Moisture flow in concrete under steady state non-uniform temperature states: experimental observations and theoretical modelling

Abstract The migration of water in concrete at temperatures up to 400 °C is controlled dominantly by pore vapour pressures. The free pore volume is as-cast concrete serves as a reservoir during the migration process and plays an important role in the mitigation of high pore vapour pressures in the hottest regions. Experimental results are presented for two concretes containing limestone and basalt aggregates. They illustrate two sets of overheat conditions in reactor containment walls: (i) long-term service conditions at steady state liner temperatures in the range 105–200 °C, with and without pressure venting close to the liner; (ii) short-term transient behaviour for an accident with temperatures to 400 °C. The results show the distributions of free and bound water in walls of two thicknesses (1.55 and 3.1 m) after approximately 1.5 years from the imposition of a temperature crossfall. The vented experiments confirm significantly higher rates of drying and the ability of water to migrate towards higher temperature locations when driven by pore pressure gradients which are in opposition to the local temperature gradients. A theoretical model, based on pore pressure gradients as the driving potentials, is introduced and used to predict water migration in a concrete wall of 5 m thickness, heated at the inner face to 200 °C. It is suggested that thick walls will take many years to dry significantly, eventhough they dry simultaneously near to the liner and at the exposed cold face. Finally it is demonstrated that the theoretical model is capable of predicting this special behaviour and therefore has an advantage over diffusion-based analyses which cannot model this feature.