Air-entraining cements and admixtures are primarily used for what purpose?

Air-entraining cements and admixtures are utilized mainly to enhance the durability of concrete in freezing and thawing environments. When air-entraining agents are introduced into the concrete mix, they create microscopic air bubbles within the cement paste. These tiny air pockets serve as pressure relief mechanisms, allowing water that expands upon freezing to have a space to occupy without causing damage to the concrete structure. This is particularly important in climates where freeze-thaw cycles are prevalent, as the expansion of freezing water can lead to cracking, spalling, and overall degradation of the concrete.

Improving workability is an effect seen with some admixtures but is not the primary purpose of air-entraining agents. Similarly, reducing cost and increasing strength are not direct benefits of air-entrainment; while there may be associated economic impacts or strength considerations in specific contexts, the immediate focus of air-entraining agents is on enhancing freeze-thaw resistance. Therefore, choosing the option related to resisting freezing and thawing correctly identifies the main function of air-entraining cements and admixtures in concrete applications.