Complex sentence: Modern concrete reinforcing structures however, may contain non-steel materials with high tensile strength, and also may be permanently stressed before or after the mass sets, so as to improve the behaviour of the final structure under working loads.
Split into simpler sentences: Modern concrete reinforcing structures however, may instead or in addition contain non-steel materials with high tensile strength. They also may be permanently stressed before or after the mass sets, so as to improve the behaviour of the final structure under working loads.