## 4 Results of the Calculations
In this zone, the apparent chloride diffusivity values of concretes C2, C4, and C5 are intermediate ones between the first and third zones, which may be the result of an increased content of cement paste compared to the third zone identical to the bulk part of concrete. However, the content of the cement paste is already smaller in the second zone compared to the first one. In the case of concrete C3, in its second zone, the value of apparent diffusivity is only twice lower than the effective one (see Table 7), which may indicate that this is a zone with a structure comparable to the inner third zone and the process of chloride binding in this layer is much closer to completion after 180 days of the experiment. 3. The third zone is located at a depth of more than 3-5 mm from the outer surface. The values of apparent chloride diffusivity in this zone characterize the bulk part of concrete not disturbed by the occurrence of the \"skin effect\" and when the chloride binding process is just beginning. As a result of the calculations made, it can also be concluded that the \"skin effect\" resulting from the changed concrete structure near the external surfaces (not in contact  
Figure 5: Variability of apparent chloride diffusion coefficients in function of depth in the samples after 180 days of diffusion process based on the results from Table 5 for concrete C1 and Table 7 for concretes C2–C5. Coordinate \(x=0\) relates to the specimen external surface exposed to the chloride solution. with the walls of the forms as in the experiment) occurs with the greatest intensity up to a depth of approx. 1-2 mm, however the cement matrix must also be capable of absorbing chloride ions, thereby significantly reducing the apparent diffusion coefficient. It should be emphasized at this point that the \"skin effect\" understood in this way under the conditions of immersing the concrete in a solution with chlorides can only last until the outer layer is fully saturated with them. The \"skin effect\" associated with the reduction in the apparent diffusivity of chlorides is also noticeable at a depth of about 1-2 mm to 3 mm in the case of concretes C2, C4, and C5, but to a lesser extent. Figure 6 shows the concentration profiles of total chloride ions (\(C_{\text{t}}\)) in the concrete samples up to a depth of 2 cm at 180 days from the start of the diffusion process. To obtain them for concrete C1, material parameters from Table 5 were used, and for concretes C2-C5 from Table 7. Importantly, also in the diagrams of Figure 6b-e, the occurrence of the \"skin effect\" in the case of C2-C5 concretes can be noticed. As a result of changes in the slopes of chloride binding isotherms in the individual concentration subintervals (Figure 4b) and the related changes in the apparent diffusivity of chlorides in individual sample zones (see Figure 5 with the zones distinguished after 180 days of the experiment), we also observe discontinuity points in the slopes of the curves describing the distribution of total chloride concentrations. In Figure 6, for illustrative purposes, the 180-day average concentrations of total chlorides in the samples' successive layers with a thickness of 2 mm, determined experimentally (\(\overline{C}_{\text{t exp}}\)) and calculated according to the model (\(\overline{C}_{\text{t model}}\)), were also plotted. As a result of the comparison, a satisfactory agreement between the measurement and model data can be noticed. Figure 6: _Cont._  
Figure 6: Mean concentrations of total chlorides after 180 days of diffusion in the samples’ successive layers with a thickness of 2 mm determined experimentally (\(\overline{C}_{\text{t exp}}\)) and calculated according to the model (\(\overline{C}_{\text{t model }}\)) with use of the material parameters presented in Table 5 for concrete C1 and Table 7 for concretes C2–C5. In the background, the continuous curves \(C_{\text{t}}\) are also shown, on the basis of which the values of \(\overline{C}_{\text{t model}}\) were calculated.