## 1 Introduction
However, some loosely bound chloride ions may be released into the solution under pressure. The chloride concentration could be 20% higher than the true value. In the diffusion cell method after the steady state diffusion experiment, the specimens are frozen in nitrogen and subsequently ground to powder samples [15]. The free chloride concentration is estimated by assuming that the concentration gradient is linear under steady state diffusion conditions. It should be emphasized here that the methods discussed above are quite complicated in use, and it is also difficult to compare the research results obtained on their basis [13]. A way out of this situation is a formulation of inverse coefficient problem, in which, based on the knowledge of chloride profiles in concrete, the parameters of binding isotherms and the diffusion coefficient can be identified, minimizing a sum of square errors between the results of initial-boundary diffusion problem and experiment in non-stationary conditions [16]. Currently, models of chloride diffusion are also being developed, taking into account the division of bound chlorides into reversibly bound (as a result of physical interactions) and irreversibly bound (as a result of chemical reactions) [17; 18]. For example, by splitting the bound chlorides into chemically-bound and physically-bound ones, the original problem of diffusion with chloride binding is transformed to the problem of diffusion with a moving boundary for which an analytical solution can be developed [18]. Another important issue in the analysis of chloride diffusion penetration into concrete is the so-called \"skin effect\" which leads to significant differences between the behaviour of concrete at the external surface up to a depth of 5-20 mm and in the bulk part of this material. Its occurrence has several reasons in practical conditions: there is the \"wall effect\" when pouring the concrete mix in the formwork, so that more paste and less aggregate accumulate at the surface of concrete; and/or bleeding occurs; and/or carbonation occurs; and/or brucite precipitation occurs when hydrated cement comes into contact with sea water; and/or when the concrete surface is covered with a protective layer--e.g., with paint [19]. The work of [PERSON] et al. [19] is one of the first to present this problem from the mathematical point of view where the modification of the classical model of chloride diffusion in a half-space is proposed, taking into account two layers with different apparent diffusion coefficients. The \"skin effect\" may also arise in the case of chloride diffusion under unsaturated conditions in the pores of concrete, and then it typically consists in shifting the maximum concentration of chloride under the concrete surface due to the alternating phenomena of moistening and drying, washing out chlorides from the surface by rainfall, etc. [20]. Therefore, the effective prediction of the progress of chloride penetration into concrete and the assessment of the durability of reinforced concrete structures from this point of view should take into account this effect depending on the method of surface finishing. An unequivocal evaluation of the process and the related selection of appropriate concrete durability models is further complicated by the fact that the penetration of chlorides into concrete depends on the additives of waste materials to cements commonly used today. For example, based on [13], it can be stated that adding ground granulated blast furnace slag or fly ash to cement increases the chloride binding capacity while adding silica fume decreases it. Taking into account the facts discussed above, the authors, to the best of their knowledge, did not find in the literature review an attempt to identify the binding isotherm parameters together with the diffusion coefficient for chlorides in concrete based on the knowledge of their profiles from diffusion tests, which will lead to a quantitative assessment of the \"skin effect\" including different types of cement.