## 3 Results and Discussion  
Figure 3 shows the results obtained in the compression testing of the cylindrical specimens, displaying the most representative graph for each dosage with polypropylene fibers, recycled carbon fibers, and without fibers. As can be observed, the concrete containing recycled carbon fibers displayed a slightly lower compressive strength than the concrete without added fibers. It was observed that in the specimens containing fibers, adding a greater amount of fibers did not reduce the compressive strength and similar performances were noted for both addition amounts. Figure 2: Tests performed: (**a**) compression; (**b**) flexural; and (**c**) impact. The results of the compression testing of cubic specimens are displayed below (Figure 4) and they exhibited similar behaviors to the cylindrical specimens in that the addition of fibers did not substantially change their compressive strength but rather improved their ductility. As can be seen in Figures 3 and 4, the behavior of concrete with polypropylene fibers and recycled carbon fibers was very similar, not offering different behaviors, with both addition percentages. Figure 5 shows the most representative mean values of the compression tests of the cylindrical and cubic specimens, which are the following: maximum strength (\(\sigma_{\text{max}}\)), strain associated with the maximum strength (\(\epsilon_{\text{max}}\)), ultimate strength (\(\sigma_{\text{u}}\)), ultimate strains (\(\epsilon_{\text{u}}\)), maximum strain energy density (E\({}_{\text{max}}\)), and ultimate strain energy density (E\({}_{\text{u}}\)). As can be seen in Figure 5, the behavior shown by concretes was similar in the cylindrical and cubic specimens, with the difference corresponding to the geometry of the specimens. The ultimate strengths were lower in the concretes containing recycled carbon fibers than in the concretes containing polypropylene fibers or those without fibers. Where the maximum  
Figure 4: Strength–strain graph from the compression tests on the cubic specimens. Figure 3: Strength–strain graph from the compression tests on the cylindrical specimens. strains were concerned, the values were similar for all concrete types, while the ultimate strains were higher for the concretes containing fibers. The highest ultimate strains were achieved in concretes containing recycled carbon fibers in a quantity of 6 kg/m3. With regard to the strain energy density, the values were similar for all concrete types, except for those obtained for the cubic specimens, for which the ultimate strain energy densities were higher for the concretes containing fibers. The analysis of all the variables that define the behavior of concrete in compression shows that the addition of fibers does not improve its characteristics, but the loss of resistance experienced by the addition of recycled carbon fibers was compatible with the requirements of concrete. According to the EHE standard (Spanish Structural Concrete Code), there is a correlation between the compressive breaking strengths obtained for the cylindrical and cubic specimens, and the strengths in standard conditions, which would be associated with the compressive strength test performed on cylindrical specimens measuring 15 cm in diameter and 30 cm in height after 28 days [57]. The theoretical conversion coefficients for the cylindrical specimens measuring 15 cm in diameter and 30 cm in height would be 0.8 and 0.97 for both the cubic specimens with 10 cm sides and for the cylindrical specimens measuring 10 cm in diameter and 20 cm in height, respectively. Table 5 displays the results obtained in the laboratory tests along with their conversion in line with the EHE, as well as the real coefficient for each mix compared to the theoretical value for the standard specimens without additions. Figure 5: Most representative mean values of the compression tests: (**a**) maximum and ultimate strengths; (**b**) maximum and ultimate strains; and (**c**) maximum and ultimate strain energy densities.