Study On The Local Damage Behavior Of Asphalt Concrete Upper Roadbed

In the recent years,with the development of China's high-speed railways,especially the extension of high-speed railway networks to cold regions,the problems of roadbed stability caused by freeze-thaw damage have become increasingly prominent,which seriously affects the ride comfort and structure stability of slab track.The poor waterproof performance of the existing slab track structure is one of the major causes of freeze-thaw damage.The application of asphalt concrete upper roadbed can significantly improve the waterproof performance of the slab track structure and reduce the dynamic stress in roadbed,which has a positive effect on improving the stability of the slab track,reducing the maintenance cost and prolonging the service life.At present,the research and application of the asphalt concrete upper roadbed is mainly based on the research results and application experience of asphalt pavement.The service environment of asphalt concrete upper roadbed in the slab track system is not clear,especially the in-depth research on the damage behavior of the asphalt concrete upper roadbed is needed.Therefore,in this study,the local damage behavior of the asphalt concrete upper roadbed was systematically studied based on its service environment in the slab track system.In this thesis,the dynamic analysis model and thermal analysis model of the slab track incorporating asphalt concrete upper roadbed were established and verified by the dynamic test data and temperature monitoring data from the test section in Zhengzhou-Xuzhou dedicated passenger line.Then,the developed dynamic analysis model and thermal analysis model were utilized to study the stress and temperature environment of asphalt concrete upper roadbed under service conditions,and the local damage mode of asphalt concrete upper roadbed was analyzed based on field investigation and simulation results.Next,considering the special service environment and performance requirements of asphalt concrete upper roadbed,a series of static and dynamic creep experiments were conducted to evaluate its long-term deformation behavior using time-temperature-stress superposition principle,and a new method named modified stress incremental method was developed and verified to estimate the thermal stress within asphalt concrete upper roadbed,so as to study its crack behavior under cooling conditions.Finally,the life-time deformation behaviors of asphalt concrete upper roadbed werepredicted based on the simulated service stress conditions and creep experiment results,and the cracking potentials of asphalt concrete upper roadbed under different cooling conditions were also predicted by combining the thermal analysis model.The simulation results show that,under high-speed train load,the maximum stress of asphalt concrete upper roadbed is vertical compressive stress on the top surface,but longitudinal tensile stress on the bottom surface.When the high-speed train passes at a running speed of 350km/h,the vertical compressive stress on the top surface is 22.5 KPa,and the maximum longitudinal tensile stress on the bottom surface is 34 KPa.The temperature of asphalt concrete upper roadbed fluctuates with the periodic change of air temperature,but the changing amplitude and phase angle vary with positions.The shoulder and middle area is the most sensitive to weather changes and the daily fluctuation of its temperature is very large,while the respond in the area directly beneath the base plate is much slower,there are roughly 5-hour and 30-hour delay for the positions at the edge and the center,respectively.The slab track system requires long design life and good stability.Although the stress in asphalt concrete upper roadbed is very small,the service time is long enough to accumulate the creep of asphalt concrete.Therefore,the potential permanent deformation is critical to the stability of the slab track system.At the same time,the thermal-structure coupling analysis results revealed that the cracking near the thermal expansion joint of the base plate is the other main damage mode of asphalt concrete upper roadbed.Utilizing the time-temperature-stress equivalent principle of viscoelastic materials,the creep compliance main curves of two asphalt mixtures,name SBS modified asphalt mixture and high viscosity rubber modified asphalt mixture,were obtained by short-term creep test,and the time-temperature-stress shifting parameters were regressed.Accordingly,based on the simulated stress environment and field temperature data,the long-term creep behavior of the asphalt concrete upper roadbed during its service life was equivalently converted to be achieved by short-term creep test under higher temperature and larger stress conditions,for example,50? and 0.5MPa,so as to predict the permanent deformation of asphalt concrete upper roadbed.The thermal-structure coupling analysis revealed that,under cooling conditions,the shrinkage of superstructure accumulates at the expansion joint of base plate,which results in extra displacement load on asphalt concrete upper roadbed and increases the potential of cracking.To study the effects of extra displacement load,an analytical method named modified stress increment method was developed on the basis of the Burgers model to calculate the thermal stress in asphalt concrete upper roadbed under cooling condition.Meanwhile,the modified TSRST test in which the HDPE-AC composite specimen was used to automatically load temperature-relating extra displacement on asphalt concrete during test progress was designed to validate the developed analytical method.A series of TSRST tests,including the standard TSRST test using AC specimen and the modified TSRST test using HDPE-AC composite specimen were conducted.The test results showed that the extra displacement load has critical effects on the thermal stress which asphalt concrete,also,the thermal stresses calculated by the analytical method were proved to be in good agreement with the results of modified TSRST test.Combined with the thermal-structural coupling analysis model,the low-temperature cracking behavior of asphalt concrete upper roadbed under different cooling environments were predicted,so as to provide a reference for the optimization design of asphalt concrete upper roadbed.