Behaviour of RC beams strengthened in flexure with hybrid CFRP-reinforced UHPC overlays

Ultra-high performance concrete (UHPC) overlays have become increasingly used for retrofitting concrete bridge decks, beams and slabs. A particular concern for the strengthening system is the localized cracking that develops within the overlay which can lead to failure and reduced strength and ductility. Fibre reinforced polymer (FRP) rebar can be used as a hybrid overlay reinforcement to overcome this issue and result in much thinner and lighter overlays. In the present study, the behaviour of reinforced concrete (RC) beams strengthened with carbon-FRP (CFRP)-reinforced UHPC overlays is examined through a robust finite element (FE) model. The model was validated against two existing experimental campaigns, resulting in excellent predictions for load-deflection and load-slip responses, cracking, yielding, and ultimate loads as well as the failure modes. A parametric study, comprising 68 models, was conducted on 5 key parameters, namely: reinforcement ratios for the beam and overlay, concrete compressive strength for the beam, overlay thickness and the interface between cast-in situ overlay and the beam. In general, the system resulted in significant increases in ultimate load and ductility compared to the control beam and those strengthened with un-reinforced or steel-reinforced overlays and eliminated the overlay cracking failure. Varying the CFRP reinforcement ratio in the overlay for the strengthened beam results in a significant increase in ultimate load in range of 112-463%, compared to the control beam. An analytical procedure was also undertaken, using parametric study results and regression analysis, resulting in the development of an analytical model for estimating the capacity of strengthened beams and can be used for design purposes.