## 6 Conclusions  
Results of the initial experimental and theoretical investigation on the axial compressive behavior of reinforced concrete columns strengthened with steel tube jacket and fill  
Figure 21: Comparison of axial compressive strengths ratios of the predicted and the test results within the experimental range 0.344–0.586 of \(\pounds\).  
Figure 20: Comparison of the plastic sectional capacity between predicted and test results.  
concrete are presented in this paper. The results of the experimental research greatly depend on the prior assumptions made before the design and creation of test samples, which imply that the reached conclusions refer to those defined boundaries. Based on the above results and within the chosen test parameters range, the following conclusions are drawn:  
(1) The strengthened column models had a ductile behavior and showed larger deformation capacity compared to plain RC column models. The failure of the strengthened columns was a result of the local buckling of the steel jacket in an outward direction and the crush of the basic RC column concrete. In the specimens with a higher confinement factor, the failure with the shear plane and diagonal cracks in fill concrete was observed.  
(2) The strain analysis revealed that the steel tube and the infill concrete are engaged for load-carrying, even if the load is only applied onto the core RC column. The load distribution on all three parts of the cross-section is achieved over the bond layer through the friction of adjacent materials. Strains observed on the core RC column and the steel tube are almost identical in the domain of exploitation loads, which implies that the composite action of the cross-section is preserved. Moreover, in the range of exploitation loads, hoop stress effects were registered and observed, i.e., biaxial state of stress formed in the steel jacket and triaxial state of stress formed within the concrete core.  
(3) The axial force versus axial deformation relationships for the strengthened column can be separated by three distinctive points: steel tube yielding, inner core column crushing, and ultimate load-bearing capacity. The shape of the force-deformation curve is directly related to the specimen's behavior and to the compressive strength of the fill concrete.  
(4) The compressive axial load-bearing capacity of a strengthened column increases almost linearly with the increase of the fill concrete compressive strength. The ultimate strength of the RC columns strengthened by the steel jacket is improved by 2.7 to 3.2 times, relative to the ultimate strength of the plain nonstrengthened RC column. The compressive strength of the fill concrete has a relatively low impact on the increased column's strength.  
(5) All three considered codes give very good predictions of the compressive axial strengths of strengthened columns. Eurocode 4 predictions have proven to be the best; therefore, it could be recommended for the design strength calculation, but the effective compressive strength of high-strength concrete must be used with the reduced value.  
Conceptualization, M.B. and A.L.; methodology, M.B. and A.L.; software, A.L.; validation, M.B. and A.L.; formal analysis, A.L.; investigation, A.L.; resources, A.L.; data curation, A.L.; writing--original draft preparation, A.L.; writing--review and editing, A.L.; visualization, A.L.; supervision, M.B.; project administration, M.B.; funding acquisition, A.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data sharing is not applicable to this article.  
For the support in the realization of the experimental research, the authors are thankful to the colleagues from the Laboratory for Material and Structure Testing at the Faculty of Civil Engineering Subotica, University of Novi Sad, Republic of Serbia.  
The authors declare no conflict of interest.