## 1 Introduction
These criteria include maximum percentages of RA content, as well as stipulations on their type, origin, dry density, water absorption, and acceptable levels of contaminants [43]. Furthermore, there are limitations concerning the compressive strength of concrete and its class of exposure [44]. In Spain, the Structural Code [45] permits the use of RAC in both mass and reinforced concrete, provided that the maximum compressive strength does not exceed 40 N/mm\({}^{2}\). Additionally, it caps the RA content at 20% of the total coarse aggregate weight, highlighting that any surpassing of this threshold necessitates further studies and supplementary experimentation. The Structural Code also states that RA must be sourced solely from sound or high-strength structural concrete, limiting the water absorption of the RA fraction to 7% and the complementary NA fraction to 4.5%. Moreover, the UNE-EN 12620 [46] stipulates a maximum permissible proportion of 2% for lightweight particles, along with restrictions for asphalt and other materials (such as clay, glass, plastics, and metals) set at 1% and 0.5%, respectively. Other countries have also incorporated the use of RA in structural concrete production [47]. Each national standard is tailored to the characteristics of its construction sector and aligns with local CE roadmaps. In the EU, member states develop their sustainable policies in accordance with the CE framework established by the EC [6]. Consequently, several regional standards share similar requirements regarding RA use. Notably, with the exception of Denmark, most EU member states restrict RA utilisation to the coarse fraction [44]. At a broader European level, the percentage of RA content is a key regulatory criterion. Standards such as LNEC E471 [48] in Portugal, NEN 5905 [49] in the Netherlands, the Structural Code [45] in Spain, and BS 8500-2 [50] in the United Kingdom limit the RA content to 20%. However, the maximum allowable concrete strength varies across these standards. While LNEC E471, the Structural Code, and BS 8500-2 cap structural concrete production at 40 MPa, NEN 5905 permits a higher threshold of 55 MPa. Conversely, there are standards with specifications that surpass these common criteria [44]. For instance, the Guideline of the German Committee for Reinforced Concrete (DAfStb) [51] stipulates that only RA classified as Type 1 (concrete waste) or Type 2 (demolition waste) can be used in the production of structural concrete. According to this guideline, for Type I, the maximum RA content is 35% of the total aggregate volume, and the concrete strength is limited to less than 25 MPa. For Type II, the RA content is capped at 25% (by volume), with permissible concrete strength of up to 35 MPa. Meanwhile, there are stricter limitations regarding the acceptable levels of contaminants. Thus, for RA classified as Type I, the maximum allowable rates of mineral contaminants are set at 2%, 0.2% for non-mineral contaminants, and 1% for asphalt. Otherwise, the percentages permitted for RA classified as Type II are 3% for mineral contamination, 0.5% for non-mineral, and 1% for asphalt. Several studies have examined the impact of RA content on both the structural performance and manufacturing process of concrete [52]. These studies suggest that RA can be incorporated regardless of its fraction or substitution percentage, provided that the required concrete quality is maintained [42]. [PERSON] et al. [53] reported that Recycled Coarse Aggregate (RCA) can be used in concrete mixtures with strengths ranging from 20 to 45 MPa, despite potential strength variations of up to 25%. Limiting concrete strength in this context is crucial, as excessive cement content could undermine the economic and environmental benefits of using RAC [42]. These findings align with the criteria established in European standards. However, several studies [47; 54] highlighted the need to supplement these results with specific clauses in structural design codes to enhance the understanding of the structural performance of RAC elements.