Behavior and reasonable design of steel-UHPC composite beams under negative moment

Abstract

After using Ultra-High Performance Concrete (UHPC), the mechanical properties of the composite beams have significant differences compared to those of the steel-normal concrete composite beams. To investigate the effect of reinforcement ratio on the force behavior of steel-UHPC composite beams, tests on the flexural performance of steel-UHPC composite beams with different reinforcement ratios under negative moments were carried out, and reasonable design of the composite beams was analyzed by using a numerical simulation method. In addition, the existing crack calculation methods for composite beams were validated. The results showed that the failure modes of the composite beams with different reinforcement ratios were similar. Increasing the reinforcement ratio from 2.01% to 2.98%, the ultimate capacity of the steel-UHPC composite beams was increased by 4% and the ultimate deflection was reduced by 7%, while the ductility was reduced by 11%. The cracking load was not enhanced, but the higher reinforcement ratio had a controlling effect on crack development and crack width in the late loading stage. For steel-UHPC composite beams, the appropriate reinforcement ratio should be less than 4%. Furthermore, the crack calculation formulas in the existing steel-concrete composite beam codes do not apply to steel-UHPC composite beams, and the calculation results were much smaller than the measured values. Among the related formulas for reinforced UHPC members, the calculated value of the formula in NF P 18–710 was closer to the measured value but the overall error was still large, so further research is needed.