Thirty dog bone-shaped specimens were tested to study the effects of steel fiber mixed with seven kinds of non-metallic fibers on the tensile properties of ultra-high-performance concrete (UHPC). Through experiments and micromorphological analysis, the effects of hybrid fibers on the compressive strength, tensile strength, peak strain, fracture energy, and characteristic length of UHPC were analyzed. The results showed that the hybrid fiber-reinforced UHPC showed good ductile failure characteristics, which reflected the good crack resistance and toughening effect of different fibers. The tensile stress–strain curves of UHPC with different hybrid fibers can be divided into two parts: ascending section and softening section, and the softening section was greatly affected by the type and content of fibers. Basalt fiber with a diameter of 0.02 mm had the most obvious strengthening effect on UHPC, and polyvinyl alcohol fiber with a diameter of 0.2 mm and length of 8 mm had the best toughening effect. The mixing of steel fiber and non-metallic fiber cannot only reduce the preparation cost but also improve the fracture energy and toughness of UHPC.

To research the flexural performance of ultra‐high performance concrete (UHPC) filled high‐strength steel tube (UHPCFST) and UHPC filled double skin high‐strength steel tube (UHPCFDST) under bending load, an experimental investigation was presented. In this paper, four UHPCFST specimens, one UHPCFDST with square tube inner specimen and three UHPCFDST with circular tube inner specimens were designed to evaluate the influence of steel ratio, steel yield strength, and cross‐section combination form on the flexural performance of the specimens. Experimental results indicated that the composite action between UHPC and high‐strength steel tube was efficient, all specimens showed ductile failure under pure bending load. As the steel ratio and steel yield strength increased, the ultimate flexural capacity of specimens significantly increased, but the changes of flexural stiffness were not obvious. The results also demonstrated that when the hollow section ratio was 0.535 to 0.567, the UHPCFDST with circular tube inner specimens had higher ultimate flexural capacity and ductility than the UHPCFST specimens. Compared with square inner tube, when the steel ratio was the same, circular inner tube was recommended for UHPCFDST specimens. An analytical model of ultimate flexural capacity for UHPCFST and UHPCFDST specimens was proposed. Compared with the three current codes, the accuracy of the model was proved.

Seventy eight dog‐bone shaped ultra‐high performance concrete (UHPC) tensile specimens were designed to study the effects of single steel fiber and hybrid steel fiber with polyolefin (PP) fiber, polyvinyl alcohol (PVA) fiber, glass fiber (GF), polyester (PET) fiber, and basalt fiber (BF) on the compressive strength, tensile strength, peak strain, and fracture energy of UHPC. The results showed that hybrid fiber reinforced UHPC had good toughness damage characteristics, which reflected the strengthening and toughening effects of different fibers. The tensile stress–strain curves of hybrid fiber reinforced UHPC could be divided into the rising section and softening section, and the softening section was significantly influenced by the type and volume content of fibers. Among the microfiber (PVA fiber, GF, and BF), GF had the most significant strengthening effect, PVA fiber had the best ability to improve the deformation of UHPC, and BF had the most significant effect on the improvement of fracture energy. Among macrofiber (PP fiber, PET fiber), PP fiber had a better ability to improve UHPC deformation than PET fiber, but both were lower than microfiber. When the total volume content of steel fiber and PVA fiber was 2.5%, the best toughening and crack resistance effect was achieved when the volume content of steel fiber was 1.5% and the volume content of PVA fiber was 1.0%. Combined with the experimental results, the UHPC tensile σ–ω curves of single steel fibers, hybrid steel–PVA fibers, and hybrid steel–PVA–PET fibers were established, and the predicted curves were in good agreement with the experimental curves.

To study the uniaxial compressive behavior and constitutive relationship of high‐strength spiral stirrups confined circular ultra‐high performance concrete (UHPC) columns, an experimental investigation was presented. In this research, 14 confined UHPC columns and seven unconfined UHPC columns were tested to investigate the effects of the stirrup spacing, stirrup yield strength, and steel fibers volume content on the compressive behavior of the UHPC columns. The test results indicated that transverse reinforcement can effectively improve the deformation capacity of the UHPC columns, and all the confined UHPC columns showed ductile failure. The results also demonstrated that the high‐strength stirrup was more effective than normal‐strength stirrup for improving the post‐peak behavior of the confined UHPC. Compared with the stirrup strength, the volumetric ratio of transverse reinforcement had a more significant effect on the post‐peak ductility and toughness of the confined UHPC columns. Transverse reinforcement and steel fibers had a combined effect on improving the post‐peak behavior. Reducing the stirrup spacing or increasing the steel fibers volume content can better improve the axial compressive behavior of columns. A new constitutive model for confined UHPC was proposed. Compared with four existing models, this new model was in good agreement with the experimental curves.

In this paper, seven ultra-high performance concrete (UHPC)-filled double skin high-strength steel tubular (UHPCFDST) columns and four UHPC filled high-strength steel tubular (UHPCFST) columns axial compression tests were carried out. The effects of hollow ratio, width-to-thickness ratio of the outer steel tube, and cross-section combination form on the axial compression performance of UHPCFDST were studied, and the failure mode, axial load-strain curve, and strain response were discussed. The results showed that the highstrength steel tube had good compatibility with UHPC, and all specimens finally had serious buckling deformation. The peak load of UHPCFDST specimens decreased with the increase of the hollow ratio, the ductility and toughness were improved. The width-to-thickness ratio of the outer steel tube had the most significant effect on the axial compression performance of the specimens. With the increase of the width-to-thickness of the outer steel tube, the ductility of the specimens tended to increase, and the toughness index first increased and then decreased. Finally, a unified calculation model for the ultimate compressive capacity of UHPCFDST and UHPCFST specimens was proposed, and the predicted value was in good agreement with the experimental value.