Research On Upwarp Of CRTSⅡ Ballastless Track Slab End And Its Renovation

The CRTS Ⅱ slab ballastless track is widely used in China’s high-speed railway,and its overall performance of running safety and stationarity is in good condition.However,under the influence of complex external factors,different kinds of diseases have occurred in the track structure.Among them,the upwarp at the end of the track slab is one of the most prominent diseases of CRTS Ⅱ slab track.Upwarp disease reduces the smoothness of the line and directly affects the safe and smooth operation of high-speed trains when it is serious.Therefore,it is urgent to carry out research on the mechanism and remediation of upwarp disease at the end of track slab.The upwarp at the end of track slab is often accompanied by interlayer interface damage and joint defects.The evolution process is complex and has obvious non-linear characteristics.However,there are still some shortcomings in the current simulation methods for the upwarp at the end of the track slab,which can not take into account the interlayer damage during the upwarp process and the diseases of the connection between adjacent slabs,thus restricting the study of the disease mechanism and influence law of the upwarp at the end of the track slab.Based on the typical structural characteristics of CRTS Ⅱ slab track,cohesion and concrete damage plastic model are introduced into the analysis of slab upwarp.The mechanism,influence law and treatment scheme of slab end upwarp are systematically studied.The main contents and conclusions are summarised as follows:(1)Based on the theory of cohesion and concrete damage plasticity,a non-linear finite element model of CRTS Ⅱ slab ballastless track upwarp is established,which can take into account both interlaminar interface damage and adjacent joints.Based on the structural characteristics of CRTS Ⅱ slab track,concrete damage plasticity model is used to consider the generation and evolution of slab indirect joint damage,cohesion model is used to simulate the bonding characteristics of interlayer interface and adjacent slab interface.Finally,a finite element model for upwarp non-linear analysis considering both interlayer interface bonding damage and adjacent slab joint disease is established.Compared with the existing methods and test results,the correctness of the non-linear parameters of the model and the non-linear analysis model of the upwarp are verified.(2)The damage characteristics of indirect joints of track slabs under integral heating and cooling loads are studied.Based on the thermo-mechanical coupling theory,the evolution process of the upwarp at the end of track slabs under high temperature in summer is studied.Based on the damage plastic constitutive relation of concrete,the damage evolution law of indirect joints of slabs under the action of integral heating and cooling load is studied.On this basis,the thermal-mechanical coupling theory is introduced to study the evolution process of upwarp at the end of track slabs under high temperature in summer.The results show that the wide-narrow juncture will be damaged at the position of the tension lock under the larger overall heating and cooling load,and the interface of the indirect joints will be separated by the cooling load,and the damage of the indirect joints will be more affected by the overall cooling.The worse the construction quality of slab indirect joint concrete is,the lower the critical temperature rise caused by damage and destruction.When slab indirect joint concrete is reduced to C35,the critical temperature rise caused by crushing decreases to 38.2℃.The construction quality should be strictly controlled when pouring slab indirect joint.The damage location of the track structure at the indirect joint of the slab is a "flat spot"upwarp centered on the joint.The maximum vertical upwarp deformation of track structure occurs between the time when the slab surface and the slab reach their maximum temperature.In site renovation and maintenance,attention should be paid to the upwarp deformation of ballastless track under the high temperature extreme period of slab surface and slab central section.(3)Based on the non-linear analysis model of the upwarp,the influence of different factors on the deformation of the upwarp at the end of the track slab is studied.Based on the non-linear analysis model of the upwarp,the effects of external temperature load conditions,interlayer interface bonding state and interplate connection state on the deformation of the upwarp at the end of track slab were studied.The results show that the overall temperature rise is the main temperature load condition of the upwarp on the track slab,and the positive temperature gradient will aggravate the occurrence of the upwarp disease on the track slab.Increasing the interfacial bond strength and toughness is beneficial to controlling the upwarp deformation and interlayer interface damage of track structure,and increasing the interfacial stiffness will accelerate the interlayer damage.Interlayer separation joints can accelerate the occurrence of upwarp on track slab,which is more disadvantageous than edge separation joints.The narrow joint defect will make the track structure in the state of upward eccentric compression,directly leading to the occurrence of the upwarp,and the greater the degree of the defect,the greater the upwarp amplitude,especially when the height of the narrow joint damage exceeds 60mm,the change speed of the upwarp has a significant increase trend.Vertical defect height of narrow joints is more disadvantageous than transverse defect length.When the damage height of narrow joints is 20mm,the overall temperature rise of 14℃ has reached the maintenance limit of track off-joint.When the overall temperature rise of 29℃,the speed-limiting treatment is needed;when the damage height of narrow joints is 100mm,the overall temperature-increasing range of speed-limiting treatment is reduced to 18℃.The site should pay attention to the state of narrow joints.Once the damage of narrow joints is found,it should be repaired in time.Strengthen the interface bond between adjacent plates can restrain the upwarp deformation at the end of track slab to a certain extent,and timely maintenance and repair of the interface crack disease between slabs should be carried out.(4)Based on different ideas of upwarp renovation,different schemes of upwarp renovation are put forward,and the feasibility and effect of different schemes are studied.Based on the three renovation ideas of longitudinal connection,semi-unit semi-longitudinal connection and unit,different renovation schemes of upwarp are put forward,and the feasibility and effect of different renovation schemes are studied.The results show that the upwarp volume of track slab can be reduced by 64%when two steel bars are implanted at the end of the slab.Continuous increase in the number of steel bars implanted will not improve the effect of upwarp treatment.It is suggested that two steel bars be implanted at the end of the slab.The closer the planted steel bars are to the end of the slab,the higher the treatment efficiency.In semi-element semi-longitudinal renovation,the slit at the indirect joint position can effectively release part of the temperature force.Under extreme temperature load,the maximum upwarp displacement at the end of the track slab can be reduced to less than 2mm,and the range of temperature rise can be increased to more than 34℃.The renovation effect is obvious.However,special attention should be paid to the problem of the slit interface close to the side of the slit under cooling load.The force of track slab is better than that far away from the slit.It is suggested that the slit should be arranged equidistantly along the track.In unit renovation,the limit of planting steel bars in track slab can be used as a transitional treatment scheme;the limit of buckling type can be used as a temporary or auxiliary measure;the limit of circular and rectangular convex platform makes the track structure in good state of stress and deformation,but the stress concentration of rectangular convex platform is relatively higher than that of circular convex platform.