Measurements and modelling of combined diffusive and advective radon transport in porous building materials

Measurements of combined diffusive and advective radon transport in aerated concrete have ken made and compared with model calculations. The experimental set-up consists of a leaktight vessel conlaining a hollow aerated-concrete cylinder that divides the vessel into an inner (inside the cylinder) and outer (outside the cylinder) cornpartmenl [I]. The measurements were conducted with a radon source in the inner compartment, for a range of ventilation rates in the inner and outer compartment and for various flows through the concrete. Pressure, temperature and relative humidity in the set-up were kept constant. It was found that a combined diffusion-advective flow (Fick-Darcy) model does not describe the data. A much better fit is obtained by assuming an advective-velocity-dependent diffusivity in the model calculations. The dependence of the diffusivity on the flow velocity may result from mechanical andlor hydrodynamic dispersion. Alternatively, a model assuming two parallel porous media with distinct volume and permeability also describes the data quite well. The results of this parallel-media model indicate a preferential airflow. About 90% of the airflow is transported through 7% of the material.