Prefabricated <scp>GFRP</scp> ‐reinforced <scp>UHPC</scp> overlays for strengthening <scp>RC</scp> beams in flexure

Abstract Field‐cast ultra‐high performance concrete (UHPC) overlays reinforced with fiber reinforced polymer (FRP) bars have been evaluated for retrofitting reinforced concrete (RC) structures, but limitations exist such as the complexity of UHPC mixing and the need for specialized equipment. To mitigate these limitations, prefabricated overlays can be used as an alternative, bonded to the member soffit by mechanical or adhesive means. In this study, experimental, numerical, and analytical investigations were carried out to evaluate the effectiveness of prefabricated FRP‐reinforced UHPC overlays for strengthening RC members in flexure. Eight beams were tested under monotonic load, including two control and six strengthened samples. The studied parameters were the (a) overlay casting type (prefabricated vs. field‐cast), (b) reinforcing the overlay with glass‐FRP (GFRP) bars, and (c) attachment of the prefabricated overlays, using either adhesive bond only or in combination with mechanical anchorage. The increase in the ultimate load ( P u ) in strengthened specimens over the control beam was 30%–265%. Reinforcing the overlay with GFRP bars was highly effective, leading to a 67%–200% increase in P u over plain layers, good ductility, and minimizing localized cracking in the overlay. Prefabricated overlays behaved in a similar fashion compared to the cast‐in‐place peers and were in some cases superior in terms of P u and ductility. While adhesively bonded overlays showed good bond, mechanically anchored counterparts were prone to drilling damage. A validated three‐dimensional finite element model was used in a parametric study on the effects of GFRP reinforcement ratio, overlay reinforcing bar type (steel, GFRP, carbon‐FRP (CFRP)), and compressive strength of the RC beam. An analytical model is also presented to estimate the load capacity of the strengthened beams.