## 5 Conclusions  
The present experimental investigation aimed at evaluating the rutting potential of different asphalt materials, which were designed based on thermal considerations only during a previous study on asphalt solar collectors. A comprehensive rheological characterization through DSR tests at the unconventional fine-mortar scale, along with an innovative modeling of the experimental data, were performed to this aim. Based on this approach, the following main findings can be summarized:  
* with respect to the selected binder (plain 50/70 pen grade non-modified bitumen), the \(G^{*}/sin\delta\) parameter indicated a PG upper limit almost equal to 52 \({}^{\circ}\)C, which can be considered acceptable for such a bitumen;
* the progressive inclusion of lithtic fractions, passing from binder to mastic (bitumen + filler with size \(<\) 0.075 mm) and fine mortars (bitumen + filler + fine aggregates with size up to 0.18 mm), led to a clear stiffening effect and reduced rutting potential; \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{\(\tau\) [kPa]} & \multicolumn{3}{c}{**s**} & \multicolumn{3}{c}{**d**} \\ \cline{3-8}  & & **A** & **B** & **R\({}^{2}\)** & **m** & **q** & **R\({}^{2}\)** \\ \hline \multirow{2}{*}{BIT} & 0.1 & 0.0035 & 0.1461 & 0.9875 & \(-\)0.1937 & 16.586 & 0.9775 \\  & 3.2 & 0.0943 & 0.1496 & 0.9869 & \(-\)0.3225 & 23.863 & 0.9776 \\ \hline \multirow{2}{*}{MAS-L} & 0.1 & 0.0009 & 0.1411 & 0.9886 & \(-\)0.0365 & 6.675 & 0.9553 \\  & 3.2 & 0.0155 & 0.1509 & 0.9907 & \(-\)0.092 & 9.6559 & 0.8854 \\  & 10 & 0.0383 & 0.1560 & 0.9910 & \(-\)0.0601 & 6.7586 & 0.8569 \\ \hline \multirow{2}{*}{MOR-L} & 0.1 & 0.0006 & 0.1324 & 0.9898 & \(-\)0.0177 & 6.338 & 0.9524 \\  & 3.2 & 0.0069 & 0.1469 & 0.9973 & \(-\)0.0947 & 11.213 & 0.9149 \\  & 10 & 0.0077 & 0.1651 & 0.9979 & \(-\)0.1058 & 11.217 & 0.9675 \\ \hline \multirow{2}{*}{MOR-S} & 0.1 & 0.0008 & 0.1224 & 0.9885 & \(-\)0.0183 & 6.6292 & 0.8581 \\  & 3.2 & 0.0058 & 0.1413 & 0.9995 & \(-\)0.0816 & 10.759 & 0.9529 \\ \cline{1-1}  & 10 & 0.0055 & 0.1619 & 0.9994 & \(-\)0.1087 & 11.698 & 0.9966 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Physical parameters describing creep-recovery strain evolution. the use of steel-slag fine aggregates at the fine-mortar scale led to an even higher stiffening effect with respect to conventional limestone fine sand, thus suggesting an enhanced rutting resistance; - slag-based mortar was characterized by a reduced accumulation of high-temperature permanent strains with respect to conventional limestone-based materials, regardless of the testing conditions. Therefore, it can be asserted that the effective use of steel slags should be promoted to achieve environmental, economical, and mechanical benefits; - the \(R_{\%}\), \(J_{nr}\), and \(J_{nr}/J_{tot}\) parameters calculated from MSCR test results were not sufficient to fully understand the rutting potential of materials, since the creep-recovery responses could be characterized by different evolutive recoverable strains after load removals; - a fractional model, constituted by a springpot and a dashpot in series, reliably fitted the evolution of the permanent deformation not only at binder scale, but also at mastic and fine-mortar scale; - the creep-recovery mechanisms could be also accounted for with simple physical parameters: in particular, the delay in time of the visco-elastic recovery could be a reliable index of the rutting issues, and could be also linked to the material stiffness (the higher the delay, the higher the stiffness and the related rutting resistance). Based on the above considerations, at this research stage it can be supposed that there are no significant rutting issues for the investigated materials. Indeed, the research contributed to advancing the knowledge about the creep-recovery rheological analysis, since DSR testing can be effectively used for mastic and fine mortar characterization, even dealing with unconventional materials such as steel slag.