How does increased air entrainment affect the compressive strength of concrete?

Increased air entrainment generally reduces the compressive strength of concrete. Air entrainment is the intentional incorporation of microscopic air bubbles into the concrete mix. These air bubbles, typically ranging in size from 10 to 1000 micrometers, improve the concrete's resistance to freeze-thaw damage. However, the introduction of air bubbles also reduces the amount of solid material (cement, aggregate) per unit volume of concrete. Since these solid materials are the primary contributors to compressive strength, the reduction in their volume leads to a decrease in strength. For each 1% increase in air content, the compressive strength can decrease by approximately 5%. The reduction in strength is more pronounced at higher air contents. The air voids created by air entrainment are essentially empty spaces within the concrete matrix, and they do not contribute to the load-carrying capacity. While air entrainment reduces compressive strength, it significantly improves the concrete's durability in cold climates. The air bubbles provide space for water to expand during freezing, relieving internal pressures and preventing damage to the concrete microstructure. Therefore, the amount of air entrainment is carefully controlled to balance the need for freeze-thaw resistance with the desired compressive strength. For example, concrete used in pavements in cold regions typically has an air content of 6% to 8%, which provides adequate freeze-thaw protection while still maintaining acceptable strength.