Local surface rust staining
Localised rust coloured stains on the concrete surface may be caused by small amounts of contaminants in the aggregate, such as pyrites. These can be avoided by careful selection of the aggregate and control of its production and storage prior to use. Similarly local stains can be caused by pieces of tying wire left in the formwork prior to casting the concrete. In both cases this is a cosmetic problem only and will not affect the long-term performance of the concrete structure.
A more general pattern of staining may appear on the soffit of a slab. This will be caused by rust stains from reinforcement, or other contamination, forming on the soffit formwork and being transferred into the surface layer of the concrete. Again this will not affect the long-term performance of the concrete.