What is the purpose of using admixtures in concrete?

The use of admixtures in concrete primarily serves the purpose of enhancing specific properties of the concrete mixture. Admixtures are ingredients added to the concrete mix during batching to modify its behavior and performance characteristics without changing the fundamental constituents of the concrete (cement, water, and aggregates).

For example, certain admixtures can improve the workability of the mixture, making it easier to place and finish, while others might increase the strength or durability of the hardened concrete. Some admixtures are designed to alter the setting time, allowing for better control over the construction schedule, especially in varying temperatures and humidity conditions. Additionally, specific admixtures can enhance resistance to water ingress, freeze-thaw cycles, or chemical attack, addressing various environmental challenges the concrete may face.

This targeted enhancement is why choice B accurately reflects the primary purpose of using admixtures, as opposed to merely reducing costs, improving aesthetics, or removing air bubbles. Those other options might be incidental effects or benefits of certain admixtures, but they do not capture the broader and more fundamental role that admixtures play in the performance optimization of concrete.