## 3 The Specimens and the Experimental Protocols  
In this study, advantage is taken of the data provided by two experimental protocols. The first one included intact beam-shaped specimens of relatively small dimensions (i.e., length equal to 250 mm and square cross section equal to 55 \(\times\) 55 mm\({}^{2}\)). The specimens were subjected to three-point bending adopting load-control conditions at two rates equal to 66 N/s and 80 N/s (Figure 1a). The distance between the supporting rollers was equal to 220 mm (Figure 1a). A photo of a typical specimen while loaded is shown in Figure 1b.  
Figure 1: (**a**) The geometry of the specimens used. Upper row left: Intact specimens. Upper row right: Notched specimens. The distance of the center of the acoustic sensor from the bottom face of the specimen was equal to h = 35 mm. (**b**) Photo of an intact specimen just before loading. (**c**) Photo of a notched specimen during the loading procedure.  
The specimens were prepared using Ordinary Portland cement and fine aggregates with average dimensions ranging from 0.6 mm to 3 mm. The \"cement/aggregate/water\" ratio was equal to 2/6/1 (by weight). The compressive strength was equal to about 32 MPa, while the respective tensile one (as determined by splitting tests, i.e., Brazilian-disc tests) was equal to about 3.0 MPa.  
In the next section the data gathered during two typical tests of this protocol will be analyzed: The first one is the test encoded as LR1: The loading rate was equal to 66 N/s, and fracture occurred at a load level equal to Lmax,LR1 \(\approx\) 3.74 kN. The second one is the test encoded as LR2: The loading rate was equal to 80 N/s, and fracture occurred at a load level equal to Lmax,LR2 \(\approx\) 3.87 kN.  
The second protocol included beam-shaped specimens of significantly larger dimensions (length equal to L = 700 mm and square cross-section equal to hxb = 150 \(\times\) 150 mm\({}^{2}\)). The specimens were notched at their central section (normally to their longitudinal axis) by means of a rotating disc. The depth of the notches was equal to c = 25 mm while their width (breadth) was equal to about d = 5 mm. The specimens were subjected to three-point bending (Figure 1a) adopting displacement-control conditions at a rate equal to 0.08 mm/min with the aid of a stiff servo-hydraulic INSTRON-SATEC frame of capacity equal to 300 kN. The distance between the supporting rollers was equal to L0 = 600 mm. A photo of a typical specimen while loaded is shown in Figure 1c.  
Again, two typical tests of this protocol will be analyzed in the next section. In the first one, the specimen (encoded as R0) was unreinforced (similar composition to the specimens of the previous protocol), and the maximum load attained was equal to Lmax = 11.70 kN, while in the second one the specimen (encoded as RM) was reinforced with short metallic fibers at a portion of 25 kg/m\({}^{3}\). Round steel fibers were used, the diameter and length of which were equal to about 1.30 mm and 50 mm, respectively. Their tensile strength is around 690 MPa. The maximum load attained for this specimen was equal to Lmax = 14.05 kN.  
For both protocols the acoustic signals were detected using an acoustic sensor, properly mounted at the plane along which crack propagation was expected, using a layer of silicon grease to optimize contact. The acoustic sensor used in the experimental protocol was the R15\(\alpha\), the resonant frequency and the operating frequency of which are 75 kHz and 50-400 kHz, respectively. The PCI-2 AE system with a preamplifier of 40 dB (Mistras Group, Inc., West Windsor Township, NJ, USA) was used to record and store the experimental data. A threshold of 40 dB was set, together with a low-frequency cutting filter, in order to exclude signals of frequency lower than 20 kHz.