PSI - Issue 64

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

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wraps). The remaining three beams were strengthened with two plies (254 mm wide) of longitudinal flexural CFRP sheets, and with three combinations of 102 mm wide U-wraps: 0U, 1U, and 2U, corresponding to no U-wrap, 1-ply and 2-ply U-wraps, respectively (see Fig. 2). The spacing (204 mm on center) and width (102 mm) of the U-wraps were identical in all strengthened specimens. 2.3. Test Setup and Instrumentation The specimens were tested in a four-point bending setup using two servo-controlled 650 kN hydraulic actuators at the Structural Engineering Laboratory of the University of Delaware. Loading was applied in force-controlled mode at 45 kN/min up to the cracking load to ensure the same initial loading in both actuators. Following cracking, actuator control was switched to displacement control at a rate of 5 mm/min. To facilitate 3D Digital Image Correlation (DIC) imaging, the specimens were tested in an inverted position; however, DIC data is not included in this paper as it is still being processed. Additionally, to validate the DIC data, specimens were instrumented at critical locations with uniaxial electrical strain gauges (Kyowa Electronic Instruments Co., Ltd) and linear variable displacement transducers (LVDTs; RDP electronics Ltd). The strain gauges (60-mm gauge length, 120 Ω resistance) were installed on the longitudinal CFRP sheets to determine the strain profile along the beam length (locations shown in Fig. 2). Additionally, on the web face of the beam not facing DIC cameras, one strain gauge (60-mm gauge length; 120 Ω) was placed on each U wrap along the anticipated critical shear crack line to measure U-wrap strain. To investigate the strains on the internal steel tension bars, two strain gauges (5-mm gauge length; 350 Ω) were placed on the middle two bars at mid-span (see section view in Fig. 2). Additionally, a strain gauge (5-mm gauge length; 350 Ω) was installed on one leg of each internal steel stirrup (when present) along the expected shear crack line to measure the stirrup strain. The midspan displacement of the beam was measured using 150 mm stroke LVDTs installed at midspan. A Vishay data acquisition system with 50 channels was used to collect data from all sensors. 3. Results and Discussion 3.1. Load-Displacement Response The load-displacement plots for all 12 tested specimens are shown in Fig. 3; tests results are summarized in Table 2. As expected, all specimens of Group A (no stirrups), and B (#2@200), failed in shear while Group C (#4@200), specimens failed in flexure. All flexural-controlled beams exhibited typical tri-linear behavior. In Group A (no stirrups), the behaviors of the unstrengthened control beam and the strengthened beam without U wraps (0U) were quite similar: both failed in a brittle mode due to formation of a single primary diagonal tension crack at approximately the same shear capacity. The strengthened specimens with U-wraps (1U and 2U) increased the shear capacity by 70% and 76%, respectively, and exhibited approximately 4% greater stiffness following cracking compared to the control specimen. The addition of second ply of U-wrap (2U) did not improve the shear capacity significantly (only 6% greater than 1U specimen). This confirms the finding that increasing the unanchored U-wrap cross-sectional area alone does not proportionally increase the shear capacity, primarily due to a decrease in debonding strain (Khalifa et al., 1998). In Group B (#2@200), the internal steel stirrups increased the shear capacity of the control specimen; however, the improvement was insufficient to mitigate the brittle failure behavior also seen in Group A. Strengthening the specimen with 1-ply (1U) and 2-ply (2U) U-wraps, increased the shear capacity by 47% and 60%, respectively, compared to the control specimen. In this group, the U-wrap shear contribution apparently was less significant compared to Group A. In Group B, the internal shear reinforcement resists a portion of the shear while behavior remains dominated by the single dominant shear crack. The overall shear capacity is greater but the proportional contribution of the FRP U-wrap is reduced (see Table 2). The control specimen of Group C (#4@200) failed in a ductile mode: concrete crushing after extensive yielding of the steel tension reinforcement. The flexurally-strengthened specimen with no U-wraps (0U) failed due to premature IC debonding exhibiting a moment capacity 11% greater than that of the control specimen. The specimen anchored with 1-ply U-wraps (1U) increased the flexural capacity by 28% (compared to control specimen) and also shifted the