Ioanna Dakanali

Acoustic Emissions versus Pressure Stimulated Currents during bending of restored marble epistyles: Preliminary results

The efficiency of two modern sensing techniques, namely the “Acoustic Emissions” and the “Pressure Stimulated Currents” ones, when they are used as Continuous Structural Health Monitoring tools, is assessed experimentally. The protocol includes multi-point bending of an accurate copy of a fractured marble epistyle of the Parthenon’s Temple on the Acropolis of Athens, under a scale of 1:3. The integrity of the epistyle is restored with three pairs of bolted titanium bars, according to the pioneering technique developed by the scientists of the “Committee for the Conservation of the Acropolis Monuments”. The data provided by the above techniques are considered in juxtaposition to each other and also in comparison to data provided by the “Digital Image Correlation” technique. It is concluded that, at least from a qualitative point of view, the data of all three techniques are in good mutual agreement. Combined exploitation of the various sets of experimental data enlightens interesting aspects concerning the succession of failure mechanisms activated during the loading procedure, revealing the critical role of the internal interfaces characterizing the restored epistyle. Moreover it is definitely indicated that both the “Acoustic Emissions” and the “Pressure Stimulated Currents” techniques provide clear signs of upcoming failure well before macroscopically visible damages are detected at the external surface of the specimen.

A COMBINED EXPERIMENTAL AND NUMERICAL STUDY OF THE PULL-OUT MECHANISM OF THREADED TITANIUM BARS EMBEDDED IN MARBLE BLOCKS

The present study describes the first stage of an ongoing research project aiming to investigate the failure mechanisms activated when reinforcing bars are pulled-out from the body of the reinforced element. Attention is given to the progressive failure of the interfaces between the constituent elements of the marble-cement paste-titanium (MCT) complex, which appears in case marble structural members of classic stone monuments are restored according to the pioneering technique used on the Acropolis of Athens worksite. Experience from already implemented projects indicates that, when restored structural members are subjected to tension or bending, the failure of these interfaces leads to debonding and finally to catastrophic pull-out of the reinforcing bar from the body of the marble volume. In this direction, a series of pull-out experiments were implemented, with specimens made of Dionysos marble, in the form of prisms of various dimensions with threaded titanium bars inserted into predrilled holes which were then filled with suitable cement paste. The results obtained from these experiments, by employing both conventional and innovative sensing techniques, were used for the calibration of a Finite Element model, which will be used for the thorough investigation of the parameters affecting the overall response of the restored element. The commercial package ABAQUS was used for the implementation of the numerical analysis. The surface contact features of the package were properly exploited in the direction of comparatively exploring and evaluating of possible modeling approaches for simulating the actual interfaces of the marble-cement-titanium complex. A reliable performance law, allowing for the incorporation of damage initiation and evolution criteria, was determined permitting satisfactory modeling of the pull-out behavior at the interface between marble and cement paste. Critical parameters, material properties and modeling choices that have a significant impact on the final outcome were considered. The numerical model finally designed approaches in a very satisfactory manner the experimentally obtained load-displacement curve, providing an easy-to-use, flexible and reliable tool for further study of the pull-out phenomenon.