Structural behavior of RC beams strengthened with UHPFRC layers incorporating a new tension stiffening factor

This study investigated experimentally and analytically the performance of reinforced concrete (RC) beams strengthened with reinforced ultra-high-performance fiber-reinforced concrete (R-UHPFRC) layers at both ultimate and service limit states. Four large-scale beams were tested: one unstrengthened and three strengthened with R-UHPFRC layers incorporating different fiber configurations (2 % steel fibers, 2 % ultra-high-molecular-weight polyethylene (UHMW-PE) fibers, and a hybrid of 1 % steel + 1 % UHMW-PE fibers). Digital image correlation (DIC) was employed to monitor curvature, crack width, and crack propagation. The beam with 2 % UHMW-PE fibers demonstrated a 47 % gain in ductility and a 112 % increase in strength, while the beam with 2 % steel fibers achieved improvements of 34 % and 137 %, respectively. The beam with hybrid fibers experienced premature cover delamination. All strengthened beams exhibited substantial reductions in the average crack width at service loads (15–38 µm) compared with the control (330 µm). To capture the contribution of the R-UHPFRC layer, Bischoff’s tension stiffening factor was modified using DIC-derived data, and a simplified equation was proposed to predict average crack width. The revised factor accurately reproduced both load–deflection responses and crack widths under service conditions, confirming the validity of the proposed modification. • Full-scale RC beams (2.2 m span) were strengthened with 50 mm R-UHPFRC overlays containing steel, UHMW-PE, or hybrid fibers. • Strengthened beams showed significant gains in load capacity (+137 %), stiffness (+253 %), and ductility (+47 %). • UHMW-PE fiber overlays reduced average crack width from 330 μm (control) to 38 μm under service load. • A new tension stiffening coefficient was derived and validated, improving prediction of load–deflection and crack response.