Microstructures evolution and chloride migration characteristics of concrete under ultra-deep underground environment

Understanding the microstructures evolution and chloride migration characteristics of concrete is essential to solve the durability problem of reinforced concrete structures in ultra-deep buried rail transit projects (40 m–100 m). Herein, we developed a novel experimental system that can simulate the corrosion process under high hydraulic pressure and stray current coexistence environment. The changes in microstructures, phases, elements, minerals, and the Ca-leaching were comprehensively investigated. Further, the mechanisms of pores and cracks propagation, and migration characteristics of chloride ions were revealed. The results showed that stray current and hydraulic pressure significantly altered the microstructures of concrete, both of which led to the increase of specific surface area, porosity, pore diameter and more ink-bottle-shaped pores. The development of dominant microfractures and three types of crack propagation patterns caused by hydraulic pressure were found, and the mechanical framework for “local fracturing” was established. Migration characteristics in micro-nano scale showed chloride ions were mainly transported along the pores, and the decrease of adsorption sites was the main reason.