Issue 50

V. Saltas et alii, Frattura ed Integrità Strutturale, 50 (2019) 505-516; DOI: 10.3221/IGF-ESIS.50.42 514 Figure 9 : The normalized cumulative distribution of the inter-event times (blue circles) of hits recorded in channel 6, during the two sub- stages of region B of the monitoring test. The fitting curves (red and dashed green lines) are also shown. Note the logarithmic scale in both axes. The fitting parameters are noted in each case. reveals that fracture is suddenly observed after 12 h, without any clear indication of pre-failure signals. The absolute energy and the inter-event times of the recorded hits have been used to distinguish the effective period of action of the cracking agent and the quasi-static behavior in the fracturing process, respectively. The correlation of the AE parameters (average frequency and rising angle) implies that the fracturing process is dominated by tensile cracks (mode-I type of fracture). The localization of the AE sources allows investigating the formation and propagation of micro-cracks around the pre-drilled hole. In this direction, we propose that the expansive agent placed into a hole of a solid material could be used as an artificial source of acoustic signals, in analogy to the lead pencil break tests (LPBT) for studying the propagation of elastic waves in materials. However, we have to point out that this proposed method, as a destructive test in contrast to the LPBT, cannot be used directly in the test specimens but should be applied separately to other suitably prepared specimens. Additionally, the use of the expansive agent may not be appropriate in the case of the high anisotropic rocks and should only be used in artificial materials. The statistical analysis of AE data in the frame of NESP provides evidence that the SCDA-induced fracture of concrete is a complex process where the long-range interactions of the micro-cracks should play a significant role to any possible model of the fracturing process. The present investigation could be further extended to other brittle materials such as rocks and to complicated arrangement of holes that are used in practical applications, in order to monitor the evolution of the complex fracture network that will be developed under the effect of SCDAs. The influence of the essential parameters for the optimum usage of SCDAs, i.e. ambient temperature, water content, hole diameter and spacing between holes should be also investigated to reveal any possible correlations with the AE parameters. A CKNOWLEDGEMENTS e acknowledge support of this work by the project ‘‘HELPOS – Hellenic System for Lithosphere Monitoring, Greece’’ (MIS 5002697) which is implemented under the Action ‘‘Reinforcement of the Research and Innovation Infrastructure’’, funded by the Operational Programme ‘‘Competitiveness, Entrepreneurship and Innovation, Greece’’ (NSRF 2014–2020) and co-financed by Greece and the European Union (European Regional Development Fund). R EFERENCES [1] Gambatese, J.A. (2003). Controlled Concrete Demolition Using Expansive Cracking Agents, J. Constr. Eng. Manag., 129(1), pp. 98–104, DOI: 10.1061/(ASCE)0733-9364(2003)129:1(98). [2] Laefer, D.F., Ambrozevitch-Cooper, N., Huynh, M.P., Midgette, J., Ceribasi, S., Wortman, J. (2010). Expansive fracture agent behaviour for concrete cracking, Mag. Concr. Res., 62(6), pp. 443–452, DOI: 10.1680/macr.2010.62.6.443. [3] Dessouki, A. El., Mitri, H. (2011). Rock Breakage Using Expansive Cement, Engineering, 03(02), pp. 168–173, DOI: 10.4236/eng.2011.32020. 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 -3 10 -2 10 -1 10 0 exponential fitting (R 2 = 0.997) 0.471(4)e -0.657(6)  (a) ch#6: 71 - 470 hit data points q -exponential fitting (R 2 = 0.978)  = 4.26(7) q = 2.58(5) normalized cumulative distribution, (P>  ) inter-event time,  (s) 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 10 -3 10 -2 10 -1 10 0 data points curve fitting (R 2 = 0.996)  = 0.0482(3) q = 1.12(3) normalized cumulative distribution, (P>  ) inter-event time,  (s) ch#6: 471 - 1340 hit (b) W