*2.3. Description of the Test Stand*

The tests were conducted on three specimens. The results obtained for each of the three specimens separately confirm the conclusions contained in the paper.

**Figure 1.** Four-point bending test [40].

The loading process was carried out with a controlled, constant displacement speed equaling 0.05 mm/min. Deflection was recorded by means of two LVDT sensors located under the beam, using a "Yoke" clamp (Figure 2), [40]. During the test, the bending load and deflection of the specimen were measured. The testing procedure corresponded to the requirements of the ASTM C 1018 standard.

At the end of the test, a 5 mm deflection of the specimens was recognized, which was determined in relation to the neutral axis. The acoustic emission sensor was fixed to the top surface of the beams with the use of an elastic band and the surface was coated with coupling graphite grease (Figure 1).

The broadband AE sensor manufactured by Physical Acoustic Corp. enabled the recording of the AE signal within the frequency range of 10–1000 kHz.

The AE signal was recorded with the use of the ADLINK 9112 card with the sampling rate of 88.2 kHz, the 12-bit resolution, and the function of a continuous recording on a computer disk. Source files were saved in the format (.wav), which makes it possible to listen to the recorded signal with the use of the computer sound card speakers.

Thanks to the recorded data, it is possible to plot a load-deflection curve, record AE events, and aggregate them (total AE).

The acoustic emission effects were presented as a 2D and 3D acoustic spectrum (amplitude of the frequency depending on sound intensity). The 2D sound spectrum was achieved with the use of the Audacity program and the 3D spectrum using SpectraPLUS-SC (Pioneer Hill Software LLC, Poulsbo, WA, USA).

The quasi-brittle cement composites (ESD—Eng. elastic range, strengthening control, deflection control) were characterized by higher load and absorbed energy in the elastic range compared to the sample without reinforcement (Figure 2) [35]. The reinforcement effects may be presented by characteristic points *fx* (*Fx*-load, ε*x*-deflection, *Wx*-work) and areas AX under the load-deflection curve.

Figure 2 presents the mechanical effects of the quasi-brittle cement composites with the corresponding acoustic effects and compiled acoustic spectra with various amplitudes corresponding to different mechanical effects (reinforcement breaking, pull-out, macrocracks, microcracking, and micro-events).

**Figure 2.** The quasi-brittle composite: (**a**) load-deflection curve, (**b**) AE, acoustic emission effects, (**c**) 2D acoustic spectrum (frequency amplitude depending on sound intensity) based on [36].
