Durability Of Ballasteless Slab Track Subjected To Dynamic Load And Prediction Of Fatigue Durability

Non-ballasted track, which is smoother, more stable and requires lower maintenance, has been proven to be the best track structure type for high-speed railway. With the large scale construction of Chinese high-speed railway, non-ballasted track has been rapidly developed and widely used. Generally speaking, non-ballasted track has good stability and durability, its maintenance is much less than that of ballast track during the operation stage. But as the infrastructure guiding the vehicle running, non-ballasted track is always exposed to natural environment and subjected the high frequency dynamic load of vehicle. With the increase of service time, environment and dynamic load both accelerate the deterioration of non-ballasted track and affect the driving comfort and safety finally. The fatigue durability life of non-ballasted track is the main factor to guarantee the driving safety during its design life. Due to its various materials, complex service environment and repeated load, the life prediction of non-ballasted track should differ from that of ballast track.According to the mechanical and material property of non-ballasted track, this paper focused on the durability research of non-ballasted track subjected to dynamic load with taking the durability factor and coupled action of temperature gradient and train into account. The main research contents and results are as follows:(1) in terms of the mechanical characteristics and material property of non-ballasted slab,the chloride permeability test of non-ballasted track under cyclic load was conducted. The chloride distributions in concrete specimens with different stress level and loading frequency were analyzed. Based on the chloride diffusion theory and genetic algorithms, the different surface chloride ion concentrations and apparent chloride diffusivities of concrete specimens subjected to different cyclic load were fitted with Matlab. The test results showed that under flexural cyclic load, the concrete damage gradually accumulates, the porosity of concrete increases, micro cracks continuously generate and propagate, the connectivity of pores becomes better, thus the chloride diffusion speed increase obviously. When the frequency of cyclic load is higher than 10Hz, it would accelerate the chloride accumulation on concrete specimen surface. The apparent chloride diffusivity increases with the increasing loading frequency, only when the loading frequency is in the range of 10 to 15Hz. it almost keeps at the same level. The apparent chloride diffusivity performs well in binomial curve with stress level.(2) choosing the concrete porosity under cyclic load as coupled parameter and taking the effect of frequency on chloride diffusion performance, the one-way coupled cyclic load and chloride diffusion model was established to analyze the effect of cyclic load on chloride diffusion in concrete. In this model, the dynamic coefficient was introduced to describe the influence of frequency, meanwhile the relationship of porosity and chloride diffusivity was carried out based on the related researches. Then the chloride diffusivity can be obtained through the calculation porosity of concrete after N cycles loading. In terms of the test in chapter 2, the mechanical model and chloride diffusion model were built using Abaqus to analyze the impact of cyclic load on the chloride diffusion in concrete. Comparing the calculation results with test results to verify its correctness.(3) on the base of miner's linear damage accumulation criterion, in terms of the structural characteristics, material and mechanical property, the life prediction method of non-ballasted track under the coupled action of train load and temperature gradient which considering the durability problems was established. Firstly, it was considered that the concrete carbonation,the steel corrosion, the freeze-thaw, the prestress loss and the CA mortar aging have impact on the material performance and structural dimension. Secondly the statistical characteristics of train load and temperature gradient were taken into account, and the interval between two adjacent wheel passing through was regarded as one cycle time. Then the reasonable concrete fatigue equation was chose according to the non-ballasted loading condition to predict its working life.(4) based on the non-ballasted track service life prediction method, the fatigue durability life of CRTS I non-ballasted slab track applied at Beijing was predicted. Thus the fatigue life of slab, concrete roadbase and CA mortar were obtained. The results show that, the track slab of non-ballasted slab track can meet the requirement of 60 years' service life, while the annual prestress loss of slab is 20% of total prestress, the track slab may failure because of fatigue.So in order to guarantee the 60 years' service life, the damage which may result in the total prestress loss should be repaired immediately. The fatigue life of concrete roadbase after operating 60 years is only 4.6 times as many as the train load cycles and its safety margin is not large enough, therefore the concrete roadbase may have fatigue failure. The coupled action of dynamic train load and temperature gradient would significantly accelerate the concrete carbonation of non-ballasted slab track. The carbonation depth of track slab is much less than its concrete cover thickness, and the carbonation depth of roadbase is approximately 21.600mm which is still less than its concrete cover thickness. The coupled action of dynamic train load and temperature gradient would rapidly degrade the frost resistance of track slab.The track slab would end in freeze-thaw failure after serving 17 years, which cannot meet the requirement of 60 years' service life.