## 2 Research Goals and Approach  
Given this background, this paper presents an experimental study aimed at evaluating the rutting potential of different asphalt blends. In particular, the study is addressed to the small-case analysis of such blends since, according to literature [21; 22], mastic and mortar phases of bituminous mixtures have a crucial influence on the mechanical performance of asphalt mixtures [23]. In this regard, several researchers report that small-scale samples including fine aggregate particles could reliably reflect the physical-chemical interactions between the asphalt binder and the aggregates [24]. Starting from this assumption, this research study evaluates specific rutting-related features at such a scale of analysis, investigating special constituent materials composing the asphalt blends. In fact, the study builds upon a previous step of a wider research project which investigated the thermal efficiency of a road energy harvesting system constituted by a steel-pipe-based asphalt solar collector (ASC) and various asphalt mixes [25]. In such a study [25], special constituent materials and mix-design proportions were selected for thermal purposes mainly. The peculiar characteristics of the ASC were (Figure 1):  
* thanks to its conduction properties, a manufactured steel slag aggregate was included in the lithic skeleton (composed by natural limestone aggregate) in order to enhance the thermal conduction within the asphalt layer, and the related solar collector efficiency [26];
* a rather fine aggregate gradation (with nominal maximum aggregate size of 8 mm) was designed to make the mixture structure compatible with the presence of the collector without compromising the functionality of the 4 cm thick covering asphalt layer;
* a low target air-void content was selected for enhancing the thermal transmission within the asphalt layer, since air voids can act as insulators. The above characteristics allowed an efficient energy collection, effectively converting the solar radiation to thermal energy, but they can lead to unacceptable rutting potential. Therefore, the present paper reports the high-temperature mechanical properties of the fine mortar phase of the abovementioned asphalt mixes produced with energy harvesting purposes. Overall, such a research study can enhance the knowledge in the field since it is specifically addressed towards the study of unconventional materials (i.e., materials containing steel slag aggregates and characterized by special proportions of constituents) through the use of quite common testing methodologies, applied at unconventional scale (i.e., fine mortar scale) and using unconventional methods of data analysis. Figure 1: Background from the previous research [25]. In particular, the rutting performance at bitumen-, mastic-, and fine-mortar scale was analyzed both using conventional (limestone) or steel slag aggregates and a plain (non-modified) bitumen according to the original mix design for the asphalt layer of the solar collector [25]. The Dynamic Shear Rheometer (DSR) was used to accomplish the objective by testing the asphalt binder, mastics (bitumen + filler with size < 0.075 mm), and fine mortars (bitumen + filler + fine aggregates with size up to 0.18 mm), reproducing the constituent proportions of the original bituminous mixtures [25]. Such a testing approach could be considered quite innovative, since few studies exist on the use of DSR for studying the asphalt blends at fine-mortar scale [27]. After a preliminary characterization of the physical properties of the binder and the fine aggregates, several rheological tests were performed analyzing the Linear Visco-Elastic (LVE) properties and the rutting performance of the materials. Binder, mastics, and fine mortars were tested after short-term laboratory aging (Rollin Thin Film Over--RTFO--test according to EN 12607-1) to simulate the real in-service conditions after material's production and paving.