At it's least harmful rust can mean stains on your concrete and at worst it can cause structural failure.
Mix designs meant to compensate for freeze/thaw conditions also increase potential for corrosion through oxidation. The same qualities of your mix meant to give water a place to expand in your concrete also allow that water to enter more easily. The presence of water and oxygen in the alkaline environment of cement are a recipe for reinforcement corrosion.
These fibers will not to rust unlike rebar or welded wire mesh and provide the structural reinforcement you need.