## 3 Results and Discussion
### Beam Failure Modes  
Figure 16 shows the failure mode of the control beam. The failure of the beam is caused by the formation of a plastic hinge, i.e., by reaching the ultimate strain of steel reinforcement during tensioning, first in the cross-section above the middle support. By reaching the ultimate strain of the steel reinforcement during tension in the cross section in the span a plastic hinge is formed and thus the failure mechanism of the beam. The maximum realized force was 100 kN, after which the deformations increase without increasing the force. In the case of the beam girder B-G1, which is reinforced with a GFRP bar above the middle support, the cross section in the span fails first, reaching the ultimate strain of the steel reinforcement, which is followed by concrete crushing (Figure 17)--by forming a plastic joint in that cross section. The formation of the failure mechanism occurs due to crushing of concrete in the cross-section over the middle support, without debonding of the FRP reinforcement system, which indicates the complete utilization of this system. The load-bearing capacity of this beam is 122 kN, and exceptional deformability and failure without major damage to the beam were observed. Figure 15: Load vs. microstrain curves for concrete for the section: (**a**) At the middle of span—Section I; (**b**) Above the middle support—Section III. In Figure 18 the shape of the failure of the beam strengthened with GFRP reinforcement in the zone of positive moments (bottom side of the beam)--beam B-G2--is shown. A failure of the section above the middle support was first observed due to reaching the limit strain of the steel reinforcement during tensioning (Figure 18a). The formation of the failure mechanism occurs due to the failure of the cross section in the span, caused by separation of the concrete at the level of the longitudinal steel reinforcement (Figure 18b). Crushing of concrete was observed in the zone of force application (Figure 18b). Transverse cracks in the concrete on the bottom side of the beam intersect the slot with the epoxy filling. The load bearing capacity of this beam is 148 kN, the deformability is good and the failure occurs with damage to the beam (Figure 18b). Beam fracture B-G3 is caused by debonding of epoxy and concrete, as a result of which concrete separates at the level of the protective layer (Figure 19a). The load-bearing capacity of this beam is high (F\({}_{\text{u}}\) = 167 kN), but the failure is sudden with a loud crack and with a considerable damage to the beam. Crushing of concrete was observed in the  
Figure 16: Control beam: (**a**) Failure of the cross-section at mid-span of control beam; (**b**) Failure of the cross-section above the support of control beam. Figure 17: Beam B-G1: (**a**) Disposition of cracks and failure of the cross-section at mid-span; (**b**) Disposition of cracks and failure of the cross section above the support. zone of force application as well as transverse cracks in concrete both on the bottom and on the top side of the beams intersecting the groove with the epoxy infill (Figure 19b). Deformability is lower compared to other beams in which strengthening was accomplished using GFRP reinforcement. Figure 20 show the failure shape of a beam strengthened with CFRP reinforcement in the zone of negative moments above the middle support (top side of the beam) and in the zone of positive moments (bottom side of the beam) caused by incremental application of test load until failure. It is observed that the failure occurs by debonding of epoxy and concrete, as a result of which concrete separates at the level of the protective layer (Figure 20a). The load-bearing capacity of this beam is the highest (\(\mathrm{F_{u}}=182\) kN) in comparison with other tested beams, but the deformability is extremely low, so that failure occurs suddenly with considerable damage to the beam.