## 2 Materials and Methods
### Life Cycle Assessment (LCA)-Methodology and Scope of the Study
#### 2.3.4 System Description and Boundaries  
The LCA models the life cycle system of a product with one or more defined functions. The systems of the products are divided by unit processes linked with the environment by elementary fluxes [38]. The system boundaries refer to the borders between the environment and the technological system [38]. Their definition is crucial to identifying the system's mass and energy flows that enter and exit. In the \"cradle to gate\" (A1-A3) approach used in this study of asphalt mixtures, the gate is defined as the point from which the asphalt mix is transferred to the truck for transportation to the consumer. The calculation of environmental impacts included all the primary inputs and outputs for the identified unit processes. According to the standard EN 15804:2012 + A2:2019/AC:2021, this LCA study covers the product stage (A) of the asphalt mixture's life cycle, as well as the extraction and processing of raw materials and waste/by-products (A1), the transport to the production plant (A2), and the production (A3) of the asphalt mixture. Figure 2 presents the product life cycle phases and processes within the system boundaries of the asphalt mixtures studied in this work. However, the system boundaries of AC14-NA do not include the RCA processes. The composition of an asphalt mixture refers to the specific arrangement and proportion of constituent materials within the mixture, including natural, artificial, or recycled aggregates, and the asphalt binder. The composition of the AC14-NA and AC14-RCA can be observed Table 4. Figure 3 presents the natural and recycled concrete aggregates and bitumen storage (using vertical tanks, which are the most suitable). Regarding module A1 of the product stage (raw material supply), it is essential to note that the AC14-NA only incorporated natural aggregates (NAs). In contrast, the AC14-RCA incorporated 55.1% RCA and only used 44.9% NA (percentages without bitumen). All the environmental impacts regarding the upstream processes of natural aggregates (extraction, crushing, and screening) were accounted for in module A1 of this LCA study. However, some specific allocation rules were considered for RCAs, which must be explained in detail. The upstream impacts of RCAs' previous life cycles, including product/manufacturing, transport, and use, are excluded from the system boundary. Therefore, only the impacts associated with the RCA preparation processes for use in the asphalt mixture are considered within the system boundary. Thus, the processing of the RCAs (reception, crushing, screening, and stocking) to become ready for use as aggregate (Figure 4) was included in module A1 of the system boundary. Module A2 includes transporting the raw materials to the asphalt plant. Moreover, the transportation between the RCA processing site and the asphalt plant must be included. However, the company has a waste treatment and processing facility within its premises, so RCAs' transport-associated environmental impact was considered null. Module A3 represents the asphalt mixture manufacturing process in a batch asphalt plant (Figure 5). This module includes the production of the asphalt mixture and all waste processes up to its end of state or disposal during the product stage.