbles that absorb expansion stress caused by freezing water.
Step-by-step explanation:
The resistance of hardened concrete to freezing and thawing can be significantly improved by using air entrainment. Air entrainment involves the inclusion of air bubbles within the concrete mix, which can absorb the stress caused by the expansion of water when it freezes. This process provides room for the ice to expand without causing significant damage to the concrete. Adding more cement, sand, or water will not have the same beneficial effect on freeze-thaw durability as air entrainment does.