Interface-controlled flexural and shear strengthening of reinforced concrete beams with prefabricated eco-designed ultra-high-performance fibre-reinforced concrete

This study investigates an ultra-high-performance fibre-reinforced concrete partially refined by industrial by-products (recycled steel fibres and spent refinery catalyst), denoted Eco-UHPFRC, as a material for strengthening RC beams. Twenty-four beams were examined in four setup configurations: monolithic flexure, monolithic shear without stirrups, flexural strengthening using prefabricated Eco-UHPFRC soffit plates, and shear strengthening using Eco-UHPFRC side panels. Two connection systems mechanical anchorage and flexible polyurethane bonding were applied. Results from four-point and three-point bending tests demonstrated that Eco-UHPFRC enhanced stiffness and load-carrying capacity, with the greatest improvements observed in shear-critical configurations. In strengthened beams, delayed crack initiation was primarily associated with continuous polyurethane bonding, whereas mechanical anchorage did not improve the cracking load relative to the unstrengthened RC reference and, in the flexural configuration, also produced a lower ultimate load than the unstrengthened beam; this specimen set is therefore excluded from the strengthening-efficiency comparisons. For strengthened beams, polyurethane bonding enabled more uniform load transfer and led to higher cracking and ultimate loads than mechanical anchorage, underscoring the importance of compatible interfaces to fully mobilize crack-bridging mechanisms. A code-based evaluation employing Eurocode 2 and ACI (for the RC baseline) alongside fib/RILEM formulations (for the UHPFRC reference) showed that conventional RC codes underestimated the behaviour of Eco-UHPFRC, whereas fib/RILEM approaches closely reproduced the monolithic and adhesively-bonded responses (within about 6–18%), while substantially over-predicting the mechanically anchored configurations (by about 40–78%), reflecting incomplete interface mobilisation. The findings suggest that Eco-UHPFRC, when used with a flexible polyurethane interface, provides an effective method for strengthening RC beams in terms of both flexural and shear performance