Which of the following is a benefit of using synthetic fibers in concrete?

The use of synthetic fibers in concrete primarily enhances its properties, contributing to decreased permeability. When synthetic fibers are incorporated into concrete, they create a more uniform and dense matrix. This density reduces the size and number of voids within the concrete, leading to a lower permeability. Lower permeability is critical in concrete applications, as it helps to prevent the ingress of harmful substances, reduces the risk of freeze-thaw damage, and minimizes corrosion of embedded reinforcement, ultimately extending the lifespan of the structure.

The other options do not accurately reflect the benefits provided by synthetic fibers. For instance, while fibers can improve performance in specific contexts, they are not typically characterized by increased flexibility or higher thermal expansion. In fact, synthetic fibers can increase impact resistance rather than reduce it, contributing to the overall durability and performance of concrete under stress.