Study On Dynamic Characteristics Of Asphalt Concrete Substructures Under CRTS ? Type Ballastless Track

Substructure undertakes the dynamic load of high-speed train as the foundation of ballastless track.With the development of railway transportation to the trend of high speed and heavy load,the dynamic effect of substructure will be further intensified.The waterproof layer of current ballastless track normally use cement concrete,and the corresponding structures will face problems of easy cracking,high rigidity,difficult maintenance etc.Asphalt concrete is a kind of viscoelastic material,which has the advantages of both strength and flexibility,and also has merits like waterproof,shock absorption and crack resistance.In this paper,from the viewpoint of dynamics,the feasibility of asphalt concrete using as the foundation layer of the rail is studied by combining the theoretical research and the numerical simulation method.The main work and conclusions are as follows:1)The mechanical properties and basic models of viscoelastic material are summarized,and the generalized Maxwell model is used to simulate the asphalt concrete.Discussing the forms and characteristics of the vertical load of train,it turns out sinusoidal excitation load should be applied.Considering the type of asphalt substructures,the mechanics of viscoelastic layered system is taken as the theoretical basis.And the selection of mechanics index for the dynamic characteristics analysis is discussed.2)The finite element software ABAQUS is used to model the asphalt concrete substructure,and the viscoelastic parameters of asphalt concrete are obtained by fitting the stress relaxation modulus master curve.The validity of the model is confirmed by comparing the calculated results of standard model and the measured data in the test section of ZhengXu.The time history curve of each index shows that the vertical acceleration and displacement firstly reach the maximum value and then fall to stable.The vertical displacement is symmetrically distributed in saddle shape,and there is a maximum value beneath the inner rail.Except for the stress concentration at the edge of the base,the dynamic stress level is higher under the rail.The dynamic stress of subgrade gradually decreased along the depth direction,and the surface layer of subgrade is reduced by 43%,and only 25%left at the top of the soil base.3)The modal analysis results show that the natural frequency range of the subgrade system is 3.21-5.55Hz,which is far lower than the vibration frequency of the vehicle body.Analyzing the advantages and disadvantages of asphalt concrete and cement concrete structures from the angle of mechanics,the calculation results reveal that the asphalt concrete structure has smaller vertical displacement and acceleration,and the stress conditions are improved,hence the asphalt concrete structure is as good as cement concrete structure in the mechanical properties,even better in some respects.4)The influence of parameters such as layer thickness,layer temperature,train speed and elastic modulus of subgrade on dynamic characteristics of asphalt concrete substructure is analyzed.It turns out vertical displacement and acceleration decrease with the increase of the thickness of asphalt layer.Balancing the performance and cost,the thickness of 15cm is appropriate.Optimal performance of asphalt concrete structure appears near-6?.The influence of vehicle speed on vertical acceleration is enormous,but little has shown on the displacement.The elastic modulus of surface subgrade has limited influence on each index,but it should be improved from the perspective of the whole structure damping.5)A design method of the asphalt waterproof layer used in CRTSIII ballastless track is carried out as a reference.By considering the environmental parameters,material parameters and traffic parameters and using mechanical response as design index,the relationship between mechanical properties and structural damage of the subgrade can be established.The main mechanical control index are the strain of the bottom of the asphalt layer and the vertical compressive strain of the top surface of subgrade.Based on the test section of ZhengXu,a design example is given.