Curing is a crucial process in concrete testing that refers to the maintenance of adequate moisture and temperature to allow for proper hydration of the concrete. This stage is vital because it ensures that the cement particles within the concrete mix fully react with water, leading to the development of strength and durability. Proper curing helps in preventing issues such as surface cracking, which can occur if the concrete dries out too quickly or experiences temperature fluctuations.
Effective curing methods may include keeping the concrete damp, using curing compounds that form a moisture-retaining film, or covering the concrete with wet burlap or plastic sheets to retain moisture. The process must be sustained for a sufficient period, usually at least seven days for most concrete mixes, to achieve optimal strength and resistance to environmental factors.
In contrast, sealing the concrete primarily focuses on preventing moisture loss after curing has been established and is not the same as curing itself. Adding water to the concrete mix enhances workability but is done before the curing process and can adversely affect the final strength if done incorrectly. While hardening in extreme temperatures can affect curing negatively, it does not define the act of curing. Hence, maintaining the right moisture and temperature during the initial hydration phase is what makes option C the correct choice.