Issue 50

V. Saltas et alii, Frattura ed Integrità Strutturale, 50 (2019) 505-516; DOI: 10.3221/IGF-ESIS.50.42 507 Figure 1 : (a) The concrete specimen with the AE sensors mounted on it. The arrow indicates the pre-drilled hole filled with the expansive mortar and the dashed circles indicate the positions of the visible AE sensors; (b) The concrete block at the end of the monitoring where the fracture plane is parallel to the y-z plane; (c) View of the two splitting pieces of the concrete block after its ultimate rupture. White arrows indicate main visible macro-cracks. Figure 2 : A representative recorded AE signal (hit) of moderate amplitude (61.4 dB), recorded in channel 6, at relative recording time, t = 13675.24175475 s. Rising angle (RA=1.2 μs/dB) is defined as the ratio of rising time (RT=74.2 μs) to the corresponding amplitude. tudes up to 90 dB starts to be observed, while afterwards, the recorded hits and their amplitudes are gradually decreased until the end of the monitoring test. This observed peak (refer to Fig.3) is attributed to the initiation of cracks generation into the concrete specimen, which occurs when the expansion stress of the cracking agent exceeds the tensile strength of the concrete specimen. The initiation of the fracturing process is manifested by a concentrated AE activity, that is, 3.5 % (or 205 hits) of the total number of hits, over a short period of 0.5 sec. Subsequently, a considerable AE activity is recorded (~45% of the total activity) that lasts for a long period of about 5.1 h. This is related to the additional generation of micro-cracks and their coalescence which leads to the formation of the major macro-cracks in the concrete specimen, as it is shown in Fig.1(b, c). It should be noted here that the crack initiation was observed after 12 h which is far from the corresponding value suggested by the manufacturer, i.e. 2-8 h after the injection of SCDA. The latter, however, should be considered as the optimum value under certain operating conditions which are related to the ambient temperature, the water content and the diameter of the borehole. A low AE activity is occasionally recorded in all channels before the observed onset of the fracturing process and from the beginning of the monitoring test, corresponding to ~4% (i.e., 243 hits) of the total number of hits and to amplitudes ≤ 61 dB. This activity should be associated with micro-cracks and friction effects, caused during the thermal expansion of the cracking agent within the pre-drilled hole of the concrete block. It is worth noting that there is no clear pre-failure activity, indicative of the upcoming fracturing process which started suddenly after about 12 h of the beginning of the monitoring test. The high recorded AE activity that is related to the initiation of the cracking process in the concrete block is also reflected to the duration of the recorded hits in each of the 7 channels, as it is illustrated in Fig.4. According to Fig.4, the hits re- corded around the time ௜ ൎ 43618 sec have duration as high as a few ms, while afterwards and for the rest of the monitoring