Calcium Leaching Of Ballastless Track Under Train Load And Stray Current

Ballastless track has become the main track structure of high-speed railway and urban rail transit due to its high smoothness,high stability and less maintenance.Although it has many advantages,the ballastless track still has some damage under the long-term effect of train load and environmental factors,such as the common interlayer mud-pumping in the area of abundant precipitation,groundwater enrichment and poor drainage.The mechanism of interlayer Mud-pumping is complex.On the basis of sufficient literature review and field investigation,firstly this paper analyzed the chemical components of mud in each stage of damage,and determined that the main chemical cause of mud-pumping is the concrete calcium leaching under the action of the dynamic water environment,fatigue of train load,stray current and the repeated erosion of interlamellar water.Then the equipment of calcium leaching coupled with fatigue load of ballastless track concrete were developed.The influence of train load on calcium leaching of ballastless track concrete was experimentally studied.Secondly,considering the influence of aggregate,the existing calcium leaching model of cement was extended to concrete,furthermore,the calcium ion migration equation under the action of electric field,the calcium ion convection equation under the action of pore solution convection,the acceleration coefficient of fatigue action and the acceleration coefficient of dynamic water erosion were supplemented and improved,and the theoretical model of multi-factor calcium leaching of ballastless track concrete was established.Finally,the finite element model of calcium leaching on ballastless track under the action of train load and stray electric field was established,and the influence of train load on calcium leaching was analyzed.The distribution characteristics of stray DC electric field on ballastless track and its influence on calcium leaching were studied.1.The characteristics of mud-pumping of the ballastless track and the equipment of calcium leaching coupled with fatigue loadThe mud between the layers of the ballastless track has different characteristics at different stages of development and at different construction sites.X-ray diffraction analysis found that the main component of the mud from the more seriously damage sections was silica,and the color was darker,and the mud from the mild section was mainly white-yellow calcium carbonate.The the dynamic water environment,repeated erosion of interlamellar water,the fatigue effect of train load and the effect of stray current lead to serious calcium leaching damage of ballastless track.According to the stress characteristics of ballastless track under train load and the frequency of train load,the stress level 0.4,stress ratio 0.5 and 25Hz loading frequency were taken as the loading targets.A equipment of calcium leaching coupled with fatigue load were developed,which are mainly composed of unbalanced mass block,forced synchronous gear box,moment distribution beam,counterweight block,Teflon bearing,frequency conversion motor and polypropylene thermoplastic（PP）environment box.The effectiveness of the loading device is verified by testing the concrete beam under the loading device.2.Accelerated calcium leaching test of ballastless track concrete under fatigue load6mol/L ammonium chloride solution was prepared,and the self-developed device were used to carry out the calcium leaching test of C40 concrete of track slab and C15 concrete of support layer under the action of fatigue load.The cumulative calcium leaching amount and the concrete strength before and after corrosion were detected.The microstructure characteristics and calcium content of concrete were detected by tungsten filament electron microscope and mercury injection method,and the reason of calcium leaching difference between the track slab and the supporting layer was analyzed.The results show that the cumulative calcium leaching amount of track slab concrete is proportional to the square root of time.The calcium leaching rate of track slab concrete is 80.7g/day^0.5/m²,and the calcium leaching rate under train load is 133.2 g/day^0.5/m²,which increases by 65%.The calcium leaching rate of supportting layer concrete is 19.2 g/day/m²,which is 42.9 g/day/m² under train load,increasing by 123%.The rebound test shows that calcium dissolution will weaken the concrete strength,and fatigue calcium dissolution will weaken the strength more.The pores of cement matrix of C15 concrete are mainly concentrated in the pore size of 77-434nm,with high porosity and large pore size,which is easier to be eroded by water and other media,while the pores of C40 concrete are mainly concentrated in the pore size of 20-120nm,which has stronger resistance to water and other encroachment and better durability due to smaller pore size.3.Theoretical model of multi-factor calcium leaching of ballastless track concreteBased on the existing diffusion-calcium dissolution model of cement and cement mortar,the concrete was homogenized into isotropic porous medium considering four effects of aggregate dilution,torsional,interfacial transition zone and percolation,and the nominal solid phase calcium concentration,nominal capillary porosity and diffusion coefficient of concrete were calculated.Based on the equivalent parameters,the solid-liquid equilibrium curve and pore evolution equation of concrete are established,and the Nernst-Planck law is applied to complement and perfect the calcium ion migration equation under the action of electric field and the calcium ion convection equation under the action of pore solution.The acceleration coefficient of fatigue action and the acceleration coefficient of hydrodynamic scouring are determined based on the experimental and literature results.The theoretical model of multi-factor calcium dissolution of ballastless track concrete is established,and the validity of the theoretical model is verified by the results of experiments and literature.4.Calcium leaching characteristics of ballastless track under the action of train load and stray currentIn order to illustrate the harm of calcium leaching on ballastless track and highlight the effect of train load and stray current on calcium leaching,it is assumed that the ballastless track is always in an extreme state of being eroded by environmental water,and the maintenance and repair during operation are not considered.Based on the multi-factor calcium leaching theory model of ballastless track concrete,a series of ballastless track calcium leaching finite element model is established in the finite element software COMSOL.Based on Maxwell’s equations,the potential characteristics of track slab and support layer are determined,and the calcium leaching characteristics of ballastless track under the action of train load and stray current are analyzed.The results show that the calcium leaching depth of track slab is directly proportional to the square root of time,while the calcium leaching depth of support layer is linearly related to time under train load.Under the action of train load,the cumulative calcium leaching amount of track slab in 60 years is 7.13kg,the leaching depth is6.9mm,the cumulative calcium leaching amount of support layer is 38.65kg,the leaching depth is 70.6mm.The train load increases leaching depth of the slab by about 1.8 times and the leaching depth of the supporting layer by about 2.5 times.Under the joint action of train load and stray current,the calcium leaching depth of track slab and support layer is proportional to time.The cumulative calcium leaching amount of track slab and support layer in 60 years is 19.1kg and 17.46mm,and the cumulative calcium leaching amount of support layer is 90.36kg and 166.1mm,respectively.The train load does not change the calcium leaching mode of the ballastless track concrete,but greatly promotes the leaching rate.The stray current changes the leaching mode of the track slab,increases the calcium leaching rate of the supporting layer,and increases the leaching depth of the ballastless track.The leaching rate increases linearly with the electric field strength.