Research On The Temperature Field Distribution And Cracking Characteristics Of Continuous Non-ballasted Track

Compared with the unit non-ballasted track,the continuous non-ballasted track has better integrality and smoothness,while its track slabs and concrete roadbeds are very likely affected by the temperature change.There is very large temperature stress when the cooling load is applied on the continuous structure,and it would inevitably cause the track slabs and concrete roadbeds cracking because of the low tensile strength of concrete.Cracks propagating to a certain extend would affect the durability of non-ballasted track,even more it would have negative impact on the stability of track structure and the driving safety of high speed vehicle.However,little systematic research has been conducted to show the cracking mechanism and its influence of continuous non-ballasted track.According to the mechanical and material property of continuous non-ballasted track,this paper focused on the temperature field distribution and crack characteristics.The theoretical and experimental researches of temperature load,crack characteristics,flexural cracking mode,and the influence of steel corrosion were conducted.The main research contents and results are as follows:1.The long-term test of continuous non-ballasted track temperature field distribution.Based on the meteorological factors and temperature field distribution test of continuous non-ballasted track applied on shanghai-hangzhou high speed railway in whole year,the temperature field distribution change rules and its relationship with meteorological factors such as air temperature,solar radiation,wind speeds and so on were obtained.By statistics analysis,the occurrence time of the summer maximum temperature on the top of track slab is roughly proportional to the longitude.It gradually delays with the longitude decreasing from east to west.For the non-ballasted track applied on Shanghai,the summer maximum temperature on the top and middle of track slab can reach at 57.3? and 50.8?,the winter minimum temperature on the top and middle of track slab are 0.15? and 3.7? separately.The maximum temperature gradient in track slab is 109.78 ?/m,and the minimum temperature gradient is-74?/m.Exponential function can well describe the temperature gradient distribution in the track slab.2.The research of temperature field distribution of track slab in early age.The temperature field FEM model of non-ballasted track was established,and its correctness was validated by the experimental results.Then taking the hydration heat of double-block track into account,the temperature field distribution of track slab in early age was analyzed,meanwhile the zero-stress temperature and the maximum temperature drop range in whole year were carried out to lay the basis for the cracking study of track slab.The results showed that the zero-stress temperature of continuous non-ballasted track which applied on Shanghai can be 15? higher than the air temperature.Therefore the influence of temperature field distribution in early age on the cracking behavior of track structure cannot be ignored.3.The research of temperature stress and crack computation method of continuous non-ballasted track.Firstly,according to the material discreteness of concrete,the cracking axial force with 95% of insurance rate was obtained.Then the temperature stress and crack computation method of continuous non-ballasted track,which takes the longitudinal resistance into account,was established to analyze the main effect factors of track slab cracking.With considering the bond slip relationship between steel and concrete,the change rule of longitudinal stiffness during the cracking process of continuous non-ballasted track was carried out,and then the tensile cracking model of continuous non-ballasted track was put forward.The results showed that,for the concrete structure,the designed and standard tensile strength in the specifications will tend to be unsafe when the external load effect is calculated by the tensile strength of concrete,and it will underestimate the actual external load when the action effect is calculated with designed tensile strength.When the longitudinal resistance is taken into consideration,the concrete tensile strength,reinforcement ration,steel bar diameter and longitudinal resistance all have impact on the critical temperature drop range,crack spacing,crack width and stress of steel,among them the reinforcement ration and longitudinal resistance have negative influence,and the crack width is inversely proportional to the square of reinforcement ration.During the whole cracking process of track slab,the average stiffness of track slab sharply decreases,while it slightly decline with the strain increase when the cracks are stable,at this moment,it is twice the steel stiffness and one-tenth of the original stiffness.4.The cracking model experimental research of continuous non-ballasted track.Based on the analysis of the relationships between the minimum crack spacing,crack width,the temperature boundary between the stable and unstable crack,stress on steel bar,cracking axial force and the effect factors of crack propagation such as strength of concrete,reinforcement ration and so on,the following results can be drawn: when the reinforced concrete model specimen is under the action of axial tensile force,the crack propagation process can be divided into unstable stage and stable stage.The minimum unstable crack space is slightly smaller than that of theoretical calculation value and distributed in a large range as normal distribution.The cracking axial tensions of most specimens are smaller than that of theoretical calculation value.During the unstable cracking stage,the stresses on the most part of steel bar are close to the yield strength.The 3 times of crack width calculated by the railway design rule can be well consistent with the surface crack width on the specimen without the bottom friction resistance,and it has large difference with that on the specimen with the bottom friction resistance,whereas the theoretical calculation results can fit well with the tested value with the consideration of bottom friction resistance.The good consistency between the tested cracking propagation rules and the theoretical results verifies the accuracy of the crack calculation method in this paper.For the specimen made of the concrete with compression strength of 40 MPa and the ribbed bar with reinforcement ration of 1.14%,the boundary temperature drop range between stable and unstable cracking stage is about 70?,while it is only 60? when the specimen is supported by the hydraulic layer.In order to effectively control the crack width,the crack of track slab should be limited in the unstable stage.5.The research of the track slab and concrete roadbed integral flexural cracking mode and the corrosion cracking characteristics of track slab.The track slab and concrete roadbed integral flexural cracking meso-scale FEM model was built.With this model,the influence of the loading strain rate on the flexural failure mode,the flexural-tensile strength and the stress-displacement relationship were studied.Meanwhile the meso-scale FEM model of track slab cracking induced by corrosion was built to study the mechanical properties and damage modes of the slabs with different rebar diameters,spacing and cover thicknesses,and an analysis was made on the effect of train and temperature loads on the cracking characteristics of reinforced concrete track slabs with corrosion.The results showed that under the static flexural load,most of the concrete cracks propagate along the aggregates edge and locate around the weak interface.The loading rate has a significant impact on the dynamic failure mode of concrete simple-supported beam.With the increased loading rate,there is an increase in the flexural strength,the single form cracks propagated along the weak interface transfer to the complicated multi form cracks,the crack width increases sharply and the concrete damage expands to the whole tension area.The interfacial integrity of non-ballasted track is very important to keeping the stability of track structure,so the interface damage should be paid more attention.The cracking mode of track slabs induced by rebar corrosion mainly depends on the thickness of rebar cover but rarely relies on the diameter or spacing of rebar.When the cover thickness is small,the rebar corrosion displacement about 70 ?m would result in the internal coalescence cracks.In the event the internal cracks of the slabs connect together,the expansion displacement of rebar increases with the increase of the rebar spacing.The train load has little effect on the slabs with corrosion crack.