## 4 Test on Beams
### Failure Tests on the Retrofitted Beams
#### 4.3.3 Experimental Strains of the CFRP Reinforcement  
Strains of the CFRP U-strips were monitored during the beam failure test by means of three strain gauges at bottom, middle and top position along each U-strip side (Figure 9a). In Figure 9, the strains measured by the strain gauges on the diagonal CFRP strips FRP-U1, FRP-U2 and FRP-U3 (Figure 9b-d) near the support H of the DB1-R beam are plotted for five different values of the vertical load (F). These five load values are smaller than the maximum vertical load (F\({}_{\text{max}}\)) applied on the beam and are: F = 0.25F\({}_{\text{max}}\) (107 kN), the load that produced the beam flexural cracking (F =0.24F\({}_{\text{max}}\), on average from the literature); F = 0.50F\({}_{\text{max}}\) (212 kN); F = 2/3F\({}_{\text{max}}\) (285 kN), that usually corresponds to the onset of strips debonding [53; 54]; F = 0.85F\({}_{\text{max}}\) (360 kN) and F = 0.95F\({}_{\text{max}}\) (400 kN). The strains measured at F\({}_{\text{max}}\) (425 kN) are not included in Figure 9 because the strips detached from the beam surface and so the strain gauge measurement is not interesting at complete debonding of strips. The strains measured before complete debonding for F = 0.95F\({}_{\text{max}}\) were about equal to 0.3% (Figure 9b-d).  
Given the linear elastic behavior of CFRP fabric, CFRP stress can be obtained directly by the measured strain and the Young's modulus of the CFRP fabric (E\({}_{\text{f}}\) = 240 GPa). The stress value corresponding to the maximum strain is 720 MPa. The CFRP effective strength (f\({}_{\text{fed}}\)) that produces debonding of the CFRP reinforcement is 695 MPa by Equation (18) in CNR-DT 200 R1/2013 [66]. This stress is function of the debonding strength (f\({}_{\text{fed}}\)) calculated by Equation (23) in CNR-DT 200 R1/2013 [66] that is equal to 715 MPa assuming the tensile strength of the CFRP fabric equal to 2400 MPa, the mean cubic compressive strength of the restored concrete cover equal to 35 MPa and  
Figure 9: Retrofitted beam DB1-R: strips locations on the beam side (**a**); strains distribution along the diagonal CFRP U-strips FRP-U1 (**b**), FRP-U2 (**c**) and FRP-U3 (**d**).  
the unit value for the partial safety coefficients in [66]. The experimental CFRP stress at the strips debonding is very similar to the predicted value by the code equations [66].  
The behavior of the CFRP strips of the beam DB2-R was similar to the one observed for the beam DB1-R. The strains measured before complete debonding for F = 0.95F\({}_{\text{max}}\) were about equal to 0.2-0.3%, except for one U-strip close to the midspan that showed locally strain values of 0.3-0.4%. A modest strain of about 0.1% was measured on the CFRP fabric region close to the beam extrados at top and middle positions along the U-strips, where strips debonding due to concrete cracking started to propagate. The applied load was about 2/3 of F\({}_{\text{max}}\) and this behavior was already observed by [PERSON] et al. 2005 [53; 54].  
The CFRP strains values measured at the beam failure due to strip debonding were smaller than the maximum values given on the CFRP fabric datasheet or obtained by direct coupon tests on CFRP specimen.  
Furthermore, the experimental evidence showed that the maximum CFRP strain at strip debonding was smaller than the design one indicated in many design guidelines [66; 67; 68; 69]. Design guidelines limit the value of the maximum CFRP strain to assure the integrity of concrete and prevent aggregate interlock failure. The value of the maximum strain is 0.005 and 0.006 in CNR-DT 200 R1/2013 [66] and fib Bulletin 14 [67], and is 0.004 in ACI 440.2R-08 [68] and TR 55 [69].  
However, it is important to underline that the experimental measurement of the maximum FRP strain is difficult because strain gauges measurement gives the local value of strain. For that reason, the maximum strain can arise in a point along the strip, where strain is not measured during the test.  
For that reason, the maximum CFRP strain can be a few greater that the measured one.