Issue 40

M. Mentzini, Frattura ed Integrità Strutturale, 40 (2017) 95-107; DOI: 10.3221/IGF-ESIS.40.09 96 because usually the reinforcing titanium bars required for reaching the initial carrying capacity can not be fitted due to the shape of the sections of the members, the relatively small size of the remaining authentic parts of the member or the great loss of authentic material [3]. The material used for the erection of the monuments of the Athenian Acropolis was Pentelic marble, well known as the building material of the classical period’s masterpieces. The needs of the restoration project in progress (for the construction of “patches” and a few new members) are covered nowadays using Dionysos marble (quarried from mountain Dionysos in Attica), which has similar properties to the authentic one. In general, the data reported in literature concerning Dionysos marble vary between very broad limits. This scattering can be attributed to the different conditions under which the tests are performed but mainly to the anisotropy of Dionysos marble. There are three different anisotropy directions (parallel to the layers, along the width of the web and along its thickness). After a long series of direct tension and uniaxial compression tests, it was concluded that the mechanical pro- perties along the two of the anisotropy directions are very similar to each other [4-6]. Thus, this marble can be considered as a transversely isotropic material described with the aid of five elastic constants: two elastic moduli, in the plane of trans- verse isotropy and normal to it, two Poisson’s ratios characterizing the lateral strain response in the plane of transverse isotropy to a tensile stress acting parallel and normal to it, and the shear modulus in the planes normal to the plane of iso- tropy. From these tests it was also concluded that the material appears to be slightly bimodular, i.e. its elastic moduli in tension and in compression are not equal. The combination of these mechanical characteristics with the special internal structure and composition of the material (mainly calcite with very small amounts of muscovite, sericite, quartz, chlorite and areas with imperfections) is responsible for the complicated form of the cracks observed. The shape and extent of the damages are also affected by the particular role of the drums as structural members [7]. In general, the damage of the monument is due to either natural phenomena or human interventions. Among the natural phenomena one could mention aging (and, as already pointed out, the special nature and the imperfections of the material), decay due to physico - chemical/biological actions, freezing, seismic action etc. Catastrophic human interventions include fire, bombing, explosion, vandalism, the problems caused by previous restoration (in the eve of the 20 th century) by engineer N. Balanos (thoughtless use of iron elements, which after oxidization expand becoming the origin of a destructive process for the marble surrounding iron elements) and finally the effects of pollution [8, 9]. The first serious damage in Parthenon was caused by the great earthquake of 426 B.C.. The most serious destruction came at 267 A.D., by a German tribe, the Erouli, who burned the place down. During the siege of the Acropolis by the Morosini’s Venetians (1687 A.D.) an explosion blew up three of the four walls of the cella, six columns on the south, eight ones on the north (the area of interest in the present paper) and the remains of Pronaos collapsed except one column [9]. Parthenon Temple is a structural system with special characteristics. The form and the stability of the structure are attained by the perfect contact between the members (absence of connecting material) which leads to the development of friction and the use of horizontal (clamps) and vertical (dowels) metallic connectors that resist tensile and shear forces. Thus a complex construction is formed, the dynamic response of which is governed by the sliding and the rocking of the individual stones, either independently or in groups [10]. The process followed to restore the structural integrity of each drum (in order to make it behaving again as an intact member) is based on the principle of avoiding overturning of the fragment (that is to be joined) from the member (in case friction forces are not sufficient) [11, 12]. The method is improved by taking also into account the critical stress state on its surface. It has the advantage of flexibility and therefore it can be applied in many different cases. The reinforcement required is calculated based on the fragment’s volume which sustains the load, its position relative to the drum and its height. Therefore the stress field developed can be estimated taking into account the overturning lever arm and the load applied [3]. Dynamic actions, in this case earthquake and explosion, can lead to drums’ raising and loss of their proper position, while the rocking effect produces impact phenomena among adjacent members. This impact damages particularly the edges of the contact area between them, which is linear and consists schematically an arc [13, 14]. As a result fragments are produced, which usually form wedges and the presence of converging strata make it easier for fracture to start [7]. To cope with these problems a special procedure was developed, in collaboration with late Pr. I. Vardoulakis, of the Department of Mechanics (National Technical University of Athens) to determine the stress state and therefore the reinforcement required [1, 15]. The restored drums are usually classified with respect to the corresponding column (4 th to 11 th ) with codes representing their original position. Each drum is denoted by two numbers: The first one indicates the column on which they belong (the columns counting starts from East to West; therefore the eight columns of the North’s Colonnade Restoration program have codes 4 to 11) and the second number indicates their exact position on the specific column (the drums counting starts from the lowest one, i.e. the one standing on the stylobate). For example the drum with the code “5.8” belongs to the 5 th column and stands on the 8 th row.