## 1 Introduction  
The construction sector is considered one of the main players in the generation of environmental impact. Besides cement production, which is responsible for more than 6% of all global CO\({}_{2}\) emissions, the total amount of construction and demolition waste (CDW) represents a third of all waste generated in the European Union. However, there are barriers to its reuse, one of the main of which lies in the lack of trust in the quality of CDW [1].  
The incorporation of recycled aggregates (RA) in concrete, by replacing a percentage of natural aggregates (NA), usually leads to poor mechanical and durability-related performances [2, 3].  
It is therefore important to understand to what extent the behavior of the RA differs from that of the NA. According to [PERSON] et al. [2], the main differences between these aggregates are:  
* Higher water absorption capacity of the RA, leading to concrete mixes with lower workability and effective water/cement ratio;
* Lower compactness and thus lower resistance to crushing, lower modulus of elasticity and higher short- and long-term deformability.  
[PERSON] [4] and [PERSON] et al. [2] evaluated the performance of recycled aggregate concrete (RAC), finding that the nature of RA strongly influences their binding capacity to the new cement paste, thus affecting the mechanical properties of the resulting concrete. Nevertheless, these authors also concluded that the use of these aggregates in concrete production is feasible. However, considering that their use affects the mechanical and durability-related properties of RAC, imposition of limitations and cautions on their inclusion is required.  
Shotcrete is widely used in the underground mining and tunneling industry, slope stability, rehabilitation works, and in many other areas.  
The tunneling industry is growing because the available space in dense urban environment tends to be used for nobler purposes than transit subways, parking lots, electric lines, water supply and sewer lines [5].  
Nowadays, the widespread use of the New Austrian Tunneling Method (NATM) has increased the use of shotcrete for linings [6]. In recent years, several studies on the use of fibres to replace the traditional electro-welded meshes have been published [7; 8; 9; 10].  
To the authors' knowledge, the incorporation of recycled aggregates from CDW in deep tunnels has not yet been studied in detail. This work studies the viability of using CDW in urban tunnels, because CDW is usually generated in urban environments. A technical and economic assessment of different dry-mix shotcrete compositions, incorporating coarse recycled concrete aggregates (CRCA) in substitution of coarse natural aggregates (CNA), intends to contribute to the sustainability of the construction sector.  
For generalization purposes, the analysis is made for a reference case of a deep tunnel where analytical equations proposed by [PERSON] and [PERSON] [11] are valid (see Appendix A), based on the convergence-confinement method, which is broadly used to describe the ground-support interaction in design practice [12; 13].  
It is demonstrated that limited incorporation ratios of CRCA can be a valid alternative from a technical and economic perspective.