When is an air entrained admixture added to a batch of concrete?

An air-entraining admixture is added to a batch of concrete primarily with the water. This is because the admixture needs to be well-dispersed throughout the concrete mix to effectively introduce air bubbles in the correct formation and distribution. Adding it with the water ensures that it can mix thoroughly, allowing the air bubbles to form uniformly during the mixing process.

Incorporating air-entraining admixtures helps improve the concrete's resistance to freeze-thaw cycles and increases its workability, contributing to overall performance and durability. This precise timing in the addition of the admixture is crucial for the desired enhancement characteristics of the concrete.

The other options do not provide the optimal conditions for dispersing the air-entraining agent effectively. For example, adding it before mixing cement could result in incomplete integration, while mixing it with the aggregates may lead to uneven distribution. Adding it after hydration would be ineffective, as the concrete would already be setting and the desired properties would not be achieved.