## 4 Concrete Samples
### Preliminary Consideration of the Photothermal Detectability  
In order to estimate the influence of moisture on effusivity, the correlation between moisture and the separate physical properties has to be evaluated. The final values were calculated with the following assumptions:  
1. The density rises linearly with the sample mass (the volume remains constant);
2. The heat capacity is the sum of capacities of concrete and water weighted by their respective mass parts (water in the micropores adds additional heat capacity to the heat capacity of dry concrete); and
3. The thermal conductivity rises linearly with increasing water content, with the slope depending on the specific concrete microstructure. A slope value of 4 was selected relative to the mass percentage of water in the concrete.  
It is important to note that different relationships between thermal conductivity and moisture were reported in the past. [PERSON] and [PERSON] noted a slope value of 5% for concrete as a general rule [24]. [PERSON] reported a series of measurements in different kinds of  
Figure 7: Variation of water content during several soaking/drying cycles. Red font indicates the corresponding photothermal measurements; day data in the boxes refer to type 3.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Date**} & \multicolumn{2}{c}{**State of Type 1**} & **State of Type 3** & **Photothermal** \\  & **(Lightweight Conc.** & **(Lightweight Conc.** & **Investigation** \\  & **Red Expanded Clay)** & **Black Expanded Clay)** & **Investigation** \\ \hline
21 November 2021 & \multicolumn{2}{c}{Saturated after 19 days} & Almost saturated after & \multirow{2}{*}{Exp. 1} \\  & in water & 19 days in water & & \\ \hline
21 November 2021 & 5 h drying in air & 5 h drying in air & No exp. \\ \hline
5 February 2022 & Soaking & Soaking & No exp. \\ \hline
13 April 2022 & Drying in air & Covered with foil & No exp. \\ \hline
4 May 2022 & In air & Drying in air & No exp. \\ \hline
10 May 2022 & After 27 days drying & After 6 days drying & Exp. 2 \\ \hline
10 May 2022 & 5 h in sun & 5 h in sun & No exp. \\ \hline
12 May 2022 & After 1 month drying & After 8 days drying & Exp. 3 \\ \hline
13 May 2022 & \multicolumn{2}{c}{Partially saturated after } & Partially saturated after & \multirow{2}{*}{Exp. 4} \\  & 20 h soaking & 20 h soaking & & \\ \hline
7 June 2022 & Drying in air & Drying in air & No exp. \\ \hline \hline \end{tabular}
\end{table}
Table 3: Chronology of soaking/drying cycles and the related photothermal investigations.  
concrete and found values between 4% and 8%, with values up to 15% for specific ternary mixtures [25]. The German IBP published several values for several materials in a table including expanded clay concrete [26]; thus, a value of 4% was selected here. In the case of the type 1 sample, a maximum mass increase of 8.8% was reached after soaking. With the aforementioned assumptions, the effusivity of the wet concrete would increase by 37% (from 1630 to 2200 \(W\)-\(\sqrt{s}\)/K\(\cdot\)m\({}^{2}\)). As shown later at the end of Section 5.1, this difference should be easily detectable with the photothermal method. If the thermal diffusivity with the same material parameters is considered, a reduction of only 5% would be expected (from 7.1 to 6.84, both in 10\({}^{-3}\) cm\({}^{2}\)/s).  
However, this is only a preliminary consideration based on Equation (1) with the six mentioned simplifications, which was successfully applied in the case of dry concrete. In particular, the assumptions of isotropic material properties and a homogeneous temperature distribution within the material are probably not fulfilled, taking into account moisture and the resulting effusivity gradients within the material and the convective cooling taking place at the surface. However, it is unknown how strongly these effects influence the photothermal response.