Fatigue Performance Evaluation Method And Improvement Technology Of Concrete For Ballastless Track Of High-speed Railway

By the end of 2021,the operating mileage of China’s high-speed railway has reached 40,000 kilometers,with a maximum operating speed of 350 km/h.Ballastless track structure made with concrete is the key position for bearing the high-frequency fatigue loads caused by high-speed train.Thus,the damage of concrete is accelerated,leading to the adverse effects on the durability issue and operation safety.To improve the fatigue life of ballastless track structure concrete,in this paper,the key parameters of ballastless track concrete fatigue test,fatigue performance evaluation method and improvement technology were studied,the following main conclusions could be drawn:（1）The load characteristics and service performance requirements of ballastless track structure have been investigated and analyzed,the influence of test parameters on the fatigue performance of concrete has been also studied.The stress of ballastless track concrete is complex and the bending tensile stress is the common type of loading.The rate of fatigue damage of concrete exposed to bending fatigue loads is significantly faster than concrete exposed to compressive fatigue loads.When the load frequency of high-speed train is higher than 15 Hz,the possibility of fatigue damage of concrete is increased.Combined with the influence of key parameters such as stress level,loading frequency and loading waveform on concrete fatigue performance,the fatigue performance test system of ballastless track concrete is quantified as follows:the loading mode is three-point bending loading,the stress level is 0.6,the stress ratio is 0.1,the loading frequency is 20 Hz,and the loading waveform is sine wave.（2）The feasibility of using surface resistivity,ultrasonic wave velocity and impact elastic wave velocity for evaluating fatigue performance were studied.The results show that the surface resistivity,ultrasonic wave velocity,impact elastic wave velocity decreases with the increasing loading rate for concrete under static loading.Under the bending fatigue loading,the damage parameters show the typical three-stage development stages of fatigue damage,and the proportion of each stage in the whole fatigue life with different evaluation method is roughly the same,the first and third stages account for 10～15%of the fatigue life,and the second stage accounts for70～80%of the fatigue life.It is proved that the surface resistivity,ultrasonic wave velocity and impact elastic wave velocity can be used to evaluate the fatigue damage of concrete.（3）Based on the linear cumulative damage theory,the fatigue damage factor（D）of concrete is defined.It is found that the change of damage factor also meets the three-stage change.Combined with the development characteristics of damage factor and residual fatigue strength under fatigue loads,the critical state of concrete fatigue failure s was determined as D_ρ=0.25,D_U=D_I=0.2,respectively,where D_ρis damage factor based on surface resistivity,D_U is damage factor based on ultrasonic wave velocity,D_Iis damage factor based on impact elastic wave velocity.（4）The effects of strength grade,alkali resistant glass fiber bridging and polymer-modified technology on the fatigue resistance of concrete were investigated.The results show that all these technologies can improve the fatigue resistance of ballastless track concrete,though the improvement mechanisms are different.Based on Weibull function,the effects of fatigue life improvement of concrete by three methods were statistically analyzed.Under the same assurance rate,the fatigue life of polymer-modified concrete is the most obvious,which is about 40～55%higher than that of C60 concrete.The second is alkali resistant glass fiber reinforced concrete,which is about 30～40%higher than the C60 concrete.Finally,with improving the strength grade of concrete,the fatigue life of C80 concrete is about 20～25%higher than that of C60 concrete.Considering the technical economy,the optimum content of polymer is 10%and that of fiber is 0.5%.