Anomalous diffusion in unsaturated flow: Fact or fiction?

The standard model of unsaturated flow widely used in building materials is essentially the extended Darcy law and the Richards equation of soil physics and hydrology. Much of the evidence for its validity comes from the measurement of water absorption (imbibition) kinetics and the analysis of water content distributions. While this model clearly captures the essential features of unsaturated water transport in some materials, several anomalies are known. In cement-based materials, strong deviations are sometimes observed. These are reviewed and assessed in relation to the wider interest in anomalous diffusion phenomena.