What is the difference between entrained air bubbles and entrapped air bubbles?

The distinction between entrained air bubbles and entrapped air bubbles is significant in concrete technology and impacts the performance, durability, and workability of concrete mixtures.

Entrained air bubbles are intentionally introduced into the concrete mix through the use of air-entraining agents during batching. This process creates small, stable air bubbles that serve to improve freeze-thaw resistance, increase workability, and reduce the risk of cracking due to expansion from freezing water. The presence of these deliberately created air pockets helps provide a cushion for the water in the mixture, allowing for volume changes without significant damage to the concrete.

In contrast, entrapped air bubbles occur unintentionally, typically as a result of the mixing process where air becomes trapped within the mixture. These bubbles can be larger and more irregularly shaped compared to entrained air bubbles and do not provide the same benefits. Entrapped air is generally considered undesirable as it can adversely affect the strength and durability of the concrete.

Thus, understanding this distinction is critical for civil engineers when designing concrete mixtures, as the intentional incorporation of entrained air can lead to high-performance concrete, while unintended entrapped air complicates the structural integrity and performance of the final product.