Most specifications call for cleaning the rust off reinforcing steel before patching a deteriorated structure. How much cross-sectional area of the rebar can you afford to lose before you jeopardize the strength of the structure?
The Technical Guidelines Committee of the International Association of Concrete Repair Specialists (IACRS) addresses this issue in the "Surface Preparation Guidelines for the Repair of Deteriorated Concrete Resulting from Reinforcing Steel Oxidation." According to IACRS, if a bar has lost more than 25% of its cross section, or if two or more consecutive parallel bars have lost more than 20% of their cross section, a structural engineer should be consulted to determine if repairs to the reinforcing steel are required. If repairs are required, the bar should be replaced or a new bar should be mechanically spliced to the old bar or placed parallel to and approximately 3_4 inch from the old bar.