The slab specimen was constructed in a local precast concrete plant and transported to the laboratory. The slab was inspected before testing, and there were two initial cracks with widths less than 0.1 mm (due to handling and transportation or shrinkage), which were considered irrelevant.
The damage caused by the P = 80 kN load were determined, based on standard measurement of crack openings, as needing an alarm. It should be mentioned that the amplitude of signals C2 and C3 when the structure was under the influence of force were bigger than prior to loading (P = 0 kN). This relationship was also observed in the next loading cycles, which hindered assessment of the condition of the structure. Examples of registered signals after the alarm state and with and without loading are shown in Figure 7. Easy designation of the level of signal amplitude below which a decrease would result in information about alarm/damage occurance was impossible. 2b1af7f3a8