Figure 5 shows the sorption data for an exposure period of over 300 days for the two concrete mixtures at different curing times. We first note that there was a moderate difference in the total water taken up for the different curing periods (about %). However, there was more rapid sorption in the samples that had been cured in limewater the shortest amount of time. Table 1 shows the sorptivity coefficient obtained from fits of the data in Figure 5 to Equation 1. Clearly the longer the curing period the smaller the sorptivity indicating that the additional curing has reduced the pore size. Previous studies  have correlated other transport properties like the diffusivity of chloride ions and permeability with curing period. For roughly equivalent curing periods it was found that the chloride diffusivity decreased by about an order of magnitude while the permeability decreased by about two orders of magnitude. Our measured sorptivities were less sensitive to the curing period, decreasing by a factor of about 2 to 3. Scaling arguments may be used to show why permeability decreases much more rapidly with longer curing than sorption does. It has been shown that for cement mortars , diffusivity scales as rc and permeability scales as rc3, where rc is a critical pore radius based on percolation ideas . Sorption in tube models (see appendix) predicts that the sorptivity scales as rc1/2. If effects due to the tortuosity of the porous medium are included, it can be shown that permeability scales as rc3 . Then sorption should scale more closely with rc. However, since the initial suction is controlled by the microstructure near the surface it is likely that tortuosity effects are less important explaining why sorption appeared less sensitive to the curing period than diffusivity.
It also appears that the sorption has nearly stopped after 200 days, or so, for the conventional concrete sample. Visual examination of such samples, broken after 400 days of sorption measurements, indicated that they were not fully saturated as the water level was far from the top. However, these samples could have been in equilibrium with the air since they were not taped on the top. Such finite size effects will be the subject of further studies. We also note that the amount of sorption can depend on mixture design. For instance, Mixture 2 was designed to produce lower porosity concrete which should result in a lower absorption.
FIG. 5. Comparison of water sorption for different curing periods and concrete design mix. Mix # 1: normal concrete; Mix # 2: HPC. All specimens were bench drived before exposure to water.