# Performance of CRTS-II Ballastless Track-Bridge Structural System Rebars under Fatigue Loading Test
## 1 Introduction  
High-speed railways have the advantages of strong transportation capacity, high speed, safety and high efficiency, and have been widely used in China in recent years [1, 2, 3]. At present, the length of the high-speed railway bridges that have been built and are under service in China accounts for nearly 50% of the total operating mileage. Among them, the length of simple-supported box girder bridges with a 32 m standard span accounts for about 95% of the total length of constructed bridges [4]. The repeated train load is the main load on the high-speed ballastless track simple-supported box railway girder bridge structural system [5]. Under the long-term train load, the change in the mechanical properties of longitudinal rebars has an important impact on the mechanical performance of the track-bridge structural system. Therefore, it is of great significance to study the real deformation process, mechanical characteristics and damage evolution of the tensile rebars of the ballastless track simple-supported box girder under a repeated train load. The fatigue performance of steel bars in the track-bridge structural system is different from that of concrete bridges, taken individually [6, 7], and most of the current research focuses on the fatigue performance of steel bars in concrete beams [8, 9]. [PERSON] [10] carried out fatigue tests on pre-stressed RC box girders and found that the fatigue strain of stirrups,deflection changes of box girders and fatigue diagonal crack damage have similar evolution trends; fatigue damage increases significantly at the initial loading stage, increases slowly with the increase in fatigue loading cycles at intermediate loading stages before growing rapidly at the final stage. [PERSON] [11] carried out experimental and numerical analyses on the residual strain of non-pre-stressed reinforcing steel in concrete beams under fatigue load. The test results show that the fatigue residual strain of non-pre-stressed rebars gradually increases during the fatigue process, and its evolution presents a \"three-stage\" development trend. He highlighted that the existence of residual strain will lead to the increase in the stress level of the steel rebars in the pre-stressed concrete beam and finally, proposed a model to analyze the residual strain of non-pre-stressed reinforcement under fatigue load. [PERSON]. [12] studied the effect of reinforcement on reinforced concrete beams under bending fatigue load, using acoustic emission technology. The experimental results show that the steel rebar provides remarkable ductility to reinforced concrete beams in which crack opening displacement and mid-span vertical displacement increase with the increase in rebar strain under increasing fatigue load, thus increasing their fatigue life. They concluded that the fatigue life of reinforced concrete beams depends on the size and quantity of the reinforcing steel, which plays an important role in improving the fatigue life of reinforced concrete beams. [PERSON] [13] carried out fatigue tests on marine sand concrete beams, using fiber-reinforced polymer reinforcement. It was found that when the applied load reaches the cracking load, the micro-cracks in the concrete in the tensile zone continue to occur and expand, and the stress in the cracked zone of the concrete shifts to the longitudinal rebar. Therefore, the expansion of micro-cracks is characterized by strain increase in the longitudinal rebar. The consequence is that the stiffness of the reinforced concrete beams decreases, and the damage of the interface stiffness increases with the increase in the load level and number of cycles. These studies improve the fatigue theory in corresponding fields and promote the development of various bridges, to a certain extent. Many scholars have carried out fatigue test research on reinforcing steel in railways and highway pre-stressed concrete bridges [14]. [PERSON] [15] conducted a constant-amplitude fatigue loading test on the simple-supported T-beam of heavy-load railway pre-stressed concrete.