# Mechanical Performance of Gilsonite Modified Asphalt Mixture Containing Recycled Concrete Aggregate
## 1 Introduction
### Recycled Concrete Aggregate Background  
During the construction process of roadway projects, large quantities of natural origin aggregates (NA) are consumed. This negatively impacts the environment. According to the authors of [39], roadway construction produces approximately 28% of the world's energy consumption and 22% of global CO\({}_{2}\) emissions. A way of reducing these impacts (reduction of the carbon footprint in the construction industry, conservation of natural resources, and reduction of hazardous emissions, among others) is replacing NAs with recycled concrete aggregates (RCAs). The latter is obtained from construction and demolition (C&D) waste (CDW), which has been increasing because of growth in the construction industry [40; 41]. The authors of [42], referring to other researchers, mention that an average between 30% and 40% of the total solid waste material comes from C&D activities. Additionally, they mention that RCAs represent between 50% and 70% (in weight) of the total CDW. For these reasons, several studies have been carried throughout the world to evaluate the use of RCAs as a substitute for NAs. Some factors that limit the use of RCAs are (i) the need for using large spaces for their storage; (ii) experience in residue recycling operations, supervisors, and trained personnel are all required; (iii) it requires knowledge about waste material markets; and (iv) it requires a high degree of awareness regarding environmental and safety regulations [43]. The broadly identified problem in most RCA studies is that these are materials with a high degree of heterogeneousness, which present greater absorption and lower mechanical resistances than those of NAs [44; 45; 46; 47; 48; 49]. This is mainly because of the adhered mortar attached to the RCA particles [50; 51; 52]. Additionally, RCAs present impurities such as bricks, roof tiles, ceramic, and organic materials, among others. Because of this, most part studies have focused on evaluating the behavior of RCAs as a substitute for NAs in the lower layers of pavement (e.g., subgrade, subbase, and base), which are exposed to less of the pressures produced by traffic [53]. However, the amount of RCA produced in the world already surpasses its common use (even in its use in pavement base and subbase), and therefore there is an existing need to broaden its use in asphalt mixes [47]. Additionally, differently to the use of RCAs in bases, subbases, or subgrades, in the context of asphalt mixes, there is an absence of the problem regarding the possible generation of lixiviates [54]. Despite the identified problems, some authors have mentioned that RCAs present particles with ideal shapes (fractured faces with low flattening and elongation indexes) that can help to generate a compact granular skeleton with good intergranular friction [55]. Furthermore, their greater porosity in comparison with NAs can help to improve adhesion to asphalt binders. RCAs have not only been experimentally characterized, but the complex failure processes of these materials have been simulated using the discrete-element method (DEM) [56]. Regarding asphalt mixes, studies have mainly focused on substituting (generally by mass) NA with RCA in different proportions (even replacing the totality of the aggregate),and then measuring the changes that took place in the mixes, as well as their mechanical resistance. The literature is ambiguous regarding drawing conclusions related to the obtained results. While some studies have reported that the evaluated properties improve, others have concluded the opposite [48; 57; 58]. Furthermore, there is a great disparity about the recommendations regarding the replacement levels of NAs for RCAs in asphalt mixes [58]. Likewise, the behavior of mixes may change depending on the source from where the RCA was obtained [48].