PSI - Issue 64

Muhammad Ishfaq et al. / Procedia Structural Integrity 64 (2024) 1540–1548 Ishfaq et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Group C (#4@200) specimens were flexure-controlled, and the strain levels in flexural CFRP sheets exceeded the ACI 440.2-prescribed debonding strain from 32% to 118%. The specimen strengthened with no U-wraps (0U) failed due to IC debonding at a strain level 32% greater than the ACI 440.2 predicted strain value, confirming, in this case, that ACI 440.2 underestimates the debonding strain. The specimens strengthened with U-wraps (1U and 2U) were able to further increase the strain utilization in the longitudinal CFRP sheet beyond the material design strain level (9000 µε), resulting in an increased 118% compared to the ACI 440 .2-prescribed debonding strain. It is noted that in practice, strain in the flexural FRP would be limited to 0.9 ε = 8100 με . The U-wrap anchoring effect increased the strain utilization from 46% to 69% for 1U and 2U specimens, respectively, compared to the unanchored strengthened specimen (0U).

10000 12000 14000 16000 18000

Strain Gauge Debonding Strain (ACI 440) Design Rupture Strain Ave Rupture Strain

U-wrap Debonding

Maximum Strain (  ) 0 2000 4000 6000 8000

U-wrap Rupture

IC Debonding

Shear Control

0U 1U 2U 0U 1U 2U 0U 1U 2U

no stirrups

#2@200

#4@200

Fig. 5. Maximum strain recorded in longitudinal flexural CFRP sheet at ultimate load.

3.4. Strain in U-Wraps The maximum strain values in each U-wrap at ultimate load were evaluated and plotted to compare with the ACI 440.2-prescribed U-wrap effective strain value (Fig. 6). In shear-controlled specimens, two U-wraps were fully engaged at the critical shear crack, and thus, their strain values were considered and reported. For the flexure controlled specimens, strains recorded in all fourteen U-wraps at ultimate load are shown (and averaged) and are compared with the ACI 440.2 U-wrap effective strain. The experimentally-determined U-wrap strains in shear-controlled specimens exceeded the effective strain values obtained from ACI 440.2 equations, indicating the latter’s conservative nature. A drop of 20% to 30% in peak strain values was observed for the Group B (#2@200) specimens compared to Group A (no stirrups) beams. This reflects the interaction between internal steel stirrups and U-wraps, i.e., the inclusion of internal steel stirrups decreases the ultimate debonding strain values of U-wraps although the total shear resisted by both steel and CFRP is increased. This observation for unanchored U-wraps was also noted in other studies (e.g., Chen et al., 2022; Chen et al., 2010; Pellegrino & Modena, 2002). The trend of decreasing strain values within each group from 1U to 2U specimens suggests that as the cross-sectional area/axial stiffness of the U-wrap increases, debonding strain decreases, consistent with the ACI 440.2 equations. The average U-wrap strain value for flexural-controlled specimens was close to the ACI 440.2 effective strain value (within a 10-12% variation). However, the variability of individual U-wrap strain values is higher due to the random nature of flexural cracking distributions in the beams.