Seismic performance of UHTCC columns reinforced with equal-strength replacement of traditional reinforcement by high-strength NPR steel bars

A novel steel reinforcement exhibiting a negative Poisson’s ratio (NPR) effect has been developed, combining high strength with high elongation. Leveraging these properties, the seismic performance of Ultra-High Toughness Cementitious Composites (UHTCC) columns reinforced with NPR steel was investigated. Specifically, this study represented a preliminary evaluation of NPR steel bars equal-strength substituting traditional reinforcement to reduce steel consumption. Pseudo-static tests were performed on six UHTCC columns. The influence of the equal-strength design principle based on NPR steel bars on failure modes, ductility, and energy dissipation capacity were analyzed. Results demonstrated that NPR reinforcement enhanced the seismic performance while maintaining shear capacity comparable to that of specimens with traditional reinforcement. Compared to pure-HRB400-reinforced specimens, hybrid-reinforced specimens and pure-NPR-reinforced specimens exhibited increases in ductility, plastic deformation capacity, ultimate drift ratio, and energy dissipation capacity. However, the underlying micro-mechanisms responsible for its superior mechanical properties remained unclear. Therefore, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analyses were conducted on NPR steel after quasi-static tensile testing. These analyses revealed that the synergistic interaction between deformation twinning and dislocation slip during deformation endows NPR steel with its superior mechanical properties. These findings confirm the feasibility of NPR reinforcement technology, providing a scientific basis for enhancing the seismic ductile design of structures.