Experiments in which heated nitrogen/steam mixtures are passed through model cracks in concrete with thermocouples embedded along the inner surfaces of the cracks provide insights about the thermodynamic processes occurring within them. The thermal behaviour that is observed results from several interlinked processes that influence one another and the resulting transfer of nitrogen, steam, water and heat. By correlating the temperatures measured at different times and positions within cracks with different separations, one can observe how the flow is modified by the formation of condensation. When pure nitrogen is passed through the cracks, only a small amount of water evaporates from the concrete and partial obstruction of the cracks by condensed water only occurs in the cracks with small separations (∼30 µm). As the proportion of steam in the mixture is increased, the thermal fluctuations caused by the formation of condensation become more important and evolve over time as the concrete is heated.