In the absorption experiments cylindrical bars of initially dry material are allowed to freely absorb water trough one end. At the start of a measurement, the lower end of the bar was inserted in a small water reservoir to about 1 mm below the surface. The resulting moisture profiles, each measured at a different time, are given in figure 1 for one type of fired-clay brick.

absorption brick
Figure 1: Moisture profiles measured during the absorption of water in a fired-clay brick

As can be seen from the figure a very steep wetting front is formed. If the moisture transport can be described by a diffusion equation, all experimental profiles of one type of material can be related by the Boltzmann transformation, l= x t^-1/2. The transformed moisture profiles for various materials are plotted in figure 2 as a function of l.

 
Figure 2: Boltzmann transformation of measured moisture profiles during absorption

For all materials the Boltzmann transformation indeed yields distinct curves, on which the data from the various profiles collapse for the individual materials. The behaviour of D describing the liquid transport is commonly approximated by an exponential function: D = D0 exp( q). The results of simulations of the moisture transport based on this exponential behaviour of the moisture diffusivity are represented by the full curves in figure 2. In all cases the computer simulation gives an adequate description of the observed moisture profiles.