## 2 Overview of the Experiment
### Mechanical Testing of Materials
#### 2.3.2 Ultra-High-Performance Concrete  
The material testing was conducted in accordance with the standard for ultra-high-performance concrete test methods [18]. Compressive strength and modulus of elasticity tests were carried out on 100 \(\times\) 100 \(\times\) 100 mm cubic concrete specimens and 100 \(\times\) 100 \(\times\) 300 mm prismatic concrete specimens that were cast concurrently with the steel pipe UHPC specimens. The UHPC mix proportions, and the 28-day strengths of the concrete specimens are presented in Table 4. The specimen naming convention is as follows: C indicates the specimen, t is the steel pipe thickness, X is the length-to-diameter ratio, m is the volume fraction of coarse aggregate, and n is the volume fraction of steel fibers.  
As shown in Table 4, the cubic compressive strengths of the steel slag micro-powder UHPC specimens with different mix designs were all above 125 MPa, with an average of 135.2 MPa. The axial compressive strengths of the prismatic specimens reached over  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Test No.**} & **Aggregate** & **Steel** & **Prism 28d** & **Cube 28d** & **Prism to** & **Modulus of** \\  & **Mix** & **Fiber** & **Strength** & **Strength** & **Strength** & **Strength** & **Elasticity** \\  & **(kg\(\cdot\)m\({}^{-3}\))** & **(kg\(\cdot\)m\({}^{-3}\))** & **(MPa)** & **(MPa)** & **(MPa)** & **Ratio** & **Ratio** \\ \hline C4-X-0/1 & 0 & 78 & 120.4 & 135.8 & 0.88 & 45.7 \\ C4-X-20/2 & 500 & 117 & 128.8 & 139.7 & 0.91 & 43.2 \\ C4-X-40/0 & 1000 & 0 & 108.6 & 130.4 & 0.83 & 40.5 \\ C6-X-0/2 & 0 & 117 & 125.5 & 141.7 & 0.88 & 42.2 \\ C6-X-20/0 & 500 & 0 & 110.5 & 129.8 & 0.85 & 44.2 \\ C6-X-40/1 & 1000 & 78 & 117.9 & 133.4 & 0.89 & 46.8 \\ C8-X-0/0 & 0 & 0 & 111 & 126.1 & 0.88 & 41.3 \\ C8-X-20/1 & 500 & 78 & 133 & 143.6 & 0.91 & 49.1 \\ C8-X-40/2 & 1000 & 117 & 116.7 & 136.7 & 0.85 & 47.2 \\ Average Value & / & / & 119.1 & 135.2 & 0.88 & 44.4 \\ \hline \hline \end{tabular}
\end{table}
Table 4: UHPC mix ratio table and measured strength.  
Figure 1: Completed specimen preparation.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Caliber** & **Thicknesses** & \(f_{y}\)**/_MPa_ & \(\epsilon_{y}\) & \(f_{u}\) & \(E_{s}\) & \(\mu_{s}\) \\ \hline
108 & 4 & 287 & 1.5\(\times 10^{3}\)\(\mu\epsilon\) & 438 & 2.06 \(\times\) 10\({}^{5}\) MPa & 0.283 \\
121 & 6 & 285 & 1.5\(\times 10^{3}\)\(\mu\epsilon\) & 445 & 2.06 \(\times\) 10\({}^{5}\) MPa & 0.284 \\
146 & 8 & 282 & 1.5\(\times 10^{3}\)\(\mu\epsilon\) & 440 & 2.06 \(\times\) 10\({}^{5}\) MPa & 0.283 \\ \hline \hline \end{tabular} Note: \(f_{y}\) for yield strength, \(\epsilon_{y}\) for yield strain, \(f_{u}\) for ultimate tensile strength, \(E_{s}\) for modulus of elasticity of steel, \(\mu_{s}\) for Poisson’s ratio.  
\end{table}
Table 3: Mechanical test values of steel.  
108 MPa, with an average of 119.1 MPa. The highest compressive strength was observed in the C8-X-20/1 group, while the lowest strength was in the C4-X-40/0 group, with a difference of approximately 25 MPa, indicating that the dosage of coarse aggregate and steel fibers had a significant influence on the UHPC strength. The ratio of prismatic to cubic compressive strength ranged from 0.85-0.91, which is higher than the typical range of 0.77 to 0.82 for ordinary high-strength concrete [19]. This is attributed to the higher modulus of elasticity of the steel slag micro-powder UHPC, which reduces the transverse deformation of the prismatic specimens under axial loading, thereby bringing the failure modes of the prismatic and cubic specimens closer together, resulting in a higher strength ratio.