Issue 50

V. Saltas et alii, Frattura ed Integrità Strutturale, 50 (2019) 505-516; DOI: 10.3221/IGF-ESIS.50.42 510 As it is clearly observed in Fig.1c, additional smaller macro-cracks were also propagated from the interface of the hole with the concrete, without however reaching the outer surface, due to the higher surrounding confinement. These cracks should be of shear and mixed-type and they are observed in a smaller percentage, as it is evident in Fig.6a. Their presence in random directions could be attributed to the heterogeneity of the concrete specimen. In both cases of tensile and shear cracks formed in the concrete specimen during the agent-induced fracturing, the different waveform features are clearly indicated in Figs.6(b,c). The low value of rise-time and the broad frequency content are indicative of tensile cracks (see Fig.6b). As for the shear cracks, rise-time is much longer and the frequency spectrum mainly contains low frequencies, as it is shown in Fig.6c. Figure 6 : (a) Average frequency versus rising angle of the hits recorded in channel 6 with amplitudes greater than 45 dB, during the two stages of the fracturing process (stages A and B) of the concrete specimen. The dashed grey line separates the two cracking modes; (b), (c) Two typical recorded waveforms [indicated by black arrows in (a)] attributed to tensile (b) and shear (c) cracking modes and their cor- responding frequency spectra. The waveform characteristics are noted in each case. TCT stands for the threshold crossing time; red dashed lines indicate the threshold of detection (39 dB). The use of 7 sensors in the monitoring test has enabled the determination of the location of AE sources. The principle of AE source localization is based on the minimization of the following quantity, chi square ( χ 2 ), by using multiple regression analysis [22,23]: ଶ ൌ ∑ ൫ ௜,௢௕௦ െ ௜,௖௔௟௖ ൯ ଶ ௜ (1) where ௜,௢௕௦ ൌ ௜ െ ଵ is the observed time difference calculated from the known arrival times in the first hit sensor and the ith sensor, and ௜,௖௔௟௖ is the calculated time difference, according to the following time-distance relationship ௜,௖௔௟௖ ൌ ቀඥሺ ௜ െ ௦ ሻ ଶ ൅ ሺ ௜ െ ௦ ሻ ଶ ൅ ሺ ௜ െ ௦ ሻ ଶ െ ඥሺ ଵ െ ௦ ሻ ଶ ൅ ሺ ଵ െ ௦ ሻ ଶ ൅ ሺ ଵ െ ௦ ሻ ଶ ቁ/ (2) In Eq.(2), ሺ ௜ , ௜ , ௜ ሻ and ሺ ௦ , ௦ , ௦ ሻ are the coordinates of the ith sensor and the unknown AE source location, respectively, in a 3D Cartesian coordinate system, and υ is the acoustic wave velocity in the material under test, which is considered isotropic and homogenous. Three-dimensional location patterns of AE events at different selected times of the monitoring test are illustrated in Figs. 7(a-d). The AE data were filtered to discard events with amplitudes lower than 45 dB, in order to improve the overall location pattern. We observe that the locations of events before the initiation time of the fracturing process, i.e. in the range A, are rather concentrated at the bottom of the hole filled with the expansive mortar (refer to Fig.7a and Fig.5). During the evolution of the fracturing process (range B), new micro-cracks were formed and propagated around the hole, possibly at the interface of the concrete with the expansive mortar [refer to snapshots (b) and (c) of Fig.7]. These are reason- able findings, considering that the expansive pressure generated by the cracking agent is not uniformly distributed along 0 200 400 600 800 1000 -0.2 -0.1 0.0 0.1 0.2 0 200 400 600 800 1000 0 2 4 6 8 10 12 14 16 18 Amp (V) time (  sec) Duration: 1062.8  s Amplitude: 66.8 dB Risetime: 19.2  s TCT: 43623.7782835 sec (b) FFT magnitude frequency (kHz) Frequency Centroid: 373.1 kHz Peak Frequency: 143.7 kHz Average Frequency: 160.9 kHz 0 400 800 1200 1600 2000 0 100 200 300 400 500 600 tensile cracks stage A stage B average frequency, AF (kHz) rising angle, RA (ms/V) shear cracks (a) 0 200 400 600 800 1000 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0 200 400 600 800 1000 0 1 2 3 4 5 6 Amp (V) time (  sec) Duration: 632.2  s Amplitude: 49.4 dB Risetime: 126  s (c) FFT magnitude frequency (kHz) TCT: 57549.507567 sec Frequency Centroid: 318.9 kHz Peak Frequency: 148.3 kHz Average Frequency: 68.0 kHz