ABSTRACT
The first models of moisture transport in concrete were initiated by the necessity to describe the drying shrinkage processes. Therefore, the researchers were primarily concerned with the description of concrete drying. The models were mostly inspired by Fick's law, and employed the linear diffusion equation for relative humidity ip,
~QI~ ®v div grad (p, (4.1)
or water content w,
dw _ / 4 . — = Dw div grad u>, (4.2)
where D^, Dw are the respective diffusion coefficients (diffusivities). Examples of such applications can be found, e.g., in Carlson (1937), Pickett (1942), Hancox (1967), Hilsdorf (1967), and many other papers up to the 1960s. However, these models gave a very poor fit to experimental data over long periods of time. Already Carlson (1937) had observed that drying becomes much slower than given by an extrapolation of the initial drying curve, which a linear diffusion theory would predict. Therefore, Pickett (1942) proposed considering the diffusivity as function of time. This, however, was a very crude approximation, which did not give a satisfactory fit to data for various specimen thicknesses and shapes.
