They differ in many ways, but as reinforcement for concrete, these differences are especially notable. Polypropylene, nylon and the other synthetic fibers used in concrete serve only one purpose: to restrain plastic shrinkage during the first 24 hours after concrete is poured. Because of their high elongation, or "stretchiness", these fibers are essentially useless for the drying shrinkage and other sorts of cracking that all concrete suffers from. These cracks, in addition to plastic cracking, is what PVA is designed to handle. In that regard it is a structural fiber like steel and AR glass.