## 2 Materials and Methods
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test** & \multicolumn{3}{c}{**Nomenclature**} \\ \hline \multirow{3}{*}{Compressive strength testing} & SA-P1 & GF-1\%.P1 & GF-2\%.P1 \\  & SA-P2 & GF-1\%.P2 & GF-2\%.P2 \\  & SA-P3 & FA-1\%.P3 & GF-2\%.P3 \\ \hline \multirow{3}{*}{Fire + compressive strength testing} & SA-P4.F & GF-1\%.P4.F & GF-2\%.P4.F \\  & SA-P5.F & GF-1\%.P5.F & GF-2\%.P5.F \\ \cline{1-1}  & SA-P6.F & GF-1\%.P6.F & GF-2\%.P6.F \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Nomenclature and tests performed on the mass concrete test pieces. In order to study the behavior of mass concrete, with and without additions, when exposed to fire, two types of test have been performed on the test pieces: a test exposing them to the direct influence of a real fire, carried out at the Madrid Fire Department, and a subsequent test involving compression to breaking point, which will enable us to compare the mechanical capacity of mass concrete, before and after thermal aggression. With the aim of reproducing the type of concrete exposure in a real fire, provoking real temperature differences between the different zones of a structural element, in the direct fire exposure test an average fire of a potential heat of 40 kg/m\({}^{2}\) was achieved, which is an average potential heat value which is produced in a fire inside a building. The heat value of dry wood is 19 MJ/kg and 40 kg of wood was used, generating heat energy of 181,640 kcal in the test [7]. In order to perform the fire testing, the test pieces were placed vertically to simulate the pillars of a building, their entire surface being in direct contact with the fire. All the test pieces were arranged on a steel grid placed on top of a test may with a surface area of 1 m\({}^{2}\), loaded with 40 kg of chopped wood in order to enable greater oxygenation of the fire and encourage combustion at the source. The test lasted for 1 h, following the method standardized by ISO 834-11:2014 [49] and measuring the temperature in the upper, middle and lower sections of each test specimen every 5 min, using a compact infrared thermometer, \"Testo 845\"model. Once the hour-long fire exposure time had passed, the test specimens were slowly cooled down, dispersing their heat until reaching room temperature (20 \({}^{\circ}\)C). The fire test can be seen in Figure 2.  
Before proceeding to perform the compression testing to breaking point, the test pieces were capped with sulfur mortar in accordance with regulation UNE-EN 12390-3:2009 [44]. In order to carry out the compressive strength testing, the test pieces were placed in the IBERTEST MIB-60/AM universal press and a preload of 10% of the maximum test load was applied, with the intention of ensuring that the upper plate was pressing uniformly on the upper surface of the test piece. Once the adjustment had been made, the compressive strength test was carried out using a piston in order to obtain the force and stroke data and then analyze the results which are relevant to the research objective, as shown in Figure 3.  
In order to characterize the test pieces chemically and microscopically before and after the fire testing and after being subjected to compressive strength testing, the Scanning Electron Microscopy (SEM) technique was used. Using the X-ray detector (EDX), one-off and zonal microanalysis was carried out on the major elements in the samples showing natural breakage. The equipment used was the JEOL JSM-820 scanning electron microscope with microanalysis and the software was EDX Oxford ISIS-Link. In the study of the samples prepared using fresh concrete, CNF accumulation was  
Figure 1: Process of manufacturing cylindrical test pieces with and without addition: dosage, standardization, mixing and molding. not detected due to the fact that perfect dispersion of the nanofiber addition was achieved, as such the test pieces also underwent thin-layer analysis, during which it was possible to observe the presence of nanofibers at 500 magnifications.