Florida DOT Concrete Batchman/Operator Practice Exam

What is a reason for slightly reducing the concrete's strength when introducing air?

1. The introduction of water

2. The creation of air voids

3. The delay in mixing

4. The use of additional admixtures

The correct answer is: The creation of air voids

When air is introduced into concrete, it typically creates air voids within the mix. These air voids serve to improve some properties of the concrete, particularly its resistance to freeze-thaw cycles and overall durability. However, the creation of these air voids also fills up some of the space that would otherwise be occupied by solid concrete material, which can slightly reduce the overall density of the concrete. This reduction in density can, in turn, lead to a decrease in compressive strength when tested, because the strength of concrete is generally proportional to its density. Therefore, while the air-entrained concrete exhibits enhanced durability in certain conditions, the trade-off is that the concrete's strength may be slightly compromised due to the presence of these air voids.