## 2 Materials and Methods
its behavior with test pieces that were not exposed to thermal aggression, and establishing results relating to tensile strength, strain and maximum and ultimate strain energy densities. In order to carry out the analysis, 18 cylindrical test pieces were made, measuring 100 mm in diameter and 200 mm in height, in line with UNE-EN 12390-1:2013 [43]. The testing program was organized using a single mix for the purpose of having the concrete manufactured at the same time and with the same characteristics. Six mass concrete test pieces were made without the addition of CNFs, 6 test pieces were made with the addition of 1% CNFs to the cement weight and 6 test pieces were made with the addition of 2% CNFs. The addition of CNFs represents percentages of 1% and 2% of the weight of the cement, which is equivalent to 2.3 kg/m\({}^{3}\) and 4.6 kg/m\({}^{3}\) of concrete respectively. For each type of test piece, 50% of them directly underwent compression testing to breaking point, according to regulation UNE-EN 12390-3:2009 [44], whilst the remaining 50% previously underwent direct fire testing in line with the fire resistance tests for materials used in the Fire Prevention and Extinction Service of the Madrid region and with regulations UNE-EN 1363-1:2012, UNE-EN 1363-2:2000 and UNE-EN 1365-4:2000 [45; 46; 47], and once they had cooled down, they also underwent compression testing to breaking point. The nomenclature and tests performed on the test pieces can be seen in Table 3.  
Firstly, and before starting to manufacture the concrete, the materials required to make the test pieces indicated in Table 1 were maintained in laboratory conditions for 24 h. The gravel was sifted using a sifting machine, with the aim of guaranteeing that there were no aggregates which were larger than 12 mm. Then, each of the materials to be used was weighed separately: cement, sand, gravel, addition of nanofibers and water, using industrial dial scales or digital scales, depending on the accuracy required. The initial concrete mixing was performed by combining the dry materials by hand, adding the materials in the following order: gravel, cement, sand and finally the addition of CNFs, in order to obtain a perfectly homogeneous mixture and greater dispersion of the nanofibers. The dry mixture was subsequently transferred to the IBERTEST vertical shaft planetary mixer, model CIB-701 updated to IB32-040V0 (Ibertest Advanced testing solutions). The mixer combined the dry materials once again for 2 min using its rotary blades, before gradually adding the water, taking into account the dampness of the aggregates used, until a plastic concrete consistency was achieved after 5 min of mixing. Once the concrete was in a plastic, cool state, the test piece molds were filled in accordance with the fresh concrete testing regulations [48]. The molds were filled in sections, compressing each layer of concrete using a steel tamping rod to reach every part of the mold, before filling it up. Once the test pieces had been made, they were stored at a laboratory temperature of approximately 22 \({}^{\circ}\)C \(\pm\) 3 \({}^{\circ}\)C and at an approximate relative humidity of 60% for 24 h, before being removed from the molds. After 24 h, the test pieces were demolded and cured in a humidity chamber at a temperature of 20 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C and a relative humidity of \(\geq\)95% for 28 days. During this time, the test pieces set and harden until they reach a minimum level of resistance that enables the compression to breaking point and fire tests to be performed, which will be detailed in the following section. The process of producing the mass concrete test pieces can be seen in Figure 1.