EXPERIMENTAL STUDY ON TEMPERATURE DISTRIBUTION OF CRTSⅡSLAB BALLASTLESS TRACK

Zhong MA，Hong XIAO，Liang GAO，Zhuoran MA，Zhongying GAN

School of Civil Engineering，Beijing Jiaotong University，Beijing 100044

Abstract：Ballastless track is a complex system under the action of multiple factors.The reaction of temperature loads could lead to track slab delamination，gap，warpage and many other problems.Taking CRTSⅡ ballastless track slab in Beijing Jiaotong University as study subject，through the study of the temperature data of track structure of site inspection，obtain the temperature distribution of CRTSⅡ ballastless track slab and the mathematical relation of slab and atmospheric temperature，and intend to provide theory support for the track plate actual maintenance.The results show that the track slab，slab base，the CA mortar layer temperature and the atmospheric temperature have the roughly identical rules，and follow well to each other；with depth increasing，the temperature variation exists delay phenomenon；At the most of time，track slab is in the situation of negative temperature gradient distribution，and the maximum of negative temperature gradient is twice bigger than the maximum of positive temperature gradient.

Keywords：ballastless track，temperature field，temperature gradient，CRTSⅡ ballastless track

Email：13274085@bjtu.edu.cn

1　Introduction

With the rapid development of high speed railway and urban rail transit in our country，ballastless track slab has been widely used.Compared with the traditional ballast track，ballastless track has the characteristics of high stability，high smoothness and less maintenance.

Ballastless track is directly exposed to the natural environment，in addition to be subjected to repetitive dynamic vehicle load，it is also affected by atmospheric temperature，solar radiation，rainfall and other environmental factors.Among these environmental factors，temperature load effect plays an important role.Temperature load can make the longitudinal consecutive ballastless track have expansion deformation and contraction stress，and the temperature gradient along the track slab depth direction can make the track slab warp and produce warping stress，which lead track slab separating from the CA mortar layer，or CA mortar layer separating from subgrade supporting layer.That is called track structure delamination.CRTSⅡ slab ballastless track structure is a kind of vertical continuous structure，temperature effect is an especially significant harm.Therefore，grasping the temperature field distribution of CRTSⅡ slab ballastless track accurately is meaningful to the temperature stress calculation，the designation and site operation of ballastless track structure.

Nowadays，the researches in factors of temperature at home and abroad is mostly about the temperature field study of road and the temperature stress study of ballastless track.And the study results on the temperature field distribution of track structure is less，and most studies use a few days in a particular area or a seasonal data as an object.However，ballastless track is widely distributed in our country.Because of regional climate differences，the track structure temperature field distribution is not the same in different parts，and the temperature distributions of the track slab and the base slab is not the same，either.Although ballastless track slab is a kind of concrete structures，it is obviously different from the road surface，bridge when affected by solar radiation and environmental factors，so we can’t apply the conclusions of these studies to ballastless track directly.

This thesis take CRTSⅡslab ballastless track as the research object，by measuring the environmental temperature and ballastless track structure temperature for a long time，acquire the temperature field distribution of ballastless track structure.

2　Test Measurement Points Introduction

Test worksite is the ballastless track model in Beijing Jiaotong University，as shown in Figure 1.The model has good light conditions and ventilation performance，accord with the actual road conditions.Due to considering summer temperature changing biggest，and having worst impact on track structure，we have a long term field monitoring on the track structure temperature from March 2015 to August 2015，every 3 minutes at a time.

Arrangement of measuring points as shown in Figure 2.In the experiment，we mainly observe the temperature of the track surface，internal and bottom，the temperature of CA mortar layer inside and bottom，supporting layer internal temperature and environmental temperature.We set up testing points in the night side，sunny side and central section of track structure，named the# 1，# 2，# 3 monitoring points，and in each monitoring point arrange 9 sensors along the depth direction of track structure，including five in track slab，one in the CA mortar layer and three in the base slab.

Moreover，in order to prevent the central point breaking down which lead to the gap of monitoring data，we arrange the # 4，# 5，# 6points as control that can do further study about the temperature field on the edge of the track slab.These three points are only arranged sensors in track slab and CA mortar layer.We also have two sensors to test environmental temperature.The total is 48.The vertical layout depth of temperature sensors are shown in Table 1.

3　Distribution of Temperature Field and Change Rule

3.1　Slab temperature changing with temperature in a single day

Selecting high temperature environment（July 9）and low temperature environment（March 20）to study temperature change.Figure 3 is daily environment temperature，slab surface temperature，the slab base temperature change rule.

Figure 3 shows：

（1）Whether the environment temperature is high or low，the environment temperature variation regularity is approximately the same.Single day temperature variation amplitude is large，the features of extremum is obvious.

（2）Daily temperature of the slab surface and the base changes with the changes of environmental temperature.Track slab surface temperature change is big，while single day extremum characteristic is obvious.And the slab base temperature change is small，while single day extremum characteristic is not obvious.

（3）Compared with the environment temperature，daily maximum temperature of the slab surface appears the time lagging of 1-2 h，while the tine of daily minimum temperature is different.The highest temperature of slab central surface is 5.5 degrees higher than the environment’s.The highest temperature of slab sun side surface is 6.75 degrees higher than the environment’s.The highest temperature of slab shadow side surface is 4.69 degrees higher than the environment’s.Under the influence of the track slab and the mortar layer’s cover，heat transmission speed slow down.There is hysteresis in daily maximum temperature of the slab base appearing，which is about 7 h.

3.2　Track slab vertical temperature changes

Temperature change of the central track slab（measuring point 2）is more stable than sun side and the shadow side，so take central track slab to study vertical temperature changes.Figure 4is for the track slab temperature change of different depth over time.

It can be seen in Figure 4：

（1）Temperature change trend of different depth of track slab is basic same with environment temperature change trend.But with the depth，temperature change trend will have hysteresis，and the daily maximum temperature difference gradually decreases，in other words the temperature gradient decreases.

（2）Due to track slab surface is directly exposed to the environment，heat exchange with the environment is quicker，so it is influenced by environmental temperature more greatly.Thus it’s warming faster during the day，cooling faster during the night，and the temperature difference is large.Slab base is difficult to exchange heat with the environment，so the temperature transformation is relatively slow and steady.Affected by the sun during the day，slab surface heats up rapidly；temperature of slab top is higher than the bottom of the plate.The heat transfers from the surface to the base，forming a positive temperature gradient；at night，the heat of slab top loses quickly，temperature is below the bottom of the plate，heat transfers from the base to the track slab surface，forming a negative temperature gradient.

（3）With the depth of the track structure，changes of temperature amplitude decreases.When the depth of the slab is more than 20 cm，temperatures don’t change basically.

Figure 5 is for track slab temperature changing with depth of different time in March 20 and July 9.

Comparing two figure lines in Figure 5 can be found：

During 8 o’clock to 12 o’clock，slab top temperature gradient is roughly same，but in the bottom of the plate，the high temperature environment temperature gradient significantly greater than low temperature environment；During 12 o’clock to 14 o’clock，change of temperature with depth is basically the same，but in the low temperature，environment temperature difference is bigger，so the temperature gradient is bigger.About 16 o’clock，due to track slab surface in high temperature circumstances has cooled，it begins to appear negative temperature gradient in the top.And in the low temperature environment，the top is still in the positive temperature gradient，a temperature change is larger than the environment of high temperature，which is a kind of hysteresis.During 18 o’clock to 22o’clock，the change of temperature with depth trends basically，but at the top of the slab temperature gradient is bigger in the environment of high temperature，affected by the environment more obvious.

4　Temperature Gradient Change Rule

4.1　Temperature gradient analysis

Using 3-8 month temperature detection data of track structure to calculate the temperature gradient of different parts of track structure，and the 3-8 month temperature gradient change rule is shown in Figure 6.

From Figure 6，can be found：

（1）The highest positive temperature gradient of track slab is 97.5 ℃/m，appearing in the April 12；Maximum negative temperature is 44.6℃/m，appearing in the April 6.And absolute value of maximum negative temperature gradient is about twice the absolute value of positive temperature gradient.

（2）In July，temperature gradient is larger than other month，while the time of larger negative temperature gradient appearing concentrating in April.

Through the statistical analysis of track slab temperature gradient，track slab temperature gradient statistical analysis diagram can be get，as shown in Figure 7.

From Figure 7，can be found that the vertical temperature gradient are mainly distributed in-35 to 15℃/m，accounting for 72.59％.The vertical temperature gradient under 0 ℃/m accounting for 68.04％，including distribution in-35 to-10℃/m accounted for 55.51％.The frequency of the vertical temperature gradient greater than 0℃/m is 31.96％，in particular，the positive temperature gradient are mainly distributed in 0 to 15℃/m，and in 15 to 85℃/m range distribution more uniform.Temperature gradient greater than 85 ℃/m is only accounted for 1.14％.Visible，negative temperature gradient is priority in the track slab temperature gradient，which effects more on the track structure.

4.2　Comparative analysis of track slab temperature gradient at different positions

By the gradient variation and statistical figure of sun side，shadow side and the central track slab，temperature gradient distribution similarity in the three parts can be found：

（1）Minimum and maximum temperature gradient of track slab is appearing on April 12and April 6.And during March 2015 to August，the time larger temperature gradient is concentrated in June and July，the time of smaller temperature gradient is concentrated on April.

（2）Negative temperature gradient is larger than positive temperature gradient.The proportion of sun side，central，and shadow side of negative temperature gradient is accounted for 65.26％，62.12％，59.27％.And the temperature gradient of three parts is more in the distribution of interval-35 to 10 ℃/m，which frequency is more than 70％.

（3）The temperature gradient of upside is larger than the under part，which is a kind of characteristics of three parts.For example，the maximum temperature gradient of upside of track slab is 97.5 ℃/m，while maximum temperature gradient of under part of track slab is only 71.77℃/m.There are the same rules in the sun side and the shadow side，which shows that temperature gradient at the bottom of the track slab，is smaller than the upper.

（4）Temperature gradient distribution of maximum range is the same.For example maximum temperature gradient distribution range of sun side，central and shadow side is all-20 to 25℃/m，temperature gradient distribution of slab surface to slab base has a similar rule.

By the gradient variation and statistical figure of sun side，shadow side and the central track slab，temperature gradient distribution differentia in the three parts can be found：

（1）The ratio of maximum positive temperature gradient and maximum negative temperature gradient is different in three parts of track slab.The maximum temperature gradient of three parts can be seen in Table 2.

As shown in Table 2，maximum positive temperature gradient of sun side is 1.56 times that of maximum negative temperature gradient，while maximum positive temperature gradient of shadow side is 3.01 times that of maximum negative temperature gradient.It can be seen from that there is difference between different parts horizontally.

（2）The maximum temperature gradient of sun side is largest while the maximum temperature gradient of shadow side is smallest.For example，the maximum temperature gradient of sun side，central，and shadow side is 121.4 ℃/m，105 ℃/m，97.5 ℃/m.Similarly，temperature gradient of slab surface to slab base has the same rule.

（3）The proportions of three parts is different.The proportion of negative temperature gradient of sun side，central，and shadow side is 65.26％，62.12％，59.27％.It can be seen that negative temperature gradient distribution is reduced gradually，and at the same time，positive temperature gradient distribution is increased gradually

5　Conclusions

（1）The temperature of slab track，CA mortar layer，slab base with a good following and periodic performance changes in the consistency with the rule of the air temperature，which keeps the one-day variation period .With the increase of the depth of the track structure，the fluctuation of temperature and the temperature gradient decrease gradually.Temperature change takes on a kind of delay phenomenon in the change of the depth of the slab.If the depth is beyond the track plate thickness（20 cm），the temperature change is stable，and the fluctuation is very slight.And so do the temperature of CA mortar layer and base layer.Temperature load effect has a big influence on the surface of the rail plate .When the depth of the track plate is more than 20cm，the load effect could be ignored.

（2）The negative temperature gradient accounted for 70％ of temperature gradient，and the maximum of negative temperature gradient doubles the maximum of the positive one.Therefore，at most of the time the track plate appears in a negative temperature gradient.Also，inner temperature is higher than the surface temperature.The negative temperature gradient is the main factor affecting the track structure.

（3）Distribution law of track plate temperature gradient is similar in sunny，the central，shaded side，but there are also differences.The positive and negative temperature gradient on the sunny side is higher than that in the central and the night side .The negative temperature gradient also takes for a bigger proportion.So temperature load effect has a greater influence of the sunny side.

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China（No.51578055），the Science and Technology Research and Development Plan of China Railways Corporation（No.2014G003-F），the National Training Program of Innovation and Entrepreneurship for Undergraduates（No.201510004041）.

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ICRE2016-International Conference on Railway Engineering