What characterizes entrained air in cementitious mixtures?

Entrained air in cementitious mixtures is characterized primarily by the presence of tiny air voids, which help improve the workability and durability of the concrete. These air voids are usually very small, typically measuring around 1 millimeter or less in diameter. The primary function of these entrained air voids is to provide space for the expansion of water when it freezes, thus minimizing the risk of damage known as freeze-thaw failure.

The process of creating these small air voids involves the use of air-entraining agents, which lead to a stable network of microscopic air pockets throughout the concrete matrix. This enhances the performance of the concrete by making it more resistant to shattering, spalling, and other forms of deterioration that can occur in extreme weather conditions.

While factors such as the water content of the mix, aggregate size distribution, and larger air voids do play roles in the overall concrete performance, they do not specifically define the characteristic of entrained air, which is defined by the size and distribution of the air voids themselves.