## 5 Conclusions  
In this paper, based on a three-span simply supported high-speed railway bridge with a CRTS II ballastless slab track structural system prototype, a 1/4-scale specimen was created, and repeated thermal loading tests were carried out. The development law of the track structure interlayer gap under repeated thermal loading and the study influence of the interlayer gap on the structural strain, displacement and stiffness have led to the following conclusions:  
(1) Under the thermal load action, the constraint of shear slot at the fixed end causes a large temperature stress inside the track structure, so the gap between the track slab and the CA mortar layer begins from the shear slots and gradually extends to the mid-span. The gap expansion experiences three stages (initiation, expansion and stabilization), and its length increases with the number of thermal loading cycles in a \"stepped\" manner. Therefore, at the design stage, it can be considered to strengthen the adhesion between the track slab and the CA mortar layer near the shear slot;  
(2) When the thermal load is applied to the track structure, the temperature inside the track slab is the largest. With the increase of the repeated thermal loading cycles, the interlayer gap between the track slab and the CA mortar appears and gradually extends. The interface bond effect of the track structure gradually decreases, and the thermal load transferred from the track slab to the lower structure through the interface bonding decreases. Finally, under the action of the same thermal load, the camber of the track structure increases gradually, and that of the concrete box beam decreases gradually; \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Structural Parts** & **Before Thermal Test** & **Before Thermal Test** & **Stiffness Degradation Rate** \\ \hline Track structure & 33.82 & 29.71 & 14.96\% \\ Track-bridge system & 329.61 & 321.29 & 2.52\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Stiffness degradation after thermal test. (3) With the increase of the number of thermal loading cycles and under the same temperature load, the track slab strain gradually increases, while that of the CA mortar and the track bed gradually decreases, with a change rate that is substantially consistent with the expansion rate of the gap. Near the fixed end section where the gap is initiated, the influence of the interlayer gap on the track structure strain is the largest. As the distance to the interlayer gap increases, the influence of the interlayer gap gradually decreases. At the sliding end section away from of the interlayer gap position, the strain change amplitude is insignificant;  
(4) At the end of the repeated thermal loading, under the same vertical static load, an increase is observed in the deflection of the concrete box beam and the strain of the track structure layers, and thus, the stiffness of the structure is degraded. The stiffness degradation of the ballastless track structure is relatively significant (14.96%); the overall stiffness reduction of the ballastless track-bridge structural system is relatively small (2.52%);  
(5) For the ballastless track-bridge structural system, the stiffness of the track system is much smaller than the stiffness of the bridge, so the contribution of track structure to the overall structural system is relatively small. Based on the above conclusion, the cohesive force model could be used to simulate the complex interface bond failure relationship. With the help of finite element method and theoretical method, further research could be carried out on the mechanism, the gap expansion law and effects of the interface bond failure of ballastless track under repeated temperature load. Conceptualization, L.Z. ([PERSON]), T.W., G.Z., M.A.D.G., Y.Z., L.Z. ([PERSON]) and W.G. ; Data curation, G.Z. and M.A.D.G. ; Formal analysis, L.Z. ([PERSON]), T.W., G.Z., Y.Z. and W.G. ; Funding acquisition, L.Z. ([PERSON]), T.W., Y.Z., L.Z. ([PERSON]) and W.G. ; Investigation, L.Z. ([PERSON]), T.W., G.Z., Y.Z., L.Z.