## 1 Introduction  
Concrete is currently the most commonly used material in the construction industry, be it in civil works or building projects, due to its good mechanical properties and versatility, but it represents a broad field for improvement based on the addition of different types of fibers, including steel, natural, glass, etc., with these being either new or recycled [1, 2, 3, 4, 5, 6, 7, 8, 9]. The addition of steel fibers, the most commonly used fibers, produces a better mechanical behavior of concrete compared to concrete without fibers or with the addition of polypropylene fibers, but they have other disadvantages, which makes the addition of polypropylene fibers rise in importance [10, 11]. Polypropylene is a thermoplastic obtained by the polymerization of propylene, a gaseous by-product of petroleum refining, which is transformed by extrusion into continuous and discontinuous polypropylene fibers assembled in a plastic matrix. Its incorporation into concrete leads to improvements in the non-linear behavior of concrete, increasing its resistance to crack propagation and preventing the formation of new cracks. This is because the incorporation of discontinuous fibers in a brittle material such as mass concrete ensures that they adhere internally to the composite, allowing it to resist higher stresses and increasing its ductility. The greater amount of polypropylene fibers per kg compared to steel fibers causes a better stress redistribution to be generated, increasing the total contact surface and reducing the proximity between them, which is why the polypropylene fibers offer concrete greater ductility and energy absorption capacity than steel fiber [5, 12, 13, 14]. The addition of these fibers also improves the impact resistance and tensile strength of concrete [7, 15, 16, 17]. With this type of addition, the quantity, type, and size of the fibers play an important role, wherein the higher percentage of the addition does not lead to better results [5]. The addition of polypropylene fibers also improves the fire resistance of the concrete as, beyond its melting point of approximately 170 \({}^{\circ}\)C, internal channels are created that allow the water vapor to escape, reducing the pressure within the mixture. This reduces the \"spalling\" effect, an effect that does not occur when steel fibers are incorporated [18; 19; 20]. In addition, it improves the mechanical properties of concretes that do not contain fibers at a cheaper price than if steel fibers were incorporated [21]. Another important aspect is that the results of the bending and impact tests depend to a great extent on the amount of fibers located in the rupture region of the specimens, hence the importance of distributing the fibers throughout the mixture [22]. It was also observed that the incorporation of polypropylene fibers produced a reduction in the diffusivity of chlorides, the opposite effect to that produced with the incorporation of steel fibers [23]. Another important aspect is that polypropylene fibers are chemically inert, which avoids the problem that could cause the corrosion of steel fibers. Within the types of polypropylene fibers, the use of macrofibers is increasingly common, since it has been observed that their addition increases ductility and produces better energy dissipation, serving as a seam between the cracks and increasing the stress resisted by polypropylene macrofibers with increasing fissure opening. Furthermore, it is observed that once the cracks are stabilized, these fibers resist an increase in the residual load, even in large crack openings [24; 25; 26; 27]. The use of carbon fiber reinforced polymers (CFRC) has increased significantly when used in numerous fields, such as industry, construction, transportation, etc., with the US and Mexico, with 36% of production, followed by Japan with 20%, as the largest producers of this type of material [28]. Projected estimates of CF demand reach an annual growth rate of 11%, with a total global CF demand of up to 117,000 tonnes estimated for 2022 [29; 30].