## Economic Viability  
Adopting the equivalent thicknesses determined in Section 3, the economic viability analysis of CRCA shotcrete is presented in this section. Based on data collected from Portuguese tunneling companies, the estimation of the current costs range to produce dry-mix shotcrete is shown in Table 8.  
Figure 2 shows the unit costs, in \(\xi\)/m\({}^{3}\), of the different shotcrete scenarios, assuming three possible costs for CRCA to assess its impact:  
* Scenario 1 (50% CNA)--the recycled aggregates' cost is taken as equal to 50% of the natural aggregates (7.5 \(\xi\)/m\({}^{3}\));
* Scenario 2 (NULL) --the recycled aggregates' cost is zero (taking into account that CDW producers are ready to pay to get rid of it);
* Scenario 3 (SUB) --there is a subsidy equal to 7.5 \(\xi\)/m\({}^{3}\) to promote recycled aggregates' use. Because the cost of CRCA is smaller than that of CNA, the unit cost of shotcrete with incorporation of CRCA is smaller. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **IR0** & **IR20** & **IR50** & **IR100** \\ \hline Yield Stress: \(p_{s}^{mtx}\) [MPa] & 1.43 & 1.32 & 1.21 & 1.39 \\ (\(\Delta p_{s}^{mtx}\) [\%]) & (-) & (\(-8\%\)) & (\(-15\%\)) & (\(-2\%\)) \\ \hline Equivalent Thickness: e [mm] & 200 & 206 & 214 & 283 \\ (\(\Delta\)e [\%]) & (-) & (3\%) & (7\%) & (42\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Variation of yield stress and equivalent thickness when a similar stiffness criterion is adopted. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **IR0** & **IR20** & **IR50** & **IR100** \\ \hline K\({}_{s}\) [MPa/m] & 636 & 697 & 766 & 653 \\ (\(\Delta\)K\({}_{s}\) [\%]) & (-) & (10\%) & (20\%) & (3\%) \\ \hline Equivalent Thickness: e [mm] & 200 & 224 & 255 & 290 \\ (\(\Delta\)e [\%]) & (-) & (12\%) & (28\%) & (45\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Results for the maximum pressure criterion. Figure 2 shows that incorporating recycled aggregates in shotcrete generates small variations is cost per cubic meter (<8%), for the three scenarios analyzed. Table 9 shows the cost of the lining per unit length of the tunnel for the two cases for which equivalent thickness was determined: similar stiffness and similar yield stress. In all cases, for simplicity's sake, rebound losses equal to 25% of the shotcrete's theoretical volume were assumed. For the case in which CRCA cost is 50% of the cost of CNA, the lining's cost per unit length of the tunnel with CRCA is slightly higher than without CRCA for IR20 (2.3%) and IR50 (5.3%) for Case 1. For the other IR's and cases, the increase in cost is higher than 10%. Because the recycled aggregates' cost represents less than 15% of the total production costs, an increase in thickness due to the lower mechanical properties leads to higher consumption of the other components, and thus costs increase. \begin{table}
\begin{tabular}{c c c c} \hline  & **Cost Range** & \multicolumn{2}{c}{**Cost Adopted**} \\ \cline{3-4}  & **[\(\ell\)/m\({}^{3}\)]** & **[\(\ell\)/m\({}^{3}\)]** & **[\%]** \\ \hline
**Equipment** & 16 to 20 & 18 & 13 \\ \hline
**Cement and Fly Ash** & 45 to 50 & 47.5 & 34 \\ \hline
**Labour** & 30 to 40 & 35 & 25 \\ \hline
**Additives** & 17 to 23 & 20 & 14 \\ \hline
**Plasticizers** & 4 to 5 & 4,5 & 3 \\ \hline
**Natural Aggregates** & 13 to 17 & 15 & 11 \\ \hline  & **Total** & **140** & **100** \\ \hline \end{tabular}
\end{table} 
Table 8: Material costs per m\({}^{3}\) of dry-mix shotcrete produced. Figure 2: Cost of shotcrete for each composition, assuming different cost scenarios of the recycled aggregates.