## 2 Materials and Methods  
### Experimental Research Methods and Materials Used  
In the experiments, samples of cement beams that imitated a monolithic reinforced concrete structure were used. This structure can be a foundation. The size of the samples was X = 50 mm, Y = 50 mm, and Z = 400 mm. Five samples of cement beams were made of PC-400D cement, steel reinforcing bars, and sand with the addition of water. Inside the beams, four steel reinforcing bars with a diameter of 4 mm were inserted, on which an optical fiber was fixed with plastic ties. The number of fiber optic sensors was selected arbitrarily and ranged from 1 to 2 for each steel reinforcing bar. Conditions of mortar hardening were chosen naturally during 28 days in water, according to the GOST 310.4-81 [47] cement methods for determining bending and compressive strength. The beams were tested and brought to failure using a hydraulic measuring press (PGI-500) (Saratov, Russia). PGI-500 is periodically checked and has a certificate of verification (VL-2-03-18000654). This press is used in the building materials laboratory for strength and for testing concrete beams in compression by creating a variable force value from 0 to 500 kN (50,986 kgf). A liquid crystal display is available to monitor the compression force and other parameters. The tests were carried out according to state GOST 310.4-81 [47] conditions for cement methods for determining bending and compressive strength. The temperature in the laboratory room varied within 24-25 \({}^{\circ}\)C. Movement of the beam along the axes OX = 0 m; OY = 0 m; and OZ = 0 m at pressure application. Before starting the experiments, the integrity of fiber-optic sensors was checked, and parameters of additional losses were set using the optical power meter Smart Pocket OLP-38 (San Jose, CA, USA) and the optical radiation source Smart Pocket OLS-34/35/36 (San Jose, CA, USA) was used. All fiber-optic sensors remained undamaged after filling the beams and drying. As a result, each beam necessarily had 4 FOSs. The rest were reserved and were not used in the experiment, as they were needed to replace the main ones in case of damage. Next, a 9/125 \(\upmu\)m Corning (Corning, NY, USA) single-mode quartz fiber (G.652.D standard [48]) with a 248 \(\upmu\)m vinyl coating was used as a fiber optic sensor. The top layer of the protective plastic coating was between 1 and 2.5 mm. A wooden mold was specially made to fill the beams, into which the sand-cement mortar was poured. The manufactured cement beams gained strength within 28 days. Figure 2 shows the manufacturing process of concrete beams and placing fiber optic sensors in them. The optical fiber used in the studies has a shell made of glass with a density of n\({}_{1}\) = 1.4570, which is lower than the core density of n\({}_{2}\) = 1.4625. The refractive indices correspond to the condition n\({}_{1}\) < n\({}_{2}\). Then, the beam deviates from the OF axis by an angle of not more than (90\({}^{\circ}\)-\(\alpha\)), and the ratio of n\({}_{1}\) and n\({}_{2}\) can be expressed as sin \(\alpha\) = n\({}_{1}\)/n\({}_{2}\). At the same time, there is an equality of the angles of incidence and refraction \(\mathrm{\acute{c}in}\) = \(\mathrm{\acute{c}out}\), which provides a total internal reflection of the pulse light wave (mode) [46]. Using a DVP 740 welding machine (Hunan, China), SC-type optical connectors were mounted on each fiber optic sensor. The SC connectors used a 2.5 mm UPC (ultra-physical contact) polished ferrule. Figure 3 shows the process of mounting optical connectors using a DVP 740 welding machine and the tools used to remove the sheath (Hunan, China). Figure 3: The process of mounting optical connectors with the use of the DVP 740 welding machine. Figure 2: The process of pouring concrete beams. The experiments were carried out using a hydraulic press for testing building materials (P-125) (Lida, Russia). The press is a set that consists of a loading device and a control panel. The measuring unit contains a torsion force meter and a diagram apparatus.