**5. Experimental Study**

As part of the tomographic experiments, studies were carried out on RC elements after cracking initiation—three beams on a laboratory scale and one prefabricated beam on a natural scale. Since the calculation methodology presented in point 3 concerned the detection of elastically degraded zones with a course perpendicular to the beam axis, the experiments illustrating the possibilities of its practical application also focused on the evaluation of this type of defects. Hence, the laboratory beams were bent until the first perpendicular cracks were formed in the middle area. In the second case, a prefabricated industrially manufactured beam, that was damaged during transport to the construction site, was inspected. There were cracks perpendicular to the beam axis and visible to the naked eye. This beam was specially selected for an assessment to also test the presented calculation method in near-real conditions. An important aspect of this study was also the willingness to check whether ultrasound tomography in the presented approach could potentially be used for quality control of prefabricated RC elements in industrial conditions.

The cross-sections of beams for tomographic imaging were selected so that the measurements were disturbed as little as possible by their reinforcement (between longitudinal bars and vertical arms of stirrups) [1,53]. In the case of laboratory beams, it was also decided to show how changing the care method can affect tomographic detection of brittle damage. For this purpose, three samples were stored under water for 1 to 28 days from the time of forming. Two of them were tested after 28 days and the third one after 35 days (after its removal from water at the age of 28 days and storage at the room conditions for the next 7 days). The storage temperature of all the beams was about 20 ◦C. Taking into account also disturbances which may be caused by uneven distribution of humidity [53], the beams were examined in conditions in which the distribution of humidity in them would be as homogeneous as possible. The 28-day laboratory beams were tested for up to about 1 h after the removal from the water bath, and the 35-day beam, after the removal from the water, was protected by polyethylene film against moisture exchange with the ambient air until the test. In turn, the prefabricated beam was stored about 7 months before the test inside the laboratory at an average relative humidity of about 50% and a temperature of 20 ◦C. The age of the prefabricated beam at the time of testing was about 9 months.

On the basis of theoretical considerations discussed in previous points, a detailed course of tomographic measurements and method of processing data obtained this way was also established and used during own experiments. The general scheme of this procedure is shown in Figure 17.

**Figure 17.** Flowchart of the calculation process.
