3.1. Test Scheme
- (1)
Dynamic modulus test
The dynamic modulus test is carried out in accordance with T 0738-2011 of JTG E20-2011. The test specimen is a cylinder with a diameter of 150 mm and a height of 170 mm. The dynamic modulus and phase angle of PUM-13, PUM-20, SMA-13, and AC-20 are determined, and three specimens are tested for each mixture. The test temperature is adjusted between 10 °C and 50 °C, and the loading frequency is adjusted between 0.1 Hz and 25 Hz.
- (2)
Hamburg rutting test
The Hamburg rutting test is carried out in accordance with the provisions of AASHTO T 324-2014. The test is carried out in a 50° water bath environment. The width of the test wheel is 47 mm, the load is 705 N, and the loading rate is (52 ± 2) times/min. The maximum permissible loading iterations are 20000 cycles, and the maximum permissible rutting depth is 20 mm. The test is stopped when either of the above two conditions is met. The Hamburg rutting tests of PUM-13, PUM-20, SMA-13, and AC-20 are then carried out. The specimens are cylinders with a diameter of 150 mm and a height of 60 mm, which are formed by rotary compaction. There are two specimens in each group, and two specimens are tested for each mixture.
- (3)
Accelerated loading test
To evaluate the long-term performance and interlayer shear resistance characteristics of typical pavement structures under the coupled action of high temperatures, water, and loading, indoor accelerated loading tests are carried out on the three typical composite pavement structures shown in
Table 3. The all-environment pavement accelerated loading system (ALT-S100) developed by Shandong Jiaotong University, Jinan, China is used for testing [
29], as shown in
Figure 4a. The wheel load is 1000 kg, the rolling speed is 4.5 km/h, the effective working length of rolling is 1 m, and the loading rate is 4000 times/h.
The composite rutting plate specimen is formed with two iterations of rolling. When the lower plate is removed, the bonding layer is treated, and then it is put into a double-layer rutting test mould to roll and form the superstructure. Three composite rut plates are formed for each structure and arranged in the effective working area [
29]. A double-layer rut board is placed on each side of the effective working area, which mainly plays the role of fixing and cushioning, and no specific test is carried out on it; the layout of the test specimens is shown in
Figure 4b. During the test, there is 50 ℃ water circulating in the test tank, and the liquid level is slightly higher than that at the top of the specimens, as shown in
Figure 4c. The accelerated loading test does not commence until the specimens are insulated in the water tank for 8 h. After 20,000 test iterations, the rutting depth of the three specimens is tested at certain intervals of load times, and the test is terminated after more than 300,000 rolling iterations.
- (4)
Interlaminar inclined shear test
Previous studies have shown that the direct shear method cannot simulate practical road conditions. Herein, the friction coefficient (δ) between the wheel and the road surface is taken as 0.5, considering the horizontal load generated by the emergency braking of driving vehicles. Further, considering special circumstances, such as the emergency braking of vehicles in downhill sections, the 45° inclined shear test is more practical [
27,
28]. The 45° inclined shear test device developed by the project team was introduced in previous study [
25,
30]. The inclined shear strength is calculated using Equation (1), where τ represents the shear interlaminar strength of the interface,
F represents the maximum vertical load, and
S represents the contact area between two layers.
The inclined shear specimens are cut from the composite rut plate. To evaluate the interlaminar shear characteristics of typical composite pavement structures under different working conditions, the specimens at room temperature (15 °C), with freeze–thaw cycles and after accelerated loading tests, are tested. The freeze–thaw conditions are implemented in accordance with the provisions of T 0729-2000. The specimens with accelerated loading tests are cut from the wheel trace belt position of the double-layer rutting plates after 300,000 loading iterations. The size of the inclined shear specimen is 50 mm × 50 mm × 100 mm, and five parallel specimens are tested under each test condition, whose average value is taken as the test result [
30,
31].
3.2. Theoretical Calculation
The numerical models of the three aforementioned pavement structures are established by using the ABAQUS finite element software, whose model size is 10 m (length) × 6 m (width) × 6 m (height). The elastic layered system is used to calculate the interlayer shear stress. It is assumed that each structural layer is homogeneous, completely elastic, and isotropic. The structural layers of the model are connected by bonding. The three-dimensional eight-node linear hexahedron element of C3D8R is used for mesh construction. The parameters of asphalt and PU mixtures are tested using the dynamic modulus test, and the material parameters of each structural layer are shown in
Table 4. During the analysis, the boundary conditions of the model are as follows: the subgrade bottom is completely fixed, the road laterally constrains the displacement in the Z direction, longitudinally constrains the displacement in the X direction, and the Y axis is the direction of road thickness. The grid is densified in the load action area. The road structure calculation model is shown in
Figure 5.
The interlaminar shear stress of the three typical pavement structures under the four working conditions described in
Table 5 are calculated. In the theoretical calculation process, the contact shape between each wheel and pavement is equivalent to a rectangle, and the equivalent length and width of the rectangular action area are 22.65 cm and 15.6 cm, respectively.