## 1 Introduction
\[\frac{\partial c}{\partial t}=D_{cl,RCMT}\left(\frac{\partial^{2}c}{\partial x ^{2}}-\frac{zFU}{RTL}.\frac{\partial c}{\partial x}\right) \tag{1}\]  
\[D_{cl,RCMT}=\frac{RTL}{zFE}\frac{x_{d}-\alpha\sqrt{x_{d}}}{t},\text{ where }E=\frac{U-2}{L}\text{ and }\alpha=2\cdot\sqrt{\frac{RTL}{zFE}}\cdot erf^{-1} \left(1-\frac{2c_{d}}{c_{0}}\right) \tag{2}\]  
From the analytical solution of Equation (1), [PERSON] [23] derived Equation (2) that determines the non-steady migration coefficient \(D_{cl,RCMT}\), where \(z\) is the absolute value of ion valence for chloride (\(z=1\)), \(F\) is the Faraday constant, \(U\) stands for the absolute value of the applied voltage, \(R\) is the gas constant, \(T\) is the average value of the initial and final temperature in the anolyte solution, \(L\) is the thickness of the specimen, \(x_{d}\) is the average value of the penetration parameters, \(t\) is the test duration, \(erf^{-1}\) is the inverse of error function, \(c_{d}=0.07\) N (chloride concentration at which the colour changes), and \(c_{0}=2\)N (chloride concentration of catholyte solution). The wide acceptance of the RCMT led to the development of service life models based on this test, namely Duracrete [27], fib 34 [31] and LNEC E 465 [32]. [PERSON] [33] suggested a classification for the chloride penetration resistance of concrete, according to different ranges of measured \(D_{cl,RCMT}\) (Table 1). However, some limitations are attributed to the current RCMT. Regarding the theoretical background of the RCMT model, various authors have reported that real chloride concentration profiles from the RCMT greatly deviate from those theoretically predicted from the analytical solution of Equation (1), characterised by a sharp front [3; 26; 28; 29]. Actually, the RCMT chloride profiles follow the same shape of long-term diffusion tests in saturated concrete [28; 29]. According to [PERSON] et al. [26], the equation to determine the \(D_{cl,RCMT}\) fails in not considering the non-linearity of the chloride binding isotherm and non-equilibrium condition between free and bound chlorides. Regarding the free to bound chloride concentration, the equilibrium should not be attained during the short RCMT period [26]. This equilibrium is reported to be achieved only over 7-14 days [1; 33]. For this reason, the diffusion coefficient determined in common long-term IT is not comparable to the RCMT. Therefore, \(D_{cl,RCMT}\) tends to be overestimated, since it cannot explore the real chloride binding capacity of concrete [26]. Other simplification is related to the non-consideration of the interaction of other ionic species during migration. In fact, the pore solution is a multicomponent electrolyte with other ions participating in the process, such as Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\) and OH\({}^{+}\), with a given activity and diffusivity in different directions [3; 15; 26; 29]. [PERSON] et al. [26] showed that, \begin{table}
\begin{tabular}{c c} \hline \hline
**Chloride Penetration Resistance of Concrete** & \(D_{cl,RCMT}\) (\(\times\)10\({}^{-12}\) m\({}^{2}\)/s) \\ \hline Low & \(>\)15 \\ Moderate & 10–15 \\ High & 5–10 \\ Very high & 2.5–5 \\ Extremely high & \(<\)2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chloride penetration resistance of concrete based on the RCMT (based on the results of [PERSON] [33]). during the RCMT, the resistivity of the sample increased, which was attributed to the densification of the pore system caused by chloride binding and the partial change of OH-ions by free chloride ions in the pore solution. These factors make the modelling of the RCMT migration a complex process. Some more accurate models have been proposed, but their complexity is barely compatible with their simple practical application [15; 29]. The RCMT is also affected by the test conditions. The calculation of the \(D_{cl,RCMT}\) requires the exact value of the electrical field across the concrete specimen.