What will happen if the air content of fresh concrete increases by 1 percent?

When the air content of fresh concrete increases by 1 percent, it generally leads to a decrease in compressive strength. The correct understanding here is that additional air within the concrete mix creates voids that reduce the overall density and structural integrity of the hardened concrete. This reduction in density and strength can typically result in a decrease of about 3 to 5 percent in compressive strength. This relationship is crucial for ensuring that concrete mixtures perform adequately for their intended structural applications.

Variations in air content can significantly affect the physical properties of concrete. While higher air content can improve certain characteristics, like workability and potentially some aspects of freeze-thaw durability in specific contexts, it adversely impacts the mix's structural performance. Therefore, while air entrainment is often utilized to enhance certain features, it needs to be balanced against the need for strength in the applications of reinforced concrete.

The other options do not correctly reflect the impact of increased air content on concrete. For instance, increasing air content will not increase density, and water demand would generally increase due to the need for additional water to maintain workability and ensure proper mixing. Similarly, while some durability aspects can benefit from controlled air content, excessive voids do not necessarily improve overall durability.