## 1 Introduction  
The China railway track system II (CRTS II) ballastless slab track has been widely applied to high-speed railway due to the superior stability, smoothness, reliability and durability [1, 2]. CRTS II slab track consists of rails, fasteners, prefabricated slab, the emulsified cement asphalt (CA) mortar layer, concrete base and subgrade [3]. The main functions of CA mortar layer are the leveling, support and amortization. However, CA mortar is one of the weakest parts for its particular material, harsh natural environment, repeated temperature loads and high-frequency train loads [4, 5, 6, 7, 8]. Specifically, to equip the CA mortar with a semi-rigid high performance with the combined merits of high strength and good viscosity, the constituent materials of it are sand, cement, emulsified asphalt and admixtures [3]. For the CRTS II ballastless track system, CA mortar is directly poured into the gap between the slab and base with a thickness of 30 mm. As an organic-inorganic nonmetal composite material, CA mortar is prone to aging damage when exposed to the air and sun [4]. Meanwhile, the repeated loads induced by the temperature difference between day and night is crucial to the fatigue damage evolution of CA mortar [7]. During its service period, CA mortar is subjected to the high-frequency train loads and is prone to fatigue failure [8]. Therefore, the interface damage between slab and mortar, known as the mortar disengagement, is a common and incident track disease in CRTS II slab track, as shown in Figure 1[9]. Mortar disengagement directly causes the insufficient support under the slab and worsens the track dynamic evenness, which further affects the vibration performance of vehicle systems [5,9]. It is necessary to repair the mortar disengagement before it develops to the stage that is detrimental to the track service life and the operation safety and riding comfort of the operation vehicle. However, the main disadvantage of CRTS II slab track is the difficulty in maintenance [10]. If mortar disengagement is repaired once detected, the workload of track maintenance will be increased greatly due to the wide application of the CRTS II ballastless slab track. Therefore, it is necessary to study the impact of mortar disengagement on the vehicle and track performances under the high frequency train loads for preliminarily determining the critical value of mortar disengagement size. Then, on the premise of not endangering track service life, vehicle operation safety and ride comfort, the appropriate track maintenance criteria can be put forward to alleviate the pressure of track maintenance. Numerous studies on the mortar disengagement in ballastless slab track mainly focused on the mechanism of interface damage. [PERSON] et al. [10] investigated the time-dependent behaviors of track system under the train loads with a 3D finite element (FE) model, which considered the damage deterioration of track structures. The author also explored the initiation and evolution of interlayer damage under the temperature loads by an experimental study [11]. [PERSON] et al. [12] analyzed the interface parameters between the slab and mortar layer based on the experimental results to provide reference for the design of CRTS II slab track. [PERSON] et al. [5] employed a special anti-fatigue device to study the damage mechanism and the degradation process of mortar disengagement in CRTS I slab track. [PERSON] et al. [13] analyzed the viscoelastic deformation of the CA mortar by building a solid (FE) model of CRTS I track using ABAQUS software. [PERSON] et al. [14] probed into the damage evolution and dynamic performance of CA mortar layer under the train loads via a vehicle-slab track coupled model established in ABAQUS. Then, studies on the influences of mortar disengagement on the track system were conducted. [PERSON] et al. [8,15] researched the influence of mortar disengagement on the concrete damage distribution and mechanical responses of slab through developing a CRTS I slab track system.