This paper focuses on a reinforced concrete corbel strengthening method using post-tensioning cables, or just using the prestressing strands themselves, which are led through additionally drilled cable ducts. The text describes a highly efficient method that has also other advantages besides efficiency, which are: the character of the structure of concrete columns with corbels does not change, the prestressing reinforcement is protected by the concrete both mechanically and against elevated temperatures, and it is easy to apply. Firstly, the paper briefly describes some of the other available methods. However, these methods usually utilize the surface of the concrete, which is often impaired. Additionally, the alternative methods cannot be used or are considered in practice to be insufficiently reliable in specific situations. The paper then describes the design, the actual implementation, and verification of the post-tensioning by measuring the concrete deformation on real columns and full-scale corbels. It also discusses the preliminary design using Strut-and-Tie method Other sections of the text describe the measurements on full-scale corbel specimens and the subsequent numerical analysis performed using advanced methods. The concordance of the results confirms the correctness of the implementation and the design of the described strengthening method.