## 1 Introduction
In this context, [PERSON] and [PERSON] [55] argued that, despite variations in behaviour, real structures incorporating RAC comply with the requirements set forth in structural standards. This suggests that recycled aggregates can be used in structural concrete, even if their quality is slightly lower. Nevertheless, integrating these advancements into structural design codes is crucial, as it would mitigate uncertainties associated with their application and promote their adoption in a sector where designers and concrete producers strictly adhere to established guidelines [47]. The impact of standards on the transition towards a CE in the construction sector was highlighted in a report by the European Aggregates Association (UEPG) [35]. The reported results revealed that the production of recycled and reused aggregates in European countries is approximately 9.3%. Among them, Belgium (29.3%), the Netherlands (24.7%), and the United Kingdom (23.9%) (Figure 1) stand out as having the highest percentages of RA in their total annual production. Meanwhile, Germany leads Europe in the total output of RA, by producing 72 million tonnes per year [35]. The increased integration of secondary raw materials in these countries coincides with their standards, allowing a higher RA content or the inclusion of RA in concretes with a higher strength than most standards. These developments are supported by established concrete-recycling industries that ensure compliance with the percentage of contaminants and the water absorption capacity indicated in the standards [43; 56]. In turn, progress in the construction sector's transition to a CE entails a macro-level impact, since it is classified as a strategic sector in the Circular Economy Action Plan [6]. Indeed, studies examining the transition toward circularity in EU member states [57; 58] reveal a slight improvement over the past two decades. This is evidenced by the Resource Productivity Indicator (RPI) (Figure 2), which demonstrates the increasing efficiency of material use over time. This increase is driven by the growth of Gross Domestic Product (GDP) relative to Domestic Material Consumption (DMC) [59]. The Resource Productivity Indicator analysis shows that the pace and intensity of this transition vary across different countries, with the Netherlands, Italy, France, Spain, and Germany at the forefront. However, according to the third and fourth reports of the CE Spanish Strategy (EEEC: acronym in Spanish) [57; 58], the reduction in material usage in Spain is still closely tied to economic fluctuations. Thus, the notable rise in the RPI since 2008 resulted from reduced building sector participation in value generation. Figure 2 illustrates that this trend has moved upward in recent years; however, absolute decoupling has yet to be achieved. The national CE reports [57; 58] emphasise that the current focus of the Spanish productive  
Figure 1: Percentage of RA production in European countries (Authors’ own based on [35]). Figure 2: Resource Productivity Indicator (RPI) (2000–2023) (Authors’ own based on [59]). system is on implementing circularity strategies at the End of Life (EOL). While this approach facilitates circularity, it does not effectively promote a transition to a CE model with less resource-intensive development. Consequently, these reports underscore the necessity of integrating circularity strategies from the initial stages of the production process. This shift could enhance the national Circular Material Use Rate (CMR), of 8.5%, as recorded in 2023 (Figure 3) [60]. This percentage, defined as the portion of materials recovered and reintegrated into the economy relative to overall material use, remains significantly below the EU target of 23.4% by 2030 [58]. In response to this need, the present study aimed to assess the circularity potential of RAC in the structure of newly built multi-family residential buildings in Spain. For this purpose, the Spanish residential building stock was analysed to identify the most representative structural system.