## 5 Conclusions  
The most important results achieved in the work can be formulated in the following points:  
1. Using a spline function to a description of chloride binding isotherm for 180-day diffusion experiments enables an assessment of the \"skin effect\" in terms of depth of its occurrence and change in apparent chloride diffusivity, which results from a different concrete composition and structure and thus different chloride binding properties in subsurface layers compared to a bulk part of material. 2. From five types of concretes tested in the study, concrete C1 containing high early strength Portland cement (CEM I 42.5 R) showed the best protective properties in terms of chloride corrosion due to the lowest effective diffusion coefficient of order of 10\({}^{-13}\) m\({}^{2}\)/s (Table 5). There was no simultaneous binding of chlorides during the 180 days of diffusion. 3. The remaining C2-C5 concretes were characterized by one to two orders of higher effective diffusion coefficient than concrete C1, but at the same time they demonstrated chloride binding which led to a reduction in the apparent diffusion coefficients in the near-surface layers also to the value of order of 10\({}^{-13}\)-10\({}^{-14}\) m\({}^{2}\)/s for concretes C2, C3, and C5, and for C4 to a value of order of 10\({}^{-12}\) m\({}^{2}\)/s (Table 7). 4. The highest chloride binding occurred in concrete C3 with blast furnace cement (CEM III/A 42.5 N-LH/HSR/NA) and concrete C5 with pozzolanic cement (CEM IV/B(V) 32.5 R-LH/NA). The concentrations of bound chlorides in the solid phase could maximally reach values comparable to the concentration of free chlorides in the pore liquid. Less chloride binding occurred in concrete C4 with low-alkali normal early strength Portland cement (CEM I 42.5 N-SR3/NA) and concrete C2 with ash Portland cement (CEM II/BV 32.5 R) where the concentration of bound chlorides in the solid phase could only be about five times lower than the concentration of free chlorides in the pore liquid. 5. Taking into account the chloride binding properties of concretes C2-C5, it can be stated that they also showed protective properties against chloride corrosion in the near-surface layers. However, in the case of concretes C2-C5 and in particular concretes C3 and C5, it should be remembered that after the initiation of chloride corrosion of the steel reinforcement, consumption of free chlorides in corrosive reactions and reduction of their concentration in the porous liquid may be related to re-releasing the bound chlorides and further \"driving\" the corrosion process [34]. 6. The authors expect that thanks to the proposed mathematical model and calculation procedures, it is possible to assess the \"skin effect\" for different types of concrete, chloride compounds ingressing into it, and the methods of its curing and surface finishing what is important in view of the fact that such an effect basically always occurs due to executive and environmental factors in practical conditions. At the present stage of the research, the precise quantitative results presented in the article can be used to assess the chloride corrosion resistance of ordinary concrete only treated with solution of sodium chloride, with similar recipes using the same type of cement, aggregate, and methods of curing and finishing. For concretes with other recipes, qualitatively (not quantitatively) similar results can be expected, if the same types of cements, aggregates, and curing and finishing methods are used, and if sodium chloride solution is a source of chloride ions. It should be emphasized that due to the complexity of the problem, an unequivocal confirmation and evaluation of the protective properties in case of concrete with a given recipe, curing and surface finishing method, and source of chloride ions are only possible when the entire procedure proposed in the article is carried out anew. 7. The presented research requires continuation in the context of conclusion no. 6.