## 3 Mechanical Properties  
Regarding the mechanical properties, as stated, the bond strength (by pull-off), compressive and splitting tensile strengths, secant modulus of elasticity, abrasion resistance and ultrasound pulse velocity were determined. Table 4 provides a summary of the hardened state mechanical properties. Regarding bond strength, no clear variation of this property with the incorporation of CRCA in concrete was found. As for compressive strength, it decreased as the percentage of the replacement of CNA with CRCA increased. Maximum reductions between 20% and 30% were obtained. This result is in agreement with those of several authors who analyzed cast RAC and obtained maximum reductions within this range. Even so, at seven days, the strength values of (RAC) shotcrete compared to cast RAC were lower, which supports the hypothesis that hardening is slower in the former than in the latter. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **RC** & **C20** & **C50** & **C100** \\ \hline \multirow{3}{*}{**Pull-off bond [MPa]**} & \multirow{3}{*}{\(1.11\)} & 0.8 & 1.11 & 1.14 \\ \cline{3-5}  & & \(-\)2.91\% & \(-\)0.15\% & \(+\)2.45 \\ \hline \multirow{3}{*}{**Compressive strength [MPa]**} & \multirow{3}{*}{**7-day**} & \multirow{3}{*}{\(18.27\)} & 17.72 & 15.99 & 14.09 \\ \cline{3-5}  & & & \(-\)3.01\% & \(-\)12.48\% & \(-\)22.88\% \\ \cline{3-5}  & & & \(-\)33.83 & 30.35 & 27.25 \\ \cline{3-5}  & & & \(-\)9.01\% & \(-\)18.37\% & \(-\)26.71\% \\ \cline{3-5}  & **56-day** & \multirow{3}{*}{\(43.24\)} & 36.58 & 35.77 & 31.91 \\ \cline{3-5}  & & & \(-\)15.40\% & \(-\)17.28\% & \(-\)26.20\% \\ \hline \multirow{3}{*}{**Splitting tensile strength [MPa]**} & \multirow{3}{*}{\(3.33\)} & 3.1 & 3.01 & 2.84 \\ \cline{3-5}  & & & \(-\)6.84\% & \(-\)9.61\% & \(-\)14.77\% \\ \hline \multirow{3}{*}{**Ultrasound pulse velocity [km/s]**} & \multirow{3}{*}{\(4.82\)} & 4.79 & 4.54 & 4.48 \\ \cline{3-5}  & & & \(-\)0.45\% & \(-\)5.66\% & \(-\)6.97\% \\ \hline \multirow{3}{*}{**Modulus of elasticity [GPa]**} & \multirow{3}{*}{\(26.14\)} & 25.41 & 24.34 & 18.08 \\ \cline{3-5}  & & \(-\)2.79\% & \(-\)6.89\% & \(-\)30.83\% \\ \hline \multirow{3}{*}{**Abrasion resistance [\%]**} & \multirow{3}{*}{\(8.13\)} & 7.49 & 7.73 & 7.06 \\ \cline{3-5}  & & & \(-\)7.87\% & \(-\)4.92\% & \(-\)13.16\% \\ \hline \hline \end{tabular} Legend: Green—Better result in comparison with reference concrete (RC); Yellow—Worse result than RC, with reductions lower than 10%; Red—Worse result than RC, with reductions higher than 10%.  
\end{table} 
Table 4: Mechanical properties of shotcrete mixes (adapted from [PERSON] et al. [25]). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **RC** & **C20** & **C50** & **C100** \\ \hline
**Coarse Natural Aggregates** & 0.331 & 0.264 & 0.164 & 0.000 \\ \hline
**Coarse Recycled Aggregates** & 0.000 & 0.066 & 0.164 & 0.327 \\ \hline
**Fine (Natural) Aggregates** & 0.354 & 0.352 & 0.350 & 0.348 \\ \hline
**Total Aggregates** & 0.685 & 0.681 & 0.678 & 0.674 \\ \hline
**Type I 42.5 R Cements** & 0.115 & 0.115 & 0.115 & 0.115 \\ \hline
**Water** & 0.175 & 0.179 & 0.182 & 0.186 \\ \hline
**Voids** & 0.025 & 0.025 & 0.025 & 0.025 \\ \hline
**Total** & **1.000** & **1.000** & **1.000** & **1.000** \\ \hline
**w/c Ratios** & **0.46** & **0.47** & **0.50** & **0.50** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Composition of the concrete mixes studied (m\({}^{3}\)/m\({}^{3}\)) and estimated w/c ratios. With respect to tensile strength, the incorporation of CRCA led to non-significant influence, as also reported by other authors, with maximum decreases of 15%, which are clearly lower than those in compressive strength. The reason for this is that the bond between the cement paste and CRCA is improved (compared to CNA) due to their shape and higher water absorption capacity. The ultrasound pulse velocity decreased as the CRCA incorporation increased, which is consistent with the higher porosity of these aggregates that leads to a concrete mix with lower quality.