## 4 Monitoring Result of Asphalt Concrete in the Test
### Responses Induced by the Temperature  
Figure 8 shows the time history curves of the ambient air temperature, the temperature-induced strain in the middle of the base plate, and the temperature-induced strain near the expansion joint, respectively. In general, the strain of the asphalt concrete layer caused by the diurnal variation of the ambient temperature is positive for most of the time near the middle position of the base plate, indicating that the surface of the asphalt concrete is in a tensile state, while it changes alternately between positive and negative values near the expansion joint, indicating that the surface is in the state of alternating tension and compression. This is because the diurnal variation of the temperature leads to the alternate changes in the temperature gradient inside the base plate and asphalt concrete substructure. When the temperature gradient is positive (the temperature was higher in the top than that in the bottom), the edge of the base plate warps downward and the center is relatively convex. Subsequently, the surface of the asphalt concrete in contact with the base plate is subjected to the compression near the expansion joint and the tension near the middle position of the base plate. Conversely, the state is exactly the opposite when the temperature gradient is negative. Besides, according to the comparison between the results in Figures 7 and 8 above, it can be seen that the mechanical responses caused by the ambient temperature changes are larger than those caused by the train load.  
Figure 8: \(Cont\).  
By comparing Figures 7c and 8b, it can be seen that the maximum tensile strain and diurnal variation range of the asphalt concrete near the expansion joint are much larger than those near the middle of the base plate. For example, the maximum tensile strains monitored by the grating fiber strain gauges A1 (near the middle position) and C1 (near the expansion joint) are 76 \(\mu\varepsilon\) and 582 \(\mu\varepsilon\), respectively. Further, the variation ranges are 86 \(\mu\varepsilon\) and 1210 \(\mu\varepsilon\), respectively. Therefore, the asphalt concrete near the expansion joint is more significantly affected by changes in the ambient temperature, which need to have careful attention paid to them.  
To study the coordinating deformation effect of the base plate and the asphalt concrete substructure, a comparison structure with the geotextile set at the end of an expansion joint has been constructed. Grating fiber strain gauges C1 and D1 were mounted near the expansion joint without the geotextile, and C2 and D2 were mounted near the expansion joint with the geotextile. According to the results shown in Figure 8c, the maximum strains obtained by C1, D1, C2, and D2 are 582 \(\mu\varepsilon\), 3 \(\mu\varepsilon\), \(-\)168 \(\mu\varepsilon\), and \(-\)162 \(\mu\varepsilon\), respectively. The minimum values are \(-\)628 \(\mu\varepsilon\), \(-\)563 \(\mu\varepsilon\), \(-\)299 \(\mu\varepsilon\), and \(-\)438 \(\mu\varepsilon\), respectively. The daily variation ranges are 1210 \(\mu\varepsilon\), 566 \(\mu\varepsilon\), 131 \(\mu\varepsilon\), and 276 \(\mu\varepsilon\), respectively. It can be concluded that the isolation layer, such as the geotextile, can significantly reduce the strain level near the expansion joint, which has a positive effect on resisting the passive tensile failure induced by the base plate.