Pull-out of threaded reinforcing bars from marble blocks

AbstractRejoining fragmented marble structural members of the Acropolis of Athens monuments is achieved by inserting threaded titanium bars into holes pre-drilled in the body of the members. Then the holes are filled with liquid cementitious material. This study aims to investigate the extraction of the bars from the marble volume (pull-out) in an effort to enlighten the mechanisms activated before and during the phenomenon. The study is implemented experimentally. The main problem hard to overcome was the fact that the weak link of the marble-cement-titanium complex, which is the marble-cement interface, is inaccessible for traditional sensing techniques. In this context innovative techniques were employed (Acoustic Emission and Pressure Stimulated Currents), which can detect failure and damages at the interior of the complex. Traditional sensing techniques were used in parallel mainly for calibration / validation reasons. The specimens, prepared by experienced personnel of the Parthenon’s worksite, were prisms made of Dionysos marble. Threaded titanium bars were inserted into through holes filled with a liquid cement paste. The quantities recorded during the tests were the load, the displacement, the bar’s axial strain and relative slip with respect to the marble, the electrical signals emitted and the acoustic emissions produced. Conclusions are drawn concerning correlations between the above quantities. The data gathered were then used to validate numerical models which will be used for parametric analyses

Mechanical response of notched marble beams under bending versus acoustic emissions and electric activity

An experimental protocol, including the combined application of both innovative and traditional sensing techniques, is described aiming to explore the mechanical response of marble and also to check the possibilities of detecting precursor phenomena designating upcoming catastrophic fracture. The protocol consisted of three-point bending tests with notched prismatic beams made of Dionysos marble, the material extensively used for restoration of the Acropolis of Athens monuments. The sensing system improvised included techniques relying on completely different physical foundations, which permit simultaneous detection and recording of the Pressure Stimulated Currents, Acoustic Emissions, three dimensional displacement fields and Notch Mouth Opening Displacements. Analysis of the results revealed interesting features of the mechanical response of Dionysos marble and indicated, also, that classical Continuum Fracture Mechanics fails to describe accurately the response of marble, at least in the presence of notches. In addition, strong correlations between the Pressure Stimulated Currents, the rate of acoustic hits and the rate of change of the opening of the pre-existing notch have been enlightened. Moreover, the onset of catastrophic crack propagation appears following distinguishable changes of the Pressure Stimulated Currents recorded. Therefore (and taking into account the very small size of the respective sensors as well as the simple complementary equipment needed), it is concluded that the specific technique could be considered as a simple and reliable tool for an alternative approach to the in-situ Structural Health Monitoring of classical stone monuments

Acoustic Emission Analysis of Cement Mortar Specimens During Three Point Bending Tests

Abstract This work discusses the experimental results of Acoustic Emission (AE) recordings during repetitive loading/unloading loops of cement mortar beams subjected to three point bending. Six repetitive loading cycles were conducted at a gradually higher load level until the failure of the specimens. The experimental results clearly show the existence and dominance of the Kaiser effect during each loading loop. Regarding the AE data, alternative analysis was conducted using the improved b-value, and the cumulative energy behaviour. Both quantities considered, show qualitative and quantitative characteristics that could be used as pre-failure indicators. In addition, a novel statistical physics analysis involving the AE interevent times was conducted by calculating the cumulative probability function P(>δτ) that follows a q-exponential equation. The entropic index q and the relaxation parameter βq of this equation show systematic changes during the various stages of the failure process. The last cycle led to a q value equal to 1.42, implying the upcoming fracture which is in good agreement with previous results obtained from a wide range of fractured materials