Issue 50

I. Dakanali et alii, Frattura ed Integrità Strutturale, 50 (2019) 370-382; DOI: 10.3221/IGF-ESIS.50.31 371 positioned on the monument and the addition of new elements to be limited as much as possible. From that moment, the term “anastylosis” was established as a reconstruction technique whereby a ruined monument is restored using the original architectural elements to the greatest degree possible. From an archeological point of view, the intervention by Balanos was successful as the monument was preserved without largely altering its condition as a ruin (due to the less additions of new marble). Unfortunately, from a structural and chemical point of view, using contemporary - for the time - material without verifying compatibility with the ancient material or chemical resistance against the extensive air pollution, while using scattered ancient fragments as ordinary building material, caused significant damage to the monument [1]. The establishment of the Committee for the Conservation of the Acropolis Monuments (ESMA) in 1975 inaugurated a new approach to the restoration of the monuments of the Acropolis of Athens and tried to heal the injured monument [1]. The committee developed a pioneering technique for the restoration of the monuments’ structural integrity according to which the fractured marble elements are connected by inserting titanium bars into pre-drilled holes. The adhesion between marble and bars is achieved by a suitable white cement paste [2]. The aforementioned technique is based on some basic principles: i) Reversibility (if needed, the monument could be brought to its state prior to the intervention), ii) Minimization of the interventions to the extent that guarantees protection of the authentic material from further damage (for this reason the number of titanium bars required by the connection’s design, should be the smallest possible) and finally iii) Compatibility between the materials used for the restoration and the authentic ones [3]. After the structural elements are repositioned to their original place, the loads that have to be sustained are their self- weight, the weight of overlying architectural members and the potential impending dynamic loads (using an increasing safety factor). The main goal is to assure that none of the assembled materials gets to be fractured. The only potential failure can be the bars’ sliding relatively to the marble. This phenomenon is the so-called ‘pull-out’. The pull-out phenomenon is a matter of concern for the engineers in many structural problems. It has been examined thoroughly in concrete structures. The testing specimens for extracting steel bars or reinforcing fibers from a concrete block have been improved lately in the direction of isolating the tested area from the influence of the experimental set-up. The main drawback of the typical concrete pull-out tests (Fig.1a) is that the stress field of the tested area is not the same with the respective one of the reinforced concrete member. In most concrete structures, the bar and the surrounding concrete are under tension (bending beam) while in pull-out tests, the bar is in tension and the concrete around it is in compression [4]. For this reason, several alternative test designs were proposed (Fig.1(b, c)). Figure 1: (a) Typical pull-out specimen [5]; (b) Alternative pull-out test [6]; (c) Push-in concrete specimen [7]. The present work is part of a wider research project for investigating the parameters influencing the pull-out phenomenon in restored marble structural elements [8-10]. As there is no standard protocol for testing the connection used for the restoration of fragmented marble epistyles, the typical pull-out tests of concrete structures have been adopted. The main goal is not only to simulate experimentally the actual stress field of the assembled parts but also to isolate the intermediate surface between the bars and the marble from the experimental set-up. The concern in monuments after restoration work takes place, is how through thorough monitoring of the structure and ‘on time’ intervention, there will be no stress field excess. In most cases in structures, before the final failure, there are microfractures to their interior which are invisible. When collapse is about to occur, there is no time for intervention. Consequently, the continuous monitoring of the structural condition of monuments, considering their sensitivity and cultural importance, is of paramount importance. In this direction, the Acoustic Emission (AE) technique is proposed as a combined innovative non-destructive health monitoring method. (a) (b) (c)