Shear behaviour of composite beams with permanent UHPC formwork and high-strength steel rebar

• The shear behaviour of UHPC-NC composite beams reinforced by high-strength rebars was discussed, where the permanent UHPC formwork and no web reinforcement were adopted. • Test parameters included the presence of UHPC and steel grids, type and thickness of formwork, strength of the NC, size of the steel grids and shear span ratio. • Based on the superposition principle, the ultimate shear capacity of the UHPC-NC composite beam was formulated. This study investigated the shear behaviour of ultrahigh-performance concrete (UHPC)-normal concrete (NC) composite beams reinforced by high-strength rebar. In the proposed UHPC-NC composite beam, the UHPC was used as a permanent formwork, and no web reinforcement was adopted. A total of twelve beams, including one reference NC beam, nine beams with pure permanent UHPC formworks and two beams with steel grid-reinforced permanent UHPC formworks, were examined via a four-point bending test. The examined parameters included the presence of UHPC and steel grids, type and thickness of the formwork, strength of the NC, size of the steel grids and shear span ratio. Three failure modes were observed depending on the shear span ratio, i.e., diagonal compression failure for a shear span ratio < 1, shear-compression failure for a shear span ratio = 1–3, and diagonal tension failure for a shear span ratio > 3. No obvious debonding was found between the UHPC formwork and NC, indicating that the interface processing method adopted in this study was effective. The test results showed that under the same shear span ratio, the shear cracking loads and ultimate shear capacities of the UHPC-NC composite beams increased by approximately 11.8–94.1 % and 16–66.6 %, respectively, compared with the reference NC beam. Among the bottom, side and U-shaped UHPC formworks, the U-shaped formwork provided the greatest improvement in the ultimate shear capacity. Based on the superposition principle, the ultimate shear capacities of the UHPC-NC composite beams were predicted.