INFLUENCE ON TRACK STRUCTURE CAUSED BY CRACK AT WIDE-NARROW JUNCTURE OF CRTSⅡ SLAB-TYPE BALLASTLESS TRACK

Yanan LIU1，2，Liang GAO1，2，Xiaopei CAI1，2，Yanglong ZHONG1，2，Dacheng LI1，2

1School of Civil Engineering，Beijing Jiaotong University，Beijing 100044

2Beijing Key Laboratory of Track Engineering，Beijing 100044

Abstract：Wide-narrow juncture is the connection of old and new concrete between track slabs of CRTSⅡ slab-type ballastless track，which results in the crack tending to arise in this place frequently.To determine the influence of crack at wide-narrow juncture，this article built damage model using the finite element software ANSYS，studied the impacts of the crack with different widths and depths on track structure mechanical properties under temperature dropping condition.Results show that the crack depth has great influence，but the crack width had no obvious effects.Under temperature dropping condition，when the depth of cracks is 0.1 m，width is greater than or equal to 0.1 mm，the slab and mortar are broken because of their longitudinal stress exceeding the ultimate strength.When the width of cracks is 0.1 mm，depth is greater than or equal to 0.06 m，the slab and mortar is also broken.So this thesis suggests the depth of crack is used to be the control element，while its values is greater than or equal to 0.06 m，the maintenance measure，such as low pressure grouting for example，should be taken in time.

Keywords：CRTSⅡ slab-type ballastless track，wide-narrow juncture，crack，temperature drop

Email：962683498@qq.com

1　Introduction

CRTSⅡ slab-type ballastless track consists of rail，elastic fastener system，precast track slab，cement and emulsified asphalt mortar（CA mortar），supporting layer and so on.The precast track slabs are connected to each other by six Φ20 mm finishing rolling rebar and then form longitudinal continuous structure[1].Among track structure parts，the wide-narrow juncture is a connection of two track slabs，and is cast-in-site concrete.So the wide-narrow juncture and track slab form the interface of old and new concrete，which is easy to bring shrinkage and drying crack due to poor bond strength.In addition，under temperature condition concrete will produce expansion deformation and warping，once the deformation is too large and tensile stress exceeds the tensile strength of the concrete，the interface will produce temperature crack which is shown in Figure 1.If the crack at wide-narrow juncture can’t be repaired in time，it will continue to degrade.When the rain water and harmful medium penetrate the track structure through the crack，the material of concrete and mortar will be possible to produce greater damage and degradation，even the track structure can be destroyed under the coupling actions of traffic load-water-temperature.According to all that said，the crack at wide-narrow juncture directly affect the service performance and durability of track structure[2-4].

2　CRTSⅡ Slab-type Track Model

2.1　Calculation parameters and model

Based on the finite element theory of beam-solid[5]，the CRTSⅡ slab-type ballastless track model was established using structural parameters in Table 1.A schematic diagram of the model is shown in Figure 2.In modeling，rail is simplified as a continuous elastic point support beam，and simulated by BEAM188，fasteners can be equivalent as spring with considering its longitudinal，lateral and vertical drag，in order to select the parameters easily，the track slab，wide-narrow juncture，mortar layer and supporting layer are simulated by solid element because of their regular shapes，subgrade is simulated by continuous and uniform linear spring and its equivalent vertical stiffness is obtained from foundation coefficient[6].And the birth-death element method is used to simulate the cracks at wide-narrow juncture.Given that CRTSII slab-type ballastless track is a longitudinal continuous structure，the model length is 97.5 m，as the same length as 15 pieces track slab.In order to ignore the influence of boundary，the crack position is at the interface of middle track slabs and wide-narrow juncture with fixed constraint at both sides of model.

2.2　Calculation loads and conditions

The crack at wide-narrow juncture leads to the failure of track slab longitudinal connection，which changes the mechanical properties of track structure under temperature，especially at high temperature and in cold condition.The crack will close at high temperature and have small impact on track.But the crack will continue to expand in cold condition and then exacerbate the impact on track.Therefore，this paper just considers the whole dropping temperature，and while making calculations，the dropping temperature of 40 ℃ is applied to track structure.

The survey on site indicates that cracks at the interface of track slab and wide-narrow juncture are common phenomenon.Most of cracks are lateral through，the width is generally 0.1 mm-2 mm，the maximum can reach 2 mm to 3 mm and changes with the temperature.Thus，while making calculations，an assumption is made that the crack throughout the whole width direction and the calculation just consider the variation of crack width and depth.Calculation conditions are as follows：

（1）While considering the impact of crack width，the depth of crack at wide-narrow juncture is 0.1 m and values of width are 0 mm，0.1 mm，0.2 mm，0.5 mm，1 mm，2 mm，3 mm，4 mm.

（2）Because the limit value of crack width is 0.2 mm in calculation，while considering the impact of crack depth，the width of crack at wide-narrow juncture is 0.2 mm and values of width are 0 m，0.06 m，0.08 m，0.1 m，0.15 m，0.2 m.

3　Influence of Crack Widths

In this section，the influence of cracks with same depth but different widths on the mechanical properties of track structure is calculated under the dropping temperature of 40 ℃.The results of rail are shown in Figure 3-Figure 5，results of track slab in Figure 6-Figure 9，and other results in Table 2.

The calculations results of rail show that after crack showing up，the rail longitudinal force and displacement change suddenly near the crack，but with the value of crack width increasing，the values of rail longitudinal force and displacement are almost the same.Concrete calculation results of rail are described as below.When the track structures are in good condition，the longitudinal force is 765.12 kN，longitudinal stress 98.79 MPa，vertical displacement 0.256mm.After the crack existing，the longitudinal force and displacement of rail change.While the width is 0.1 mm，the longitudinal force is 766.49 kN，the maximum vertical displacement is 0.256 mm，minimum is 0.193 mm which shows that rail has a vertical irregularity.While the width is 4 mm，the longitudinal force is 766.55 kN，the maximum vertical displacement is 0.282 mm，minimum is 0.197 mm.In a word，crack width increasing has little influence on the rail mechanical properties.

The calculations results of track slab show that after crack showing up，the track slab longitudinal stress and displacement change suddenly near the crack，but with the value of crack width increasing，the track slab mechanical properties changes unobviously.Concrete calculation results of track slab are described below.When the track structure are in good condition，the longitudinal stress distribute uniformly，is 14.402 MPa and the vertical displacement is 0.259 mm.While the width of crack is 0.1 mm，the longitudinal stress tense is-8.760 MPa，the maximum vertical displacement is 0.282 mm，the minimum 0.076 mm.While the width is 4 mm，the longitudinal stress tense stress turns to impress stress，and the value is-8.945 MPa，the maximum vertical displacement is 0.285 mm，the minimum 0.076 mm.Under dropping temperature，the crack will continue to expand，but the value changes are very small in different widths.After calculation，the width of crack which was 0.1 mm changes to 0.243 mm，and the width of crack which was 4mm changes to 0.249 mm.These above results show that crack width increasing has little influence on the mechanical properties of track slab.

The results in Table 2 show that ballastless track stress and deformation change suddenly after the crack shows up at the wide-narrow juncture.The longitudinal stress，lateral stress and vertical stress of track slab increases near the crack，and the longitudinal stress near crack appears stress concentration，causing a larger tensile stress，which exceeds the tensile strength of the concrete，and then the crack will continue to expand.The longitudinal stress，lateral stress and vertical stress of mortar layer suddenly increase after crack existing，and when the crack depth is 0.1 m and the width is equal to or larger than 0.1 mm，the longitudinal stress of mortar layer is about 4 MPa，which has exceeded the limit of tensile stress，about 3 MPa，so the crack will lead to collateral damage of mortar layer.The stress and deformation of supporting layer suddenly increase after the wide-narrow juncture appears crack，but the amplitude is smaller than track slab and mortar layer.

As is known from the results in the table and figures，the crack width has small impact on the stress deformation of track structure.But when the depth of cracks is 0.1 m，width is greater than or equal to 0.1 mm，the crack will continue to expand and mortar layer is broken because of their longitudinal stress exceeding the ultimate strength.

4　Influences of Crack Depths

This section calculates the impact of crack with the same width but different depths on force and deformation of ballastless track during temperature drop 40 ℃.The results of rail are shown in Figure 10 to Figure 12，results of track slab in Figure 13 to Fiture 16，and other results in Table 3.

As is known from the rail calculation results，when the wide-narrow juncture cracks and the widths of crack are unchanged，the increasing of crack depth has an obvious impact on the rail longitudinal force and displacement during temperature drop.The longitudinal force，longitudinal displacement and vertical displacement of rail increase with the increasing of the crack depth，the rail arches near crack.When the crack depth is 0.02 m，the rail longitudinal force is 765.2 kN，rail vertical displacement of arch is-0.254 mm，sinking displacement is-0.256 mm；when the crack depth is 0.2 m，the rail longitudinal force is 775.63 kN，rail vertical displacement of arch is 0.173 mm，sinking displacement is-0.405 mm，relative displacement is 0.578 mm，vertical irregularity of rail is obvious.Overall，the crack depths have a greater impact on the rail.

The track slab calculation results show that when the crack shows up and the widths of crack are unchanged，the increasing of crack depths has a large impact on the track slab longitudinal stress and displacement under the dropping temperature.When the crack exists，the longitudinal stress and displacement of track slab surface increases with the increasing of crack depths.According to the calculated results，when the crack depth is 0.02 m，the longitudinal stress of the track slab surface suddenly reduces to 5.868 MPa，at the same time，the vertical displacement of track slab upturns at the crack，sinks near the crack，and the upturned displacement is-0.255mm，sinking displacement is-0.259 mm；when the crack width is 0.2 m，the longitudinal stress of track slab surface suddenly reduces to-20.470 MPa，upturned displacement of track slab surface is 0.673 mm，sinking displacement is-0.407 mm.In addition，crack width increases with the increasing of crack depth under the dropping temperature.While the crack depth was 0.02 m before calculation，the crack width rises to 0.029 mm and the crack depth was 0.2 m，the crack width rises to 0.744 mm.When the depth of crack is greater than or equal to 0.06 m，the change rate increases.In summary，the crack depth has an obvious impact on the force and deformation of track slab.

Ballastless track stress and deformation change suddenly in the wide-narrow juncture crack.The longitudinal stress，lateral stress and vertical stress of track slab increases near the crack，and the longitudinal stress near crack appears stress concentration，causing a larger tensile stress，which exceeds the tensile strength of the concrete，then the crack will continue to expand and the damage aggravates.The stress and deformation of mortar layer also change suddenly in the wide-narrow juncture crack and the longitudinal stress，lateral stress and vertical stress of mortar layer suddenly increase after crack existing，and the stresses increase rapidly with the deepening of the crack.The limit tensile strength of mortar layer is about 3 MPa，but when the crack depth is equal to or larger than 0.06 m，the longitudinal stress of mortar layer has exceeded the limit of tensile，resulting in the crack and damage in the vicinity of the wide-narrow juncture，so the crack will lead to collateral damage of mortar layer.The stress and deformation of supporting layer changes in the vicinity of the wide-narrow juncture crack，the longitudinal stress，lateral stress and vertical stress of supporting layer suddenly increase after the wide-narrow juncture appears crack，but the amplitude is smaller than track slab and supporting layer.

5　Conclusions

This paper analysis the influence of crack with different widths and depths on the ballastless track and CWR under the dropping temperature，the conclusions goes as following：

（1）Compared with no crack at the interface of track slab and wide-narrow juncture，the stress and deformation of track structure have marked changes after the crack appearing.

（2）This paper research shows that the crack depth has great influence on track structure，but the crack width had no obvious effects under temperature dropping condition.The concrete calculation results are that while the depth of cracks is 0.1 m，width is greater than or equal to 0.1 mm，the slab and mortar are broken because of their longitudinal stress exceeding the ultimate strength；while the width of cracks is 0.1 mm，depth is greater than or equal to 0.06 m，the slab and mortar is also broken.

（3）During temperature drop，the crack will continue to expand，even destroy the mortar layer，and crack depth is the control factor of influence.According to above results，a suggestion is made，which is that when crack shows up and its depth is greater than or equal to 0.06 m，the maintenance measures，such as grouting without press，should be taken.Because the tension fracture will appear in winter when construction is taken in summer，and the crush of track slab may happen in summer when construction is taken in winter.So the maintenance measures should choose proper temperature to avoid new problems.

Acknowledgement

The authors wish to acknowledge the motivation supported by the China Railway Corporation（No.2014G001-F）.

References

[1]Xu H.，Liu X.，Xu J.H.，et al.，2013.Influence on CRTS Ⅱ Slab Track Structure Caused by the Deboning between Slab and CA Mortar under the Action of Temperature Load[J].Railway Standard Design，9：9-12.

[2]XU H.，Xie K.Z.，Chen R.，et al.，2012.Influence on Track Slab Caused by Crack and Repairing Material at Wide Juncture of CRTS Ⅱ Slab-type Track[J].Railway Standard Design，7：30-32，37.

[3]Lou L.W.，Xie Y.J.，Xin X.Z.，Zhong X.H.，2015.Study on Crack in Interface between Track Slab and Wide Joint for CRTS Slab-Type Ballastless Track[J].Railway Engineering，1：98-101.

[4]Huang H.S，Zeng Y.，Xu G.X.，et al.，2014，Effect on longitudinally Continuous Reinforcing Bars’Mechanical Property Caused by the Cracking at Wide Juncture of CRTS Ⅱ Slab Ballastless Track on Bridge[J].Railway Standard Design，2：33-36.

[5]Liu X.Y.，Zhao P.R.，Yang R.S.，Wang P.，2010.The Ballastless Track Design Theory and Design Method of Passenger Dedicated Line[M].Chengdou：Southwest Jiaotong University Press.

[6]Wang X.M.，2007.ANSYS Numerical Analysis of Engineering Structure[M].Beijing：China Communications Press.