Rust on reinforcing steel is often not a concern where it is created by fresh water, and study has shown that a small layer of rust may actually aid to enhance the connection with concrete. However, if you work with saltwater rebar, or old rebar that has been in concrete for a long time, then this metal will likely be contaminated with sodium chloride and other additives used in cement to make it more flexible when in water. In this case, the iron should be removed before using it in new concrete. This can be done by either cleaning it with wire wheel or grinding it off with a grinder. After removing any residue of concrete from the surface of the bar, it can be used in your next project. However, old rebar is usually discarded after being used in one construction site because it is very expensive to remove all the additives from it. Therefore, if you plan to reuse it, you should definitely clean it first so that you do not have any negative effects on your concrete workers.