## 2 Materials and Methods
### Experimental Research Methods and Materials Used
The measuring range is from 125 to 625 kN, and the indication range is from 0 to 625 kN; the division value is 1.25 kN; the admissible load measurement error in forward stroke starts at 20% of the largest value of the measurement range but is not lower than 10% of the maximum limit load; and the percentage of the measured load is \(\pm\)2. The concrete beam was placed on two supports, and the third support was used to develop pressure on the center of the beam. To control the displacement of the plates, a vernier caliper with a digital indication in the 0-150 mm range and with a resolution of 0.01 mm and accuracy of \(\pm\)0.01 mm (Sichuan, China) was used. Figure 4 shows the process of putting pressure on a concrete beam and its destruction. As mentioned earlier, the optical fiber was pre-embedded in the body of the reinforced concrete beam. With pressure on the beam in the central part, its expansion occurs due to stress concentration and the formation of cracks. When the beam is destroyed, it is deformed, so the steel reinforcement to which the fiber-optic sensors are attached is also deformed. The put-forward hypothesis was tested during the experiment that the optical fiber could work as a sensitive sensor for identifying damage to a monolithic reinforced concrete structure. The method is based on the control of changes in the properties of light passing through the optical fiber under mechanical action, which makes it possible to establish the numerical value of mechanical stresses or deformations in the beam body as well as to determine the limiting value at which a crack can occur. The proposed method makes it possible to establish the locations of local hidden defects (cracks) in reinforced concrete structures' foundations and use them in the future on other long-building structures. The experiment allows simulation of complicated operating conditions, for example, during flooding and destroying a monolithic reinforced concrete foundation. When the foundation concrete is destroyed, dislocation and cracking can be established. Figure 5 shows the scheme of the experiment: Several fiber-optic sensors (2) are located in the body of beam 1, which are fixed on steel reinforcing bars, and rod (3) presses on beam (1) from above. The other two rods are located on the bottom of plate (4). The location of rod 3 makes it possible to destroy the beam during bending. The top of plate 5 is lowered onto the beam providing load (6), and is transmitted through rod (3). This is how the standard tests of reinforced concrete beam samples are carried out to determine their bending strength. In the center of the beam, the mechanical stress concentrator and the zone of irreversible deformation are formed, where cracks (7) are generated. Fiber-optic sensors are connected to data processing unit (9) using optical patch cords with SC connectors (8). Each optical patch cord is equipped on both sides with connectors of the SC type. Selecting  
Figure 4: Carrying out the experiment. these connectors is arbitrary, and they can be replaced by others without any problems, for example, by the FC type. The data processing unit has four main parts: (10)--module containing laser and optical splitter; (11)--module of photodetector, the number of which corresponds to the number of channels; (12)--microprocessor module of video image pre-processing and coordination with the computer; and (13)--DC source for power supply of photomatrixes and laser. The coherent source of laser radiation has a wavelength in the visible light range of 650 nm, and permissible deviations are not more than 5 nm. In the experiments, InGaAs (laser launcher level CLASS IIIB) semiconductor lasers with a power of 10 mW or more were used. The laser power was 20 mW. It is possible to use a laser with a power of up to 30 mW and a 650-850 nm wavelength. An optical splitter is used to split the beam into several channels, which distributes the light wave's energy in equal proportions between all the channels.