## 3 Test Results and Analysis
### Bond Load-Slip Curve Analysis  
The load-slip (_P-S_) curves of the eight specimens rolled out of the test specimens are shown in Figure 4. As shown in Figure 4, at the beginning of loading, the slopes of the curves increase, with a linear upward trend, and the curves of the different specimens overlap. With increasing loading, the _P-S_ body is divided into two types: Type 1 has no obvious peak point, and the loading continues to rise, such as the ZU\({}_{0}\)N\({}_{1}\)W, ZU\({}_{1}\)N\({}_{0}\)W, and JU\({}_{1}\)N\({}_{0}\)W specimens, with a rising section, a secondary rising section, and a residual section. Type 2 has no obvious peak point, and the loading tends to reach a stable level in the later stage. There is a rising section and a residual section, and most of the specimens were of this type. Most of the specimens also tended to become stable after the rising section, and the falling section was not obvious, which indicates that the welded steel ring shear connectors improve the steel tube-concrete interfacial bond performance. The residual section of the curve fluctuation proves that, at this time, the welded reinforcement shear connectors still contribute to the interfacial shear capacity. Similar to ordinary steel-tube concrete, the bond strength \(\tau_{u}\) at the interface between the steel tube and the core concrete in this test can be calculated according to Equation (1) [25]:  
\[\tau_{u}=\frac{P_{u}}{\pi D_{0}l} \tag{1}\]  
where \(P_{u}\) is the bond-breaking load, \(D_{0}\) is the inner diameter of the round steel tube, and \(l\) is the interface bond length. From the _P-S_ curves, three characteristic points were identified to analyze the _P-S_ curves directly: the starting point (\(S_{s}\), \(P_{s}\)), which is the starting point of the rapid development of the slip point; the peak point (\(S_{u}\), \(P_{u}\)), which is the maximum position of the load in the slip process; and the residual point (\(S_{r}\), \(P_{r}\)), which is the endpoint of the residual section after the peak. The characteristic values of the _P-S_ curve of each specimen, calculated by Equation (1), are shown in Table 6.  
Figure 4: _P-S_ curves. Based on the results in Table 6, it can be seen that, for the specimen with core concrete of UHPC, when the inner wall of the steel tube is not set up with any structural measures, the bond strength \(\tau_{u}\) of ZU\({}_{1}\)N\({}_{0}\)W is 4.80 MPa, which is 1.2 times of that of the specimen with core concrete of C40; the reason for this is that the steel fibers of UHPC are mixed into the concrete, and they will be distributed chaotically in the concrete interior. Moreover, they can effectively impede the development of microcracks and formation of macroracks in the concrete interior, and they promote the development of the friction coefficient of the inner wall of the steel tube. For the specimens with the same percentage of UHPC and C40 thickness in their core concrete, the bond strength of the two-ring rebar ring specimens was significantly improved by about 33% compared with the one-ring rebar ring. The bond strength of all of the specimens constructed with internally welded rebar rings was improved compared with the specimens with glossy no-ring construction measures. In addition, glossy no-ring construction can increase the bond strength by increasing the strength of the concrete, but when the shear joints were welded on the inner wall of the steel tube, the bond strength was not improved significantly with the increase in the proportion of UHPC. Meanwhile, the load was mainly transferred through the shear joints; the bond strength of the two specimens under the corrosive curing was 5% lower compared with that of the specimen under natural curing. In the existing codes, the bond strength of steel-tube concrete is usually taken as a low fixed value. The Chinese code (DBJ/T 13-54-2010) [26] and the European code [27] take the design value of the bond strength of round steel-tube concrete to be 0.23 MPa and 0.55 MPa, respectively.