GFRP-reinforced UHPC overlays for strengthening RC beams in flexure

The use of ultra-high-performance concrete (UHPC) as a repair overlay of reinforced concrete (RC) members has significantly advanced in the past decade. Two common challenges encountered in un-reinforced overlays, a localized crack that appears before reaching desired failure modes and the potential issue of excessive thickness, leading to increased dead loads and higher repair costs. In this work, glass fiber reinforced polymer (GFRP) bars are suggested to improve the performance of plain UHPC overlays. Seven RC beams were fabricated and tested under four-point bending to evaluate the effectiveness of the strengthening system. The test matrix included: an un-strengthened (control) beam; two beams strengthened in flexure with plain UHPC overlay having a thickness (t) of 30 or 50 mm; two beams strengthened with 30- or 50-mm thick overlay reinforced with 2 phi 10 GFRP bars having a full length (L) of 2250; and two beams strengthened with 30- or 50-mm thick overlay reinforced with 3 phi 10 GFRP bars having a partial length (L) of 1500. The GFRP reinforcement ratio (rho(gfrp)) varied between 0 % and 5.24 % for the four reinforced overlays. Plain overlays increased the beam ultimate load (P-ult) by 35-64 %, with localized cracking identified as the predominant mode of failure. Flexural failures occurred in beams strengthened with GFRP bar-reinforced overlays, mainly in the concrete beam structure. In these instances, P-ult showed a significant increase, ranging from 190 % to 257 % in comparison with the control sample and from 114 % to 118 % in comparison with equivalently thick plain overlays. The addition of GFRP reinforcement led to a significant increase in beam ductility, between 1.0 and 2.20 times higher than the control beam. Partial-length GFRP bars resulted in comparable P-ult as their full-length counterparts but prompted overlay detachment failure at their termination points. An analytical model previously used by the authors was evaluated in this work and found to yield acceptable predictions of the beam ultimate capacity.