## 4 Discussion  
The analysis of the results of the compressive strength tests shown in Figure 13 make it possible to observe how the test pieces incorporating 1% or 2% of CNFs in cement weight increase their maximum strengths (\(\sigma\)max) compared to a traditional concrete without additions, and even when they have undergone direct fire testing, the strengths reached are much higher than 50% in the case of an addition amount of 1% of CNFs. What happens with maximum strains is worth special mention, since concretes without addition reach lower values when they are not exposed to the action of a fire but achieve strengths which are considerably higher, enabling us to deduce that the addition of CNFs in such percentages gives the concrete greater mechanical resistance, withstanding greater workloads and barely suffering material strain. The maximum strain energy density is closely related to the maximum strength reached and its maximum strain. The more energy a material absorbs, the better its mechanical behavior. As we can appreciate in Figure 13, the test specimens which best represent this parameter are  
Figure 12: Energy dispersive x-ray microanalysis at 500 magnifications of thin-layer test pieces containing 2% of CNFs. (**a**) Before thermal aggression; (**b**) After thermal aggression. those containing 1% of CNFs. When we refer to concretes without addition and with a 2% addition of nanofibers, the strain energy density is very similar, enabling us to deduce than an increased addition to the concrete does not improve the material's behavior. Analyzing the ultimate strength values (ou), Figure 14 enables us to compare the evolution of the mean values associated with the ultimate strengths and strains calculated based on compressive strength testing, before and after being subjected to direct fire exposure, using concrete without additions which has not been exposed to fire as a reference for the comparison. It is observed that the ultimate strength values are always lower in test pieces with the addition of CNFs, both before and after the fire, and that they are practically the same in concrete without addition following thermal aggression. When we observe the strain behavior for the ultimate strengths, we detect a unique phenomenon, which is that before being exposed to fire, the concretes with the addition of CNFs reach strain values which are also lower, however, in concretes with addition and which have been exposed to fire, despite their lower ultimate strengths, the strains achieved are much higher, with values approaching 25% in some cases. When we look at the data which the test concerning the ultimate strain energy density gives us, the behavior is similar to the ultimate strain, with lower values in concretes with addition and without being exposed to fire compared to traditional concrete. Once again, the values increase considerably when these concretes with addition are exposed to heat, reaching strain energy densities of almost 80% of the value of a traditional concrete. Figure 14: Evolution of the mean values associated with the ultimate strengths and strains from compressive strength testing, in test pieces before and after direct fire testing. Figure 13: Evolution of the mean values associated with the maximum strengths and strains from compressive strength testing, in test pieces before and after direct fire testing. Figure 15 sets out the mean values of the temperatures reached by the test pieces with and without additions during direct fire testing. Generally, it can be observed that the line representing the most regular thermal behavior is that which incorporates a 1% addition of CNFs to the cement weight. The presence of excess CNFs, as in the group containing 2% of addition, causes the temperature to increase more rapidly than in the other groups studied due to its high heat transfer coefficient and significant proportion of nanofibers and as such the cooling phase is also slower, exposing the cement matrix to a higher temperature as a consequence of its thermal inertia.