# Concrete Gas Permeability: Implications for Hydrogen Storage Applications
## 2 Experimental Development
### Procedures  
The experimental procedures initiated with the oxygen permeability measurement, which was determined in hardened concrete by applying different gas pressures under laminar flow conditions. The flow rates through the test specimen per unit time were measured. The test specimens had a cylindrical geometry with flat faces, sensibly perpendicular to their axis, a thickness of 50 \(\pm\) 2 mm, and an effective diameter of 75 \(\pm\) 2 mm, 100 \(\pm\) 2 mm, or 150 \(\pm\) 2 mm. To start the test, the specimen was placed inside the cell, where it was sealed with a polyurethane collar. On one of the surfaces, the gas was applied and had to pass through the specimen in order to leave from the other surface. The gas that passed through the concrete left the test cell in a tube that was connected to the graduated tube so that the flow could be measured. Five pressure values for each gas were used. Multiple measurements were taken for each pressure until three with an error of less than 3% were obtained. Pressure values between 0.5 and 3.5 bar were used. The average flow values obtained at different pressures for the same specimen were considered. For the final calculation, average values of at least 5 measurements of the diameter and height of the specimen were taken. To calculate the permeability coefficient, Equation (1) is used, considering each gas's respective dynamic viscosities, as shown in Table 2.  
\[K=\frac{2\cdot Q\cdot p_{0}\cdot L\cdot\eta}{A(p^{2}-p_{a}^{2})} \tag{1}\]  
where:  
\(K\) is the permeability coefficient of oxygen (m\({}^{2}\));  
\(Q\) is the gas flow through the specimen (m\({}^{3}\)/s);  
\(p_{0}\) is the pressure at which Q (m\({}^{3}\)/s) is determined (considered equal to \(p_{a}\)) (N/m\({}^{2}\));  
\(L\) is the thickness of the specimen (m);  
\(\eta\) is the viscosity of the gas (N\(\cdot\)s/m\({}^{2}\));  
\(A\) is the cross-sectional area of the specimen (m\({}^{2}\));  
\(p\) is the pressure applied during the test (N/m\({}^{2}\));  
\(p_{a}\) is the atmospheric pressure (N/m\({}^{2}\)). In order to determine the conditions of the specimens properly, other tests have been performed. After testing each specimen, they were weighed, and their resistivity was measured using direct or indirect methods (depending on the specimen's size) and ultrasonic testing using both direct and indirect methods. The electrical resistivity of hardened concrete indirectly indicates the connectivity and size of its pores and the degree of liquid saturation in its porous network [36]. A higher water content in the concrete results in lower electrical resistivity. The electrical resistivity can be measured indirectly using the four-point or Wenner method and directly using a potentiostat/galvanostat [37]. The four-point method applies a stable current between the external electrodes and measures the change in potential \begin{table}
\begin{tabular}{c c c c} \hline
**Gas** & **Viscosity (10\({}^{-5}\) Pa\(\cdot\)s)** & **Atomic Mass (g/mol)** & **Density (g/mL)** \\ \hline
**Hydrogen** & 0.8400 & 1.00797 & 0.071 \\ \hline
**Helium** & 1.9460 & 4.0026 & 0.126 \\ \hline
**Nitrogen** & 1.7660 & 14.0067 & 0.810 \\ \hline
**Oxygen** & 2.0260 & 15.9994 & 1.429 \\ \hline
**Argon** & 2.2150 & 39.9480 & 1.400 \\ \hline \end{tabular}
\end{table} 
Table 2: Gas properties. difference between the internal electrodes. According to the standard mentioned, the four-point method involves applying a current between two electrodes positioned on a surface of the specimen and measuring the voltage between the two interior electrodes aligned with the former. This method allows for the determination of electrical resistance [38; 39]. For the direct measurement, a modification of the UNE-EN 12390-19 standard was followed.