## 3 Laboratory Testing  
The properties of the rail pad (SBR, styrene butadiene rubber) and assembly rail fastening system of the direct fixation track system of the Yeongjong Grand Bridge are presented in Table 3.  
The laboratory testing of this study examined the changes in the spring stiffness of new and used pads under static, dynamic, and aging spring stiffness tests. The appropriate rail pad replacement periods were also investigated in terms of their function and usability based on the observed changes in the spring stiffness of the rail pad. Figure 16 shows the rail pad samples used in the laboratory testing, and Table 4 lists the test conditions. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Static** & **Dynamic** & **Aging (static)** \\ \hline Loading rate & 50 kN/min & 50 kN/min \\ \hline Load range & 0-85 kN & 5-75 kN & 0–85 kN \\ \hline Frequency & \multicolumn{2}{c}{900 times (4 Hz)} \\ \hline Temperature (hour) & & 70 \(\pm\) 1 \({}^{\circ}\)C (72 h) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Spring stiffness test conditions of rail pad. Figure 16: Rail pad samples. (**a**) New rail pad and (**b**) used rail pad (corroded rail pad). \begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**Spring stiffness (kN/mm)**} \\ \cline{2-3}  & **Rail pad** & **Assembly rail fastening system** \\ \hline Static & 80–90 & 123.36 \\ \hline Dynamic & 120–140 & 211.86 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Design values of the spring stiffness of the rail pad for the direct fixation track. Static and dynamic vertical spring stiffness tests were conducted in accordance with the KRS (Korean Railway Standards) TR (track-rail) 0014-18(R) standard [18]. In these tests, each rail pad was fixed on top of the loading plate as shown in Figure 17. The zero-point setting was then performed after the linear variable differential transformers (LVDTs) were installed. The vertical displacement of the pad was measured at four points, and the results were used to prepare load-displacement curves. In the dynamic spring stiffness test, loading was performed 900 times in the 5-75 kN load range following a 4 Hz sine wave in accordance with KRS TR 0014-18(R) [18]. As shown in Table 5, the differences between the static, dynamic, and aging spring stiffness test results for the new and used rail pads were found to be less than \(\pm\)5% when subjected to an accumulated passing tonnage of 0.85 MGT. This indicates that the rail pads installed in the direct fixation track structure were not deformed or fractured by the temperature, period of use, or train load (accumulated passing tonnage). Periodic replacement of soft rail pads is required to reduce track load and the damage caused by the deterioration of the rail pads. However, the rail pads of the direct fixation track system of the  
Figure 18: Load–displacement curves. (**a**) Static test (new rail pad); (**b**) static test (used rail pad); (**c**) dynamic test (new rail pad); (**d**) dynamic test (used rail pad); (**e**) static test after aging tests (new rail pad); and (**f**) static test after aging tests (used rail pad). Yeongjong Grand Bridge evaluated in this study exhibited a displacement of approximately 0.5 mm when subjected to a load of 50 kN (static wheel load of AREX), and a displacement of approximately 0.94 mm when subjected to a load of 85 kN (static wheel load of KTX) based on the results of static spring stiffness tests. In contrast, the average displacement of the rail was 2.64 mm for AREX and 3.76 mm for KTX. The rail and track girder are coupled with the strong force of the fastener and exhibit integrated behavior [3]. The actual displacement of the rail was found to be 0.11 mm for AREX and 0.20 mm for KTX. This indicates that the actual displacement of the rail pad due to the wheel load of a passing train would be much smaller than that measured in the laboratory test results.