Issue 41

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 41 (2017) 536-551; DOI: 10.3221/IGF-ESIS.41.64 549 (a) (b) Figure 14: The spatial distribution of acoustic events for the second (a) and third (b) time intervals [12]. finally attributed to yielding of the reinforcing bars. The destruction of the restored structure (excluding a local fracture of a corner of one of the two fragments) appeared when the reinforcing bars of the lower layer yielded and fractured, triggering yielding and fracture also of the bars of the other two layers and causing collapse of the epistyle. The specific type of failure is not the worst scenario for scientists working for the restoration of monuments. Their worst scenario cor- responds to fracture of the marble volumes. On the other hand, the failure mode here observed is not the most “desirable” one; the latter corresponds to the gradual pull-out of the reinforcing bars since (if timely detected) it permits undertaking of actions that can prevent final collapse. It could be therefore concluded that the pioneering technique developed and used for the restoration of monuments on the Acropolis of Athens should be further considered. What is to be answered is whether the procedure nowadays used to calculate the number of reinforcing bars and their diameter as well as the choice of the material they are made of are the most appropriate ones. In addition, taking advantage of the data from all sensing techniques used, according to a combined manner, it was possible to clearly enlighten the succession of failure modes activated within the structure (which is a complex of three materials with two interfaces). It was definitely concluded that the final failure was the result of only two mechanisms: Cracking of the cement paste and marble (at different time intervals) and yielding and fracture of the reinforcing bars. The third failure mechanism, usually appearing in similar tests, i.e., the pull-out of the reinforcing bars, was totally suppressed. Concerning the monitoring of the epistyle during loading it was concluded that all techniques used provided useful data of complementary nature. For example the data concerning the opening of the fault provided by the clip gauges were compared against the respective ones obtained by the Digital Image Correlation technique and were found in excellent qualitative and quantitative agreement. Therefore, depending on the needs and restrictions of specific experimental proto- cols either of them can be used. Along the same lines the data provided by the Acoustic Emissions and Pressure Stimulated Currents techniques were found in excellent qualitative agreement (recall that each technique is used to measure different quantities). The time variation of a series of parameters, quantified with the aid of these techniques (the electric current, the energy of the electric current, the cumulative hits per second, the cumulative energy of the acoustic events, the frequency of the signals and their RA-value, and the Ib-value), were found to exhibit characteristic changes (extreme values or slope changes) almost at identical time-instants. It is thus safe to conclude that the Acoustic Emission technique can be used to quantitatively calibrate the data of the Pressure Stimulated Currents and vice-versa, again depending on the needs of specific experimental protocols. Of utmost importance is, also, the fact that both techniques (AE and PSC) provided clear signs which indicate entrance of the system to a “critical stage”, or in other words pre-failure indicators warning that final failure is impending. What is more it was concluded that various parameters could undertake the specific role: The electric current and the respective energy (concerning the PSC technique), the Ib -value, the cumulative number of hits, the RA value (concerning the AE technique). Obviously, the optimum choice (which could be a combination of more than one parameters) is something