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Air entrainment in concrete typically refers to intentionally incorporating tiny air bubbles into the mix. While this has several benefits, one notable negative effect is that it can lead to decreased bonding between concrete and steel reinforcement.

The reason for this decrease in bonding is that the presence of air bubbles creates voids within the concrete, which can act as barriers between the concrete matrix and the steel. These voids interfere with the mechanical interlocking and adhesive bonding that occurs at the interface where concrete meets steel. A strong bond between these materials is crucial for the overall structural integrity of reinforced concrete; weak bonding can lead to issues such as corrosion of the steel reinforcement and reduced load-bearing capacity.

In contrast, the other options provided highlight beneficial aspects of air entrainment. Increased workability refers to how well the concrete can be mixed, placed, and finished. Reducing shrinkage and improved freeze-thaw resistance are also advantages of incorporating air into the concrete, as entrained air helps accommodate volume changes and enhances durability during freeze-thaw cycles.