## 2 Materials and Methods  
During the process and execution of the experimental part of the research, the materials and resources detailed below were used:  
Materials used:* Cement type CEM II/B-L 32.5 according to the Structural Concrete Code UNE-EN 197-1:2011 and RC-08 [3; 40]. Its physical and chemical characteristics are shown in Table 1.
* Washed, siliceous, fine river sand with a particle size of 0-4 mm, according to regulation UNE-EN 13139/AC: 2004 [41]. * Washed, siliceous, coarse aggregate with a particle size of 4-20 mm, with a maximum aggregate size of 12 mm, according to regulation UNE-EN 12620:2003+A1:2009 [42]. * Water from the Canal de Isabel II supply system in the Madrid region, since this meets the technical specifications established for structural concrete. * Carbon nanofibers (CNFs), used with approximate diameters of 100 nm, density of 1.95 g/cm\({}^{3}\) and a surface area of 45 m\({}^{2}\)/g with a separation between the graphite planes of 0.335-0.342 nm. * No additives were used. * The dosages which were used are shown in Table 2. The content of cement, sand, gravel and water is that used in each mix. The amount of addition used in each case is expressed in two ways: according to the percentage (%) of the cement weight and in kg/m\({}^{3}\) of concrete. With the aim of achieving the objective of the present research project, the behavior of mass concrete specimens without additions and with different CNF percentages has been studied in compressive strength tests in accordance with Annex 14 of the Structural Concrete Code [3] for the manufacturing of mass concrete reinforced with fibers. A group of test pieces was initially tested in direct contact with fire, before subsequently undergoing compression testing to breaking point, with the aim of comparing \begin{table}
\begin{tabular}{l l l l l} \hline \hline  & SO\({}_{3}\) & \(\leq\)3.50\% & Compression strength at 28 days & \(\geq\)32.5 MPa \\ Cement & Cl\({}^{-}\) & \(\leq\)0.10\% & Expansion (Le Chatelier) & \(\leq\)10 mm \\  & Chrome (VI) water soluble & \(\leq\)2 ppm & Setting start time & \(\geq\)75 min \\  & Compression strength at 7 days & \(\geq\)16 MPa & Setting final time & 220 min \\ \hline \multirow{3}{*}{Sand} & Cl\({}^{-}\) & \(\leq\)0.00\% & Sand equivalent & \(\leq\)57 \\  & Lightweight particle & \(\leq\)0.50\% & Saturated density dry surface & 2.549 g/cm\({}^{3}\) \\  & Acid-soluble sulphate & \(\leq\)0.80\% & Organic material & Exempt \\  & Total sulphur compounds & \(\leq\)0.11\% & Water absorption & 3\% \\ \hline \multirow{3}{*}{Gravels} & Cl\({}^{-}\) & \(\leq\)0.001\% & Saturated density dry surface & 2.711 g/cm\({}^{3}\) \\  & Acid-soluble sulphate & Category & Lightweight organic contaminants & \(\leq\)0.50\% \\  & Total sulphur compounds & \(\leq\)0.02\% & Water absorption & \(\leq\)0.50\% \\ \hline \multirow{3}{*}{CNFs} & Density & 1.95 g/cm\({}^{3}\) & Diameter & 100 nm \\  & Surface area & 45 m\({}^{2}\)/g & Separation between the graphite planes & 0.335–0.342 nm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition and physical characteristics of the cement, sand, gravel and carbon nanofibers (CNFs). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**Concrete Type**} & \multicolumn{2}{c}{**HM-25**} \\ \hline
**Addition Type** & **Without Addition** & **CNFs** & **CNFs** \\ \hline N\({}^{\text{o}}\) specimens & 6 & 6 & 6 \\ Dosages(c/s/g/w) & & 1/2/3/0.5 & \\ Cement (kg) & 4.088 & 4.088 & 4.088 \\ Sand (kg) & 8.472 & 8.472 & 8.472 \\ Gravel (kg) & 14.616 & 14.616 & 14.616 \\ Water (l) & 2.040 & 2.040 & 2.040 \\ Addition (Kg/m\({}^{3}\)) 1 & 0 & 2.3 & 4.6 \\ \hline \hline \end{tabular} \({}^{1}\) 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.  
\end{table} 
Table 2: Dosage for the mixes and material content depending on the addition.