## 3 Materials and Methods
### Testing Methods  
Bitumen \(BIT\) (in RTFO-aged condition) was studied by performing different tests with the DSR (EN 14770) in order to assess the rheological response of the binder within the typical in-service temperature range. First, oscillatory Amplitude Sweep (AS) tests were carried out with a parallel plate geometry to identify the LVE domain, i.e., the strain domain where the response is linear with respect to the stress/strain level applied [28]. Using the 8 mm diameter \(BIT\) samples (with testing gap of 2 mm), three test replicates were executed, increasing the shear strain \(\gamma\) from 0.01 to 100% (through logarithmic ramps) at a test frequency \(f\) of 20 Hz. Tests were \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Characteristic**} & \multirow{2}{*}{**Test Method**} & \multirow{2}{*}{**Unit**} & \multicolumn{2}{c}{**Value**} \\ \cline{3-4}  & & & **Limestone** & **Steel Slag** \\ \hline Particle density & EN 1097-6 & Mg/m\({}^{3}\) & 2.76 & 3.84 \\ Shape index & EN 933-4 & \% & 12.8 & 7.8 \\ Flakiness index & EN 933-3 & \% & 10.5 & 8.3 \\ Sand equivalent & EN 933-8 & \% & 78 & 92 \\ Rigden voids & EN 1097-4 & \% & 32.7 & – \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Physical properties of the selected aggregates. \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline \multirow{2}{*}{**Aggregates**} & \multicolumn{6}{c}{**Oxide Content [\%]**} \\ \cline{2-8}  & **MgO** & **Al\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **CaO** & **TiO\({}_{2}\)** & **Cr\({}_{2}\)O\({}_{3}\)** & **MnO** & **FeO** \\ \hline L & 2.50 & 1.00 & 3.34 & 52.71 & – & – & – & 0.39 \\ S & 3.65 & 9.30 & 13.02 & 29.60 & 0.35 & 4.03 & 5.09 & 32.84 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Morphological composition of the selected aggregates. carried out at a temperature \(T\) of 10 \({}^{\circ}\)C (a pre-conditioning time of 15 min was established to ensure the thermal homogeneity on samples). At such testing conditions, the lowest LVE limit was found and then applied for all the following rheological tests carried out at higher temperatures and/or lower frequencies [29]. The complex shear modulus \(G^{*}\) and the related phase angle \(\delta\) (i.e., the delay of the response due to the visco-elastic nature of the binder/blends) were measured, allowing calculation of \(G^{\prime}\) (storage modulus) and \(G^{\prime\prime}\) (loss modulus). According to wide literature, LVE limits were identified as the strain level corresponding to a \(G^{*}\) decreased to 95% of its initial value [30]. Frequency Sweep (FS) oscillatory tests were then carried out in parallel plate geometry using a strain level within the LVE domain; a test frequency varying from 0.1 to 20 Hz was selected. Such frequency sweeps were carried out at temperatures ranging from 10 to 80 \({}^{\circ}\)C, with step of 10 \({}^{\circ}\)C using a pre-conditioning time of 15 min at each \(T\); 8 mm diameter samples (with testing gap of 2 mm) or 25 mm diameter samples (with gap of 1 mm) were tested; the geometric test configuration was selected based on expected stiffness of the tested sample (two replicates for each test). Experimental data were first represented in Black diagrams (phase angle \(\delta\) vs. norm of complex modulus \(G^{*}\)) to check the alignment of the experimental data series in a unique curve (this happens for thermorheological simple materials), and thus to verify the consistency of the well-known Williams-Landel-Ferry equation [31] and the related time-temperature superposition principle. Based on FS results for \(BIT\), preliminary basic information about its rutting potential were evinced; this was accomplished by calculating the \(G^{*}/sin\delta\) parameter at 10 rad/s (1.59 Hz) as suggested by the Performance Grade (PG) classification approach [32]. Then, the responses of the binder were finally evaluated in terms of rutting potential considering the high in-service temperatures to which the bitumen will be subjected.