| In view of the problems existing in CRTS II slab ballastless track,such as the weakening of interlayer strength between track slab and mortar layer,gap under track slab,damage of joints between track slabs,over-limit and uneven distribution of cracks at the end of track slab,by studying the longitudinal forming process of track structure in detail,it is pointed out that II slab has unique longitudinal connection characteristic and structural characteristic.The longitudinal connection characteristic makes great differences between track slab temperature T when mortar layer hardens,track slab temperature_zT when narrow joints harden,track slab temperature_lT when track slabs connected by tension,and track slab temperature_kT when wide joints harden.The structural characteristic shows that the number of longitudinally connected bars at the end of slabs is significantly less than that of in the middle.Through the systematic study of the indoor shear tests between track slab and mortar layer and the on-site track slab push plate tests,the interaction mechanism between track slab and mortar layer was obtained.Based on the longitudinal connection characteristic and structural characteristic of CRTSII slab,the initial temperature effect of track slab and its influence on cracks of track slab and the gap under track slab were analyzed,and the corresponding solutions are put forward.The main conclusions are as follows:In the longitudinal forming process of track structure,the impacts of track slab expansion in the process of the longitudinal connection of track slabs on the bonding state between track slab and mortar,the initial stress at the end of tensioned rebars when wide joints harden,and the stresses of narrow joints were analyzed.It is pointed that the constraint effect of mortar layer on the deformation of track slab expansion is small,and the principle of bonding limit of the existing CRTSII slab needs further study.In order to maintain the strong restraint of mortar layer on track slab,it is recommended to increase the critical displacement value when the constraint of mortar layer on track slab reaches the maximam.In the longitudinal forming process of track structure,action characteristics of the tensioning force of tensioned rebars are studied.The re-compression of narrow joints caused by the tensioning force cannot be ensured,if track slabs are connected by tensioning force after the concrete hardening of narrow joints,compressive stresses of narrow joints can be increased about 3.01~3.09 MPa.When wide joints harden,the stresses of narrow joints are mainly determined by the temperature difference of(T_k-T_z)and the tensioning force.In order to keep the compressive stresses of narrow joints existent when wide joints harden and the function of tensioning force not weakened,the pouring temperatures of the joints concrete can’t be controlled only in constuction,it is also suggested to take measures to control _zT,_kT and_lT,and make_kT and_lT not less than_zT.The influencing factors and the change rule of crack widths both at the end _kωand in the middleω_zof track slab are studied.The most influential factors on_kωare the initial tensioning force at the end of tensioned rebars and the initial compressive stresses of narrow joints when wide joints harden,and there is also whether track slab has a crack in the middle or not,and both the bonding state between track slab and mortar layer and the designed tensioning force of tensioned rebars have little effect on_kω.The initial stress at the end of tensioned rebars when wide joints harden is the most influential factor onω_z,and the bonding state between track slab and mortar layer has little effect on_zω.When track slab temperature decreases 30℃and 40℃respectively,the numeric ranges of_kωare 0.182~1.906mm and0.389~2.546mm respectively,the numeric ranges of_zωare 0.202~0.429mm and0.323~0.429mm respectively.Crack widths of track slab distribute unevenly as a result of the longitudinal connection characteristic,and the numerical value of crack width is large because of the weak connection between track slabs.Influenced by the longitudinal connection characteristic and structural characteristic of track slab,the variation range ofω_kis large,and _kωis generally larger thanω_z,which are consistent with the crack characteristics of CRTSⅡslab track at present.The impacts on the gap under slab by both the initially uneven loading of joints and the damage of narrow joints were studied.When track slab temperature increased 40℃and its temperature gradient increased-50~100℃/m simultaneously,the most influential factor on both the gap under track slab and rail irregularity is the crush of narrow joints due to the large bearing capacity,and secondly is wide joints bearing only,the last is the well-distributed stresses of joints or there is a certain initial compressive stress of narrow joints when wide joints harden.Among them,the gap under track slab and the rail irregularity are both small if there is a certain initial compressive stress of narrow joints when wide joints harden.This is consistent with the design concept of CRTSⅡslab,which is the compressive stresses of narrow joints should be always existent when wide joints harden.Although the value of the gap under track slab caused by the uneven initial stresses of joints is small(increased about 0.5~1mm),it will greatly increase maintenance work of the mortar layer during the later operation and maintenance period.When track slab temperature is high,in order to reduce the gap under track slab,the damage of joints between track slabs,the upwarp of track slab and rail irregularity caused by it,it is suggested to take measures during constuction to reduce the difference of_kT and_zT,and make_kT higher than_zT by no more than10℃.Therefore,when track slab temperature is high,the narrow joints and wide joints can bear capacity at the same time,moreover,the bearing of narrow joints is slightly larger,and the wide joints bearing only and the crush of narrow joints caused by bigger compressive stresses should be avoided. |
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