To facilitate handling, what is commonly allowed to be added to concrete?

The use of admixtures in concrete is a common practice to enhance its properties and improve its workability. Admixtures are chemical substances that are added to the concrete mix before or during mixing. They can serve various purposes, such as improving the flowability of the mixture, accelerating or retarding the setting time, reducing water content, and enhancing durability. Admixtures can also be used to modify other characteristics of the concrete, such as its strength, resistance to freezing and thawing, and overall performance in specific environmental conditions.

In contrast, while options like water, sand, and aggregate are essential components of concrete, they do not specifically address the enhancement for handling that admixtures provide. Water is primarily used to hydrate the cement and enable the concrete to achieve its required strength. Sand and aggregate are essential for creating the bulk of the concrete but do not have the same tailored effects that various admixtures offer. Therefore, adding an admixture is a targeted approach to improving the concrete's handling characteristics, making it the appropriate choice in this context.