## 1 Introduction  
Due to high reliability and stability, ballastless slab track systems have been widely utilized in high-speed railways all over the world. In China, the CRTS III (China Railway Track System III) slab ballastless track structure is the third type and the latest slab track system, and more than 5000 km has been built as of October 2020 [1]. The steam-cured concrete chest slab, the self-compacting concrete (SCC) filling layer, the isolation layer (geotextile material), and the concrete base plates are the main components of the CRTS III slab track, which is a laminated structure. According to field investigations [2, 3], transverse base plate cracking is one of the major forms of slab track defects. Figure 1 shows typical transverse cracks in the CRTS III type base plate. These cracks are located on the surface and at the middle portion of the base plate, and a few of them are extended to full depth. The field crack surveys conducted at the Beijing-Tangshan High-speed Railway revealed that most transverse cracks were initiated within a month after a specified 14-day wet-curing period. After about 2 months of casting, up to 50% cracking probability was reported. Undoubtedly, the formation of early cracks results in concrete deterioration and increases maintenance costs of a track structure. Therefore, the causes and prevention of early base plate cracking needs to be well understood. The concrete base plate of high-speed railway ballastless track system is designed with the aim of supporting the upper components and transmitting train loads to the subgrade or the elevated bridge foundation. The current study focuses on the base plate resting on simply-supported prestressed girder bridges, which are a widely utilized structure form because bridge foundations make up a great percentage of foundations of the highspeed railways in China. It is noted that the CRTS III base plate has varying dimensions in design and mainly depends on the dimension of the precast box-girder. The base plate investigated in this study is 5.65 m in length, 2.90 m in width, and 0.2 m in thickness, and it is reinforced by two layers of steel mesh inside. It is a well-known fact that transverse cracks in a plate structure of concrete are caused by longitudinal tensile stresses. These stresses are highly related to thermal, shrinkage, and creep strains. On the one hand, the precast bridge foundations restrain the natural thermal and shrinkage movement of the concrete. On the other hand, the temperature field of a concrete plate under natural environment is rarely uniform or linearly distributed. Drying shrinkage is also nonlinearly distributed along the plate depth. Consequently, non-uniform shrinkage and temperature deformation cause stresses even without external restraint. A numerical study has shown that shrinkage strains are caused by drops in relative humidity (RH) by 1% is 15 \(\times\) 10\({}^{-6}\) to 20 \(\times\) 10\({}^{-6}\), which appears larger than the strain caused by temperature decrease by 1 \({}^{\circ}\)C [4]. In the literature [5; 6; 7; 8; 9], a strong emphasis is given to the non-uniform distribution of drying-induced stress within the concrete, but these works are either based on indoor tests or are only focused on numerical simulations. As a result, the previous studies provide limited guidance to the problem in current research because of dissimilar humidity diffusion performance and mechanical properties between the cast-in-place fresh concrete base plate and the prefabricated concrete bridge foundation and some other special characteristics of the CRTS III slab track. Previous studies have also revealed that the most important construction-related factors affecting transverse cracking in plate structures of (reinforced) concrete involve weather and curing [10; 11; 12; 13]. Adverse weather includes strong wind, high and low temperatures, and low-humidity conditions [14].