## 2 Materials and Methods  
In order to perform the experiments required for the present research, the materials listed below were used:  
* CEM II/B-L 32.5 N cement, prepared according to regulations UNE-EN 197 [48] and RC-16 [49] with regard to conformity, composition, and specification criteria, and in accordance with the quality management and environmental management standards ISO 9001 [50] and ISO 14001 [51]. * Fine graded washed silica sand with a particle size of 0-4 mm. The aggregate used complies with regulations UNE-EN 12620:2003 + A1 [52] and UNE-EN 13139 [53]. * Coarse graded washed silica aggregate with a particle size of 4-12 mm. The aggregate used complies with regulations UNE-EN 12620:2003 + A1 [52] and UNE-EN 13139 [53]. * Drinking water from the Canal de Isabel II Madrid region's main water supply, which complies with the technical specifications for its use in structural concrete. * Macro synthetic polypropylene fibers (SikaFiber T-48). These fibers comply with the specifications of regulation UNE-EN 14889-2: 2008 [54] with regard to the conformity requirements for polymer fibers for concrete. * Recycled carbon fibers manually extracted from wooden elements measuring 80 mm \(\times\) 155 mm \(\times\) 1000 mm. It was previously tested in the materials laboratory at the Escuela Tecnica Superior de Edificacion as part of the doctoral thesis of [PERSON] and used by way of reinforcement in U-shaped flexural test [55]. * Superplasticizer based on polycarboxylates (MasterGlenium SKY 604), adding 0.7% of the superplasticizer in relation to the cement weight. The additive used complies with regulation UNE-EN 934-2:2010 + A1 [56]. The characteristics and appearance of the fibers are described in Table 1 and Figure 1.  
In order to be able to assess the performance of the concrete, 5 mixes were produced: one reference mix and four mixes containing added polypropylene or recycled carbon fibers in quantities of 3 kg/m\({}^{3}\) and 6 kg/m\({}^{3}\), and in accordance with Annex 14 of the EHE [57]. The dosages for each mix and types of specimens made are set out in Tables 2 and 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Specimen/Mix** & **Reference** & \begin{tabular}{c} **Carbon Fiber** \\ **3 kg/m\({}^{3}\)** \\ \end{tabular} & \begin{tabular}{c} **Carbon Fiber** \\ **6 kg/m\({}^{3}\)** \\ \end{tabular} & \
\begin{tabular}{c} **Polypropylene Fiber** \\ **3 kg/m\({}^{3}\)** \\ \end{tabular} &
\begin{tabular}{c} **Polypropylene Fiber** \\ **3 kg/m\({}^{3}\)** \\ \end{tabular} \\ \hline Cylindrical specimens & & 6 & 6 & 6 & 6 \\ \(\phi\)100 mm \(\times\) 200 mm & & & & 6 & 6 \\ Cubic specimens & & 6 & 6 & 6 & 6 \\
100 mm \(\times\) 100 mm \(\times\) 100 mm & & & & 6 & 6 \\ Prismatic specimens & & 6 & 6 & 6 & 6 \\
500 mm \(\times\) 50 mm \(\times\) 50 mm & & & & 6 & 6 \\ Slabs & & 6 & 6 & 6 & 6 \\
520 mm \(\times\) 100 mm \(\times\) 30 mm & & & & 6 & 6 \\ TOTAL & 24 & 24 & 24 & 24 & 24 \\ \hline \end{tabular}
\end{table} 
Table 2: Types of specimens made. Figure 1: Polypropylene fibers and recycled carbon fibers. The nomenclature used and the tests performed on the different specimen types are displayed in Table 4.  
In order to start the process of producing the concrete, all materials were prepared in the materials laboratory at the Escuela Tecnica Superior de Edificacion of the Universidad Politecnica de Madrid. Once the materials were weighed, the cement, sand, and gravel were poured into the IBERTEST vertical planetary concrete mixer, model CIB-701 updated to IB32-040V0, and dry mixed. Subsequently, with the mixer switched on, the mixing water was gradually added, followed by the superplasticizer, in accordance with the instructions on the specification sheet [58]. After ensuring that the mixture was uniform, the fibers were added. All mixes underwent slump testing in line with regulation UNE-EN 12350-2 [59] using the Abrams Cone. The cylindrical and cubic specimens were produced using standardized steel molds in accordance with regulation UNE-EN 12390-1:2001 [60].