Flexural strengthening of reinforced concrete beams or slabs using ultra-high performance concrete (UHPC): A state of the art review

•The state of research on flexural strengthening of RC beams or slabs with UHPC was reviewed.•Different failure modes of UHPC-RC composite members under flexure were identified.•The main parameters affecting the flexural strength were discussed.•Analytical and numerical models were summarized for strengthened beams or slabs with UHPC. The use of ultra-high performance concrete (UHPC) to strength existing reinforced concrete (RC) structures in flexure has been explored in recent decades. As UHPC developed in different countries performed different properties, the effectiveness of RC structures strengthened with UHPC varies. Moreover, the lacking of code provisions restricts the wide application of this novel strengthening technology. It is necessary to review experimental studies for the guidance of code elaboration. In this research, the state of research on the flexural strengthening of RC beams or slabs with UHPC was presented. From the technical literature review, an experimental database was established. In order to examine the effectiveness of strengthening schemes with UHPC, size effect, and mechanical properties of RC beams or slabs, pre-damage degree for RC, strengthening configuration, characteristics of UHPC layer, curing conditions for UHPC and interfacial preparation for concrete substrate were discussed. In the literature review presented, different failure modes of UHPC-RC composite members under flexure were also identified. Then, analytical and numerical models developed in the literature to reproduce structural response and to predict cracking and ultimate capacity of strengthened beams or slabs with UHPC were summarized, and a comparison between these models was presented. The experimental evidence showed that UHPC could be used to increase the flexural strength of RC beams or slabs. Also, a cost analysis comparing with other strengthening techniques (such as CFRP) was presented. Finally, some future work is recommended to complement this promising strengthening technique for existing RC structures under flexure.