## 3 Results and Discussion
Having observed these strengths in standard specimens in accordance with the regulations, it is important to mention that by using CEM II/B-L 32.5 N cement, minimum strengths of around 27 N/mm\({}^{2}\) were obtained for concretes containing recycled CFRP, which, according to the Spanish Structural Concrete Code, makes it viable as a concrete for building construction. Figure 6 shows the results obtained in the flexural tests. As can be seen, the addition of fibers to concrete improved its ductility, the best performance being achieved by the specimens containing 3 kg/m\({}^{3}\) of recycled carbon fibers, this type of concrete exceeding the load borne by more than 20% when compared to concretes containing polypropylene fibers or to conventional concretes. It is important to mention that greater strengths were achieved despite finding very few fibers in the region of rupture in all the specimens. The specimens featuring more fibers in the region of rupture, such as specimen P-PP6-1, were able to bear an even greater residual load. The specimens with 6 kg/m\({}^{3}\) of recycled carbon fibers reached the maximum deformation before breaking, showing the capacity that this type of fiber confers on concrete. This detail greatly expands the options for using recycled CFRP fibers, making it possible to produce concretes that are stronger, cheaper, and have a much lower environmental impact than conventional concretes or those containing polypropylene fibers by ensuring that there are a higher number of fibers in the region of rupture. Figure 7 shows the mean values for the energy absorbed in the impact test for each concrete type. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimens** & **Mean Value** & **EHE Coefficient** & **Conversion** & **Real Coefficient** \\ \hline C & 30.663 & 0.970 & 29.743 & 0.970 \\ C-PP3 & 29.251 & 0.970 & 28.373 & 0.925 \\ C-PP6 & 29.972 & 0.970 & 29.073 & 0.948 \\ C-FC3 & 28.019 & 0.970 & 27.178 & 0.886 \\ C-FC6 & 27.454 & 0.970 & 26.630 & 0.868 \\ CU & 39.812 & 0.800 & 31.849 & 0.800 \\ CU-PP3 & 38.600 & 0.800 & 30.880 & 0.776 \\ CU-PP6 & 38.280 & 0.800 & 30.624 & 0.769 \\ CU-FC3 & 37.425 & 0.800 & 29.940 & 0.752 \\ CU-FC6 & 36.805 & 0.800 & 29.444 & 0.740 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Conversion values for the compressive strength in different types of specimens. Figure 6: Load-deformation graph from the flexural tests. As can be observed, the lowest amount of energy absorbed on impact was associated with the concrete specimens without added fibers. The concrete containing polypropylene fibers displayed an improved performance of as much as 32% with the addition of 6 kg/m\({}^{3}\), although the best performance was achieved by the concrete containing 6 kg/m\({}^{3}\) of recycled carbon fiber, which displayed an improvement of 45.68%. Figure 8 shows the symptomatology of concrete when it was subjected to the different tests. In Figure 8a, associated with the compression testing of cylindrical specimens, it can be seen that the addition of fibers reduced the spalling effect, as the concrete that did not contain fibers was the only type to have suffered degradation after the test. The addition of the fibers decreased the cracking of the concrete, producing concretes with greater ductility, which translates into greater absorption of the load without detaching. Figure 8b shows the flexural test, in which it was observed that the higher the number of fibers in the region of rupture, the more ductility increases. Figure 8c shows the case of specimen P-PP6-6, where, due to the large amount of fibers concentrated in the region of rupture, the pendulum was unable to fracture the specimen. These results show the importance of fiber distribution in concrete [22]. The relationship between the most representative values obtained in the different tests is shown in Table 6, taking conventional concrete as the reference value.