Experimental study on heat transfer rules and thermal damage of steel-concrete-steel tunnel elements in fires

In this paper, internal temperature field, heat transfer forms, and macroscopic thermal damage of steel-concrete-steel tunnel elements under three thermal insulation schemes were investigated by conducting fire tests under RABT standard fire curve. The results show that steel shell temperature and internal temperature have distinct phases over time. There is a significant temperature gradient in vertical direction inside the structure. Temperature distribution in horizontal direction is also uneven. High temperature and uneven temperature distribution cause primary crack development and penetration, separation of concrete and steel shells, deformation of bottom steel shell, and a loss in rebound strength within 800mm depth. Fireproof board and fireproof coating insulation schemes can effectively lower internal temperature and its uneven degree, reducing macroscopic thermal damage. Fireproof coating insulation scheme provides more effective insulation in early stages, but there is local detachment later on, resulting in insufficient reliability and stability. Heat transfer mode from fireproof boards to the steel shell is mainly through thermal radiation, supplemented by thermal convection and thermal conduction. Radiant flux is more than 7.1 times the sum of convective flux and conductive flux during fire tests. The study helps to understand the damage mechanism of steel-concrete-steel tunnel elements in tunnel fires.