Issue 50

V. Saltas et alii, Frattura ed Integrità Strutturale, 50 (2019) 505-516; DOI: 10.3221/IGF-ESIS.50.42 513 ITs in region B1 incorporates two different patterns, i.e. two regions of small and large values of IT that could be related to the micro-cracks generation and propagation in random positions in the interface between the SCDA and the surrounding concrete. Their subsequent coalescence in the macro-scale during the fracturing process resulting to ultimate failure, obeys a mechanism closer to a Boltzman-Gibbs statistics. A similar behavior of the q -index during three-point bending tests of cement mortar beams has been reported in ref.[36], where q -index was calculated in successive stages of the test, as the speci- mens approached failure, and was found to decrease, due to the different associated fracture mechanisms. More recently, NESP analysis was conducted at inter-event times of AE during repetitive loading/unloading cycles of cement mortar beams subjected to 3-point bending [37]. The experimental data were fitted by a q -exponential function; the entropic index q was found to increase in each repeated cycle, approaching a critical value ( q =1.42) when the specimen reached ultimate failure. Figure 8 : (a) Time evolution of the inter-event times of hits recorded to sensor with the maximum number of recorded hits (sensor 6) during the monitoring test of SCDA-induced concrete fracture. The 3 stages (A, B and C) refer to the separation applied in Fig.5, according to the variation of the cumulative absolute energy of the recorded hits. (b) The same data as in (a), in sequential order. The blue line represents a smoothed curve with an averaging method of 20 points. A color scale is used to distinguish the hits with different amplitudes. Only the data of stage B have been further analyzed in the context of NESP. (c) The function F(τ) of recorded hits in channel 6, for the stage B (43618 – 62145 s) of the monitoring test. The number of consecutive hits is N =10. The inset shows in greater detail the first 6 minutes of the fracture process. Note the logarithmic scale in the vertical axis, in all cases. C ONCLUSIONS n the present work, the fracturing process of concrete specimens under the effect of a soundless chemical demolition agent was investigated by using the AE technique. Real-time monitoring of a concrete block with a pre-drilled hole filled with SCDA was performed for a period of 24 h by an array of 7 AE sensors. The analysis of the experimental data 0 1 2 3 4 5 6 7 8 9 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 channel #6 inter-event time,  (sec) time (x 10 4 sec) (a) A B C 0 200 400 600 800 1000 1200 1400 1600 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 (b) inter-event time,  (sec) sequential number of hits 20 points smoothing 39 51 63 75 87 amplitude (dB) region B 4.4 4.8 5.2 5.6 6.0 10 -1 10 0 10 1 10 2 10 3 4.36 4.37 4.38 4.39 10 -1 10 0 10 1 10 2 10 3 function F (s -1 ) time (x 10 4 sec) (c) I