The description of moisture transport mechanisms on the basis of Eq. (51) requires the determination of the function Dθ. An interesting discussion of the variation of this function with the water content of the material has recently been published by Martys [19]. The author clearly emphasizes the nonlinear character of this function.
Measurements made on a sand column [20] show that when the humidity in the medium is higher than 4% (by weight), the contribution of the vapor phase to the overall moisture transfer is negligible. In that case, one can assume that Dθ=DL.
Over the years, numerous experimental techniques have been used to determine the moisture transport properties of hydrated cement systems. A thorough discussion of this subject is beyond the scope of this paper. Comprehensive critical reviews of this problem can be found in references [21] and [22].