Final answer:
Air-entrainment is an example of an admixture in concrete, used to improve workability and resistance to freeze-thaw damage.
Step-by-step explanation:
In concrete, an admixture is a substance that is added to the mixture to improve specific characteristics of the concrete. One example of an admixture in concrete is air-entrainment. It is added to the concrete to improve workability and resistance to damage caused by freeze-thaw cycles.