## 1 Introduction  
Concrete-filled steel tubular columns can fully leverage the advantages of steel and concrete. The steel tube confines the concrete in a lateral compressive state, while the infilled concrete enhances the stiffness of the steel tube; the two materials work together, greatly improving the overall bearing capacity of the composite member [1, 2, 3]. In recent years, ultra-high-performance concrete (UHPC) has been used to replace ordinary concrete in steel-concrete composite members in order to achieve higher mechanical properties due to UHPC's exceptional durability and strength. However, the high engineering cost of UHPC has limited its practical application [4, 5, 6]. To reduce the preparation cost, some researchers have developed low-cost and sustainable steel slag micro-powder UHPC by replacing the quartz powder in UHPC with industrial waste steel slag that has been processed into micro-powder [7; 8; 9; 10]. Therefore, by injecting this steel slag micro-powder UHPC into steel tubes, the fabrication cost can be effectively reduced while still improving the mechanical properties of the steel-concrete composite member. It is extremely important to study the bearing capacity of steel pipe UHPC short columns, and a number of scholars have carried out related studies. [PERSON] et al. [11] and others showed that the ductility of steel pipe UHPC columns can be improved by restricting the concrete to a tube with high strength or small diameter-to-thickness ratio; [PERSON] et al. [12] carried out the tests on 12 steel pipe restrained UHPCs, which showed that the effect of the hoop coefficient on the damage mode of steel pipe concrete short columns and proposed a formula for calculating the bearing capacity of steel pipe re-strained UHPC; [PERSON] et al. [13] carried out 27 high-strength steel pipe high-strength scelp pipe high-strength concrete and ordinary steel pipe high-strength concrete comparative experimental research, indicating that the steel pipe on the UHPC restrained enhancement effect is stronger than the ordinary concrete, with GB2014 [14] specification to calculate the load-bearing capacity of the results of the match is good; [PERSON] et al. [15] researched the 18 carbon fiber composites of UHPC-steel pipe combination of short columns for the axial compression. The test showed that the load-bearing capacity of the combined short column is significantly higher than the simple superposition of the load-bearing capacity of each structure, and with the increase in CFRP thickness, the higher the combined load-bearing capacity enhancement; [PERSON] et al. [16] analyzed the effect of different steel content and steel strength on the axial compression strength of steel pipe UHPC through the axial compression performance testing of 9 UHPC short columns, indicating that the steel content and steel strength of UHPC axial compression strength of different steel content and steel strength of UHPC, and the effect of different steel content and steel strength of steel pipe UHPC axial compression strength. The results showed that the higher the steel content and steel strength, the stronger the restraining effect, and suggested the selection of high-strength steel to improve the ultimate load-carrying capacity of UHPC steel pipe short columns. The above studies show that the increase in the strength of steel tubes can effectively improve the ductility and load-carrying capacity of UHPC columns. The increase in core concrete strength can also, in turn, enhance the overall stiffness of steel pipe concrete members. However, in the study of steel pipe UHPC, the effect of the change in the strength of the in-fill concrete on its performance is mostly analyzed, while the effect of the type of UHPC and the mixing of other admixtures on the overall structure is less studied. As a key bearing material in steel tube UHPC composite members, the performance of UHPC can significantly affect the overall behavior of the steel tube UHPC specimens.