Stray Current Induced Corrosion Control in Reinforced Concrete by Addition of Carbon Fiber and Silica Fume
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Stray Current Induced Corrosion Control in Reinforced Concrete by Addition of Carbon Fiber and Silica Fume Zhipei Chen1, Dessi Koleva1, Eduard Koenders1 and Klaas van Breugel1 1 Faculty of Civil Engineering and Geosciences, Department Materials and Environment, Delft University of Technology, Stevinweg 1, 2628 CN, Delft, The Netherlands ABSTRACT Stray current arising from direct current electrified traction systems and then circulating in reinforced concrete near railways is known to induce corrosion on embedded steel reinforcement. The present paper will review the principles of stray current induced corrosion in reinforced concrete, which is relatively uncommon but with significant impact in practice. Within one of the approaches to ease this kind of specific corrosion in reinforced concrete, carbon fibres (CF) can be added to enhance the conductivity of concrete, subsequently reduce the stray current density and/or direct the stray current dissipation in a desired manner. The side effects (such as increasing the bulk matrix porosity) caused by CF, which can in turn reduce the general corrosion resistance of reinforced concrete, will be compensated by adding silica fume (SF). The combination of CF and SF can be a potentially feasible and original application to reduce the risk of stray current induced corrosion in reinforced concrete, without obvious negative side effects. INTRODUCTION In direct current electrified traction systems, such as electrical trains, tram systems or underground trains, the current drawn by the vehicles returns to the traction power substation through the running rails which, besides forming part of the signalling circuit for the control of train movements, together with return conductors are then used as the current return circuit path. However, owing to the longitudinal resistance of the rails and their imperfect insulation to ground, part of the return current leaks out from the running rails and returns to the traction power substation through the ground, forming the stray current [1]. As any underground metallic structure has (in general) a lower electrical resistance than the soil, the stray current flows through these metallic structures, such as reinforcing bars in concrete (figure 1). Stray current circulating in reinforced concrete may initiate corrosion or accelerate existing corrosion processes on embedded reinforcement. Additionally, stray current can also affect microstructural properties of the concrete matrix [2]. Stray direct currents (DC) are known to be much more dangerous than stray alternating currents (AC) [3]. As for reinforced concrete structure buried near the rail, in the case of DC, at the point where the stray current enters the reinforcement, a cathodic reaction occurs (generally oxygen reduction for this environment): 1 O 2 +H 2 O+2e- → 2OH (1) 2 while an anodic reaction (metal dissolution) occurs where the stray current flows out from the reinforcement to the electrolyte (i.e. soil, concrete), which is the process of corrosion:
Fe → Fe 2+ +2e −
(2)
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