## 3 Materials and Methods
### Real Measurements  
The setup for the measurements is shown in Figure 8a.  
It contained the following elements:  
* A Handyscope HS5 (TiePie, Sneek, The Netherlands)signal generator that acts as an acquirer and also as a generator of arbitrary functions.
* A FALCO WMA-300 (Falco Systems, Amsterdam, The Netherlands) transmission amplifier capable of generating voltages up to \(\pm\)150 V, with a bandwidth from DC to 5 MHz.
* A _5660B pre-amplifier_ (Panametrics, Waltham MA, USA) reception amplifier, a low-noise amplifier with a wide bandwidth from 500 Hz to 40 MHz. The gain is selectable at 40 or 60 dB. In our case, we used the 40 dB setting.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**UPV (m/s)**} & \multicolumn{2}{c}{**Mechanical Strengths**} & \multirow{2}{*}{**Density**} \\ \cline{2-2}
**Material** & **Longitudinal** & **Transversal** & & **Flexural** & **[MPa]** & **[MPa]** \\ \hline P2 & 4121.58 & 2522.86 & 9.42 & 59.82 & 2190.46 \\ \hline P1 & 4807.69 & 2882.88 & 13.32 & 90.45 & 2329.53 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of P1 and P2 mortars: ultrasonic pulse velocity (UPV), compressive and flexural strengths, and density (measured on prismatic 4 \(\times\) 4 \(\times\) 16 cm\({}^{3}\) specimens).  
Figure 7: Process of fabrication: (**a**) mixing process; (**b**) kneading with steel mold and PVC pipe; (**c**) compaction and settling of the specimen material; (**d**) final specimen (type D).  
* Motor 1 and motor 2 were used to move the probe and the receiving transducer, which were controlled by an Arduino system through a specific driver. Motor 1 and 2 were stepper motors with 200 steps per revolution (3A max).
* _Olympus Immersion Transducer Power Series U8517179_ (Olympus Europa SE & Co. KG, Hamburg, Germany) 1 MHz immersion transducers (see Figure 8b).  
Imversion ultrasound coupling was used instead of airborne ultrasound because the ultrasound absorption factor is lower in water (0.0022 dB/cm/MHz) than in air (1.64 dB/cm/MHz) [29] as it is the reflection factor between the water-cement interface. However, this will favor the creation of multiple reflections due to the size of the tomography tank.  
The transducers were excited by a burst signal, with five cycles, an amplitude of 150 V peak, and a central frequency of 1 MHz. Meanwhile, the sampling frequency was 50 MHz, with an average performed every 16 traces and a vertical resolution of 14 bits (originally 12 real bits, enhanced through averaging across traces).  
In addition, as shown in Figure 8, the number of beams and the number of directions were controlled by motor 1 (M1) and motor 2 (M2), respectively. Motor 1 sweeps from 0\({}^{\circ}\) to 180\({}^{\circ}\) with a step of 3.6\({}^{\circ}\) (52 positions), where the receiver transducer is mounted, while motor 2 sweeps from 0\({}^{\circ}\) to 360\({}^{\circ}\) with a step of 5.25\({}^{\circ}\) (70 positions), controlling the rotation of the specimen (and the transmitter).  
Therefore, for each specimen, a total of 3640 measurements were performed using 52 reception positions, with a step of 3.6\({}^{\circ}\), and 70 transmission positions, with a step of 5.25\({}^{\circ}\). Figure 9 presents a flowchart with pseudocode depicting the execution of the measurement process.  
Figure 8: (**a**) Setup for measurements. (**b**) 1 MHz immersion transducers (Olympus Immersion Transducer Power Series U8517179).