Issue 50

V. Saltas et alii, Frattura ed Integrità Strutturale, 50 (2019) 505-516; DOI: 10.3221/IGF-ESIS.50.42 509 Figure 5 : The cumulative absolute energy (in aJ) in each recorded channel, during the monitoring test of the concrete specimen. The overall monitoring period is divided into 3 stages, A, B and C (see text for details). period, the recorded hits have substantially lower duration. Prior to the onset of the intense AE activity, the duration of the occasional recording hits are limited to less than 1 ms, suggesting that the corresponding acoustic emissions have rather low energies and do not correspond to any significant precursory signal of the upcoming SCDA-induced cracking process. Further information on the cracking process caused by the expansive agent in the concrete block can be derived from the time evolution of the cumulative absolute energy of the recorded hits in each channel, during the monitoring period (refer to Fig.5). The highest cumulative energy is recorded in channel 6, which is closer to the hole filled with the expansive agent. In all sensors, however, the recorded energy variations exhibit the same characteristics. The overall monitoring period has been divided into 3 stages, depending on their specific characteristics. The “silent” stage A (0-43618 sec) that appears prior to the initiation of the cracking process, corresponds to a limited number of hits (~4% of the total number) with low amplitudes and short durations and thus to a cumulative energy close to zero. In stage B which lasts for 5.1 h, the cumulative energy of hits increases drastically exhibiting stepped-like behavior that corresponds to the formation of macro-cracks, as it is shown in Figs.1(b,c). This stage of strong AE activity can be considered as the effective period of action of the expansive agent in the fracture of the concrete specimen. Finally, in region C, although the recorded hits reach 20% of the total, the cumulative energy remains almost constant until the end of the monitoring test, due to the relatively short duration and the low amplitudes of the recorded hits. The separation of the monitoring period in the aforementioned stages, according to the values of the cumulative energy will be used later in the statistical analysis of the AE data. The characterization of the cracking mode (tensile or mode-I and shear or mode-II) during the fracturing process of the concrete specimen can be achieved by correlating specific AE parameters, namely average frequency (AF) and rising angle (RA) [16-19]. This classification is based on the fact that the slower shear waves transmit more energy than the faster tensile waves, resulting to longer rising time and to a low frequency content of the recorded waveform, in opposition to the tensile waves which correspond to short rising times and higher frequencies [16]. The average frequency as a function of the rising angle of the recorded hits in channel 6, for the stages A and B of the monitoring procedure is depicted in Fig.6a. Additionally, two representative recorded waveforms corresponding to different cracking modes and their frequency content are illustrated in Figs.6(b,c). We observe that the majority of the recorded hits are characterized by low values of RA and AF values in the low frequency range, i.e. 50-250 kHz. According to the afore- mentioned classification, these recorded hits are due to the formation of tensile cracks inside the concrete specimen. This is in accordance to the shape of the specimen after fracture, as it is depicted in Fig.1b. The compressive stresses that develop in the radial direction of the cylindrical hole and accumulate to the interface between the expansive agent and the walls, lead to indirect tensile stresses in the tangential direction of the hole which manifest as macro-cracks in the y-z plane. The latter is the predominant plane of fracture, since the distance of the cylindrical hole from the side surfaces of the specimen is minimal in the y-direction, corresponding to minimal confinement from the surrounding material. These observations are in agreement with published results stating that SCDA-induced initial cracking occurs when the mean value of the tangential stress reaches the splitting tensile strength of the specimen and the crack propagation is caused mainly by mode-I type of failure [5,11,20,21].