## 3 Experimental Results and Specim Behavior
### Strengthened Columns  
Testing of all strengthened models has been conducted according to similar experiment program phases for control group models to ensure that results could be directly compared. In that way, a direct assessment of the success of the method for strengthening RC columns with a steel tube jacket was possible. In addition, it was possible to cross-compare strengthened models, so the influence of different quality of fill concrete was evaluated. Figures 8-10 show a diagram of the dependence of compression axial force to the longitudinal and to the lateral strains, and diagrams of force to axial shortening for strengthened specimens are presented. The labels Concrete A, Steel A, and Steel L on diagrams mark the directions of main strains on the vertical column. Notation A means axial, and L means lateral. The steel yield strain limits (\(\varepsilon_{y}\)) are also shown. From the presented diagrams, it can be seen that, in the domain of exploitation loads or for forces up to 50% of crushing load, the behavior of all parts of the composite cross-section is linear. Pronounced nonlinear increase of deformations in relation to increasing of the loading was registered for forces above 80% of the crushing load. From diagrams, it can also be noticed that longitudinal strains on the steel tube (Steel A) and on the RC column (Concrete A) are equal for forces from 25% to 40% of ultimate force, and therefore, the full composite action is preserved in that load range. At higher stress states, different values of strains in concrete and steel yielded a conclusion that some slippage between parts of the composite cross-section parts had occurred. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Control Group** & **Fill 1** & **Fill 2** & **Fill 3** \\ \cline{2-5}  & **S0x** & **B2x** & **B4x** & **P0x** \\ \hline N\({}_{\text{u,exp}}\) (kN) & 592 & 1600 & 1709.3 & 1864 \\ S\({}_{\text{n}}\) (kN) & 36.7 & 5.66 & 38.02 & 11.3 \\ COV (\%) & 6.2 & 0.35 & 2.2 & 0.6 \\ ks (\(-\)) & – & 2.7 & 2.9 & 3.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Ultimate load forces of column models. Figure 7: Column models after the experiment. Figure 8: Measured axial load versus strains (**a**) and load versus deformation (**b**) relationships of the first group of strengthened specimens (B2x series). Figure 10: Measured axial load versus strains (**a**) and load versus deformation (**b**) relationships of the third group of strengthened specimens (P0x series). All strengthened specimens showed similar behavior; therefore, for one of them, the characteristic states are presented in Figure 11. It can be observed that the stress-strain curves showed a linear increase of deformations from the beginning of loading until the steel yielding started. Axial strains in strengthened specimens reached the steel yield strain before the specimen attained its ultimate strength, which indicates that the steel strength was fully utilized. The elastic steel limit was reached at around 1 mm of specimen shortening. That state was marked with point A. Further increase of the load caused stretching of the steel jacket, which had a consequence on the bonds between the steel tube, the concrete fill, and the basic RC column. Subsequently, the steel jacket could not provide efficient confinement to the fill concrete, which resulted in the crushing of the basic RC column, and the load-bearing steel plates started to push down into the concrete. When the load-bearing plates were fully embedded in the concrete, the top and bottom hydraulic press steel plates came into contact with the specimen. At that time, the load was applied to the column's full cross-section, which is marked with point B. The ultimate load on the specimen is marked with point C. After the load was applied to the whole cross-section, the specimen was only able to take a relatively small increase of the force and had great development of deformation, before reaching its ultimate load (range from point B to point C). Figure 12 shows the end plane of strengthened specimens.