Nowadays reinforced concrete bending elements are strengthened most often using post-tensioning technology with unbonded steel tendons or CFRP materials. Type of material used to strengthen of the building structure is selected based on the real conditions of structure exploitation as well as structural solution, accessibility to element and anticipated strengthening effectiveness. Values of post-tensioning forces are known based on measured jack pressure, concrete strains in selected cross sections and girder deflection. In case of strengthening the cracked elements, the cracks should be injected with epoxy resin before start to strengthen. The method of injection should be suitable to values of crack widths. In case of many cracks, some of them (small cracks) are unfilled. It can be supposed their presence have influence for strengthening effectiveness evaluation of reinforced concrete elements.