## 2 Materials and Methods
### Tested Concretes  
Information regarding the porous concrete types, identified with the letters A, B, C, and D, are summarized in Table 1. The following parameters are reported: aggregate size (according to EN 933-2:2020 [27]), aggregate particle density (according to EN 1097-6:2013 [28]), void ratio (according to ASTM C1754/C1754M [29]), flexural strength (according to EN 12390-5:2019 [30]), previous concrete density (according to ASTM C1754/C1754M [29]), and water permeability (according to ASTM D2434-19 [31]). Two different types of aggregates have been used in the mix design: normal weight and lightweight. The normal weight aggregates have been used in concrete type A and have an irregular shape with an average dimension of 4-8 mm, while the lightweight aggregates have an almost perfect round shape, i.e., spherical, with different dimensions ranging between 4 and 8 mm, 2 and 4 mm, and 0.5 and 1 mm for concretes B, C, and D, respectively. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Concretes**} & **Aggregate** & **Aggregate** & **Void** & **Flexural** & **Pervious** & **Water** \\  & **Sigze** & **Particle** & **Ratio** & **Strength** & **Concrete** & **Permeability** \\  & **[mm]** & **Density [kg/m\({}^{3}\)]** & **[\%]** & **[MPa]** & **Density [kg/m\({}^{3}\)]** & **[mm/s]** \\ \hline \multirow{4}{*}{A} & crushed & & & & & & \\  & normal weight & 4–8 & 2650 \(\pm\) 30 & 25 \(\pm\) 2 & 3.5 \(\pm\) 0.5 & 1955 \(\pm\) 20 & 9.5 \(\pm\) 1.1 \\  & aggregates & & & & & & \\  & round & & & & & & \\  & lightweight & 4–8 & 905 \(\pm\) 85 & 25 \(\pm\) 3 & 1.5 \(\pm\) 0.3 & 932 \(\pm\) 35 & 10.9 \(\pm\) 1.7 \\  & aggregates & & & & & & \\  & round & & & & & & \\  & lightweight & 2–4 & 325 \(\pm\) 35 & 25 \(\pm\) 2 & 1 \(\pm\) 0.2 & 514 \(\pm\) 20 & 7.4 \(\pm\) 0.9 \\  & aggregates & & & & & & \\  & round & & & & & & \\  & lightweight & 0.5–1 & 510 \(\pm\) 70 & 20 \(\pm\) 2 & 0.5 \(\pm\) 0.1 & 682 \(\pm\) 15 & 3.6 \(\pm\) 0.5 \\  & aggregates & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Four mix design of porous concrete characteristics with respect to: aggregate size, aggregate particle density, void ratio, flexural strength, previous concrete density, and water permeability. Figure 1: Sample (**A**): crushed normal weight aggregates 4–8 mm; Sample (**B**): round lightweight aggregates 4–8 mm; Sample (**C**): round lightweight aggregates 2–4 mm; Sample (**D**): round lightweight aggregates 0.5–1 mm. Concrete A differs significantly from the other three regarding the concrete density value, which is strongly affected by the higher values of the aggregate particle density. The four concretes present a similar void ratio. However, while this parameter is constant for concretes A, B, and C, it decreases for concrete type D, which features smaller aggregates. It can also be noticed that there is a decrease in the flexural strength for lower densities and smaller aggregate dimensions. For each concrete, three different sample types have been manufactured with three different thicknesses, i.e., 20 mm, 40 mm, and 60 mm (Figure 1); for each of them, three samples have been produced. The panels are square-shaped in plan with a side dimension of 60 cm. Three different mounting conditions were tested for the samples with a thicknesses of 20 and 40 mm, that is, coupling different samples in multiple layers (Figure 2), adding a 50 mm air gap between the sample and the room floor (Figure 3), and adding a layer of fibrous material (rock wool) in the air gap itself (Figure 4). The identification codes of the samples and mounting conditions have been summarized in Table 2. The coupling of different samples in multiple layers has been performed only within the same concrete in order to compare their performance with single layers of the same thickness and investigate any anisotropy at the back surface of each layer. The multiple layer configuration is obtained by superimposing one panel to the other, with no joint or glue connecting them.