Entrained air bubbles are introduced into concrete for what primary benefit?

Entrained air bubbles are introduced into concrete primarily to enhance its freeze-thaw durability. This technique helps prevent damage that can occur from the expansion of frozen water within the concrete. By incorporating air voids, the concrete can accommodate the pressure caused by freeze-thaw cycles without cracking or spalling. The small, well-distributed air bubbles provide space for water to expand when it freezes, which reduces internal stress and increases the longevity and performance of the concrete in cold climates.

The other options do not accurately capture the primary reason for incorporating entrained air. Increasing density, for instance, is not a goal of air entrainment; rather, the addition of air bubbles decreases the overall density of the concrete. A lower water-cement ratio can improve strength but is not directly related to the introduction of air. Additionally, installing reinforcements pertains to enhancing structural integrity, while air entrainment is solely focused on improving durability against environmental factors.