## 5 Mechanical Tests  
### Compressive and Tensile Strengths  
Performed mechanical tests, as described in Section 2.2.3, included uniaxial compression (UC) for determining compressive strength and mechanical behavior, and both splitting tensile (ST) and three-points bending (TPB), for determining indirect tensile strength. A total of 154 specimens were tested, according to Table 4. Mean strength results are summarized in Table 5.  
Based on the UC results in Table 5, Figure 9 shows the evolution of the uniaxial compressive strength \(f^{\prime}_{c}\), normalized with respect to \(f^{\prime}_{c}\) at 20 \({}^{\circ}\)C, for increasing temperatures and for each mortar type. It can be observed that compressive strength mainly decreases with the increase of the acting temperatures pre-imposed during the thermal treatments. A quite gradual reduction in compression strength can be observed in both composites (see Figure 9), i.e., at 300 \({}^{\circ}\)C (mid-level) the decrease of the compressive strength was 19% for both mortar types; while significantly larger reductions occurred under high temperatures exposures (73% and 56% for MNS and MHS, respectively, at 600 \({}^{\circ}\)C). It is important to remark that the obtained strength values of MHS at 100 \({}^{\circ}\)C were too low, indicating some setup error in the corresponding tests. Figure 8: Open (and apparent) porosity (**a**) \(p(\%)\) and (**b**) \(p^{*}\) for MNS and MHS at different temperature levels. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Temperature** & \multicolumn{2}{c}{**UC**} & \multicolumn{2}{c}{**ST**} & \multicolumn{2}{c}{**TPB**} \\
[\({}^{\circ}\)C] & \(f^{\prime}_{c}\)**[MPa]** & \(f^{\prime}_{t}\)\({}^{ST}\)**[MPa]** & \(f^{\prime}_{t}\)\({}^{TPB}\)**[MPa]** \\ \hline  & **MNS** & **MHS** & **MNS** & **MHS** & **MNS** & **MHS** \\ \hline
20 & 46.55 & 77.12 & 5.13 & 6.99 & 8.35 & 9.42 \\
100 & 46.65 & 62.32 & 4.77 & 6.90 & 7.99 & 8.88 \\
200 & 38.42 & 67.36 & 4.63 & 6.60 & 7.13 & 9.56 \\
300 & 37.58 & 62.58 & 3.34 & 5.45 & 4.99 & 8.08 \\
400 & 30.09 & 44.28 & 2.99 & 3.59 & 4.86 & 5.02 \\
500 & 21.66 & 40.68 & 1.99 & 3.00 & 3.08 & 4.57 \\
600 & 12.77 & 34.00 & 1.09 & 1.76 & 1.14 & 2.78 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mean peak stresses in UC, ST and TPB tests. From Figure 9, it can be concluded that compressive strength reductions of both normal and high strength mortars followed a similar trend under increasing temperatures. However, at temperatures above 400 \({}^{\circ}\)C, MNS presented a higher relative strength loss. The resulting indirect tensile strengths (\(f^{\prime}_{t}\)\({}^{ST}\)) obtained in the splitting tensile tests and detailed in Table 5, are plotted on the left side of Figure 10, normalized with respect to (\(f^{\prime}_{t}\)\({}^{ST}\)) at 20 \({}^{\circ}\)C, for increasing temperatures and for each mortar type. The results show the clear incidence of temperature on indirect tensile strength, causing a progressive degradation for increasing thermal effects. MHS specimens were less affected under low temperatures (up to 300 \({}^{\circ}\)C), while for high heat treatments they were greatly affected, presenting important thermal damages. Contrarily, strength loss in MNS mixtures, shows a more gradual (almost linear) dependence on the temperature level than the MHS ones. However, for temperatures above 400 \({}^{\circ}\)C, strength loss was very similar for both mortar qualities. On the right plot in Figure 10, the obtained flexural tensile strengths (\(f^{\prime}_{t}\)\({}^{TPB}\)) corresponding to the three-points bending tests are presented, normalized in this case with respect to (\(f^{\prime}_{t}\)\({}^{TPB}\)) at 20 \({}^{\circ}\)C, for increasing temperatures and for each mortar type. From this plot, it can be observed that the effect of thermal treatments leads, as a general trend, to a gradual reduction of the flexural strengths as the temperature levels increase, for both MNS and MHS.