## 3 Test Section of Asphalt Concrete for Ballastless Track Substructure
### Test Program and Arrangement of Sensors  
The in situ tests consisted of two parts. The first one was the forced vibration test, which was used to investigate the dynamic responses of the asphalt concrete substructure induced by the cyclic high-speed train load. The second part was the temperature-induced deformation monitoring test, which was mainly to study the thermodynamic properties of the asphalt concrete substructure caused by changes in the ambient air temperature. For the in situ forced vibration test, the vibration exciter SBZ30 with the sinusoidal loading form, as shown in Figure 5, was selected to simulate the cyclic train load in the asphalt concrete substructure test section from January to April 2018. According to the design parameters of the high-speed train such as the axle weight, the axle distance, and the running speed, the forced vibration tests were designed into three groups. The specific parameters of the vibration exciter for each group are shown in Table 4. The tests were conducted at the frequency of 12.3 Hz with the vibration force amplitude of 58.8 kN, at the frequency of 12.3 Hz with the vibration force amplitude of 105.2 kN, and at the frequency of 19.0 Hz with the vibration force amplitude of 155.8 kN in test-1, test-2, and test-3, respectively. They were equivalent to the actual train load with the speed of 350 km/h and the axle load of 17 tons, the speed of 350 km/h and the axle load of 19 tons, and the speed of 450 km/h and the axle load of 19 tons, respectively. The number of loading cycles in each test was two million. Figure 4: Construction process of the asphalt concrete in railway. For the temperature-induced deformation monitoring test, the field ambient air temperature and the thermal strains of the asphalt concrete caused by changes in the temperature were monitored continuously from March 4 to March 6 2018. In addition, according to the above analysis of the working conditions of the asphalt concrete substructure, the expansion and contraction of the base plate will also cause the coordination deformation of the asphalt concrete substructure. However, the coordinating effect of the base plate and the asphalt concrete substructure can be reduced, if the inter-layer bonding between them is weakened. Therefore, a comparison structure has been constructed in the test section. That is, the geotextile has been set with a width of 1.2 m at both the left and right ends of the expansion joint, as shown in Figure 6, to isolate and weaken the inter-layer bonding effect. The forced vibration test focuses on the tensile strain at the bottom of the asphalt concrete layer and the compressive stress on the surface of the roadbed. The temperature-induced deformation monitoring test focuses on the thermal strain characteristics on the surface of the asphalt concrete, especially the mechanical and thermodynamic behavior of the asphalt concrete substructure near the center of the base plate and the expansion joints. Therefore, the arrangement of sensors in this test is shown in Figure 6, including 2 earth pressure sensors (denoted by P1 and P2), 8 grating fiber strain gauges (denoted by A1, A2, B1, B2, C1, C2, D1, and D2), and 30 resistive strain gauges. More specifically, the earth pressure sensors were set at the depth of 10 cm below the surface of the roadbed, corresponding to the plane position at the center of the asphalt concrete slice 1 and slice 2. The space around the pressure sensors was backfilled with fine sand after the placement, as shown in Figure 6. The grating fiber strain gauges were arranged on the surface of the asphalt concrete. The resistance strain gauges were uniformly pasted on the bottom of the asphalt concrete slices. The adhesive was CC-33A, and a 2 mm thick epoxy coating was used to protect the strain gauges, as shown in Figure 6. The arrangement process was as follows:  
1. Cut and lift the asphalt concrete slice 1 and slice 2;
2. Locate the adhesive position of the strain gauges and polish the surface;
3. Stick the strain gauges;
4.