## 4 Analysis of the Test Results  
The reinforced concrete columns were strengthened with a steel jacket, while the compressive strength class of the fill concrete was varied in three types. Comparative relationships of load versus strains on test specimens are presented in Figure 14. Diagram of axial strains in the concrete of the core RC column for all three groups of strengthened models are presented in Figure 14. The strains of a plain RC model are also shown. The steel yield strain (\(\varepsilon_{y}\)) limits are marked with vertical dashed lines. The reinforced concrete column of the control group (S02) had nearly linear behavior up to failure load. In composite models, for all three model groups, the longitudinal strains in concrete for the last load phase are much larger compared to specimens of the control group. That behavior was expected because of the greater ductility of the composite cross-sections. Column models filled with regular fine-grain concrete (B21 and B41) showed almost identical behavior through all load phases, and they had very close ultimate strains. The specimens filled with special high-quality concrete (P01) showed much stiffer behavior and had lower ultimate deformation compared to the other two strengthen group models. The ultimate  
Figure 12: Failure modes of strengthened columns at the top end plane. Cracks in specimen P01 (**a**), embedded top bearing plate (**b**), and removed top bearing plate (**c**). Figure 13: Strengthened column models after the experiment. strains were almost half as low, which can be explained by properties of high-strength concrete, especially a lesser ductility behavior and a sudden loss of load-bearing capacity. The main difference between square and circular columns, besides greater strength, was ductility. The strengthened column had a large deformation capacity and the ability to restrain the pronounced development of cracks in concrete. Similar behavior was observed on columns strengthened by carbon fiber-reinforced polymer jacket [29] and on columns retrofitted with a steel jacket [26; 27; 28]. Figure 14b gives the comparative relationships of axial force versus axial and lateral strain relationships of steel jacket in strengthened specimens. All models showed almost linear behavior until reaching yield stresses in steel. The behavior of specimens beyond the elastic stage mainly depends on the fill concrete compressive strength. In the exploitation load domain, which corresponds to forces up to around 800-900 kN (or 0.5 Nu), it can be seen that the difference between all strengthened models is relatively low. The relationships of a measured axial force (N) versus axial deformation (\(\Delta\)) for a typical specimen from all of the groups are shown in Figure 15. As from previous diagrams, it can be noticed that strengthened specimens filled with regular-strength concrete had greater deformation and ductility compared to strengthened models filled with high-strength concrete. The characteristic points noted in Figure 15 are marked by Ai, Bi, and Ci, where \(i=1\), 2, 3. Points are A--steel tube yielding; B--inner core column crushing; and C--load-bearing capacity, the same as in Figure 11.  
Figure 14: Comparative relationships of load versus strains on test specimens. Registered strains on concrete (**a**) and on steel jacket (**b**). Figure 15: Comparative relationships of load versus deformation of specimens. The mean value of ultimate load forces for models of the control group and three groups of strengthened models are presented in Table 4. In addition, the standard deviation of results (S\({}_{n}\)) and the coefficient of variation of results (COV) are presented. The specimens that suffered a welding fracture were excluded from statistical analysis. The coefficient of enhanced bearing capacity of a strengthened column (\(ks\)) compared to the column without strengthening is also shown.