Abstract: In order to study the damage effect of concrete internal structure in the high temperature environment of tunnel fires, the micro CT scanning tests and SEM scanning tests were used to detect the microstructure of concrete samples at the temperatures range of 100 ℃ to 800 ℃ . The quantitative relationship between temperature and fracture rate, fracture length and connectivity index was established. The research showed that the cracks appeared in the concrete internal structure and gradually expanded with the fire development. With the increase of temperature, the porosity, pore throat length and width showed an exponential increase trend, which indicated the accumulation of the damage of concrete microstructure. The volume content of the largest pore keeps increasing first and then decreasing with the increase of fire temperature. It was found from SEM images that the expansion and dehydration of cement hydrate gel is an important trigger factor for the expansion of concrete cracks.

Key words: high fire temperature, concrete, microstructure, CT scanning, SEM