Issue 50

E. D. Pasiou, Frattura ed Integrità Strutturale, 50 (2019) 560-572; DOI: 10.3221/IGF-ESIS.50.47 561 Due to the extensive failures observed on the monuments of the Acropolis, caused by a series of disasters (either due to human interventions or natural phenomena), the Greek Ministry of Culture founded the “Committee for the Conservation of the Acropolis Monuments” (ESMA) in 1975 and the “journey” of restoring the monuments of the Acropolis started. At the very beginning of the restoration project, it was observed that the vast majority of the connections (i.e., the “con- nector-intermediate material-marble surrounding the groove” complex) were destroyed: Either the connectors were frac- tured and/or the surrounding marble volume was fragmented. In this context, the engineers in charge confronted a series of problems and pressing questions that had to be answered: 1. The use of iron during previous restoration projects was almost exclusively responsible for the fractures. Indeed, iron was oxidized, the connectors swelled and the surrounding marble was fractured. What type of metal could substitute iron for the construction of the new connectors? 2. Would the chosen metal affect the intermediate material in the grooves? If the answer was “yes”, which material would it be suitable and compatible? 3. Mining of marble volumes from mount Pentelicon was forbidden. What type of marble was compatible with the ancient one and could be used for the construction of supplements or new blocks? To answer the aforementioned questions, the engineers in charge for the restoration project collaborated with scientists from various universities and disciplines in order to thoroughly study the mechanical and chemical properties of the ancient materials and the candidate substitute ones. After a long period of intensive study, it was concluded that the proper substitute materials are the following: 1. Grade 2 pure titanium instead of iron [4,5]. 2. White cement mortar instead of molten lead [6], taking into account that lead and titanium produce galvanic element. 3. Dionysos marble instead of Pentelic marble [7-9]. Therefore, during the restoration project in progress of the Parthenon, the marble blocks are connected to each other with titanium connectors and the groove is filled with suitable mortar. The mechanical behaviour of the as above designed horizontal connections, i.e., those with “I”-shaped connectors, was one of the problems studied in-situ by the restoration personnel, almost at the beginning of the restoration project. More specifically, their response under tensile loading was studied experimentally [10]. However, the “I”-shaped connectors are also stressed under shear (observations of the members of the temple in-situ supports the shear loading of the connectors [10]), but the problem had not been investigated until recently. The study of the response of the new design of the con- nectors under shear started around 2008, in the Laboratory for Testing and Materials (LTM) of the National Technical University of Athens (NTUA). A series of problems, usually coming up during experimental protocols, appeared and had to be solved as it is discussed in next sections. T HE SPECIMENS AND THE EXPERIMENTAL SET - UP ionysos marble is an orthotropic, extremely brittle material [7-9]. Its mechanical response is characterized by an extremely pronounced “size effect” [11-13] dictating the use of large specimens (the dimensions of which should ideally approach those of actual structural members) in order for the results obtained experimentally to be size independent [14,15]. Simulating a typical connection of the epistyles of the Parthenon Temple under a 1:3 scale (and at the same time of marble blocks of the entablature of the temple under a 1:1 scale), the minimum length of the specimens was estimated to about 50 cm (note that the scales refer to the dimensions of the connector and the groove). The dimensions of the marble volumes, especially the distance between the web of the groove and the free surface of the blocks, were based on in-situ observations [16]. The next question to be answered was related to the exact geometry of the specimens that would enable the implementation of pure shear experiments. How should the specimen be gripped and loaded? In which loading frame of a typical laboratory could the experiments be carried out? Based on the experience of the personnel of the restoration project, it was decided to join together two Dionysos marble blocks with one titanium “I”-shaped connector (Fig.1a) and mortar. The main difference between the specimens of previous protocols [17] and the protocol here described was the shape of the marble volumes. Indeed, in previous protocols (and in the preliminary stage of the protocol here described) both volumes were prismatic (Fig.1b) and the smaller one was fixed on the table of the loading frame using a series of rigid metallic plates, rods and nuts. Two holes were drilled through the “thickness” of the larger volume enabling loading of the specimen with the aid of a couple of rigid bars. That experimental set-up is shown in Fig.2a and more details can be found in [17]. On the other hand, according to the new design of the specimens, only one of the two volumes was prismatic while the second one was “Γ”-shaped (Fig.1c). The prismatic volume was again fixed on the table of the loading frame, while on the D