Study On Interface Performance Of CA Mortar Layer Of CRTS Ⅱ Slab Ballastless Track Under The Thermal Loading

The CRTS Ⅱ slab track,with its excellent characteristics of smoothness,reliability,and long service life,has become one of the main types of track structures for high-speed railways.However,in practical service environments,the difference in thermal conductivity and linear expansion coefficient between concrete and Cement Asphalt(CA)mortar can cause the track slab and CA mortar layer to undergo non-coordinated deformation under long-term temperature effects,leading to deterioration in the bonding performance of the CA mortar layer at the interface with the track slab,and subsequently causing problems such as track slab voids and interlayer gaps,which seriously affect the safe operation of high-speed railways.In this paper,relying on the National Natural Science Foundation of China project "Study on Failure Mechanism and Damage Assessment of Layered Failure of Longitudinally Connected Slab Ballastless Track for High-Speed Railway under Coupled Thermal and Load"(No.52278311),we investigated the damage and performance degradation of the CA mortar layer interface under temperature effects and conducted normal tension and tangential shear tests on full-scale models of track slab-CA mortar layer-baseplate at different temperatures,studied the temperature variation law of the normal and tangential bonding performance of the CA mortar layer interface,established the CA mortar layer interface temperature-dependent cohesive force model,and proposed a temperature damage model and evaluation method for slab track structures.The main research contents and conclusions are as follows:Firstly,the spatial structure and load characteristics of the CRTS II type slab ballastless track in high-speed railways were analyzed,and the typical diseases and causes of the in-service slab ballastless track such as joint separation and warping of the track slab were summarized.The damage and evolution mechanism of the CA mortar layer interface was explored from the perspectives of thermodynamics theory,temperature load test,engineering examples,and numerical simulation.Secondly,the application of interface fracture mechanics in the investigation of interfacial properties of bonded bi-materials is explored.It is demonstrated that the cohesive zone model effectively captures the damage characteristics of the concrete-CA mortar interface.Building upon the bilinear cohesive zone model,an interface temperature-dependent cohesive zone model is proposed,which further aids in the examination of the mechanical properties of the concrete-CA mortar interface under temperature variation conditions.Thirdly,axial tensile and shear push tests were carried out on full-scale models of rail plate-CA mortar layer-base plate under temperature conditions of 0℃,15℃,30℃,45℃,and 60℃,and the influence of different temperature conditions on the normal and tangential bonding properties of the CA mortar layer was studied,revealing the evolution mechanism and failure mode of the interface damage of the CA mortar layer.The results showed that the bonding surface between the rail plate and CA mortar layer was highly vulnerable,and the evolution of interface damage manifested as a progressive "from edge to center" behavior,with the interface damage degree more severe near the end of the plate.At 0℃,the normal and tangential bonding properties of the CA mortar layer exhibited "brittleness",while at 60℃,the bonding properties of the CA mortar layer at the interface showed relatively "ductility".The increase or decrease of temperature by15℃ significantly reduced the interface bonding strength,with a reduced rate of up to58.5%;after the interface temperature increased to 30℃,the degradation of bonding strength was relatively slow;at 60℃,the normal and tangential bonding strength of the interface were only 1/17 and 1/15 of their initial values,respectively.During the cooling process to 0℃,the interface stiffness slightly increased,while during the temperature rise from 15℃ to 30℃,the interface stiffness decreased significantly.After 30℃,the rate of decrease in interface stiffness tended to be stable.When the temperature reached 60℃,the normal and tangential stiffness of the interface were only 1/51 and 1/46 of their initial values,respectively.Furthermore,a temperature-dependent function for the key parameters of the interface damage evaluation of the CA mortar layer was established,and the key parameter values of the temperature-dependent cohesive force model of the interface were determined.Finally,the accuracy of the finite element model was verified by comparing it with experimental results.The finite element calculation results showed that without considering the structural steel bars,the maximum temperature rise that the track slab could withstand was reached when the temperature pressure generated within the track slab reached the ultimate bearing capacity of 17036 k N.Furthermore,combining the concrete plastic damage model,CA mortar plastic damage model,and CA mortar layer interface temperature-dependent cohesive force model,a damage model and evaluation method for CRTS II type ballastless track structure was proposed,which provides a reference for the long-term performance research and design of new ballastless tracks.