**4. Conclusions**

The determination of the stiffness of columns strengthened with composite materials (PBO mesh on mineral mortar in the considered case) is a difficult and complicated task. One cannot adopt, a priori, the standard regulations dedicated to reinforced concrete or composite elements to determine

the sti ffness of such columns. The aim of the investigations presented in this paper was to assess the e ffect of the PBO–FRCM reinforcement of eccentrically compressed columns on their sti ffness depending on the level of stress intensity.

In the case of unstrengthened columns, the expected results were measured. As eccentricity increased, the longitudinal ultimate force decreased and the deflection of the columns increased. As the curvature of the columns increased, so did the bending moment at half of their height. Each of the columns failed at a correspondingly lower intensity of the stress in the whole cross-section, which was produced by applying longitudinal force on the more compressed side.

In the case of columns C\_1H and C\_2H, which were strengthened only transversely, our measurements and evidence collected point to the fact that this kind of reinforcement increases their load capacity, but at the same time, the sti ffness of the columns increases due to the confinement of the cross-section. In comparison with the reference (unstrengthened) specimens, the curvature would decrease at the initial eccentricities after the first and second PBO layers were laid. This was accompanied by an increase in the ultimate force and a decrease in the horizontal deflection of the columns. At eccentricities of 16 and 32 mm, the columns failed at correspondingly higher sti ffness values than the reference members, owing to the use of transverse composite reinforcement. Whereas in the case of columns C\_1H\_0 and C\_2H\_0, their load capacity was reached at lower sti ffness values, which indicates considerable destruction of the concrete core under axial compression.

In the considered case of quadrangular columns, the tri-axial state of stress induced by the confinement of the concrete is insignificant and the addition of another layer of transverse composite reinforcement only increases the sti ffness of the composite jacket.

Unfortunately, in the case of columns C\_1V1H and C\_1V2H, their higher ductility and greater horizontal deflectability in the bending plane do not directly translate into an increase in their load capacity in comparison with the columns without longitudinal composite reinforcement. As shown in the earlier research, longitudinal PBO–FRCM reinforcement improves the ductility of eccentrically compressed columns. The presence of this reinforcement, however, reduces longitudinal deformations at which the mesh ruptures, which is disadvantageous. Thanks to the presence of longitudinal PBO reinforcement, the columns failed at higher sti ffness values in the whole range of the eccentricities: 0, 16 and 32 mm.

This paper investigated the e ffect of PBO–FRCM reinforcement on the sti ffness of eccentrically compressed RC columns. The dependencies between the change in the elasticity modulus of the concrete and the change in the sti ffness of the tested specimens were examined. This subject is of grea<sup>t</sup> practical relevance. Few such experiments are available in the literature. The analyses presented in this paper can be used to design a wider series of tests of PBO–FRCM columns with various types of reinforcement, which are subjected to eccentric compression. The results of such tests can be used to formulate a standard relation for a column sti ffness reduction coe fficient depending on the type of reinforcement, the longitudinal force and the cross-section's stress intensity.

**Author Contributions:** Conceptualization, T.T. and M.M.; methodology, T.T.; formal analysis, T.T. and M.M.; investigation, T.T. and M.M.; writing—original draft preparation, T.T.; writing—review and editing, M.M.; visualization, T.T. and M.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflicts of interest.
