Issue 41

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 41 (2017) 536-551; DOI: 10.3221/IGF-ESIS.41.64 548 It is clearly observed from Fig.13a that there are not striking differences between the signals for the four time intervals considered. Excluding very few signals of the first interval with relatively low frequency and increased RA-value, which could indicate preliminary shear (friction) effects (perhaps of parasitic nature), the vast majority of signals recorded are characterized by a combination of high frequency and low RA-value. It is therefore safely concluded that the specific design of the epistyle’s restoration suppressed completely the pull-out phenomenon, i.e., the gradual removal of the reinforcing bars from the marble’s body without fracture of neither the marble nor of the reinforcing bars. This is very interesting since the specific failure mechanism seriously concerns the members of the scientific personnel working for the restoration of the Acropolis monuments [37, 38]. Indeed, it has been verified both in the laboratory and also in the worksite that the weak link of the three-material complex (marble-cement paste-titanium) is the interface between the marble and the cement paste which due to pure adhesion fails prematurely [8-10]. It is mentioned, however, that the specific failure mechanism is not always “unwelcome”, since it may act as a protective mean against failure (fracture) of the authentic marble body itself. Further analysis of Fig.13a leads to the conclusion that during all four time-intervals the predominant failure mechanism is cracking due to normal stresses. In combination to the data of all sensing techniques it is concluded that during the first time-interval the element suffering from cracking is the layer of cement paste between the two fragments at the lower half of the member (i.e. its portion under tension). In a second phase cracking of the layer of cement paste that fills the holes and surrounds the metallic bars appears in the immediate vicinity of the interface (recall that according to previous numerical studies it is only a small portion of the anchoring length - equal to a few cm - that undertakes the tensile force balancing the externally applied bending load [8-10]). During the remaining time-intervals the element suffering from cracking is the body of the two marble fragments. It is emphasized, however, that the intensity of this type of cracking is not constant during the whole duration of the last two time intervals. Obviously a “silent” period, i.e. a period of low acoustic activity appears during yielding of the lower layer of reinforcing bars, which is indeed observed in Fig.12 around a time instant equal to about t=1130 sec. In an attempt to further enlighten the specific issue the respective data concerning the mutual dependence of the average frequency and the RA-parameter are plotted in Fig.13b, as they were obtained from a typical pull-out test of a threaded titanium bar from a marble block [11]. It is emphasized that the specimens of the specific test (see the figure embedded in Fig.13b) were prepared, also, by experienced technicians of the Parthenon worksite and the materials used (marble, bar and cement paste that fill the hole in which the bar is driven) were exactly the same with those used for the construction of the epistyle’s copy tested here, in order for a direct comparison to be possible. As it can be seen from Fig.13b at the initial stages of the experiment the signals are characterized by high frequency and low RA-value, indicating cracking as predominant failure mode (of the intermediate cement paste layers). Gradually the signals change and during the final stage of the experiment they are characterized by lower frequency and considerably higher values of the RA parameter, indicating clearly that pull-out has started and the predominant failure mode is slip of the cement paste with respect to the marble’s body. The specific conclusion was further supported by the data of other sensing techniques as it is described in ref.[11]. The as above conclusions are further verified by the spatial variation of the acoustic signals, plotted in Fig.14 [12]. It is observed that initially the events are detected along the fault (at its lower two thirds). For the specific load level it is obvious that neither the bars yield nor the marble is fractured. Therefore the specific events are due to cracking of the cement paste between the two fragments. Afterwards the events are localized in two areas and more specifically the ones where the bars of the two lower layers intersect the fault’s plane. Again the load level does not support cracking of the marble body and therefore these events are safely attributed to cracking of the cement paste layer surrounding the reinforcing bars. D ISCUSSION AND CONCLUSIONS n accurate copy of a typical epistyle of the Parthenon’s Temple was fractured into two asymmetric fragments and it was restored with the aid of six titanium bars according to the technique developed by the scientists of the “Committee for the Conservation of the Acropolis Monuments”. The epistyle was subjected to multi-point bending with the aid of a special metallic structure consisting of I-shaped beams and cylindrical bars. The aim of the study was to comparatively assess the efficiency of two modern sensing techniques (“Acoustic Emissions” and the “Pressure Stimulated Currents”) as Continuous Structural Health Monitoring tools. From the Strength of Materials point of view it was found that the response of the epistyle to bending load was almost linear for about three quarters of the maximum load attained. A characteristic plateau of the load-deflection curve can be A