An innovative strategy of exploring high-strength engineered cementitious composites to solve the strength-ductility dilemma of UHPC structures

Ultra-high-performance concrete (UHPC) featuring high compressive and tensile strength could significantly enhance the load capacity of UHPC structures. However, due to the apparently inferior strain capacity of UHPC compared to that of steel bar, the deformation capacity of UHPC structures is substantially lower even than that of conventional reinforced concrete structures, leading to a strength-ductility dilemma for these kinds of structures. High-strength engineered cementitious composite (HS-ECC), combining the excellent tensile behavior of ECC (especially the tensile strain capacity) and the compressive performance of UHPC, could work compatibly with steel bars for the whole loading process preventing crack localization. This enhances both load and deformation capacity of HS-ECC structure. HS-ECC beams exhibited superior ductility and load capacity than those of UHPC beams with reinforcement ratio within 3.6 % in this research. When the reinforcement ratio was 1.3 %, the ductility index of HS-ECC exceeded that of UHPC 402.5 %. A theoretical model based on the layered section method was further proposed, accurately predicting the load-midspan deflection curves of HS-ECC beams. Analysis revealed that the contribution of HS-ECC to flexural strength of HS-ECC beams decreased significantly with increasing reinforcement ratio. The requirement of HS-ECC tensile strain capacity decreased, while the requirement of HS-ECC compressive strain capacity increased with increasing reinforcement ratio. Finally, a performance-based design method considering the compressive-zone depth as the indicator was proposed, which can balance the strength and ductility of HS-ECC beams, as well as increase the utilization ratio of HS-ECC. This research lay the ground for the design and practical application of reinforced HS-ECC beam.