## 3 Results and Analysis  
The results obtained in the compressive strength testing of the concrete before it underwent fire testing are shown in Figure 4, by means of the most representative graph for each type studied, reflecting the evolution of the strain in relation to tensile strength, in test pieces without addition and with percentages of 1% and 2% of CNFs. As can be appreciated, the strengths are markedly greater in the test pieces with the addition of CNFs, better behavior being achieved for percentages of 1% of nanofibers, where the strength achieved is notably greater as is the case with strain. However, the fact of incorporating a larger amount of CNFs does not improve strength in the test pieces. Figure 5 shows us how the temperature evolves as the direct exposure to fire test progresses in the test pieces without addition and with the addition of 1% and 2% CNFs to the cement weight. As can be appreciated, the test provides us with significant data whereby the test pieces reach the highest temperature 10 min after starting the fire since it is a highly developed fire, reaching values of close to 600 \({}^{\circ}\)C in test pieces containing 2% of CNFs and values of 450 \({}^{\circ}\)C in test pieces without addition and with 1% of CNFs. The high temperature of the test pieces containing 2% of CNFs is due to the high heat transfer coefficient that this material possesses and that owing to its dispersion throughout the concrete mix the test piece increases in temperature evenly. Concrete containing 1% CNF in cement weight reaches similar temperature values to those reached in concrete without addition and 25%  
Figure 3: Capping of test pieces and compression testing to breaking point. Figure 2: Fire test involving the entire surface of the test piece being directly exposed to the fire and taking measurements with an infrared thermometer. lower than those reached in concrete with 2% CNFs in cement weight. In the case of concretes with 2% of CNFs, once the maximum temperature has been reached, there is a decrease in temperature faster than in concrete with 1% of CNFs, which translates into greater cracking of the concrete. The test pieces experience different temperature gradients depending on the areas where the flames reach them, as such, the heat transfer of the material can be observed, as well its absorption and dissipation capacity. Focusing on test pieces made of traditional concrete (Figure 5a), without any kind of addition, we can see that the top of the 20 cm test piece displays temperature jumps, increasing 100 \({}^{\circ}\)C from the lower part where the fire is most direct, to the upper part, which reaches practically the same temperature as the middle section. This is due to the fact that concrete is able to dissipate the temperature on the surface of the material very well, and that in order to increase its internal temperature significant heat is required over a long time period. Observing Figure 5b, it is worth highlighting that the temperature reached in the middle and upper areas of the test piece is practically the same, and in comparison with concretes without addition it has a lower thermal gradient. This is due to the 1% amount in cement weight of addition incorporated in the concrete, as a part of the calorific value of the fire is consumed in heating that proportion of CNFs dispersed in the cementitious mass, absorbing part of the energy and not transferring heat to the cement mass. At the lower part of the test piece, the temperature that is reached is greater, since the carbon nanofibers, due to their high heat transfer coefficient generate a temperature increase in this area of greater flame exposure. The concrete containing 1% CNFs in cement weight displays similar behavior in the cooling phase to the concrete without additions, where the temperature decrease is progressive and as such, the material becomes deformable and has less cracking. Finally, Figure 5c is that which displays the highest temperature values in the entire test piece.