Firstly remove all the weak concrete and get information about all reinforcement. Then remove all rust from steel using rust remover and rust preventers.
If previous steel corroded, then provide anchorage, i.e., Shear connectors. Use tack coat or adhesion coat to bond the old and fresh concrete. Normally, polymer and epoxy resin-based material is used.
One patching method is useful for reforming concrete to the actual surface level. Polymer is a suitable mortar because it is useful with or without accumulating.
In cracks, pores or honeycombing can be filled with cement slurry, polymer-modified slurry, or epoxy. After all, that process uses a proper protective coating.
The elementary verifying system should use for the structure. Remove all the weak concrete from the structure. Then wash the surface and remove rust from the corroded steel using rust removers and rust preventers.
According to the need, add extra steel around the structure and formwork. Add a polymer-based adhesion coat between aged and new concrete surfaces. For workability, add the admixture in concrete and position it with the actual thickness.