*3.1. Concrete Permeability*

The permeability test used nitrogen as a gas media and the Cembureau method was applied [31]. The testing set-up used is presented in detail in Figure 1.

**Figure 1.** Permeability Cembureau testing set-up of the sample chamber.

In Equation (1), the permeability (k) was determined:

$$\mathbf{k} = \frac{2\mathbf{Q}\mathbf{P}\_\mathbf{a}\eta\mathbf{L}}{\mathbf{A}\left(\mathbf{P}^2 - \mathbf{P}\_\mathbf{a}^2\right)} \text{(m}^2\text{)},\tag{1}$$

where:

Q: the measured gas flow intensity Q = V/t (m3/s); V: gas volume (m3) t: time (s) Pa: atmospheric pressure (1 bar = 105 Pa); P: absolute pressure (Pa); A: cross section area of the specimen (m2); η: nitrogen viscosity; η = 17.15 (Pa·s);

L: thickness of the specimen (m).

The initial reference permeability of the concrete in the samples at 90 days old was determined on the specimens that were not pre-dried, in order to represent the non-dried condition in the real structure. Subsequently, the samples were heated to a temperature ranging from 200 to 1000 ◦C, and after cooling the permeability was measured. Each measurement value represented the mean value from three samples.
