Understanding the phenomena of coupled heat and mass transfer in concrete structures subject to high temperatures in a short duration has essential applications in the safety assessment of nuclear reactors and of tall buildings. In this paper a mathematical model, simulating the coupled heat and mass transfer in concrete structures at elevated temperatures as fire, has been developed and numerically solved. The numerical results predict the phenomenon of ‘moisture clog’ and the explosive spalling of concrete under fire. The investigations show that the seal layer as a fire protection has significant effects on the pore pressure buildup in the concrete walls which, in turn, improves the susceptibility of fire damage.