# Application of Mastic Asphalt Waterproofing Layer in High-Speed Railway Track in Cold Regions
## 1 Introduction  
Freeze-thaw damage of subgrade is a typical cause of deterioration of road and railway structures in cold regions. The accumulation of freeze-thaw cycles reduces subgrade strength and causes uneven deformation, thus increasing train-induced dynamic force and vertical displacement of the structure [1, 2]. With the fast development of high-speed railway in China and the extension of high-speed railway networks to cold regions, avoiding freeze-thaw damage of high-speed railway subgrade is of crucial importance to ensure track stability and reduce maintenance costs. Subgrade water content is one of the key factors affecting freeze-thaw damage [3]. Penetration of surface water increases the water content of subgrade and worsens its freeze-thaw damage [4]. The waterproofing layer paved on the subgrade surface of slab track systems is typically used for the purpose of improving the drainage of surface water and avoiding water penetration into subgrade [5]. The concrete waterproofing layer is commonly used in existing high-speed railway lines in China, however, field investigation shows that it cracks easily in winter, especially in cold regions [6]. Consequently, the cracks result in a poor waterproofing performance of the concrete waterproofing layer and provide paths for surface water to penetrate. The better flexibility and permeability of asphalt materials offers the possibility to improve the waterproof performance of railway track systems [7]. Asphalt mixtures are widely used in pavement engineering as well as waterproofing projects. Meanwhile, it has also been successfully applied in railway structures. Worldwide experience has shown that the use of asphalt materials can offer a good alternative in modern railway construction [8; 9]. At present, the application of asphalt layers in railway track systems can be classified into three categories, namely asphalt sub-ballast layer in the ballast railway track, asphalt supporting layer in the ballastless railway track, and the asphalt waterproofing layer in the slab track. The asphalt sub-ballast layer positioned between the ballast and the subgrade is the most popular structure form among them, which has been practically used in many countries [10]. An asphalt layer of 12-15 cm is used in both normal-speed and high-speed ballast track in Italy, France, Spain, and the United States [11; 12; 13; 14; 15; 16; 17; 18]. While, in Japan, a 5 cm asphalt layer is employed to enhance the carrying capacity of ballast track. It is reported that the asphalt sub-ballast layer can provide a waterproofing layer effectively reduce train-induced pressure in the subgrade, thus increase subgrade stability [19]. Asphalt ballastless track is mainly applied in Germany and Japan [20]. Germany developed asphalt ballastless tracks by placing sleepers directly on the asphalt roadbed; during the past decades, a series of asphalt ballastless track systems, including ATD (Asphalttragschicht mit Direke Auflagerung--asphalt rail span with direct support), SATO (Studiengesellschaft Asphalt Oberbau--study group for asphalt superstructure), and Getrac systems, have been developed and applied in practical railway lines [21; 22]. Asphalt layers have also been used as upper roadbeds in slab tracks to mitigate train-induced vibration, as well as to improve the waterproofing performance in Japan [19]. In the recent years, with the fast development of high-speed railways in China, the asphalt waterproofing layer has been developed as a novel drainage solution of slab track [23; 24]. In 2005, a dense-graded asphalt mixture was used in the Beijing high-speed railway test track as the waterproofing layer, but plenty of cracks were found after several years of operation, and insufficient compaction and poor cracking resistance were reported to be the main reasons. Then the emulsified asphalt slurry seal was tested in Beijing-Tianjin express railway in 2008, but this test failed as well due to serious raveling and stripping of the seal [6]. [PERSON] et al.