## 1 Introduction  
Asphalt concrete has been widely used in pavement engineering because of its excellent structural and functional properties. Many countries have introduced asphalt materials into railway projects to achieve functions of roadbed waterproofing, vibration attenuation and noise reduction [1; 2; 3]. For example, asphalt concrete has been used as the subballast layer or bed course to reduce track vibration and noise in France, USA, Germany, Italy, and Spain [4; 5; 6; 7; 8; 9]. It has also been paved upon the roadbed surface to make use of the waterproofing performance in Japan [10; 11]. Moreover, in China, both self-compacted and roller-compacted asphalt concrete has been applied in several high-speed railways as waterproofing structures, including some test sections on the Harbin-Qiqihar and Zhengzhou-Xuzhou high-speed lines [12; 13; 14]. Worldwide practices have shown that asphalt concrete in railways not only has an excellent waterproofing and vibration attenuation performance, but also has a good durability and economy [5; 6; 7; 8; 12; 15].  
In recent years, asphalt concrete is gradually being popularized and used in new high-speed railway projects, and many relevant studies have been carried out around the world. For example, [PERSON] et al. performed a series of numerical analyses by using the KENTRACK simulation code to consider the maximum tensile strain and compression stress in the system [16]. [PERSON] obtained dynamic responses by simulating two kinds of hot mix asphalt (HMA) ballastless track substructures [17]. [PERSON] et al. characterized the viscoelastic behavior of asphalt concrete by the generalized Maxwell model, and developed a 3D finite element model to study the dynamic characteristics of an asphalt concrete waterproofing layer [18]. [PERSON] et al. studied the long-term settlement characteristics of hybrid asphalt railway tracks by large-scale model tests in the laboratory [19]. [PERSON] et al. designed a railway asphalt concrete for high-speed railway infrastructures and investigated its performances and mixture combination parameters through full-scale laboratory dynamic model tests [20]. It can be seen that these previous studies mainly focus on the dynamic mechanical responses of asphalt concrete in railways by numerical simulation, or the material design and long-term performance evaluation by laboratory tests. Further, these researchers have not considered the influence of air temperature changes.  
Generally, asphalt concrete is paved between the railway roadbed and the base plate as a full-section railway substructure in China [18; 21]. It can be predicted that asphalt concrete is inevitably subjected to complex working conditions such as the cyclic train load and ambient air temperature changes. However, the studies on the strain characteristics and the failure mode of this structure under these comprehensive working conditions are still limited, especially in situ tests on asphalt concrete for high-speed ballastless track substructures. Therefore, the main purpose of this research is to reveal the mechanical characteristics and failure mode of the asphalt concrete substructure for a ballastless track induced by the cyclic train load and environmental temperature changes, respectively. By conducting in situ forced vibration tests and temperature-induced deformation monitoring tests, the responses of asphalt concrete near the expansion joint and the center of the base plate were studied. Then the potential failure modes were analyzed based on the test results and field investigation. Finally, some design concerns and construction instructions useful for the application of asphalt concrete in high-speed railway infrastructures were proposed.