## 1 Introduction  
Ever since the pavement engineering society started investing in the research of recycled material incorporation into conventional pavement mixtures, a major concern is to avoid downgrading mixture quality [1, 2, 3]. Systematic experiments, primarily conducted in laboratories and, to a lesser extent, in the field, have confirmed that, when compared to control mixtures made of conventional materials, non-conventional mixtures, including waste plastic, crumb rubber, waste glass, steel slag, and reclaimed asphalt pavement (RAP), can perform as well as or even better against fatigue, rutting, and moisture damage [4, 5, 6, 7, 8, 9]. It is observed that most of the relevant studies concentrate on problems concerning these materials' mechanical performance and structural contribution. In contrast, even though surface layers are often replaced within the context of a maintenance and rehabilitation (M&R) plan, serviceability concerns and functional characteristics (such as texture, skid resistance, etc.) are more infrequently studied and hence have received less attention [10; 11]. In any case, it is expected that the diversity of waste materials' properties can indeed produce some controversial results, as per the performance of non-conventional mixtures, something that justifies why related research consistently revives. On the other hand, it might be anticipated that practitioners do follow these research trends and try to broadly apply this knowledge in the field through an extended use of these materials [12]. However, this is yet to be achieved so far. Therefore, this study tries to integrate additional critical aspects and reasons, apart from those related to the mechanical testing of these materials, about why recycled mixtures are not consistently selected and uniformly applied during construction and reconstruction activities in real practice. The motivation for this objective is two-fold. The first has to do with the frequency of asphalt layers' maintenance and/or rehabilitation. In other words, the surface layer, in particular, exhibits an increased potential for inclusion of recycled or other waste materials, because of the periodical surface interventions that can take place during a pavement's lifespan [1; 13]. In real practice, road operators are unwilling to move forward with expensive and extensive full-depth interventions. Therefore, surface treatments are typically applied to most pavements, provided that structural soundness over time is ensured. Aligned with the concept of long-life pavements (LLPs) [14], or perpetual pavements [15], frequent surface layer interventions usually include minor maintenance measures, like crack sealing, or the replacement of the surface layer. The latter explains why using non-conventional materials for the surface layers, either locally available from pavement reconstruction activities or from landfilling, does indeed make sense. Secondly, in fully serviceable pavements, it is important for the surface layer to provide a distress-resistant behavior with both structural and functional features. This indicates that the material mix design for the top layer is an essential step in both the pavement design process and the pavement's operational service life. The absence of specifications together with the related strict behavioral requirements of surface layers can account for the related diversity of research findings. Indeed, these limitations might control the maximum rate of waste materials in the composition of the non-conventional mixtures. Hence, despite the continuously revitalized research, there is still a lot to achieve, until a more broadened and consistent use of these materials at surface-layer mixtures takes place.