###### Abstract
The authors evidenced a strong cubic correlation between compressive strength and ultrasonic pulse velocity. Here, ultrasonic wave values increased as compressive strength also increased. [PERSON] et al. (2016) [21] studied steel fiber-reinforced concrete (SFRC) with hooks on the edges and polypropylene fibers. Fibers were varied in volume fraction to assess the elasticity modulus by employing non-destructive ultrasound and resonance tests. The authors concluded that the dynamic elasticity modulus measured with the ultrasound method increases when increasing volume and slenderness ratio regardless of fiber type, while the dynamic elasticity modulus measured with resonance increases when decreasing volume and slenderness ratio for both fiber types. Finally, [PERSON] et al. found that the hybrid mixture of 0.5% steel fibers with 0.2% polypropylene fibers exhibited the best results. [PERSON] et al. (2021) [22] studied concrete reinforced with polyolefin fibers using 3-point bending tests, the visual DIC (digital image correlation) method, and ultrasound. This study modified fiber weight dosages to analyze their influence on post-fracture behavior. The objective of the research was to use a non-invasive method, ultrasound, and DIC to assess the evolution of damage in concrete and the possibility of its characterization. [PERSON] et al. concluded that ultrasound results made it easier to characterize the level of damage. In addition, they state that ultrasonic waves could be used in a real-time monitoring system as an autonomous damage indicator of a fracture process. [PERSON] et al. (2013) [23] studied high-strength concrete reinforced with polyvinyl alcohol (PVA) fibers using uniaxial compression tests, split tests, bending tensile strength, and the static elasticity modulus. Here, fiber volume fractions varied between 0.25% and 0.5%, and the length of fibers ranged from 6 to 12 mm, so their influence on the dynamic elasticity modulus may be assessed. [PERSON] et al. concluded that the values of dynamic and static elasticity modulus denote a high level of consistency with each other in elements at the same age, with both values following the same trend; however, the dynamic elasticity modulus was 2.6 to 6.9% higher than the static elasticity modulus. Previous research concluded that the ultrasound method is feasible for use in concrete reinforced with steel or hybrid fibers [1; 10; 11; 12; 13; 14; 17; 18; 19; 24; 25]. The literature review determined that research already exists on concrete with steel and polypropylene fibers. However, the research has only used fiber volume as an independent variable for concrete with polypropylene fibers, and none discusses the effects of fiber length in determining its dynamic and mechanical properties. Therefore, to have a full picture, this research study works with the Reinforcement Index (_RI_), which is the product of fiber volume fraction (\(V_{f}\)) (that is, fiber volume divided by the total component volume) with slenderness (\(l\)/\(d\)) (that is, fiber length divided by fiber diameter). The objectives of this study are to evaluate (1) compressive strength as a function of ultrasonic pulse velocity (UPV) and Reinforcement Index (_RI_); (2) static elasticity modulus (\(E_{c}\)) as a function of UPV; and (3) dynamic elasticity modulus (\(E_{d}\)) as a function of compressive strength (\(f_{c}\)) and Reinforcement Index. For this, an experimental campaign was designed where the independent variable was the Reinforcement Index of polypropylene fibers through the variation in fiber volume fraction and length. In this study, we worked with fiber volume fractions of 0, 0.4, 0.8, and 1.2% and fiber lengths of 40, 50, and 60 mm. The dependent variables of this study were the mechanical properties of concrete: compressive strength and static elasticity modulus. On the other hand, the mediating variables were UPV and dynamic elasticity modulus. Finally, three methods were used for non-destructive prospecting using the ultrasonic pulse test: direct, semi-direct, and indirect methods.