Effect of stray current on stability of bound chlorides in chloride and sulfate coexistence environment

Destabilization and release of bound chlorides in reinforced concrete structures (RCS) cause accelerated corrosion of steel bars, posing an attendant risk to the durability of the whole structure. This problem is exacerbated by an aggressive environment where chloride and sulfate coexist, and stray current is present, which are commonly observed for RCS built in the subway system. This study tried to simulate the environment of underground track structure by means of intermixing sodium chloride with sodium sulfate and concurrently imposing a stray current. The chloride binding capacity of mortar samples prepared with different water-to-cement (w/c) ratio was examined. Moreover, the effect of current density on the stability of bound chlorides was also investigated. The results showed that the stability of bound chlorides was closely related to the w/c ratio and the current density. In the presence of stray current, the chloride binding capacity of cement mortars was significantly decreased by either a high w/c ratio or a high current density. Interestingly, stray current had a bigger impact on the stability of physically bound chlorides but a much smaller impact on the stability of chemically bound chlorides. X-ray diffraction (XRD) analysis and Thermal gravimetric (TG) and differential thermal gravimetric (DTG) analysis demonstrated that the destabilization of bounded chlorides under stray current and in the presence of sulfates was mainly attributed to the decomposition of the C-S-H gel and the Friedel's salts. Findings from this research shed new light on the fate of bound chlorides in RCS in complex subway environments. (C) 2018 Elsevier Ltd. All rights reserved.