stray current experiment and analysis...

STRAY CURRENT EXPERIMENT AND ANALYSIS FOR METRO TUNNEL

Yongjing Tang (1, 2) and Koujun Wei (1,2)

(1) Department of Geotechnical Engineering, Tongji University, Shanghai 200092， China

(2) Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education,Tongji University， Shanghai, 200092， China

Abstract

Deterioration of reinforced concrete structures of Metro tunnel is often caused by corrosion of steel reinforcement due to stray current. Although some measures have been taken for decreasing stray current, extension construction always changes the original protection system that steel reinforcement has been welded together for mitigating stray current. However, the difference of stray current before and after extension construction of metro line is unknown. And the challenge is how to guide the later extension construction. This paper presents preliminary experiment to simulate the Metro tunnel in the extension condition with two steel reinforcement cages weled and binded by tie wire which are given impressed current. The results provided show significant differences between the cages. The further experiment with the same steel reinforcement cages embedded in concrete will be carried on recently.

Keywords stray current; Metro tunnel; steel reinforcement cage; tie wire; weld; extension construction

1. INTRODUCTION Stray current is a kind of random current that leaks to some areas surrounding running rails

in Metro tunnels. It is inevitable that stray current occurs when train locomotives are driven by the direct current (DC) and not all of the DC goes back to power substations,as shown in Figure1(a)[1]. Simplified track-to-earth voltage/stray current circuit model is shown in Figure1(b)[2]. The stray current reduces power efficiency of the DC system, causes corrosion to adjacent ferrous or conductive objects and structures.

When the electrons find a path that offers less resistance — a steel reinforcement network, for instance — they jump the track, and follow the network instead. Therefore, the steel reinforcement embedded in the concrete is often welded to let the electron pass fluently so that the stray current in the steel reinforcement can go back to power substation. However, during extension construction of Metro tunnels, the original welded steel reinforcement network is changed, the electron in the steel reinforcement network goes out from the relative

2nd International Symposium on Service Life Design for Infrastructure 4-6 October 2010, Delft, The Netherlands

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small resistance points, such as the crack position[3]. Consequently , we take some measures to mitigate stray current[4-7]:(1)providing secondary measures of stray current collection using collector mat underneath the rails; (2) measuring the system voltage at each collector mat (Rail to Ground tests); (3) ensuring design does not inadvertently provide path to ground through items that are connected to track .i.e. from rail heaters or switch machines.

(a) stray current in Metro line (b) simplified track-to-earth voltage/stary current circuit model

RN: resistance of negative return circuit;RP: resistance of positive circuit;RL: track-to-earth resistance at the load end;RS: track-to-earth resistance at the source end;IT: train operation current;IH: current return through the rails;IL: leakage current to earth at the load end;IS: current returning to the substation through earth;VS: substation voltage;VGL: track-to-earth voltage at the load location;VGS: track-to-earth voltage at the substation location;VN: voltage developed across RN by IN

Figure1: Stray current in Metro line and simplified track-to-earth voltage/stray current circuit model

With the time, cracking and leaking water in Metro tunnel lead to resistance decrease and more stray current. It means that stray current is increased although the measurements mentioned above are taken especially when extension construction of Metro tunnel occurs.

Based on the maintenance difficulty degree and cost, steel reinforcement embeded in the tunnel concrete structure is more important than that embeded in ground level underneath running rails and the running rails themselves. The running rail can be replaced easily, but the steel reinforcement in the structure underground is very difficult to replace.

According to reports of Shanghai Metro, fixed traction current of the train is 3000A[8]. Genererally, running rails are intermittent within the range of power substation, so resistance is increased because of the joints of rails. If joint resistance is more than 0.1Ohm because of oxidization, pollution and looseing etc., potential differences of two adjacent rail is 300V, which will generate stray current. And likewise, the joint of the cathode of traction substations and the rail may produce potential differences. Part of this stray current goes through steel reinforcement embedded in concrete of Metro tunnel.

During the last decade, Metro extension construction is frequently carried out in Shanghai’s Metro system. Since designed holes have been opened in the original network of the welded steel reinforcement, stray current flowing into the tunnel structure increases. The acceleration of extension construction in the next decade will result that the welded steel reinforcement network changes more in order to add the parking and underground shopping mall which will connect to subway stations. But the steel reinforcement in the old and new structures may not at a good electric connection. Joints in reinforcements are a prime example of this, since if electrical continuity is not complete, current must enter the electrolyte (concrete) in order to pass from one reinforcing steel member to the next.

To predict the stray current and potential through the steel reinforcement network or cage embeded in the tunnel structure, two different connected ways are compared in this paper. A welded steel reinforcement cage and a steel reinforcement cage binded by tie wire carry the

stray current


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same impressed DC. Therefore, this paper focuses on how the steel reinforcement embedded in concrete for Metro tunnel by two different connecting ways influence the stray current. In the experiment, the impressed current is used to simulate the stray current.

2. EXPERIMENT

(a)steel reinforcement cage joint binded by tie wire (b)two steel reinforcement cages welded and binded by tie wire

(c) DC Power Supply

(d)digital multimeter (e) measurement point

Figure 2: Experiment specimens and equipment

Two specimens simulate subway station by 1:100 scale approximately with the dimensions of 1000mm and 400mm and 200mm. Two steel reinforcement cages that one is welded and the other is binded by tie wires are shown in Figure 2(a) and (b). The reinforcement diameter is 20mm in transverse and 16mm in longitudinal direction. The DC Power Supply(SWYJ-26005) shown in Figure 2(c) is employed to supply electric current DC. The digital multimeter in Figure2(d) is used to measure the potential and the resistance between the measurement points. In this paper, the points shown in Figure 2(e) are choosen to be measured for the potential between them when electirc current DC is 2.0A, 2.5A, 3.0A,3.5A and 4.0A, respectively.


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3. MEASUREMENT AND DISCUSSION Before the experiment, the resistance of the two cages between the measurement points is

measured and the reasults are listed in Table 1 below.

Table 1: The resistance between the measurement points (Ohm)

cage by tie wire cage welded

Point 0-1 1.3 1.2

Point 0-2 1.2 1.0

Point 0-3 1.1 0.9

Point 0-4 1.0 0.9

Point 0-5 1.2 1.0

Then the potential of Point 0-1 is recorded when the electric current is 2.0A,2.5A,3.0A,3.5A and 4.0A. Each set of data is read every other minute and the results are expressed with the diagram. Figure3 presents the potential value versus time of Point 0-1.

(a) impressed current is 2.0A (b) impressed current is 2.5A

(c) impressed current is 3.0A (d) impressed current is 3.5A


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(e) impressed current is 4.0A

Figure 3: Potential versus time of Point 0-1

Similarly , the potentials of Point 0-2,Point 0-3,Point 0-4 and Point 0-5 are measured. The results are as follows:






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1144







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From Figure3-7, we can see that the potential of the cage welded is smaller and more stable than the cage by tie wire. It proves that when the stray current goes through the steel reinforcement in the structure underground,the cage welded takes less corrosion risk than the cage by tie wire. The detailed results are shown in Table 2.

Table 2: the potential between the measurement points (V)

Point 0-1 Point 0-2 Point 0-3 Point 0-4 Point 0-5 cage current min max min max min max min max min max

I=2.0A 0.131 0.310 0.104 0.175 0.184 0.216 0.113 0.135 0.081 0.233I=2.5A 0.162 0.396 0.132 0.223 0.233 0.278 0.137 0.165 0.102 0.294I=3.0A 0.195 0.490 0.158 0.276 0.283 0.341 0.165 0.200 0.123 0.357I=3.5A 0.231 0.587 0.188 0.324 0.339 0.410 0.194 0.235 0.144 0.407

cage

by tie wire

I=4.0A 0.268 0.699 0.218 0.355 0.398 0.476 0.221 0.274 0.168 0.434I=2.0A 0.061 0.207 0.056 0.070 0.050 0.071 0.053 0.085 0.049 0.172I=2.5A 0.076 0.243 0.070 0.087 0.061 0.088 0.066 0.106 0.062 0.219I=3.0A 0.092 0.326 0.085 0.111 0.074 0.107 0.080 0.128 0.077 0.275I=3.5A 0.110 0.412 0.101 0.132 0.090 0.124 0.095 0.153 0.093 0.326

cage

welded

I=4.0A 0.126 0.498 0.118 0.146 0.104 0.142 0.110 0.178 0.112 0.353

The relationship of potential versus impressed current is also given in the experiment as shown in Figure8.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0

impressed current(A)

potential(V)

0

0.1

0.2

0.3

0.4

0.5

0.6

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0


pote

nti

al(V

)

(a) cage binded by tie wire (Point 0-1) (b) cage welded (Point 0-1)


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0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0


potential(V)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0


potential(V)

(c) cage binded by tie wire (Point 0-2) (d) cage welded (Point 0-2)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0


pote

ntial

(V)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0


pot

enti

al(

V)

(e) cage binded by tie wire (Point 0-3) (f) cage welded (Point 0-3)

0

0.05

0.1

0.15

0.2

0.25

0.3

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0


potential(V)

0

0.05

0.1

0.15

0.2

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0


potential(V)

(g) cage binded by tie wire (Point 0-4) (h) cage welded (Point 0-4)

0

0.1

0.2

0.3

0.4

0.5

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0


pote

ntial

(V)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

I1=2.0 I2=2.5 I3=3.0 I4=3.5 I5=4.0


poten

tial

(V)

(i) cage binded by tie wire (Point 0-5) (j) cage welded (Point 0-5)

Figure 8: Potential versus impressed current


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From Figure8, we can see that the value of potential shows a nearly linear relationship with the impressed DC. The larger the impressed DC, the more discrete the potential between the points. The potential of the cage welded is less than the cage by tie wire. In addition, the range of variation of the cage welded is smaller than the cage by tie wire.

The further research will put the cages embeded in the concrete and simulate the tunnel structure and measure potential again. The data from the next experiment will be compared with the result in this paper. Combined with numerical method, the corrosion risk of reinforced concrete structures of Metro tunnel will be estimated.

4. SUMMARY Stray current in Metro tunnel is increased due to extension construction. The experiment

shows that steel reinforcement cages with welded and binded by tie wire have significant difference to stray current. Welded reinforcement cage takes less corrosion risk than the steel reinforcement cage binded by tie wire. The potential is nearly linear with stray current. Potential in the cage by tie wire is larger than in the cage welded. It indicates that the weled steel reinforcement network should be protected from being destroyed during extension construction. The new welded steel reinforcement network should be built even if the original welded network have been broken due to openning new holes across it. The further experiment with the same steel reinforcement cages embedded in concrete will be carried on recently.

ACKNOWLEDGMENT This paper was supported by funds from NSFC Project titled Ancient tower structure

stability evaluation (Grant 50878153) and national Kdy Technolgy R&D Program(grant 2006 BAJ27B02-02).

REFRENCES [1] Yongjing Tang, Binde Huang. Study on stray current intensity and its effect. Proceedings of 1st

international workshop on service life design for underground structures. Publish house of Tongji University(2006). 190-196

[2] Kinh D.Pham, track to earth potentials and stray current monitoring on Portland Trimet max light rail system. Proceedings of the 2008 IEEE/ASME joint rail conference(2008). 381-392

[3] Tang Yongjing, Reducing the Risk of Reinforcement Corrosion in Concrete Structures and Stray Current Prevention in Metro Line. Proceedings of the 2nd international conference on risk analysis and crisis response(2009). 355-359

[4] Code for construction and acceptance of civil air defence works [5] W.Von Baeckmann, W.Schwenk, and W.Prinz, Handbook of cathodic corrosion protection theory

and practice of electrochemical protection processes.Determination of the state of corrosion of the reinforcing steel. Gulf Publishing,1997

[6] Ronald L.Miller and Millian H.Hartt,stray current and galvanic corrosion of reinforcing steel in concrete.Material Performance(1976). 20-27

[7] Anton Kacicnik,Daryle H.,Use of new stray current mapping technique to enhance direct assessment(DA) results on urban stray current influenced pipeline. Northern area western conference.2004

[8] Shucai YANG, Xu YANG, Evaluation of stray current corrosion resistance of concrete in metro construction, Archit. Civ. Eng. China 2008,2(3):246-252


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stray current experiment and analysis...

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