Stray current induced ITZ effect on chloride transport in concrete

Understanding the influence of interface transition zone (ITZ) on chloride migration characteristics in concrete under coupled stray current and high hydraulic pressure is essential to solve the durability problem of ultra-deep buried infrastructures (40 m-100 m). Herein, an advanced test system is developed and the chloride migration characteristics of deeply buried concrete is comprehensively investigated. Specimens were casted with the same aggregate volume content but different aggregate size distribution. The ITZ between aggregate and cement matrix was measured from BSE (back scattered electron imaging) images, and the ITZ thickness was measured by Vickers hardness. The silver nitrate spray was used to determine the chloride migration front, and the effective diffusion coefficient was calculated. The results showed the concentric expansion and overflow method based on BSE images were effective ways to determine ITZ porosity distribution. The significant increase of ITZ thickness and porosity on concrete reaction surface under stray current environment was observed and defined as "stray current induced ITZ effect", which was considered to be the fundamental reason for the distinct differences in chloride migration depth and effective diffusion coefficient of specimens with different aggregate gradations.