## 3 Experimental Phenomena and Analysis
### Failure Modes  
Observing the loading behavior of the steel pipe-steel slag micro-powder UHPC short column specimens, the failure process was generally similar across all specimens. The typical stress-strain curve is shown in Figure 4, where \(\epsilon_{y}\) and \(\epsilon_{x}\) represent the longitudinal and circumferential strains, respectively. Given the large number of specimens, a selection of typical axial compression failure specimens was made, and the damage morphology of each is presented in Figure 5.  
Figure 4: Typical stress-strain curve. Figure 5: Specim damage morphology. According to the loading process in the field and Figure 4, the axial compression damage history of the steel pipe-steel slag micro powder UHPC short column can be divided into the following three stages:  
Stage I: Prior to reaching the ultimate load, the overall deformation of the steel pipe was relatively small, and the steel pipe and concrete acted together as a composite system with no apparent pre-failure phenomena. Stage II: Near the ultimate load, the specimen entered the ductile stage. Distinct cracking sounds from the concrete could be heard, and the UHPC at the two ends was locally crushed. The specimen underwent brief oscillations due to the localized concrete damage. As the load continued to increase, the rust on the steel pipe surface began to flake off, and small bulging bags formed due to the increasing deformation from the compression and extrusion interaction. As the specimen's length-to-slenderness ratio decreased, the failure mode gradually transitioned towards shear. This was attributed to the difference in Poisson's ratios between the core concrete and steel pipe under compression, leading to extrusion friction interaction. When the interaction and binding force reached their limits, deformation damage occurred. Stage III: Upon entering the failure stage, a clear tearing sound from the steel pipe could be heard. At this point, the strong constraint between the steel pipe and concrete had largely collapsed, and the structure could no longer withstand a significant load. This was followed by the collapse of the specimen, characterized by large bulging shear damage and buckling. Comparative analysis of Figure 5 reveals that the damage morphology of the specimens varied due to different factors, which are analyzed as follows:  
From group (a) in the figure, it can be observed that as the L/D ratio of the steel pipe-concrete composite decreases (from 4 to 3 to 2), the main bulge-flexure deformation location of the steel pipe gradually shifts from the two end locations towards the central region of the specimen. When the specimen is shorter and thicker, the steel pipe has a stronger restraining effect on the UHPC, and the deformation of the UHPC is primarily concentrated at the two compressed ends, manifesting as a blunt-coarse damage pattern. As the length-to-slenderness ratio of the test specimen increases, the longitudinal bending effect of the steel pipe under loading gradually becomes more pronounced, and the specimen is more prone to undergo buckling deformation and bending damage from the middle region. Comparing the damage morphology shown in Figure 5b for the different steel content groups, we can observe the following: As the wall thickness of the steel pipe in the UHPC increases, the steel content also increases (0.166, 0.231, 0.266). The damage mode of the steel pipe transitions from a shear bulging type to a more pronounced transverse expansion type of damage. With the increase in steel content, the degree of central bulge-flexure deformation and the end shear damage phenomena become less significant. The angle of deformation also gradually changes from an oblique orientation to a more horizontal one. In other words, higher steel content leads to a shift from a localized, shear-dominant failure to a more distributed, transverse expansion-type failure. The deformation behavior becomes less concentrated at the ends and center, instead transitioning to a more uniform, horizontal expansion across the specimen.