Which of the following is NOT a type of fiber material used in concrete?

Cotton is not used as a type of fiber material in concrete. While fibers can enhance certain properties of concrete, such as tensile strength and crack resistance, the types of fiber most commonly utilized in concrete mix designs include polypropylene, steel, and carbon. These materials are selected for their durability, strength, and compatibility with concrete.

Polypropylene fibers help reduce plastic shrinkage cracking and can improve impact resistance. Steel fibers, on the other hand, provide significant reinforcement and can help manage cracking under stress. Carbon fibers, known for their lightweight and high strength-to-weight ratio, can also be used to enhance the mechanical properties of concrete.

Cotton, while it is a natural fiber, lacks the necessary characteristics that would make it beneficial in concrete applications, such as resistance to decay and sufficient tensile strength. Thus, it does not belong to the category of fibers typically used in concrete compositions.