## 3 Laboratory Tests
### Experimental Procedure  
A total of six specimens were prepared and tested to study the influence of bonding properties on the bending performance. The geometry and materials were identical. Steel tubes with a nominal yield strength of 235 MPa, ultimate tensile strength of 412 MPa, elastic modulus of 210 GPa, and Poisson's ratio of 0.286 were used, and the related parameters are shown in Table 1. These steel tubes have an outer diameter of 426 mm, wall thickness of 5 mm or 10 mm, and total length of 4 m. The inner surfaces of the steel tubes were cleaned and filled with concrete. The results from the concrete property test revealed a compressive strength of 62.6 MPa at an age of 28 days. Similarly, the reinforcing cages used in these specimens were the same. Each of them consisted of four longitudinal bars (16 mm diameter with nominal yield strength of 400 MPa) and hooping (10 mm diameter with nominal yield strength of 300 MPa). As shown in Figure 7a, the side length of hooping was 255 mm. Hooping was installed at four sections of each specimen, which were 0.2 m, 1.2 m, 2.8 m, and 3.8 m in length. Because of the construction conditions, the shape of the hoop was rectangular, which was not like the test pile used in the field test. While we focused on the interface effect of concrete and steel casing on the bending capacity of specimens, the effect of hooping shape had a limited effect on this. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material Type**} & \multicolumn{2}{c}{**Modulus of**} & \multirow{2}{*}{**Poisson’s Ratio v**} & **Yield Strength** & **Tensile Strength** \\  & **Elasticity E/GPa** & & \(f_{y}\)/MPa & \(f_{u}\)/MPa \\ \hline Q235b & 210 & 0.286 & 235 & 412 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relative parameters of the steel casing. The specifications of the constructed specimens are summarized in Table 2. Groups B1, B3, and B5 were prepared to evaluate the effect of slurry conditions on the moment capacity of piles; groups B2, B4, and B6 were prepared for the same purpose. Groups B1 and B2 were prepared to evaluate the effect of casing thickness on the moment capacity of piles; groups B3, B4, B5, and B6 were prepared for the same purpose. The test setups of specimens B1 to B6 are shown in Figure 7c. In order for a similar construction condition of the field test mentioned before to be simulated, those specimens should be merged into mud with different properties to form different bond qualities between the casings and the concrete. For the no-slurry skin condition, water was used instead; for the thin-slurry skin condition, the mud from the field test after the second hole cleaning was used, and the density was 1.15 g/cm\({}^{3}\); for the  
Figure 7: The layout of **(a)** strain gauges, **(b)** specifications and **(c)** dial gauges. thick- slurry condition, the mud after the first hole cleaning was used, and the density was 1.20 g/cm\({}^{3}\). Strain gauges (BX120-4 BB, China) and strain acquisition instruments (DH3816, China) were used to measure the strain of the concrete core and steel casing. The strain gauges were attached evenly around the reinforcement cage and the casing, which is shown in Figure 7a. A vertical load was applied through a jack (shown in Figure 7c), which introduced a pure bending moment situation at the middle of the beam. Identical loads were applied at two points on the beam. As shown in Figure 7a, a concrete pool with a size of 4 m \(\times\) 5 m \(\times\) 4 m was used as the loading table, and the specimens were 4 m long and hinged on it, which can restrict displacements of the ends. The distance between the two hinges was 3.6 m. Three dial gauges were installed at the opposite side of the loading point and at the middle of the span. The accumulation of the initial load was 10% of the ultimate load of specimens for each loading step and was kept for 2 min. When it was close to the failure state, the specimens were loaded slowly until failure.