## 4 Test on Beams
### Failure Tests on the Retrofitted Beams
#### 4.3.2 Retrofitted Beam Failure Mechanism and Damage  
Each beam showed brittle failure with diagonal shear cracks and CFRP reinforcement debonding probably due to insufficient anchorages of the new bottom longitudinal rebars close to the beam supports H and R (Figure 8a,b). The anchorages of the new bottom rebars used to improve the beam flexural strength at the midspan, were interrupted close to the beam supports in the case of the beam DB1-R and at the supports for the beam DB2-R. This interruption is not correct because [PERSON]'s truss physical model requires the contribution of continuous bottom rebars along the beam length with efficient anchorages at beam ends. The beam failure mechanism was triggered by concrete cracks at beam intrados close to the support H (Figure 8a,b). These cracks were due probably to the slip of the new bottom rebars at anchorages or to detachment of the new bottom concrete parts (the RC slab built to insert the new bottom rebars) not connected properly to the beam by means of connectors (Figure 4a). The extension of these cracks caused the detachment of the new concrete cover and the CFRP reinforcement debonding from the beam sides. Both strengthened beams collapsed for shear in correspondence to a large diagonal crack and the failure occurred with a complete debonding of the U-strips (Figure 8a,b) at about the same load (F \(\approx\) 425.5 \(\sim\) 427.5 kN). Since the beam spans and the bottom reinforcement area at the midspan are different in both beams (Figure 2), different capacities are expected if a flexure failure happens. However, the shear failure was observed at the end of the experimental tests and therefore the beam span and longitudinal reinforcement did not affect the peak force. Figure 8: Damage on the beam DB1-R (**a**) and DB2-R (**b**) at support H after failure tests (back view). Figure 7: Retrofitted beams: load-deflection responses along the beams DB1-R (**a**) and DB2-R (**b**). The failure of the beam DB1-R started from the hinge support H (Figure 8a) where a wide diagonal crack crossed the closer U-strip. The experimental behavior of this beam can be described along the load-deflection curve showed in Figure 7a. Along the initial branch of the curve (until point 1, Figure 7a), the DB1-R presented about 12% greater stiffness than the DB1 beam before retrofitting based on the measured midspan deflection: the mean deflection values for F = 92 kN were 0.66 mm and 0.74 mm for the beam DB1-R and DB1. Around the vertical load value of 370 kN (point 2, Figure 7a), the U-strips on the left of the midspan close to the support H started to debond locally. When the vertical load increased from 400 kN to 425 kN, the strips debonding became more evident proceeding until failure (point 3, Figure 7a). Deflection at the midspan increased and diagonal shear cracks, already wide at 400 kN, continued to grow becoming very large. A noticeable difference between the diagonal displacement along the U-strips near the hinge support (H, Figure 5b) and the ones near the roller support (R, Figure 5b) was evident with the displacement values close to the support H 4-5 times greater than those near the support R. When the beam was loaded up to failure, the beam shear collapse mechanism concentrated near the hinge support H. After the maximum vertical load, the beam behavior was characterized by decreasing of the load-carrying capacity due to the subsequent debonding of the remaining U-strips (point 4-7, Figure 7a) starting from the ones closer to the H support. The loss of the beam carry capacity was sudden along the load-deflection curves at each U-strips debonding from the maximum load 425 kN to the ultimate load 310 kN (point 7, Figure 7a). The rupture sequence involved four strips within a couple of seconds: each sudden fall of force along the curve (Figure 7a) represents the detachment of a strip. When the load is about 310 kN, all the U-strips broke, and the diagonal shear cracks became larger and larger until the beam failure occurred (point 7, Figure 7a).