## 3 Results and Analysis
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Addition Fibers** & \(\sigma_{\text{max}}\) & \(\sigma_{\text{u}}\) & \(\epsilon_{\text{max}}\times 10^{-3}\) & \(\epsilon_{\text{u}}\times 10^{-3}\) & \(\text{E}_{\text{max}}\times 10^{-2}\) & \(\text{E}_{\text{u}}\times 10^{-2}\) \\  & **(N/mm\({}^{2}\))** & **(N/mm\({}^{2}\))** & **(Adimensional)** & **(Adimensional)** & **(N/mm\({}^{2}\))** & **(N/mm\({}^{2}\))** \\ \hline Compressive & Without addition & 14.372 & 10.063 & 3.117 & 5.956 & 2.755 & 6.217 \\ strength tests & CNFs 1\% & 17.183 & 6.877 & 3.020 & 4.690 & 3.161 & 5.340 \\ CNFs 2\% & 15.943 & 6.382 & 3.013 & 5.420 & 3.063 & 5.781 \\ \hline Fire + & Without addition & 19.556 & 10.938 & 4.864 & 8.466 & 5.406 & 11.199 \\ compressive & CNFs 1\% & 22.187 & 2.221 & 4.068 & 7.981 & 5.393 & 9.590 \\ strength test & CNFs 2\% & 18.630 & 1.86928 & 3.750 & 7.615 & 4.230 & 8.276 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Most representative mean values of the three specimens for each concrete type subjected to compressive strength testing and direct fire testing, with the different addition percentages. Figure 6: Evolution of strength (\(\sigma\)) in relation to strain (\(\epsilon\)) in test pieces without addition, with percentages of 1% and 2% of polypropylene fibers and with percentages of 1% and 2% of steel fibers, after undergoing direct fire testing. In Figure 9, in the concrete without addition and not exposed to the fire, the presence of calcium silicate hydrate (C-S-H) gel can be observed, which is formed by hydrating cement, along with the  
Figure 8: Scanning electron microscopy at 300 magnifications in test pieces without addition SA, and with the addition of CNFs in percentages of 1 and 2% by weight of cement GF-1%, GF-2%. (**a1–a3**) Before thermal aggression; (**b1–b3**) After thermal aggression. Figure 7: Compression testing to breaking point in test pieces without addition and with 1% CNFs, before and after fire testing. formation of portlandite and the acicular shapes which are typical of ettrinigite. In the concrete containing CNFs, it can be observed that the fibers point radially towards the exterior of the nodule, constituting a perfectly locked, dense crystalline mesh. Figure 9a2 shows the nested CSH nodules in a more advanced phase, where the silicate gel matrices are grouped together, leaving increasingly closed spaces, locked by the CSH fibers and the AFf phase. The calcium silicate hydrate is a principal cement compound and the main reason for its resistant properties, by means of its branches around the nodule and pointing towards the aggregate grains it bonds the compounds, anchoring the paste-aggregate mix and providing the cementitious matrix with cohesion and rigidity. After the fire, it can be observed that many of the CSH fibers in the concrete test pieces have been destroyed, thus reducing in number. However, in the concrete containing 1% of CNFs it is observed that the bonding of ettrinigite needles is not widespread, and the arrangement of the almost perfect crystalline mesh continues to exist, demonstrating the improved behavior in compressive strength testing of the test pieces containing CNFs. Finally, in the figure containing 2% of CNFs, we find a vacuole and inside the pore the ettrinigite fibers have disappeared due to thermal aggression, although  
Figure 9: Scanning electron microscopy at 2000 magnifications in test pieces SA, GF-1% and GF-2%. (**a**) Before thermal aggression; (**b**) After thermal aggression. it has a good general appearance and its edges retain the CSH-fiber formation which confers resistance and cohesion to the structure. Figure 10 making possible to observe that the figures related to the samples exposed to the fire (Figure 10b), possess a clearly common and characteristic feature, where all of the acicular and pointed shapes have become rounded.