## Materials and Methods
### Test Procedure  
#### 2.4.1 Non-Destructive Ultrasonic Pulse Testing  
The 58-E4800 UPV equipment by Controls was used for the ultrasound tests. Said equipment consists of a receiver transducer and a transmitter transducer capable of emitting frequencies ranging from 24 to 150 kHz. The recommendation of standard ASTM C-597 was followed [28], which states that frequencies equal to or greater than 50 kHz should be used to obtain greater accuracy in time measurement. The time meter is included in the equipment and has 1-microsecond accuracy as recommended by the standard [28]. Figure 3 provides images of the equipment used.  
In the ultrasound test, the transducer transmits P (longitudinal) waves and S (shear) waves (plate waves) through the material in the axial direction. Additionally, edge waves (P and S) are produced but are very weak and carry little energy [11]. R (Rayleigh or surface) waves are also produced through the surface due to the boundary conditions [11; 17; 18]. P waves are characterized by high velocity and low amplitude, and R waves have low velocity and high amplitude [17]. The receiving transducer receives these waves; each of these waves' arrival can be recorded with an oscilloscope's help [17]. In this research study, no oscilloscope was used; because of this, only the arrival of the P wave was recorded. Since the test is very susceptible to interferences by external agents (such as machine vibrations), it was carried out without any type of vibration [24].  
Three transmission modes were applied to all specimens: direct, semi-direct, and indirect methods. These methods were performed as per the BS-1881 standard [29]. The configurations of the three methods can be seen in Figure 3. In the direct method (Figure 3a), the transducers were placed on the ends of the beam located in the centers of the opposite faces at a distance of 545 mm from each other. For the semi-direct method (Figure 3b), the transducers were placed on adjacent perpendicular faces; one transducer in the center of the end face of the beam and another on the central face of the specimen at a distance of approximately 240 mm. Finally, for the indirect method (Figure 3c), both transducers were placed on the same side of the lateral surface of the parallelepiped at a distance of 450 mm from each other.  
The test was performed three times for each specimen to eliminate possible interference factors. To do this, the center-to-center distance between transducers was measured, and the time was measured automatically with the equipment. With these results, the P-wave velocities were obtained. As a result, ASTM C-597 provides the following Equation (1) for calculating the dynamic elasticity modulus:  
\[E_{d}\ =\ \frac{V_{p}^{2}(1+v_{d})(1-2v_{d})\rho}{(1-v_{d})} \tag{1}\]  
Figure 3: Ultrasound configurations: (**a**) direct method, (**b**) semi-direct method, (**c**) indirect method.  
where \(E_{d}\) is the dynamic elasticity modulus (N/m\({}^{2}\)), \(V_{p}\) is the P-wave velocity (m/s), \(v_{d}\) is the dynamic Poisson ratio, and \(\rho\) is the concrete density (kg/m\({}^{3}\)). Note that, in Equation (1), the dynamic elasticity modulus also depends on the dynamic Poisson ratio. Therefore, the velocity of two types of waves is required to calculate the Poisson ratio and the dynamic elasticity modulus. Due to this, the ASTM C-597 standard [28] recommends using the resonance test for this purpose; however, in this research, only the velocity of the P wave was measured. Accordingly, the Poisson ratio obtained from the static Poisson ratio test was used for this research. According to the results reported by [PERSON] [24], there is a correlation between the static and dynamic Poisson's ratio with an error of 39% and 14% when using the direct and indirect UPV methods, respectively, measured in concrete specimens with polypropylene fibers. [PERSON] (2011) [18] determined a variation of \(\pm\)7% in the dynamic elasticity modulus when the Poisson ratio varies between 0.15 and 0.25.