What impact does the addition of fiber to concrete mix primarily have?

The addition of fiber to a concrete mix primarily decreases plastic shrinkage. When concrete is mixed, particularly in its plastic state (when it is still wet), it is susceptible to shrinkage due to evaporation and other factors. This shrinkage can lead to cracking, compromising the integrity of the structure.

Fibers, such as polypropylene or fiberglass, help to control this shrinkage by providing additional tensile strength to the concrete. They distribute loads and stresses more evenly within the mix, which helps mitigate the development of fissures and cracks during the curing process. This is particularly important in environments where concrete may be exposed to rapid evaporation or extreme temperatures.

While the other options offer benefits in their own right, they are not the primary impact of adding fiber. For instance, adding fibers may not significantly impact drying time, and while some types of fibers can influence color, it is not a primary function. The thermal insulation property of concrete is also not fundamentally enhanced by the addition of fiber. Thus, recognizing the role of fibers in reducing plastic shrinkage highlights their importance in improving the durability and performance of concrete structures.