Shear behavior and bearing capacity of reinforced concrete beams with a UHPC stay-in-place formwork

To investigate the shear behavior of composite beams with a UHPC stay-in-place formwork, seven composite beam specimens with a UHPC stay-in-place formwork (URB) and one reference specimen for normal reinforced concrete (RC) beam were designed in this paper. An experimental study was conducted to investigate the shear behavior of composite beams with a UHPC stay-in-place formwork under static loading. The research parameters included shear-span ratio, transverse rib spacing, and stirrup ratio. The results indicated that UHPC stay-in-place formwork can effectively improve the stiffness, cracking resistance, and shear resistance of members. Compared with the RC beam, the shear load when the first bending crack appeared (V-bcr) and the shear load when the first shear diagonal crack appeared (V-scr) of composite beams increased by 328 % similar to 422 % and 175 % similar to 206 %, the stiffness when the first bending crack appeared (EIbcr) and the stiffness when the first shear diagonal crack appeared (EIscr) increased by 30.3 % similar to 44.8 % and 4.9 % similar to 36.0 %, respectively, and the shear bearing capacity increased by 72 % similar to 89 %. The shear bearing capacity of composite beams increased significantly with the decrease of shear-span ratio. The increase in stirrup ratio had an improvement in the shear bearing capacity of composite beams, but had a small impact on the cracking resistance. The decrease in spacing of transverse ribs did not yield a substantial increase in shear bearing capacity for composite beams. Nonetheless, the difference between V-u (ultimate shear bearing capacity) and V-scr exhibited an obvious increasing trend, suggesting an enhancement in cracking resistance. The interface treatment method of setting transverse ribs improved the integrity of the composite beam and avoided longitudinal interface separation between the formwork and the core concrete. The presence of steel fibers effectively inhibited crack development through their bridging action. Finally, the calculation model of shear bearing capacity of composite beams with a UHPC stay-in-place formwork is established based on the truss-arch model. The average ratio of the calculated value to the experimental value is 0.99, with a coefficient of variation of 0.06. The theoretical calculated are in good agreement with the experimental results.