## 1 Introduction  
Concrete is one of the most widely used materials in the construction industry [1; 2; 3; 4]. Hydraulic cement concrete results from mixing and binding stone aggregates with a mixture of water and Portland cement. The elemental composition of concrete is aggregates plus a binder [5]. The main advantages of concrete are accessibility, versatility, and strength. Concrete has good compressive strength but low tensile and flexural strength [6]. Moreover, concrete is a quasi-brittle material as it does not feature good inelastic deformation capacities [1]. Its low tensile strength and fragility are especially evident in the appearance of cracks in slabs. These cracks engender tensile stresses that exceed the tensile strength of concrete [7]. In recent years, breakthroughs have been reported in diverse types of concrete, encouraged by the demands of the construction industry. Therefore, currently, the main types of concrete are high-strength [8], self-compacting [9; 10; 11; 12], permeable [13; 14; 15; 16], light [5; 17; 18] and fiber-reinforced concretes. The development of fiber-reinforced concrete (FRC) started at the end of the 1950s. In the 1960s, various fiber types were introduced: vegetal, mineral, synthetic, steel, and carbon, among others [19]. FRC is mainly designed for constructing elements prone to cracking, such as sprayed concrete in tunnels, beams, slabs, and structures, which withstand impact forces [20; 21; 22] or pavement construction. Fibers reduce slab thickness, control cracks and decay ratios due to cracking, increase joint spacing, and improve cracking properties [23; 24; 25]. Fibers mainly enhance the post-cracking behavior of concrete as they help control cracks and constrain deformations caused by tensile forces because they can withstand generated tensile forces. Consequently, fibers increase concrete toughness, i.e., the amount of energy required for concrete to fail [20]. Many fiber families exist according to classification criteria [26]. The main criteria are materials, geometry, shape, cross-section, and texture [23]. Each type of fiber behaves differently when hardened concrete is subjected to tensile stresses [27; 28]. According to their materials, fibers are classified into synthetic and metallic [26]. Synthetic fibers are classified into polypropylene, carbon, glass, and basalt polymer fibers. Given their affordable cost, some of the most used synthetic fibers are polypropylene (PP) fibers [29]. PP fibers are not strong and feature a low elasticity modulus, but they are flexible, ductile, and have high toughness due to their large inelastic deformation capacity [29]. PP fibers have lower strength, elasticity modulus, and specific weight than synthetic and metallic fibers [30; 31]. The tensile strengths of PP and steel fibers range from 150 to 700, and 400 to 1200 MPa, respectively. The elasticity modulus of PP and steel fibers range from 3.5 to 9.5 and 180 to 200 GPa, respectively [24]. The specific weights of PP and steel fibers are 0.91 and 7.86 g/cm\({}^{3}\), respectively. Moreover, PP fibers exhibit a better capacity for tensile elongation (21%) than steel fibers (3.5%) [30]. PP fibers are also chemically neutral, making them resistant to alkali and acids and non-corrosive [30; 32]. Further, PP fibers have high melting temperatures (165-170 \({}^{\circ}\)C) because they are thermostable polymers [30; 33]. These fibers increase the durability, plastic deformation, deformation capacity, toughness, cracking control, shear strength, and ductility of concrete [21]. Based on their geometry, fibers are classified into macrofibers and microfibers [26]. Macrofiber lengths range from 19 to 60 mm, and their diameter usually exceeds 0.3 mm, whereas the length and diameter of microfibers are usually under 19 and 0.3 mm, respectively [34]. According to their structure, microfibers are classified into monofilament and fibrillated [35]. Longer fibers are more efficient at improving the mechanical properties of concrete than shorter fibers [36].