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Article

The Impact of Stray Currents on Chloride Transport in the Concrete of Urban Rail Transit Structures

by
Yuancheng Ni
1,
Eryu Zhu
1,* and
Liangjiang Chen
2
1
School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China
2
China Railway Economic and Planning Research Institute Co., Ltd., Beijing 100038, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(10), 1695; https://doi.org/10.3390/buildings15101695
Submission received: 22 April 2025 / Revised: 11 May 2025 / Accepted: 15 May 2025 / Published: 17 May 2025
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

To study the chloride transport properties of urban rail transit structures under the action of stray currents, electrochemical tests were employed as part of this paper to investigate the impact of stray currents on cementitious materials and pore structure and further analyze the chloride distribution of specimens in different conditions. Results show that a stray current accelerates calcium ion precipitation in chloride solutions, reducing calcium hydroxide content compared to unelectrified specimens. This dissolution alters the concrete pore structure, increasing porosity by 26.3%, 31.2%, and 36.1% for specimens electrified at 50 mA, 100 mA, and 150 mA, respectively, after 28 days. The effect coefficient kp of stray currents on the porosity of concrete is given with the test results. Electrified specimens have a higher chloride content compared to unelectrified specimens, with free chloride increasing more than bound chloride as current and time increase. The chloride ion binding capacity of concrete electrified at 150 mA is only 60% that of unelectrified, indicating the significant weakening effect of stray currents on it.

1. Introduction

With the continuous expansion of urban rail transit networks, the impact of stray currents on the durability of structures and their ancillary facilities is drawing widespread attention. In rail transit systems, the catenary or power rail supplies power, while running rails and earth serve as the return path. However, incomplete insulation between running rails and the ground degrades over time, causing current leakage and stray current formation [1]. Stray currents significantly modify the transport of chloride ions, enhancing their migration toward metallic structures and thereby exacerbating corrosion processes [2,3,4,5]. This affects the durability of bridges, tunnels, and nearby pipelines, posing safety risks. It is important to determine the influence of stray currents on concrete resistance to chloride ion penetration for the assessment of the durability of urban rail transit structures.
Currently, research predominantly focuses on the corrosion of buried metallic pipelines and reinforcing steel in concrete structures induced by stray currents. Stray currents can cause the significant corrosion of buried metallic pipelines due to the strong polarization effect they exert on nearby metals; the severity of this corrosion is much higher than that of natural corrosion [6,7,8,9]. Xia et al. [10] conducted an on-site investigation of buried natural gas pipelines near metro lines and observed a pipe-to-soil potential disturbance of up to 200 mV, with the potential soil gradient reaching 62 mV/m. Beggs et al. [11] analyzed the localized corrosion effects of stray currents on buried metallic pipelines using data-driven approaches. Srikanth et al. [12,13] evaluated the corrosion behavior of buried water supply pipelines near metro systems and concluded that stray currents were the primary factor responsible for rapid metal corrosion in these environments. Masuda et al. [14] highlighted the severe corrosion of substructures caused by stray currents and advocated for the implementation of cost-effective technical measures to mitigate their corrosive impact. Bertolini et al. [15] conducted a corrosion experiment on reinforced concrete under current conditions, which showed that direct stray currents accelerate initial pitting on steel surfaces and increase corrosion rates, with chloride ions acting as catalysts. Chen et al. [16] demonstrated through research that the corrosive effects of direct stray currents on rail tracks surpass those of fatigue-induced and chloride salt-related corrosion. Brenna et al. [17] experimentally determined that the alternating current has a significantly lower impact on steel specimens compared to direct current and noted that even trace amounts of chlorides can substantially reduce the critical current density required for corrosion initiation. Tang [18] utilized Tafel polarization tests to investigate the corrosion behavior of steel fibers in concrete under simulated stray current exposure; the results showed that elevated levels of chloride ions (≥0.6 mol/L) increase the propensity of steel to pitting corrosion. Hernandez et al. [19,20] conducted field investigations into the corrosion of electrified rail tracks and found that stray current-induced corrosion was more severe than chloride salt-induced corrosion under identical conditions. Yin et al. [21] determined the appropriate adsorption laws for physical adsorption and chemical binding behaviors and established the chloride binding capacity equation of cement-based materials for calcium leaching. Chen et al. [22] found that the addition of FA and GGBS reduced the concentration of free chlorides in the NaCl solution and enhanced the chloride binding capacity of hardened cement paste. Liu et al. [23] discovered that the multi-ion electrochemical coupling effect can promote calcium leaching in the early stage, resulting in a coarse pore structure and accelerating the migration of chloride ions. Bahman et al. [24] found that the addition of limestone can inhibit the formation of AFm, thereby disrupting chemical binding. As the content of limestone increased, the chloride binding capacity became weaker. Zhu et al. [25] conducted an electro-accelerated experiment on plain concrete under a flat electrode electric field. The results showed that the binding capacity of concrete would decrease with the increase in the electric field, and the physical adsorption of chlorides was less than the chemical binding. Jiang et al. [26] used the Katz–Thompson (K-T) model, pore network model, and computational fluid dynamics (CFDs) to predict permeability and compared the pore structure characteristics obtained from different methods. Ding et al. [27] found that the pore volume of concrete decreased due to the carbonation of calcium hydroxide, and the pore structure coarsened due to the carbonation of C-S-H. Wang et al. [28] found that both fly ash and silica fume could reduce the porosity of concrete and increase the fractal dimension of the pore surface. Huang et al. [29] found that under the combined action of SO2 and CO2, corrosion products generated huge crystallization pressure on the silicate matrix, thereby causing microcracks and increasing the porosity of concrete. Guo et al. [30] quantitatively characterized the pore size distribution in concrete materials by X-CT and analyzed the pore shape characteristics of different sphericities.
It is evident that research on chloride ion transport in concrete under the influence of stray currents remains relatively limited. Thus, in this study, the chloride ion transport characteristics of concrete specimens placed in a sodium chloride solution were experimentally examined under varying current densities and exposure durations. Subsequently, the pore size distribution and chloride ion distribution within concrete materials were quantitatively analyzed under different testing conditions. A comparative assessment was conducted on the degradation of chloride ion transmission resistance performance. Additionally, the impact of stray currents on the concrete pore structure and concrete resistance to chloride ion penetration was systematically discussed. The results can provide a new strategy for forecasting and evaluating the durability of urban rail transit structures.

2. Materials and Methods

2.1. Materials and Specimens

Ordinary Portland Cement (PO.42.5), supplied by the Shandong Shanshui Cement Plant, Jinan, China, was used as cementitious material. The chemical composition of the cement is listed in Table 1. River sand with a fineness modulus of 2.40 and an apparent density of 2600 kg/m3 was used as a fine aggregate. Local crushed gravel with a size ranging from 5 to 20 mm and an apparent density of 2740 kg/m3 was used as the coarse aggregate. The mixed proportion of concrete is listed in Table 2.
The mixtures were cast into a mold with the dimensions of 150 mm × 150 mm × 150 mm, and steel with a diameter of 16 mm and a length of 300 mm was precast at the center of the cross-section. The concrete specimens were cured for 28 d under standard conditions (relative humidity of 90 ± 5% and temperature of 20 ± 2 °C).

2.2. Experimental Setup

To simulate the situation of stray current transmission in concrete in actual engineering environments, as shown in Figure 1, this study performed electrification tests on reinforced concrete specimens exposed to NaCl solutions. Based on the intensity of leaked stray currents during operation, which typically range from 20 mA to 100 mA [31,32], this experiment applied constant currents of 50 mA, 100 mA, and 150 mA to simulate a stray current. Electrification times were set at 1 d, 7 d, 14 d, and 28 d, respectively. The test solution was a 5% NaCl solution prepared from industrial-grade anhydrous sodium chloride. The test environment temperature was maintained at 20 ± 5 °C. In addition, a set of specimens without current was setup as the control group. Our experimental design is shown in Figure 2.
The details of the corrosion test under stray current for the specimens are listed in Table 3.

2.3. Chloride Measurement

The specimens were taken out after the chloride ion transport experiment, clamped with a fixture, and cut, as shown in Figure 3, with each layer controlled at a thickness of 5 mm. The sliced specimens were placed in a vacuum-drying oven at 50 °C until they reached a constant weight. Then, the cooled samples were ground in a ceramic mortar until they passed through a square sieve with a pore size of 160 μm. The contents of free chloride and total chloride were determined, respectively, in accordance with the JTS/T 236-2019 standard [33]. After chemical titration with silver nitrate, the concentration of chloride ions was measured by a ZDJ-600D fully automatic potentiometric titrator (Yantai, China).

2.4. Mercury Intrusion Porosimetry (MIP)

MIP was conducted using the Micromeritics AutoPore IV 9500, Norcross, GE, USA, with a maximum working pressure of 414 MPa to evaluate a theoretical pore diameter of 5 nm. Block samples approximately 3 mm to 5 mm in size, free of coarse aggregates, were carefully prepared by sawing. The samples were vacuum-dried at 50 °C until constant weight was attained and were then cooled to room temperature for later use. The measurement was divided into low-pressure analysis (0 to 0.275 Mpa) and high-pressure analysis (0.275 Mpa to 241.5 Mpa). After the low-pressure analysis began, the expansion meter was evacuated to 7 Pa, and then mercury injection began. Following the standard ASTM D4404-10, the pore size could be obtained through the Washburn equation, with a surface tension of 0.484 N/m and a contact angle of 130°, respectively [34,35]. The changes in pore diameter with the cumulative volume of invaded pores were recorded.

2.5. Thermal Gravimetric (TG) and Differential Scanning Calorimetry (DSC) Analysis

TG-DSC analysis was conducted using STA-449C, Bavaria, Germany, with a temperature range of 20 °C to 1500 °C and temperature change rate of 0.01 °C/min to 99.9 °C/min. The crucible type was alumina crucible, and the protective gas was nitrogen. The specimens were cut and placed in a vacuum-drying oven and dried at 50 °C until constant weight was achieved. Then, the dried and cooled samples were ground with a grinder until they passed through a square sieve with a pore size of 160 μm. Approximately 10 mg of the powdered sample was weighed and evenly spread out at the bottom of the crucible. The test conditions involved heating from 30 °C to 1000 °C at a rate of 10 °C/min under protective nitrogen gas.

3. Results and Discussion

3.1. The Influence of Stray Currents on Cement Hydration Products

Cement hydration products significantly influence concrete performance by affecting chloride transport. Chloride ions react with hydration products like aluminate phases and Ca(OH)2, forming Friedel’s salt via ion exchange and adsorption. The formation of Friedel’s salt refines the concrete pore structure, thereby affecting the migration of chloride ions. Additionally, hydrate silicate calcium (C-S-H) gel physically binds chloride ions, reducing their concentration. The external voltage can affect the dielectric parameters of cement-based materials, and their solid products are mainly concentrated at the pore walls, which may cause the decomposition of C-S-H in the cement under a current. The influence of the stray current on cement hydration products was analyzed by the TG-DSC test on the hydration products of specimens subjected to different currents. Figure 4 shows the thermogravimetric analysis curves for current values of 0 mA, 50 mA, 100 mA, and 150 mA after 28 days.
There are three obvious endothermic peaks in the sample. According to the theoretical atlas of thermal analysis and the related theory of cement hydration, the first endothermic peak appears near 100 °C, which is mainly due to the dehydration of C-S-H gel. The second endothermic peak appears at around 450 °C, which is due to the decomposition of Ca(OH)2. The third endothermic peak appears between 650 and 750 °C, which is due to the decomposition of CaCO3. By calculating the mass loss within different endothermic peak temperature ranges, the content of different hydration products can be obtained to reflect the hydration situation of cementitious materials. As the current value increases, the content of Ca(OH)2 in the samples gradually decreases, while the difference in the C-S-H gel is relatively small. The contents of Ca(OH)2 in four groups are 22.31%, 20.64%, 19.55%, and 18.17%, respectively, and the contents of C-S-H gel in four groups are 14.72%, 14.80%, 14.75%, and 14.78%, respectively. CaCO3 typically remains stable under a current. The observed variation in CaCO3 content may be attributed to the inadvertent introduction of a small amount of CO2 during the sample preparation process, which subsequently affects CaCO3 content in the test.
According to the TG-DSC curve of the above specimens, it can be concluded that the formation of an electric field by the stray current in concrete accelerates the precipitation of Ca2+, causing the dissolution of Ca(OH)2. As the current increases, the precipitation rate accelerates. Therefore, the Ca(OH)2 content in the specimens under a stray current is lower than that without a current.

3.2. The Influence of Stray Currents on the Pore Structure of Concrete

Moisture and chloride can only diffuse through the pores in concrete, and the larger the pore size, the greater the impact on the diffusion of chloride ions. This indicates that interconnected pores and their distribution are one of the key factors affecting the diffusion of chloride ions into concrete. The pore structure characteristics, such as pore distribution and the porosity of concrete, are related to its durability. The transport properties of uncracked concrete as a porous medium material largely depend on its pore structure and pore distribution. The pore types of concrete can be divided into three types in terms of pore size: the gel pore (less than 10 nm in diameter), capillary pore (between 10 nm and 1000 nm in diameter), and pore (more than 1000 nm in diameter) [36]. The gel pores are too small to promote chloride transport, and most of the ion transfer is through capillary pores. Capillary holes can be further divided into medium capillary holes (diameters between 10 nm and 50 mm) and large capillary holes (diameters between 50 nm and 1000 mm) [37]. Large capillaries promote and allow large amounts of chloride to diffuse. Medium-sized capillaries have little effect on the diffusion of chloride.
The MIP test was conducted on the pore structure of concrete specimens under different currents (0 mA, 50 mA, 100 mA, and 150 mA) at different times (1 d, 7 d, 14 d, and 28 d). The results of the pore structure parameters of concrete under each condition are shown in Table 4.
The pore structure in electrified concrete shows increased total porosity and more capillary pores compared to naturally soaked specimens. Based on the test results, the influence of stray current intensity and current time on concrete porosity was further analyzed.

3.2.1. Current Intensity

The changes in the pore structure of specimens with different currents (0 mA, 50 mA, 100 mA, and 150 mA) under the same current time (28 d) are shown in Figure 5. There is a correlation between the total porosity and the stray current value. As the current increases, the porosity and the capillary pore volume of concrete both increase, especially for pore diameters between 50 nm and 1000 nm.
The pore size distribution curve of the specimens without a current tends to be an unimodal curve, while the curve near the 100 nm pore size of the specimen after electricity is relatively convex. This indicates that stray currents alter the pore size distribution of cement slurry, with larger currents shifting the peak pore size to a larger area and increasing the number of capillary pores. In addition, when larger stray currents pass through the steel bars in concrete, the proportion of larger capillary pores in concrete is relatively higher. The presence of steel bars leads to the accumulation of free chloride ions in anodic regions within the concrete. According to the principle of chemical equilibrium movement, to keep the ion concentration balanced in the solution during electrochemical reactions, Ca(OH)2 near the steel bar is decomposed [38,39]. The pore volume growth with a pore size greater than 200 nm is primarily attributed to the dissolution of Ca(OH)2. The results indicate that stray currents can cause pore coarsening.

3.2.2. Current Time

The changes in the coagulation and pore structure at different current times (1 d, 7 d, 14 d, 28 d) under the same current magnitude (150 mA) are shown in Figure 6. The pore distribution curve shifts to the upper right with the current time, which leads to a gradual increase in porosity and pore size. This is because the electric field strength inside the concrete is relatively low at the initial stage of electrification, which limits the transmission speed of chloride ions. As the current time increases, Ca(OH)2 gradually dissolves, resulting in an increased degree of pore connectivity and the coarsening of pores, which leads to faster chloride ion intrusion into the concrete.
In the early stages of testing, naturally immersed specimens exhibit more refined pore structures compared to specimens under currents. For specimens with a current time of 7 days, the gel pore volume is close to that of naturally soaked specimens, while the capillary pore volume is higher than that in specimens without a current. For the specimen with a current time of 14 days, the pore volume of each aperture inside the concrete is higher than that without current. The results indicate that the impact of stray currents on the pore structure of concrete increases significantly with the current time.

3.2.3. The Influence of Stray Currents on the Porosity of Concrete

The pore structure of concrete controls the ionic transport performance of concrete materials. Due to the resistivity of the pore solution being much lower than that of the cement paste, the solution is more conductive. Therefore, during the corrosion process, the ionic transport can be regarded as dependent on the size, distribution, and porosity of the pore structure. According to the test results, the comparison of the porosity of concrete under the influence of different stray currents can be obtained, as shown in Figure 7.
The porosity of specimens under different test conditions is shown in Figure 7. It can be seen that stray currents can increase the porosity of concrete, and the greater the stray current value is within a unit of time, the greater the increase in porosity. For the specimens without a current, the porosity decreases to a certain extent with the extension of the soaking time. Based on the test results shown in Figure 7a, the ratio of the porosity of concrete under a current compared to that without a current can be defined as the influence coefficient kP, as shown in Figure 7b. Therefore, the influence coefficient kP is obtained and expressed as follows:
k p = a + b q c q = i ⋅ t
where i is the current value; t is the electrification duration; and a, b, and c are the fitting parameters determined from the test results. Based on these results, the expression for the influence coefficient kp is derived as follows:
k p = 1.0096 + 0.2368 i ⋅ t 0.6033
From Figure 8, it is clear that the effect of stray current on porosity varies with current values but follows a similar overall trend over time. At the initial stage of electrification, the effect is minimal due to incomplete concrete saturation and limited calcium ion precipitation from stray currents. Thus, the porosity of electrified and non-electrified specimens is nearly identical. As the electrification time increases, the influence of stray currents on porosity becomes more significant.
Due to the resistivity of the pore solution in concrete being much lower than that of cement paste, the ion transport can be regarded as dependent on the size, distribution, and connectivity of the pore structure. The K-T model can be used to characterize the relationship between fluid flow within the medium in pores and pore characteristics as follows [40]:
K = d c 2 226 F = d c 2 ϕ 226 τ 2
where τ is the tortuosity parameter of the pore structure; ϕ is the porosity of the concrete material; and K is the permeability of the concrete material. It can be seen that stray currents increase concrete’s porosity, raising the diffusion coefficient and leading chloride ions to penetrate concrete more quickly.

3.3. The Influence of Stray Currents on Chloride Transport in Concrete

3.3.1. Chloride Ion Concentration Distribution in Concrete Under Stray Current

The force of an electric field can alter the motion of charged particles within the field. Based on this characteristic, for concrete under the action of a stray current, it is necessary to consider the influence of the electric field formed on the internal chloride ion transport. Figure 8 shows the distribution of chloride ions inside concrete specimens subjected to currents of 0 mA, 50 mA, 100 mA, and 150 mA for 1 d, 7 d, 14 d, and 28 d. The transmission rate of chloride ions into concrete is significantly accelerated, and the chloride ion content in the shallow layer of concrete increases rapidly with the increase in stray current intensity. The content of chloride ions gradually decreases as the distance from the exposed surface of concrete increases under a current.
The transmission rate of chloride ions into concrete is significantly accelerated by increasing the stray current intensity, leading to a rapid increase in the chloride ion content in the shallow layer. The chloride ion content decreases with the distance from the exposed concrete surface due to the increasing transport resistance with depth.
The variation in chloride ion concentrations at different depths is shown in Figure 9. The chloride ion concentration in the concrete near the exposed surface (2 mm from the exposed surface) gradually increases with time. In the initial stage of the experiment (0–7 days), the increase in the chloride ion content on the surface of the non-electrified specimen over time was even higher than that of the electrified specimen. Subsequently (14–28 days), the increase in the chloride ion content on the surface of the non-electrified specimen gradually slowed down. The chloride ion content of the electrified specimen increased consistently with the specimen. At the distance of 8 mm from the exposed surface, the chloride ion concentration of the unelectrified specimen changed very little over time and remained at a relatively low level after 28 days. The concentration of chloride ions in the electrified specimen changes significantly over time, which also proves that stray currents can cause chloride ions to invade the interior of concrete more quickly.

3.3.2. The Ability of Concrete to Bind Chloride Ions Under Stray Currents

The migration of chloride ions in concrete is influenced by the pore structure of concrete and the interaction between the pore walls and ions. This interaction can be divided into physical adsorption and chemical binding, as shown in Figure 10. Physical bounding is mainly due to the exchange of Cl− ions with OH− ions at the adsorption sites in the pore structure, which is caused by the hydration product C-S-H of cement. Chemical bounding is mainly due to the reaction between C3A and Cl− in cement components, forming Friedel’s salts [41].
Among the ways in which concrete combines with chloride ions, chemical bounding is relatively stable and less susceptible to external conditions, while physical bounding can change with alterations in external conditions. This paper conducted chloride ion transmission tests under stray currents to obtain the chloride ion binding conditions of the specimens at different current values and different times, as detailed in Table 5.
Based on the test results, considering the full influence of current on the transmission of chloride ions, the total chloride ion content (Ct), free chloride ion content (Cf), and bound chloride ion content (Cb) of the specimens under 0, 50 mA, 100 mA, and 150 mA for 28 d are shown in Figure 11a. It can be seen that with the increase in the current’s value, Ct and Cf in the specimens increase, and the increases in amplitudes are basically the same, while the change in Cb is not obvious. Defining Cb/Cf as the degree of chloride ion binding in concrete, the results of different groups are statistically analyzed, and the variation in the degree of chloride ion binding in concrete with a current can be obtained, as shown in Figure 11b. It can be seen that free chloride ions increase more rapidly than bound chloride ions due to the presence of a current, indicating that stray currents can affect both total and free chloride ion levels in concrete.

3.4. Measures to Inhibit the Corrosion of Structure by Stray Current

3.4.1. Surface Coating

Surface coating technology is generally applied to the surface of steel bars or concrete and can significantly reduce the rate of chloride ion penetration and corrosion. Commonly used coated steel bars include galvanized steel bars or epoxy resin-coated steel bars. The critical threshold of chloride ions for galvanized steel bars is higher than that of bare steel, which reduces the corrosion rate of steel bars [42]. Due to the thin thickness of the galvanized layer, the coating reacts quickly and has a relatively short effective duration. In contrast, epoxy resin-coated steel bars have good impermeability and prevent steel bars from corroding. In addition to metallic and organic coatings, the surface treatment of steel bars using cement slurry inhibitors and cement polymer anti-corrosion coatings has also shown significant corrosion inhibition effects [43]. Surface coatings are mostly applied to the surface of concrete, and their anti-corrosion mechanism is to hinder the diffusion of chloride ions into the concrete or reduce the diffusion rate of chloride ions. Traditional polymer coatings mainly include epoxy resin, acrylic, and polyurethane. Polymer coatings have good hydrophobicity and can form a polymer film on the surface of concrete, preventing the flow of liquid water and providing a good physical barrier against chloride ions.

3.4.2. Electrochemical Treatment

Electrochemical treatment generally includes cathodic protection and electrochemical dechlorination. Both methods involve applying a current between the steel bars and an external anode to polarize the steel bars negatively, thereby inhibiting corrosion. Essentially, they both change the environment around the steel bars, remove chloride ions, and generate hydroxide ions on the surface of the steel bars, which maintain the passivation of the steel. Cathodic protection generally requires long-term application to effectively and persistently inhibit steel bar corrosion, and the personnel and maintenance costs are relatively high. The working principle of electrochemical dechlorination is similar to cathodic protection, but it uses a higher current density to accelerate the rate at which chloride ions leave the concrete. Although electrochemical dechlorination is effective, it may have certain side effects. For example, it can increase the alkalinity of the pore solution in concrete. When reactive aggregates are present, it may cause an alkali–aggregate reaction, leading to concrete expansion and cracking [44]. At the same time, hydrogen gas generated at the cathode may penetrate into the steel bars, causing hydrogen embrittlement and resulting in structural cracking or sudden failure [45]. Therefore, there are still controversies regarding the effects of electrochemical dichlorination.

3.4.3. High-Resistance Concrete

At present, stray currents flowing through the microstructure of concrete can be reduced by increasing the resistance of concrete. The concrete track bed and soil form the last link of the insulation circuit. As a permanent facility, the improvement of the insulation performance of the concrete track bed can greatly enhance the insulation of the entire structure and reduce the amount of stray current and subsequent maintenance work. When the resistivity of concrete exceeds 5 × 104 to 7 × 104 Ω·km, the probability of the corrosion of the steel bars in concrete will be significantly reduced [46]. Increasing the impedance of reinforced concrete is an effective measure to inhibit stray current corrosion. Adding active minerals such as fly ash and mineral powder to concrete not only saves costs but also increases the resistance by approximately seven times [47]. This is mainly due to the pozzolanic and filling effects of active minerals, which can optimize the pore structure of dense concrete and enhance its own performance. However, it should be noted that adding active minerals reduces the alkaline content in cement hydration products, which is not conducive to long-term corrosion protection. Additionally, increasing the thickness of the protective layer of the structure can effectively prevent the invasion of external harmful substances such as chloride ions.

4. Conclusions

Through the experiment of chloride ion transmission in concrete under stray current conditions, the influence of stray currents on the concrete resistance to chloride ions was explored. The main conclusions obtained are listed as follows:
(1)
Stray current causes the accelerated precipitation of calcium ions in concrete. This is primarily attributed to the stray current, which induces the accumulation of chloride ions near the steel surface, leading to the faster decomposition of Ca(OH)2 the maintenance of ionic balance.
(2)
Stray currents lead to an increase in the porosity of concrete. In our study, under the same current time (28 days), the porosity of concrete specimens with different currents (0 mA, 50 mA, 100 mA, and 150 mA) were 10.52%, 13.29%, 13.81%, and 14.32%, respectively. Under the same current (150 mA), the porosity of concrete specimens with different current times (0 d, 7 d, 14 d, and 28 d) were 11.36%, 12.78%, 13.21%, and 14.32%, respectively. As the electrification time increases, the influence of stray currents on porosity becomes more significant.
(3)
Stray currents change the distribution of chloride ions in concrete. The chloride ion concentration of the concrete on the surface of the specimen (2 mm away from the exposed surface) gradually increases with time. In the early stage of the experiment (0–7 days), the increase in chloride ion concentration at the surface layer of the unelectrified specimen over time is even higher than the electrified specimen. Subsequently (14–28 days), the increase in the chloride ion content on the surface of the unelectrified specimen gradually slows down. The chloride ion content of the electrified specimen increases to a certain extent over time.
(4)
A stray current weakens the stability of bound chlorides in concrete. For the specimens under currents (0 mA, 50 mA, 100 mA, and 150 mA), the values of their chloride ion binding capacity coefficients α are 0.2455, 0.1964, 0.1609, and 0.1470, respectively. As the current intensity increases, the chloride ion content continues to increase, and the increase in free-state chloride ions is higher than that of cured-state chloride ions. The ability of concrete to bind chloride ions gradually weakens with the increase in the stray current.

Author Contributions

Conceptualization, Y.N. and E.Z.; methodology, Y.N.; software, Y.N.; validation, Y.N. and L.C.; formal analysis, Y.N. and L.C.; investigation, Y.N.; resources, Y.N. and E.Z.; data curation, Y.N. and E.Z.; writing—original draft preparation, Y.N. and E.Z.; writing—review and editing, Y.N., L.C. and E.Z.; visualization, L.C.; supervision, E.Z.; project administration, E.Z.; funding acquisition, E.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Scientific Research Projects of Beijing Subway, grant number 51727813.

Data Availability Statement

The data provided in this study will be released upon reasonable request.

Conflicts of Interest

Author Liangjiang Chen was employed by the company China Railway Economic and Planning Research Institute Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Schematic diagram of stray current in urban rail transit.
Figure 1. Schematic diagram of stray current in urban rail transit.
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Figure 2. The setup of the chloride ion transport in the experiment under current: (a) experiment schematic; (b) experiment process.
Figure 2. The setup of the chloride ion transport in the experiment under current: (a) experiment schematic; (b) experiment process.
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Figure 3. Concrete sampling for chloride content measurement.
Figure 3. Concrete sampling for chloride content measurement.
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Figure 4. The TG-DSC curves of the specimens after 28 days: (a) 0 mA; (b) 50 mA; (c) 100 mA; and (d) 150 mA.
Figure 4. The TG-DSC curves of the specimens after 28 days: (a) 0 mA; (b) 50 mA; (c) 100 mA; and (d) 150 mA.
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Figure 5. Changes in pore structure under different current values for 28 days: (a) cumulative pore volume curve; (b) pore size distribution curve.
Figure 5. Changes in pore structure under different current values for 28 days: (a) cumulative pore volume curve; (b) pore size distribution curve.
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Figure 6. Pore structure changes under different current times for 150 mA: (a) cumulative pore volume curve; (b) pore size distribution curve.
Figure 6. Pore structure changes under different current times for 150 mA: (a) cumulative pore volume curve; (b) pore size distribution curve.
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Figure 7. Effect of stray current on concrete porosity: (a) comparison of porosity for each concrete specimen; (b) the influence coefficient kP of stray currents on the porosity of concrete.
Figure 7. Effect of stray current on concrete porosity: (a) comparison of porosity for each concrete specimen; (b) the influence coefficient kP of stray currents on the porosity of concrete.
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Figure 8. Chloride ion content under different currents: (a) 1 d; (b) 7 d; (c) 14 d; and (d) 28 d.
Figure 8. Chloride ion content under different currents: (a) 1 d; (b) 7 d; (c) 14 d; and (d) 28 d.
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Figure 9. The variation in the chloride ion concentration with time in concrete at different depths: (a) 2 mm from the exposed surface; (b) 8 mm from the exposed surface.
Figure 9. The variation in the chloride ion concentration with time in concrete at different depths: (a) 2 mm from the exposed surface; (b) 8 mm from the exposed surface.
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Figure 10. Schematic diagram of multi-scale diffusion of chloride ions in concrete.
Figure 10. Schematic diagram of multi-scale diffusion of chloride ions in concrete.
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Figure 11. The ability of concrete to bind to chloride ions under a current: (a) Ct, Cf, and Cb of samples under 0, 50 mA, 100 mA, and 150 mA for 28 d; (b) Cb/Cf of samples under 0, 50 mA, 100 mA, 150 mA.
Figure 11. The ability of concrete to bind to chloride ions under a current: (a) Ct, Cf, and Cb of samples under 0, 50 mA, 100 mA, and 150 mA for 28 d; (b) Cb/Cf of samples under 0, 50 mA, 100 mA, 150 mA.
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Table 1. Chemical composition of cement (mass percentages of oxides %).
Table 1. Chemical composition of cement (mass percentages of oxides %).
MaterialsCaOSiO2Al2O3Fe2O3MgOSO3K2O
Cement64.6518.614.654.192.333.280.93
Table 2. Mixture proportion of concrete (kg/m3).
Table 2. Mixture proportion of concrete (kg/m3).
CementSandGravelWaterW/C Ratio
40052513552000.5
Table 3. Tests group of chloride ion transport under stray currents.
Table 3. Tests group of chloride ion transport under stray currents.
GroupCurrent (mA)Time (d)GroupCurrent (mA)Time (d)
S0-101S2-11001
S0-27S2-27
S0-314S2-314
S0-428S2-428
S1-1501S3-11501
S1-27S3-27
S1-314S3-314
S1-428S3-428
Table 4. The pore structure parameters of specimens under different stray current values.
Table 4. The pore structure parameters of specimens under different stray current values.
Time
(d)		Current
(mA)		Porosity (%)Average Pore Diameter
(nm)		Pore Size Distribution (%)
<1010–5050–1000>1000
1011.3426.3135.6746.0314.234.07
5011.2226.1431.3149.1416.183.37
10011.2526.2030.3948.4917.313.81
15011.3626.3029.4248.3518.243.99
7011.0926.2234.8946.2514.254.61
5012.0726.4231.3447.0617.484.12
10012.4126.5129.4547.8218.384.35
15012.7826.8529.7746.4719.374.39
14010.9126.1136.4845.4113.424.69
5012.8327.1031.0746.1518.104.68
10013.0926.7130.1046.3619.494.05
15013.2127.2329.0546.3820.044.08
28010.5226.0135.6246.3313.814.24
5013.2927.1529.1145.2120.184.28
10013.8127.3228.2946.4921.154.07
15014.3227.6227.1846.7821.854.19
Table 5. Chloride ion content of specimens under different test conditions.
Table 5. Chloride ion content of specimens under different test conditions.
Current
(mA)		Time
(d)		Chloride Ion Content (mg/g)
CtCfCbCb/Cf
01 d6.2424.9951.2470.2496
7 d6.1814.9501.2310.2487
14 d6.2204.9681.2520.2520
28 d6.0414.8251.2160.2521
501 d6.0235.0101.0130.2022
7 d7.1976.0321.1650.1931
14 d8.4427.0651.3770.1949
28 d8.9407.4791.4610.1953
1001 d6.1335.2820.8510.1611
7 d7.5976.5451.0520.1607
14 d9.1847.9131.2710.1606
28 d9.4948.1771.3170.1611
1501 d6.1905.3870.8030.1491
7 d7.9596.9281.0110.1459
14 d9.8238.5651.2580.1469
28 d10.3659.0431.3220.1462
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Ni, Y.; Zhu, E.; Chen, L. The Impact of Stray Currents on Chloride Transport in the Concrete of Urban Rail Transit Structures. Buildings 2025, 15, 1695. https://doi.org/10.3390/buildings15101695

AMA Style

Ni Y, Zhu E, Chen L. The Impact of Stray Currents on Chloride Transport in the Concrete of Urban Rail Transit Structures. Buildings. 2025; 15(10):1695. https://doi.org/10.3390/buildings15101695

Chicago/Turabian Style

Ni, Yuancheng, Eryu Zhu, and Liangjiang Chen. 2025. "The Impact of Stray Currents on Chloride Transport in the Concrete of Urban Rail Transit Structures" Buildings 15, no. 10: 1695. https://doi.org/10.3390/buildings15101695

APA Style

Ni, Y., Zhu, E., & Chen, L. (2025). The Impact of Stray Currents on Chloride Transport in the Concrete of Urban Rail Transit Structures. Buildings, 15(10), 1695. https://doi.org/10.3390/buildings15101695

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