Moisture dependence of radon transport in concrete: measurements and modeling

The moisture dependence of the radon-release rate of concrete was measured under well controlled conditions. It was found that the radon-release rate almost linearly increases up to moisture contents of 50 to 60%. At 70 to 80% a maximum was found and for higher moisture contents the radon-release rate decreases very steeply. It is demonstrated that this dependence can be successfully modeled on basis of the multi-phase radon-transport equation in which values for various input parameters (porosity, diffusion coefficient, emanation factor, etc.) were obtained from independent measurements. Furthermore, a concrete structure development model was used to predict at any moment in time the values of input parameters that depend on the evolution of the concrete microstructure. Information on the concrete manufacturing recipe and curing conditions (temperature, relative humidity) was used as input for the concrete structure model. The combined radon transport and concrete structure model supplied sufficient information to assess the influence of relative humidity on the radon source and barrier aspects of concrete. More specifically, the model has been applied to estimate the relative contributions to the radon exhalation rate of a 20-cm-thick concrete slab of radon produced in the concrete slab itself and due to diffusive transport through the slab of radon from soil gas.