#### 1.1.3 Damage Status of the Base Plate of CRTSIII SBT  
At present, many lines in China have adopted CRTSIII SBT. However, China has a large area and complicated geological conditions, leading to many external environmental factors that may result in cracking of the base plate. Currently, cracking is the principal malfunction of CRTSIII SBT base plate. When base plate cracks appear, they can lead to muddying or frost-heave damage, as well as a reduction in the resilience of the base plate concrete and corrosion of the steel bars inside the base plate. To ensure that the durability of the structure is not affected, and the possibility of effectively rectifying the crack that appears in the ballastless track structure, the crack propagation characteristics of the track structure must be understood for prevention and repair of the cracks. Treatment of the problem is particularly critical [19] in meeting the serviceability requirement of the track. At present, China has laid CRTSIII SBT on many lines, such as the Cheng-Guan line and Beijing-Shenzhen passenger dedicated line [20]. The track structure is in good general service condition, maintenance-free, and has high stability. However, several portions of the base plate revealed some durability issues, primarily in the form of cracks in the grooves of the base plate. Some of the cracks occurred about a week after the base plate was poured during the construction period. Close examination of the track structure revealed that there is the appearance of transverse through cracks at the base plate during the operation stage. The appearance of these cracks not only affects aesthetics but if not controlled, cracks may affect the force/load bearing capacity. Figure 2a shows the cracking problem of the CRTSIII SBT base. Cracking of the base plate concrete is the principal problem of CRTSIII SBT and occurred frequently. After extensive investigation, it was discovered that the cracks were most concentrated in the groove part and progressed in transverse and lateral directions. As shown in Figure 2b, most of the cracks appeared in the construction stage, particularly between one week to one month after the pouring of the base plate concrete. These cracks have seriously affected the safety and durability of CRTSIII SBT and were also the main cause of other problems of the base plate, such as mud tumbling and frost-heave damage. During operation, it is impossible to visually see whether new cracks have occurred since the groove part of the base plate is completely covered by the upper structure. Aside from the lateral force of the wheel load the track is subjected to during operations, there are longitudinal forces such as the braking force that impact on the track under many weather conditions. This loading condition has a long-term effect when cyclically acted on the track structure and accumulates over time. The cracks might buckle, and expand and, consequently, penetrate one another, leading to the emergence of lateral and lateral  
Figure 2: Cracking problem of CRTSIII SBT base. (**a**) Overall view of cracking; (**b**) cracking form 1.  
through cracks. The emergence of cracks in the ballastless track allows other track problems to occur. For heavy-haul railways, the track structure is prone to fatigue damage owing to the lengthy impact of large train loads and the external temperature and humidity environment. In recent years, much research has been done on structural damage detection methods and detection principles. In [21], [PERSON], based on a genetic algorithm, proposed a vibration-based damage identification method, which is used to identify the location and severity of structural damage under temperature change and noise. A new damage identification optimization algorithm is proposed in [22], called bare bones particle swarm optimization with double jump (BBPSODJ), which has good adaptability. In [23], a method combining an autoregressive (AR) time series model and two-step artificial neural networks (ANNs) is proposed to identify damage under temperature change.