## 2 Materials and Methods
### Solar Reflectance Index  
The solar reflectance index (SRI) represents the \"coolness\" of a material surface, since it accounts for the two major characteristics, i.e., solar reflectance and thermal emittance. This index is determined according to the procedure described in the American standard ASTM 1980 [44]. Firstly, the steady-state surface temperature under the sunlight (\(T_{s}\)), the apparent sky temperature, \(T_{sky}\), the solar absorbance, \(\alpha\), the irradiance, \(I\), and the thermal emissivity, \(\varepsilon\), are evaluated (Equation (3)). \[\alpha~{}I=\varepsilon~{}\sigma~{}\Big{(}T_{S}^{4}-T_{Sky}^{4}\Big{)}+h_{c}~{ }~{}(T_{S}-T_{a}) \tag{3}\]where \(\sigma\) is the Stefan-Boltzmann constant (5.670367 \(\times\) 10\({}^{-8}\) W m\({}^{-2}\) K\({}^{-4}\)) and \(h_{\rm c}\) is the convective coefficient (W m\({}^{-2}\) K\({}^{-1}\)). In addition, the steady-state surface temperature can be calculated by using Equation (4). \[T_{s}=309.07+\frac{\left(1066.07\ \alpha-31.98\ \varepsilon\ \right)}{\left(6.78\ \varepsilon+h_{c}\right)}-\frac{\left(890.94\alpha^{2}+2153.86\ \alpha\ \varepsilon\ \right)}{\left(6.78\ \varepsilon+h_{c}\right)^{2}} \tag{4}\]  
Finally, the solar reflectance index (SRI) of a surface can be calculated by iteration from the solar reflectance and thermal emittance testing measurements for some defined convection conditions and \(T_{s}\) (Equation (5)), where \(T_{w}\) and \(T_{b}\) are the steady-state surface temperatures of the white and black reference surfaces, respectively. \[SRI=100\ \frac{T_{b}-T_{s}}{T_{b}-T_{w}} \tag{5}\]  
The conditions are described in the American standard ASTM 1980 [44]. Considering medium wind conditions, the standard black material surface with zero solar reflectance index (\(SRI\) = 0) is characterized by solar reflectance and thermal emittance values of 0.05 and 0.9, respectively, whereas the standard white material surface has solar reflectance of 0.8 and thermal emittance of 0.9. Accordingly, high values of thermal emittance and solar reflectance induce lower surface temperatures and higher SRI results. The solar reflectance index (\(SRI\)) could be considered as an indirect coolness scale where the fully reflective white material corresponds to an index equal to 100, and the fully absorptive black surface is characterized by an index equal to zero. Nevertheless, some new materials recently available in the building sector could be lower than 0 or higher than 100. Table 3 shows the albedo average results (Rm), solar absorptance (\(\alpha\)), thermal emittance (\(\epsilon\)), temperature that each sample would reach (\(T_{s}\)) and the solar reflectance index (\(SRI\)) for three convection coefficients (\(h_{c}\)), corresponding to three wind speeds, obtained in the tested precast pavements. Average albedo values of all the samples range from 0.28 to 0.57, while solar absorptance values range from 0.42 to 0.72. In addition, solar absorptances for the white and black bodies were 0.2 and 0.95, respectively. High-albedo concrete surfaces tend to keep the pavement environment cooler than low-albedo concrete surfaces by promoting a further takeaway in energy efficiency enhancement of the heat pumps because of the lower thermal gap [45]. Only three samples have albedo values below 0.30 (Glacier 1, Glacier 2 and Glacier 4). Glacier 1 and Glacier 2 correspond to the concrete slabs aged in a natural manner, while Glacier 3 and Glacier 4 are concrete samples without ageing. These paving materials were made with dark grey Portland cement and fine siliceous aggregates. The ageing effect is quite low, that is, the albedo fell from 0.30 to 0.28 in the worst-case scenario. Furthermore, the new mix design provides albedo values that can go as high as 0.36-0.40 (Glacier 5 and Glacier 6). In any case, concrete pavements have a much higher albedo than asphalt pavements due to their light gray color compared with black, but unfortunately, concrete albedo decreases over time because of weathering and the accumulation of dirt.