## 2 Experimental Procedure
### Experimental Methods
#### 2.3.2 Chloride Penetration Depth and Chloride Ion Diffusion Coefficient  
Figure 4 shows the method for testing the chloride penetration depth, which was measured in accordance with NT Build 443. Concrete specimens were fabricated with dimensions of \(100\times 100\times 400\) mm and  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Mix No. 1**} & \multirow{2}{*}{**W/B 2**} & \multirow{2}{*}{**S/A 3 (\%)**} & \multicolumn{4}{c}{**Unit Weight (kg/m\({}^{3}\))**} & \multirow{2}{*}{**NP (B\(\times\%\))**} & \multirow{2}{*}{**PC (B\(\times\%\))**} \\ \cline{3-3} \cline{5-8}  & & **W 4** & & & & & & & \\ \hline C35\_O10 & 0.42 & 48.5 & 185 & 444 & & 908 & 919 & 0.7 & \\ C24\_O3\_E5\_G2 & 0.50 & 48.5 & 165 & 102 & 170 & 68 & 910 & 921 & 0.7 \\ \hline \hline \end{tabular}  
* See Table 3; 2 water/binder ratio; 3 sand/aggregate ratio; 4 water, 5 crushed sand; 6 gravel.  
\end{table}
Table 4: Mixing proportions of concrete.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Evaluation Item**} & \multicolumn{1}{c}{**Test Method**} \\ \hline Slump (mm) & ASTM C143 \\ Air contents (\%) & ASTM C231 \\ Compressive strength (MPa) & ASTM C873; ASTM C39 \\ Chloride penetration depth (mm) & NT Build 443 \\ Chloride ion diffusion coefficient (m\({}^{2}\)/s) & NT Build 492 \\ Porosity (\%) & ASTM D 4404 \\ \hline \hline \end{tabular}  
* See Table 5: Test methods for each evaluation item.  
\end{table}
Table 5: Test methods for each evaluation item.  
immersed in a 2.8 M NaCl solution for 3, 7, 14, 28, 56, and 91 days. To ensure that chloride penetrated the concrete in only one direction, the side and bottom surfaces were coated with epoxy resin. To measure the chloride penetration depth, a 0.1 N AgNO\({}_{3}\) solution was sprayed after the specimens were fractured at each age, and discoloration was measured.  
Figure 5 shows the method for testing the chloride ion diffusion coefficient. Specimens for the chloride diffusion test were fabricated by cutting a certain portion of \(\mathcal{O}100\times 200\) mm cylindrical specimens to a 50 mm thickness using a diamond cutter. In accordance with NT Build 492, a 0.3 N NaOH solution (approximately NaOH 12 g in 1 L of water) was used as the anode, and 10% NaCl solution (100 g of NaCl + 900 g of distilled water) as the cathode.  
Figure 4: Test method of chloride penetration depth. (**a**) experimental setup of specimens; (**b**) test method.  
Figure 5: Test method of chloride ion diffusion coefficient. (**a**) experimental equipment; (**b**) test method; (**c**) measurement zone.  
The applied voltage and test duration were selected based on the current value when a 30 V voltage was applied. The chloride ion penetration depth was measured using the same method as above, and the chloride ion diffusion coefficient was calculated using Equation (1).  
\[D_{nssm}=\frac{RT}{zFE}\frac{X_{d}-\alpha\sqrt{X_{d}}}{t}, \tag{1}\]  
\[E=\frac{U-2}{L}, \tag{2}\]  
\[\alpha=2\sqrt{\frac{RT}{zFE}}\cdot\mathrm{erf}^{-1}\left(1-\frac{2C_{d}}{C_{0}} \right). \tag{3}\]  
where \(D_{nssm}\) (m\({}^{2}\)/s) is the non-steady-state migration coefficient; \(z\) is the absolute value of the ion valence, and for chloride, \(z=1\); \(F\) is the Faraday constant, \(F=9.648\times 10^{4}\) J/(V\(\cdot\)mol); \(U\) (V) is the absolute value of the applied voltage; \(R\) is the gas constant, \(R=8.314\) J/(K\(\cdot\)mol); \(T\) (K) is the average value of the initial and final temperatures in the anolyte solution; \(L\) (m) is the thickness of the specimen; \(X_{d}\) (m) is the average value of the penetration depths; \(t\) (s) is the test duration; \(\mathrm{erf}^{-1}\) is the inverse of the error function; \(C_{d}\) is the chloride concentration at which the color changes, \(C_{d}=0.07\) N for OPC concrete; and \(C_{0}\) is the chloride concentration in the catholyte solution, \(C_{0}\approx 2\) N.