Which of the following fibers is commonly used to increase impact resistance in concrete?

Polypropylene fibers are commonly used to increase impact resistance in concrete due to their unique properties. When added to concrete mixes, polypropylene fibers help to enhance the overall toughness and durability of the material. They do this by creating a matrix within the concrete that helps to hold the aggregate together, thereby reducing cracking and controlling shrinkage as the concrete cures.

The ability of polypropylene fibers to absorb and disperse energy also improves the concrete's resistance to impact loads. This makes it particularly beneficial in applications where the concrete may be subject to sudden impacts or heavy loading, such as in industrial floors, pavements, or precast concrete products.

While nylon, glass, and acrylic fibers are utilized in different applications or for varying purposes in concrete and construction materials, they do not possess the same degree of effectiveness in enhancing impact resistance as polypropylene fibers do. Each of these materials has its own set of properties and uses, but for increasing impact resistance specifically, polypropylene is the material that stands out.