## 3 Experimental Phenomena and Analysis
### Load-Deformation Curve  
The measured load-displacement curves of 27 steel pipe-steel slag micronized UHPC short columns are shown in Figure 6, with the values of each curve taken as the average of two displacement gauges of the same group. The shapes of the load-vertical displacement curves of the specimens are related to the steel fiber and coarse aggregate dosage, as well as the type of steel pipe. The specimen load-deformation curves can be roughly divided into two types: Type I curves undergo three stages in the loading process--an elastic stage, an elasto-plastic stage, and a falling stage; while Type II curves exhibit a second rising stage after reaching the peak value, showing a tendency of falling first and then rising. Figure 6: Load deformation curve. 1. Elastic phase: In the elastic phase, as the specimen displacement increases, the load exhibits a straight linear relationship with the growth until it reaches about 90% of the ultimate load. During this stage, the UHPC and the outer steel pipe undergo uniform expansion, and the core UHPC and the steel pipe do not experience strong interactions. The hoop action of the steel pipe is not yet significant, and the core concrete begins to exhibit some small cracks. 2. Elastic-plastic stage: As the load approaches around 90% of the ultimate load, the slope of the load-deformation curve starts to decrease, and the curve deviates from its initial linearity, gradually flattening out until reaching the peak. At this elasto-plastic stage, the core concrete and the steel pipe begin to deform under compression and exhibit increasing friction, and the restraining effect of the steel pipe starts to become more pronounced. The Luders slip lines appear on the surface [22], and the internal UHPC cracks begin to extend and develop further. 3. Decline phase: As the displacement increases, the load begins to decrease from the peak load, and the decreasing section of the curve flattens out until the curve remains near horizontal. The steel tube shows bulge-flexure deformation at this point, with increasing ability to constrain the core UHPC as the tube displacement increases. 4. The second rising stage: For the specimens with steel pipe thicknesses of 6 mm and 8 mm, a second rising stage is observed after the initial decline in the load-deformation curve. This is because the thicker steel pipe provides stronger constraint and limits the relative slip between the internal components, allowing the specimen to transition from the plastic flow stage into a reinforcement stage, where the load-bearing capacity experiences a second increase. The curve then maintains a slowly rising slope as the steel pipe undergoes more severe drum-like bending deformation, and prominent cracks begin to appear in the internal UHPC. After analyzing Figure 6, it can be seen that the specimens with L/D ratios of 3 and 4 show more pronounced falling sections, followed by two rising sections, which can be categorized as Type II curves. In contrast, the specimens with an L/D ratio of 2 exhibit a more gradual descent after reaching the peak, without obvious falling and strengthening sections, and can be classified as Type I curves. Comparing specimens with the same L/D ratio, the higher the thickness of the steel pipe, the more pronounced and prominent the recovery section after the initial decline. This indicates that the Type II curve demonstrates better ductility than the Type I curve. The steel content remains the primary factor affecting the post-peak ductility, and an increase in steel content leads to an overall increase in the hoop coefficient, which enhances the confinement on the concrete and subsequently improves the overall ductility of the specimen. For the steel fiber-reinforced concrete specimens, the axial load drop is more moderate, and the curve exhibits a more rounded shape, which helps maintain the resistance to axial load. At the same time, the presence of steel fibers also significantly affects the ductility of the specimens.