## 4 Experimental Results and Discussion  
Figure 2 shows the temporal evolution of the PSC electrical signal (blue line) and the imposed stress in juxtaposition with the corresponding axial deformation. In addition, the linear fitting of the axial deformation is depicted with a dark green line to make the transition from the linear to the non-linear elasticity region more visible. This transition phase occurs at \(t\approx 120\) s. For \(t<120\) s the PSC values are low, showing a small trend to increase. In the time period from \(t\approx 120\) s to \(t\approx 139\) s (the moment the load stabilizes at 54.5 MPa), a strong increase in the PSC values is observed. Such an observation is compatible with previous works [16; 26], since, in this phase, the progressively increasing uniaxial stress exceeds the linear elasticity limit. In the time interval 139 s \(\leq t\leq 195\) s, the imposed uniaxial stress remains constant and the PSC shows a well-known decreasing trend which has been repeatedly observed in cementitious materials [16; 49], and in geomaterials [21; 49; 50], while a small increase in axial deformation is observed, due to the increased accumulation of internal damage. Then, during the 195 s \(<t<220\) s time interval (yellow shaded area), the PSC exhibits a clearly visible temporary increase, which is in agreement with the steeper increasing trend of the axial deformation (see Figure 2). Similar excitations of the PSC were reported in previous related works [16; 26], in which, however, no acoustic emissions were recorded, so that there was a comparison between the PSC and the AEs. The concurrent AE recordings in this work and the study of the behavior of some statistical quantities, as seen later, will strengthen the opinion that the appearing peak of the PSC is not random, but is directly related to physical parameters such as deformation and the accumulation of internal damage, and at the same time is an indication that the final fracture is imminent. Indeed, the PSC, after first showing a rather slow weakening, begins its sharp increase 20 s before the final fracture, which is a consequence of the im  
Figure 1: The experimental set-up for the concurrent recordings of the axial deformation, the PSC and the AE. pending final fracture. Also, it is combined with the highest rate of increase in the axial deformation. Finally, it should be noted that the incremental changes in the PSC signal that are synchronized with corresponding increased rates of axial deformation, confirm the hypothesis of the MCD model, according to which there must be a ratio between the PSC and the rate of deformation. In order to identify qualitative similarities between the PSC signal and the AE technique, the AE data collected during the experiment were also used. A total of \(N=3215\) AE hits were recorded. Figure 3 presents the temporal variation of the AE hit amplitudes in juxtaposition with the axial deformation. Subsequently, the temporal evolution of the AE hit occurrence rate expressed through the \(F\)-function, and the AE hit energy rate expressed through the \(P\)-function during the entire experiment, were calculated and will be presented. It is noted that, for the needs of  
Figure 3: The temporal variation of the AE hit amplitudes and the axial deformation variation, depicted with respect to time. Figure 2: The PSC, the stress and the axial deformation variations, depicted with respect to time. the present analysis, both the \(F\)- and \(P\)-functions were calculated using a group of \(n=100\) consecutive AE hits. Each subsequent value of the \(F\)- and \(P\)-functions, was calculated by shifting the n AE hits window by 50 hits. Each calculated value of the \(F\)- and \(P\)-functions is associated with a specific time instance, \(\tau\), for presentation reasons. The time instance, \(\tau\), corresponds to the average value of the occurrence times of the AE hits of each group.