Synergistic effects of steel and polymer steel-like fibers on the interfacial pore structure and performance of ultra-high-performance concrete

To overcome the durability-related limitations and workability constraints associated with high steel fiber (ST) contents in traditional ultra-high-performance concrete (UHPC), this study systematically investigated the effects of single-fiber systems (ST or polymer steel-like fibers (PSF), with volume fractions of 1%-3%) and hybrid fiber systems (with a total fiber volume fraction of 3%) on the macroscopic properties and microstructure of UHPC. Experimental results show that PSF has a limited effect on improving the compressive strength of UHPC, but it is highly effective in enhancing tensile deformation capacity, controlling autogenous shrinkage, and enhancing resistance to chloride ingress. The hybrid fiber system helps alleviate the insufficient flowability caused by high ST contents while maintaining flexural performance comparable to that of UHPC reinforced with high ST content, and enhances resistance to chloride ion transport under the tested conditions. Microstructural analysis suggests that a more compact effective fiber-matrix interfacial transition zone (ITZ) is formed around PSF, which contributes to improved interfacial energy dissipation and stress transfer efficiency in the hybrid fiber system. Furthermore, the hybrid use of ST and PSF helps reduce the formation of large pores and defects within the matrix. The hybrid use of ST and PSF provides a promising strategy for the design of UHPC with balanced mechanical performance and chloride ion transport resistance under the tested conditions.