###### Abstract  
Compressive strength and elasticity modulus are the main mechanical properties of concrete. The non-destructive ultrasound pulse test can be used to determine these properties without compromising the structure's integrity. This study seeks to assess whether a correlation exists (1) between the Reinforcement Index (\(RI\)) and the mechanical properties, (2) between the \(RI\) and the dynamic properties, and (3) among the dynamic properties of polypropylene fiber-reinforced concrete. The \(RI\) was modified through fiber volume fraction (0, 0.4, 0.8 and 1.2%) and fiber length (40, 50 and 60 mm). The dynamic properties were assessed through dynamic elasticity modulus and ultrasonic pulse velocity (UPV), which were determined by direct, semi-direct, and indirect prospect methods. Finally, compressive strength, static elasticity modulus, and Poisson's ratio were assessed through destructive tests. Their relationship with UPV and the dynamic elasticity modulus is also subsequently studied. The results reveal a correlation between \(RI\) and compressive strength and UPV; however, the static elasticity modulus only exhibits a correlation with UPV in one of its measurement methods. Finally, empirical models were developed for predicting compressive strength, elasticity modulus as a function of ultrasonic pulse velocity and \(RI\), and dynamic elasticity modulus as a function of compressive strength and \(RI\). static elasticity modulus; compressive strength; dynamic elasticity modulus; fiber-reinforced concrete; non-destructive testing; UPV +
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There are two types of tests to measure the properties of concrete laid on-site: (1) destructive testing (DT) and (2) non-destructive testing (NDT). DTs need one or several specimens that are mechanically brought to failure to measure their properties (for example, compressive or bending strength test). In the case of existing structures, specimens need to be removed from them, causing structural damage. On the other hand, NDTs determine material properties without bringing them to failure. These tests include ultrasonic pulse, resonance, or sclerometer [13,14]. The use of NDTs is vital to determine the state of a structure without having to cause structural damage. In addition, NDTs are less expensive, require less time, are repeatable, and have a greater scope than DTs [13,14]. On the other hand, they require well-trained operators and engineers to obtain and interpret reliable results, a calibration procedure in the laboratory is mandatory, and a large number of variables can affect the test readings, so the knowledge to avoid these and the correct preparation of the specimen is fundamental, other limitations corresponding to each of the NDTs can be found in the literature [15,16]. [PERSON] and [PERSON] [12,17] state that the ultrasonic pulse test is the most reliable, simple, and accessible non-destructive method for assessing material properties. The interest in this method has increased in recent years due to its use in assessing concrete structures [18]. However, this method needs a previous UPV study of the laboratory material to calibrate the results obtained in the field. The ultrasonic pulse method was first introduced by [PERSON] [12] in 1945 in \"The non-destructive testing of concrete.\" This paper presents the theory necessary to carry out the test, establishing a relationship between the velocity of wave propagation and the density and elastic properties of the material. This way, if density and velocity are known, compressive strength and static elasticity modulus can be determined [19]. [PERSON] and [PERSON] (2018) [20] studied concrete reinforced with steel and polypropylene fibers with the addition of ashes resulting from coal burning using simple compression tests, split tests, bending tension tests, ultrasounds, and sclerometers. In this study, fiber materials and the volumes of each type of fiber in the specimens are varied.