ZHANG Lu, WEN Bo, NIU Ditao, JI Zhiyuan

(1. Department of Civil Engineering, Xi’an University of Architecture & Technology, Xi’an 710055, China; 2. State Key Laboratory of Green Building in Western China, Department of Civil Engineering, Xi’an University of Architecture & Technology, Xi’an 710055, China; 3. State Key Laboratory of Green Building in Western China, Department of Civil Engineering, Xi’an University of Architecture & Technology, Xi’an 710055, China; 4. Department of Civil Engineering, Xi’an University of Architecture & Technology, Xi’an 710055, China)

Abstract: Based on the mechanism of stray current generation in underground structures, the concrete durability test device for stray current and sulphate in typical soil environment was designed to study the damage of concrete under the action of stray current and sulphate. The deterioration law of concrete under the action of stray current and sulphate was studied by microscopic techniques such as scanning electron microscopy (SEM)and X-ray diffraction (XRD). The microstructure of corroded concrete was observed to determine the phase composition of erosion products. The damage performances such as quality, strength, and dynamic elastic mode of corroded concrete were performed. The experimental results show that, under the action of stray current, the products of sulfate-eroded concrete are mainly gypsum, ettringite, and thaumasite; the stray current accelerates the hydration process of cement and the erosion of concrete by sulfate; when the concrete pores are filled with the erosion product, there is an increase of approximately 10% in the concrete compressive strength and dynamic elastic modulus; and the concrete compressive strength is more sensitive to the stray current electrification period than the current intensity.

Key words: stray current; sulfate attack; concrete damage; deterioration mechanism; degradation law

1 Introduction

In the deep soil, stray current leaked from underground structures or subways trail to the soil is a common feature. The combination of stray currents and chlorine ions in the soil leads to steel corrosion[1-3]. Subsequently, there is durability degeneration of the whole underground structure; this is particularly prevalent in groundwaters rich in sulfate ions[4]. Many researchers have focused on the influence of stray current on concrete structure, with specific focus on the corrosion mechanism of steel reinforcement under the joint action of stray current and chloride ion; however, there is limited research on concrete structure durability under the interaction of stray current and sulfate ions. Geng[5]and Zhanget al[6]studied the deterioration mechanism of reinforced concrete under the action of stray current and chloride ion as well as the influence of chloride ion transfer performance. Further, Liet al[7]studied the direct current stray current corrosion of buried pipelines in experiments. They found that the erosion rate of direct current (DC) stray current is linearly correlated with the current intensity through the metal pipe. On the other hand, Cabrera[8]studied the influence of erosion rate of reinforcement on crack and bond slip. Kimet al[9]noted that the stray alternating current (AC) erosion on the metal pipe in the soil was much better than that on the DC stray current erosion. Fuet al[10]studied the corrosion of buried pipelines by AC stray current;they found out that the higher the density of AC stray current, the faster the corrosion rate of metal pipelines from the results of weightlessness method. A study by Tang[11]evaluated the corrosion behaviour of steel fibres by voltammetry and electrochemical impedance spectroscopy. The findings indicated that high concentration of chloride ions increased the point corrosion tendency of steel fibres in simulated concrete pore solution and mortar samples. Tenget al[12]discussed the influence of different loading levels on steel corrosion of reinforced concrete members under the combined action of stray current and chlorine salt solution by adopting linear polarization resistance method to measure the corrosion current density of steel reinforcement in simulative environment of subway engineering service structures.Wanget al[13]studied the influence and mechanism of sulfate erosion on the properties of cement mortar materials under the action of electric pulses. Further,Huanget al[14]studied the factors that influence sulfate erosion on cement mortar under the action of electrical pulses. Xiaet al[15]presented a numerical investigation on the penetration of chloride ions in a saturated cement paste by considering the interaction between different ionic species in pore solution. The results showed that chloride concentration profiles are significantly different in the cement pastes of different initial concentrations. These findings were complemented by Tang[16], who investigated the action of stray current induced corrosion on steel fibre reinforced concrete; the presence of high concentration chloride ions was found to increase the pitting corrosion tendency of steel fibres in simulated concrete pore solutions and mortar specimens.

The findings from the aforementioned studies indicate that the simulation of stray current in tests is mostly constant current, which subsequently simulates stray current by applying voltage stabilized electric field or impulse electric field. Therefore, this method can only qualitatively analyse the sulfate erosion acceleration of stray current. However, during the existing tests, stray current changes concrete resistivity and soil resistivity greatly, which leads to uncontrollable stray current. Moreover, the short period transformation of pulsed electric field causes ions to vibrate; the simulation results of stray current on ion transfer patterns are thus inconsistent with the actual results. In addition, the corrosion environment modified in tests is immersed in a solution, which differs with the actual soil environment of underground structures. Based on this, to obtain the real corrosion law of stray current on concrete and steel, a new set of stray current test devices,in which soil is first used as the corrosive medium,is proposed. Furthermore, the mechanism of sulfate erosion on concrete with stray current was studied to determine the microstructure of corroded concrete and the phase composition of corroded products. Next, the performance parameters such as quality, strength, and dynamic elastic modulus of concrete were tested. Finally, based on the variation of these parameters, the degradation law of concrete under the action of stray current and sulfate was obtained.

2 Experimental

2.1 Raw materials and mix ratio

The cementing material corresponds to P·O42.5 ordinary portland cement as displayed in Table 1.The fine aggregates were made of river sand with a fineness modulus of 2.4. The coarse aggregates were basalt gravel with a particle size of 5 to 20 mm. The fine and coarse aggregates satisfied the requirements of the aggregate grading curve. The water-reducing admixture corresponded to the polycarboxylic acid superplasticizer, with a water reduction rate ranging from 25% to 30% of mass fraction, and the mixing water corresponded to tap water. The mixture ratio of the C40 concrete is presented in Table 2. The compressive test of concrete consisted of 3 specimens. The size of the specimen was 100 mm×100 mm×100 mm. The concrete quality, relative dynamic elastic modulus, and flexural strength were tested with 3 specimens in each group. The test temperature is 30 Celsius degree. The size of the specimen was 400 mm×100 mm×100 mm.

Table 1 Cement chemical composition and mass fraction/%

Table 2 Concrete mix ratios

2.2 Concrete damage test

2.2.1 Loading method

Based on the principle of generating stray current,the generation mode, transmission path and environment of stray current close to the actual situation were simulated in the laboratory. The experimental device designed in this paper is shown in Fig.1.

Fig.1 Simulated stray current experimental device

The durability test for concrete specimens subjected to the interaction of stray current and sulfate was carried out after 28 days of standard curing. During the specific test, the concrete cube was applied around epoxy resin that were put into the soil environment with 5% Na2SO4solution and 20% water content. Stainlesssteel plates were drilled into stainless steel screws and installed with wires at opposite corners, and then placed on top of the specimen. The upper surface of the specimen was not in contact with soil; however,the remaining surface was covered with soil. The stainless-steel plate was placed on the surface as a special circuit for generating stray current. Sliding rheostats were placed in the loops of the two steel plates to adjust the strength of stray current to the test design value.The current that controls the flow of concrete specimens is 0.6 mA/dm2, 1.2 mA/dm2, 2.4 mA/dm2, and 4.8 mA/dm2. As specified in the “CJJ49-1992 subway stray current corrosion protection technical regulations”[17-19],the allowable limit of leakage current density is 0.6 mA/dm2. The erosion age of the test was 15 d, 30 d, 60 d, and 120 d; when the corresponding erosion age was reached, the concrete specimen was taken out for testing. The control group adopted the same soil test environment as the test group but did not load stray current on the specimen.

2.2.2 Concrete performance experiment

In the experiment, TYA-2000 electro-hydraulic pressure testing machine was used to test the compressive strength of concrete specimen. An electronic balance with an accuracy of 1 g was used to measure the quality of concrete specimens. A DNS 300 electronic universal testing machine was used to test the flexural strength of concrete specimens, while the relative dynamic elastic modulus of concrete was measured by ultrasonic method in the Hc-f800 concrete crack defect comprehensive tester. After obtaining microscopic test samples from concrete specimens, the Quanta 600 FEG field emission scanning electron microscope and the INCA Energy 350 Energy spectrum analyser were used to observe the microstructure evolution and growth characteristics of erosion products inside concrete. The phase composition of powder samples was analysed by X-ray diffraction test with EMPYREAN X-ray powder diffractometer.

3 Results and discussion

3.1 Quality change

3.1.1 Influence of the stray current intensity

Fig.3 Concrete mass variation under different current intensities

The law of change in the quality of concrete with current conduction time under the action of stray current, is shown in Figs.2 and 3. Based on Fig.2, the quality rapidly rises early, in the middle and later sections as indicated in the smooth area. However, quality change fluctuation is not obvious. When the stray current strength is within the specified limits, changes in concrete quality of concrete under sulfate erosion can be neglected. Under the action of high strength stray current, the law of the concrete quality change shows a trend of rapid early rise, slow medium-term speed rise and a rapid decline in the later stages. When the stray current goes beyond the specified value, the effect of sulfate erosion on concrete quality is significant in soil environment. Thus, stronger stray current results in rapid changes in the quality of concrete. During the early period, the specimen’s quality rises rapidly. The rapid increase in the quality of the concrete specimen can be attribute to fact that the specimen in half-buried in the soil environment, which heaps up part of soil.

Fig.2 Concrete mass variation at 0.6 mA/dm2 current

3.1.2 Influence of quantity of electric charge

The changing rule of the concrete quality under different quantities of electric charge is shown in Fig.4(a). From the figure, an increase in the quantities of electric charge, results in an early rapid increase in the concrete quality. When the quantities of electric charge reaches 72 mA/dm2, concrete quality increase slows down; this trend continues until the quantities of electric charge reaches 108 mA/dm2, whereby there is a rapid quality decline. The changing rule of the concrete quality by the quantities of electric charge of 72 mA/dm2is shown in Fig.4(b). An increase in the intensity of electric current and the decrease of electrifying time,results in gradual smaller changes in the concrete quality. This shows that the change in quality is more sensitive changes in the electrifying time; moreover, the stray current strength has a small effect on the change in concrete quality.

Fig.4 Variation of concrete mass on amounts of electricity

3.2 Compressive strength

3.2.1 Influence of erosion age

Fig.5 shows the changing rule of concrete compressive strength under different erosion ages. The compressive strength of concrete initially increases, before decreasing rapidly. Notably, during the test period,the changes on the control group were not obvious. The results showed that the concrete’s compressive strength in the test group changes rapidly under the effect of stray current. The stray current also accelerates sulfates erosion of concrete, which subsequently shortens the time for the concrete’s compressive strength to decline.Under the action of stray current, the concrete’s compressive strength increases by 10%. Due to the sensitive effect of the current on the defect path under the action of stray current, sulfate ions are guided into the defect location, and the filling effect of erosion products leads to the improvement of the compressive strength in the concrete.

3.2.2 Influence of quantity of electric charge

Fig.6(a) shows the relative compressive strength of each test group affected by different quantities of electric charge in 60 d. Fig.6(b) shows the relative compressive strength of each test group when the quantity of electric charge is 72 mA/dm2. From the Fig.6(a),the relative compressive strength of concrete presents a law of initial increase, followed by a rapid decrease with the increase of stray current intensities, and the recorded decreasing range was 14.6%. From Fig.6(b), the compressive strength of the test group with a strength of 0.6 mA/dm2had no obvious change when the same power supply was applied. However, the compressive strength of the other test groups changed to different degrees. The results indicated that an increase in the current strength led to a decrease in the erosion age,and a subsequent gradual decrease in the concrete’s compressive strength. Under the action of stray current and sulfate, the concrete’s erosion degree is more sensitive to the increase in erosion age than the increase of current strength.

Fig.6 Concrete relative compression strength on electric charge

3.3 Flexural strength

After 120 d of erosion, the failure load and flexural strength of concrete specimens eroded by sulfate in the soil environment under the action of different stray current intensities is shown in Table 3 and Fig.7.When the test period reaches 120 d, the flexural strength of concrete in the control group and the experimental group with a stray current intensity of 0.6 mA/dm2does not change significantly. The results indicate that under the action of 0.6 mA/dm2current intensity and sulfate in soil environment, the flexural strength of concrete remains constant, and the stray current with low current strength has little effect on the change of flexuralstrength caused by the sulfate attack on concrete. For the three test groups with stray current intensity of 1.2 mA/dm2, 2.4 mA/dm2, and 4.8 mA/dm2, the flexural strength of concrete decreased after 120 d of erosion,and the maximum reduction reached to 16%. The concrete’s flexural strength under different stray current intensities is consistent with the compressive strength trend.

Table 3 Flexural strength of concrete at 120 d erosion age

Fig.7 Concrete relative flexural strength at 120 d

3.4 Relative dynamic modulus of elasticity

The changes in the relative dynamic elastic modulus of concrete, along with erosion age under different stray current strengths is represented in Fig.8. Fig.9(a)shows the influence of the relative dynamic elastic modulus of concrete under different quantities of electric charge, while Fig.9(b) shows the relative dynamic elastic modulus of concrete under the action of constant electric charge quantity.

Fig.8 Relative dynamic elastic modulus of concrete under different stray current intensities

Fig.8(a) indicates that the dynamic elastic modulus of the control group and the concrete specimens with current strength of 0.6 mA/dm2fluctuate steadily near the initial value, without any notable change.Based on Fig.8(b), the relative dynamic elastic modulus of concrete under the action of three different stray current intensities have similar change rules, including three stages: a slow rise stage, followed by a stable stage and a fast fall stage. The first stage is attributed to the formation of cement hydration products in concrete under the action of stray current and the filling function of erosion products generated by the reaction between sulfate and concrete hydration products. In the second stage, the expansion products generated by the concrete fill the pores, hinder the path of ions into the concrete,and subsequently reduce the internal compaction. The third stage results from the increase in erosion time,the expansion of erosion products to produce more micro-cracks and the surface gel structure is destroyed,which leads to softening and denudation. Eventually,sulfate ions from the soil environment penetrate into the concrete, which accelerates its erosion and destruction, leading to the accelerated decline of the relative dynamic elastic modulus.

Fig.9(a) shows that changes in the relative dynamic elastic modulus of concrete presents a short and rapid increase stage with the increase of quantity of electric charge, followed by a stable stage. Next, the inflection point of decline appeared, and the relative dynamic elastic modulus of concrete gradually accelerated to a decline in the later period of erosion with electric charge quantity at 72 mA/dm2. Meanwhile, Fig.9(b)shows that an increase in current intensity accompanied by a decrease of electrifying time, leading to a gradual decrease of relative dynamic elastic modulus of concrete. When the experimental group with 1.2 mA/dm2of the stray current intensity was at the decline stage,the experimental groups of 2.4 mA/dm2and 4.8 mA/dm2were still in the stage of rising relative dynamic elastic modulus. These results indicate that the relative dynamic elastic modulus are more sensitive changes in electrifying time; furthermore, the strength of stray current has little influence on the change of relative dynamic elastic modulus.

Fig.9 Relative dynamic elastic modulus of concrete on the amount of electricity

3.5 Concrete microstructure evolution

The field emission electron microscope analysis revealed that the erosion products were mainly distributed in the pores, micro-cracks and the interface between aggregate and cement paste as shown in Fig.10. Fig.11 shows the microstructure of concrete under the action of different stray current intensities at the erosion age of 120 days. Fig.12 shows the internal micromorphology of concrete in the test group with a stray current intensity of 4.8 mA/dm2at different erosion ages. Based on Fig.10, under the traction of stray current, the migration of sulfate ions in the concrete are concentrated at the defect. However, the positive ions such as calcium ion and aluminium ion also were transmitted in these places. When the circulation direction is opposite to the sulfate ion, the erosion products are easily formed.

Fig.10 Distribution of erosion products

Fig.11 Microscopic morphology of corroded concrete under different stray current intensities at 120 d

Fig.12 Microscopic morphology and EDS spectra of concrete with 4.8 mA/dm2 current value

Fig.11 shows the concrete micromorphology with different stray current strength at 120 d corrosion age.From Fig.11(a), at a current intensity of 1.2 mA/dm2,the erosion products in concrete were mainly acicular ettringite crystals. Ettringite crystals were filled with a number of large micro cracks, and there were many small pores at the interface. This shows that ettringite has a great destructive effect on the inside of concrete,that is, it exerts great stress on the inside microstructure of concrete, leading to the generation and development of micro-cracks and destroying the integrity of gel structure. From Fig.11(b), when the stray current intensity was 2.4 mA/dm2, the generated ettringite crystal was mainly rod and long strip, and the erosion product crystals formed were thicker and denser near the crack position and the pore connection. No corrosion product crystal was observed at the relatively complete position of concrete microstructure, indicating that the stray current played a role in guiding the concentration of corrosive ions at the defect and promoting the occurrence of corrosion reaction, which had a great impact on the destruction of concrete internal microstructure.From Fig.11(c), when the stray current intensity was 4.8 mA/dm2, the microstructure of concrete surface was seriously damaged, and the granular calcium carbonate crystals were scattered and piled up, and the gel structure was missing. The reason is that under the continuous action of high strength stray current, the calcium and aluminum ions in the gel would be lost, leading to the gel structure decomposition. The EDS indicated that the granular crystal elements scattered on the surface were mainly elements of C, O, Si, and Ca. It was determined that the surface crystals were calcium carbonate and silicon dioxide crystals, which showed that high stray current strength had a destructive effect on the concrete gel structure.

Fig.12 indicates that in the early stage of erosion, the main form of erosion product in concrete was carbonaceous sulfate-silicone-calcium stone. At the middle stage of erosion, the main erosion product was ettringite. Additionally, large quantities of gypsum crystals were observed in concrete during the subsequent erosion. According to Fig.12(a), at the corrosion age of 30 d, there were still many clusters of crystals around the micro-cracks in the concrete, and a small amount of rod-shaped crystals formed in the cracks to fill the cracks. Carbonaceous silicalite crystals were generated in the micro-cracks, indicating that the corrosion of sulfate ions on cement matrix at concrete defects was greatly accelerated under the action of stray current. From Fig.12(b), at the corrosion age of 60 d, acicular ettringite crystals filled in the internal defects of the concrete. And calcium hydroxide crystals were damaged to varying degrees attached with a large number of actringite crystals, and a small number of gypsum crystals begin to appear. From Fig.12(c), at the corrosion age of 120 d, a large number of short columnar gypsum crystals had appeared inside the concrete,but a large number of sodium sulfate crystals had not been observed, so the sulfate crystallization pressure has no influence on the destruction of the internal structure of the concrete, indicating that under the joint action of stray current and sulfate, the concrete is mainly subjected to chemical erosion. The product element composition was analysed according to EDS (as shown in the analysis area in Fig.12(d)) and the EDS energy spectrum is represented in Figs.12(e) and 12(f). Based on Fig.12(e), the main constituent elements are Ca, C,O, S, and Si, and the molar fraction ratio ofx(Ca):x(S)is 3, indicating the presence of carbonaceous sulfoxide crystals. Fig.12(f) shows the main components as Ca, C, O, S, Si, Al, and Na, and the mole fraction ratiox(Na):x(S) as 1; this indicates that the crystal is a newly formed hydration product under the action of stray current. It also indicates that the hydration product contains sodium sulfate crystals.

3.6 Phase composition of erosion products

3.6.1 Influence of stray current intensity

The XRD diffraction analysis diagram of the test group with different current strengths at the erosion age of 60 d is shown in Fig.13. As the current intensity increased, the change trend of diffraction peak intensity of ettringite and carbonaceous sulfoxide first increased before decreasing. In the increase phase, the diffraction peak became gradually sharper, the crystallinity of erosion products was enhanced, and the crystal proportion of erosion products in polymers increased. In the later erosion stages, the diffraction peak of carbon-sulfur silicon-calcium stone gradually disappeared, the crystal surface spacing enlarged, and the crystal diffraction peak intensity of the erosion product with dense atomic arrangement gradually decreased. Greater current intensity resulted in a greater influence on the migration of OH-ions. The decline of surface pH value resulted in unstable decomposition of erosion products of carbonaceous sulfoxide[18,19]. Finally, the content of products and the intensity of diffraction peak decreased.

Fig.13 X-ray diffraction diagram on concrete surface of different current strength at 60d erosion age(where, E: Ettringite;G: Gypsum; Q: Quartz; T: Thaumasite; C: CaCO3; CH:Ca(OH)2; SS: Sodium Sulfate)

3.6.2 Effect of erosion ages

The intensity of stray current erosion in different ages of 2.4 ma/dm2, and X-ray diffraction analysis of the concrete surface is shown in Fig.14. As the erosion age increased, the diffraction peak of ettringite showed stronger trends. However, the existence of ettringite and carbon common in thaumasite diffraction peak increased as the erosion age increased. At the erosion age of 15 d, the surface layer concrete contained multiple sodium sulfate crystals. The results showed that under the action of stray current, there was a massive increase in the sulfate erosion rate of concrete. Therefore, an obvious effect of stray current on the erosion reaction speed. In addition, the sulfuric acid root ion migration rate increased significantly.

Fig.14 X-ray diffraction analysis on concrete surface layer at different erosion ages

3.6.3 Concrete phase composition at different depth of erosion products

Fig.15 shows X diffraction analysis diagram in the concrete surface layer of 6 mm when erosion age is 120 d. From Figs.15(a) and 15(b), at 0.6 mA/dm2current strength, the ettringite in the erosion products of the diffraction peak intensity increased. The intensity of the diffraction peak of calcium carbonate was also more obvious than the control group, whereas gypsum diffraction peak intensity change a little. Results indicate that the current strength of the upper limit value of specification had a greater influence on the erosion of ettringite reaction, and the produced gypsum had little effect on the erosion reaction. Based on Fig.15(c), an increase in depth resulted in an increase in crystallinity and carbon sulfur rankinite and ettringite content increased. There was also complete erosion of crystal products. However, the crystallinity of gypsum and content decreased, and there was formation of poor erosion products. The ettringite and carbon sulfur rankinite was stable and the gypsum formation was better due to the surface’s low pH. Internal high pH values favour the existence of ettringite and carbon sulfur rankinite,but it is not conducive for the formation of gypsum.

Fig.15 X-diffraction patterns of concrete layers in different current intensity groups at 120 d erosion age

4 Conclusions

Stray current is widely distributed in subways and underground structures, which is a phenomenon harmful to structures. To investigate the law of the stray current on the concrete, the microscopic damage on concrete under the action of stray current and sulfate is studied. SEM/EDS and XRD are used to observe the microstructure of corroded concrete and determine the phase composition of erosion products.The degradation mechanism of concrete sulfate erosion under stray current is observed. Furthermore, through electro-hydraulic pressure testing machine and concrete crack defect comprehensive tester test, the change rules of concrete quality, compressive strength, and relative dynamic elastic modulus under the combined action of spur-current and sulfate are obtained. The specific research conclusions for this study are:

a) Based on the distribution characteristics of stray current in the underground structure, a stray current test device is designed in line with the situation.

b) Stray current can accelerate the internal hydration of concrete and the deterioration degree of concrete, and guide the concentration of erosion products in defects, further it can lead to formation of new cement hydration products such as gypsum, ettringite,and thaumasite in micro-cracks.

c) Increase of stray current intensity had obvious influence on the number of erosion products and crystal morphology of erosion products. Increasing stray current intensity massively accelerated the rate of change in concrete quality.

d) Stray current can significantly accelerate the erosion reaction of sulfate and cement hydration products. Within a relatively short time, the pores inside concrete invaded by sulfate are filled and enter to the deterioration stage.

e) The influences of stray current on concrete flexural strength tends to be consistent with the compressive strength, and the maximum reduction value of concrete flexural strength reached was 16%.