## 4 Experimental Results
Thus, the corresponding steel-slag-based asphalt mixture can be supposed to be characterized by an overall reduced rutting potential, which is of crucial importance for preventing excessive permanent strain accumulation for the investigated solar collector at high in-service temperatures. A more representative picture of the permanent deformation resistance of the studied blends is given by the \(J_{nr}/J_{tot}\) ratios. In this sense, it is worth noting that such parameters can vary from 0% (elastic material completely recovering the accumulated strain) to 100% (viscous material not recovering any accumulated strain). As expected, decreased \(J_{nr}/J_{tot}\) when temperature and stress level increased were observed. \(J_{nr}/J_{tot}\) approaching 100% were measured at temperatures higher than 70 \({}^{\circ}\)C, regardless of the stress level. However, this ratio was lower for _MOR-S_ with respect to corresponding _MOR-L_, highlighting again the reduced rutting potential due to the adoption of steel slag aggregates. In this regard, it is worth noting that such behavior could be likely due to the superior characteristics of the manufactured aggregate (angularity, roughness, toughness, etc.) rather than to a different chemical interaction with the bitumen [38]. At any rate, such experimental findings cannot be ascribed to different constituent proportions, since the blends were designed to attain exactly the same volumetric proportion among the constituents, properly taking into account the different particle densities of the aggregates. As anticipated, a complete description of the evolutive visco-elastic behavior during MSCR tests was attained through the application of the described fractional model. The three model parameters (\(\alpha\), \(b_{1}\), \(b_{2}\)) introduced in Equation (5) were determined by a mathematical fitting procedure (i.e., the bisquare trust-region Matlab algorithm) of the creep-recovery curves over the different testing cycles. The fifth cycle for each \(\tau\)-\(T\) combination and material was analyzed as a representative case, since negligible variations over the 10 cycles were recorded at the same \(\tau\)-\(T\). Figure 6 presents an example of graphical model fitting for the recovery phases of the different materials at 60 \({}^{\circ}\)C and 0.1 kPa (experimental data and fitted model are given in the main \(y\)-axis, while residuals \(r\) are plotted in the secondary \(y\)-axis). All the model coefficients are then presented in the following Table 5, along with the fitting reliability (expressed as R-square--sum squared error--SSE); it can be noted that, overall, a very high \begin{table}
\begin{tabular}{l c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & \\ \cline{2-10}  & **15** & **20** & **25** & **30** & **35** & **40** & **45** & **50** & **55** & **60** & **75** & **80** \\ \hline \(G^{\star}\) [kPa] & 10,573 & 3610 & 1092 & 300 & 79.83 & 23.30 & 7.46 & 3.10 & 1.22 & 0.68 & 0.30 & 0.12 \\ \(\Delta\) [deg] & 51.37 & 59.18 & 66.33 & 72.20 & 76.46 & 79.80 & 82.43 & 84.51 & 86.02 & 87.00 & 88.24 & 88.50 \\ \(G^{\star}/sin\delta\) & 13,535 & 4204 & 1192 & 315 & 82.11 & 23.67 & 7.53 & 3.11 & 1.23 & 0.68 & 0.30 & 0.12 \\ \(\{\)Fe\(\}^{\star}\) & & & & & & & & & & & & \\ \hline \hline \end{tabular}  
* SHRP requirement for upper PG temperature [32]: \(G^{\star}/sin\delta>2.20\) kPa.  
\end{table} 
Table 4: RTFO-aged \(BIT\)\(G^{\star}/sin\delta\) values for PG upper temperature classification (1.59 Hz). goodness of fit was also obtained (Table 5) in the case of unconventional mastic and fine mortar tested materials. Figure 5: MSCR test results. Percentage recovery \(R_{\%}\) at 3.2 (**a**) and 10 kPa (**b**); non-recoverable creep compliance \(J_{\rm irr}\) at 3.2 (**c**) and 10 kPa (**d**); non-recoverable creep compliance related to total creep compliance \(J_{\rm irr}/J_{\rm tot}\) at 3.2 (**e**) and 10 kPa (**f**).