The primary reason for intentionally incorporating microscopically sized air bubbles in fresh concrete is to provide freeze-thaw durability to the hardened concrete. Typical concrete is a porous material with an interconnected network of capillary voids and channels that can absorb both fresh and saltwater during normal environmental exposure. At sufficiently low temperatures some or all of this absorbed water can freeze and expand, generating stresses that are high enough to crack, and in some cases disintegrate, the concrete. Incorporation of air bubbles in the fresh concrete produces air voids in the hardened concrete that act as pressure-relief zones into which ice and unfrozen water can expand during freezing. A typical cubic yard of air-entrained, freeze-thaw resistant concrete will contain about 10 billion air bubbles ranging in size from that of the coarse aggregate particles (10-20 mm) to that of the cement particles (5-10 μm).