## 5 Conclusions  
The presented study has confirmed that the initial loading has a significant influence on load-carrying capacity as well as deformability and stiffness of beams strengthened using composite materials. The initial state of RC beams should not be overlooked when analyzing the effectiveness of this type of strengthening. The observations and results of experiments and FEA results became the basis for the formulation of the following general conclusions:  
1. The increase in the load-carrying capacity of strengthened beams without preloading in comparison with reference beams with no CFRP strip was 30%, and 24% for beams relieved before strengthening to the level of their deadweight. 2. The reductions in the deflection at failure in relation to the deflection of the RC beam with no strip were 33% for beams \"reinforced\" with CFRP strips before the beginning of load application, 37% for fully relieved beams (neglecting the initial deflection of the beam), and 32% and 31%, respectively, for initially preloaded beams that were previously loaded to the design load-carrying capacity level of a beam without strengthening and overloaded by 25% in relation to the design load-carrying capacity of a beam with no CFRP strip. 3. The preload on the beam before strengthening affects the utilization rate of the composite material. The maximum deformation of the composite strip during the failure of the beams ranged from 4.58% for the beam loaded before strengthening to the level of the design load-carrying capacity of the reference beam to 6.45% for the beam unloaded before strengthening. 4. The beginning of the yielding process of the reinforcing steel for strengthened beams was observed later than for beams without strengthening. The yielding of the reinforcing steel for unloaded beams before strengthening occurred for a load 20% higher than for beams without strengthening, whereas for beams overloaded before strengthening by 25% above the design capacity, it was only 8%. 5. The curvature and stiffness of the beams depend on the load level at which the CFRP strip strengthening is realized. Strengthening of unloaded beams has a significant impact on the increase in the stiffness of this beam only from the design load capacity of the beam without the strip. From this level, a much smaller increase in the curvature of the strengthened beam is visible in relation to the beam without strengthening and a significant increase in its stiffness. 6. Failure of the tested beams was rapid and was not signaled early enough. The destruction was preceded by the delamination of the strip from the concrete surface. Based on the state of deformation of all beams, the initiation of failure took place in the zone of the constant bending moment, in which cracks began to intensively open due to the rapidly increasing strains of the reinforcing steel. \begin{table} \begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**Experiment**} & \multicolumn{2}{c}{**FEM Analysis**} \\ \cline{3-6}
**No.** & **Beam type** & **Stiffness B** & **Relative** & **Stiffness B** & **Stiffness B** \\  & & **[kNm\({}^{2}\)]** & **[-]** & **[-]** & **[kNm\({}^{2}\)]** & **[-]** \\ \hline
1 & Br & 3692.4 & 1 & 2746.9 & 1 \\
2 & Bz & 3840.7 & 1.04 & 3236.8 & 1.18 \\
3 & Boow—Stage 1 + 2 & 3666.21 & 0.99 & 2875.31 & 1.05 \\  & Boow—Stage 2 & 4483.62 & 1.21 & - & - \\
4 & Bow & 3464.7 & 0.94 & 2732.6 & 0.99 \\
5 & Bopw & 3333.9 & 0.90 & 2233.8 & 0.81 \\ \hline \hline \end{tabular}   
[FOOTNOTE:*]Footnote *: footnotemark:  
Strengthening structures with composite materials is particularly effective in the case of structures that were overloaded during their operation.