J Mech Behav Mater 20 (2011): 55–65 © 2011 by Walter de Gruyter • Berlin • Boston. DOI 10.1515/JMBM.2011.007 The problem of mechanical compatibility of natural building stones in the restoration of monuments. Part II: Specimens with modified boundaries Nikolaos L. Ninis 1 and Stavros K. Kourkoulis 2, * 1 Finance Management Fund for Archaeological Projects, Ministry of Culture and Tourism, Athens, Metsovou 33, GR 106 82, Ligourio, Greece 2 Department of Mechanics, National Technical University of Athens, School of Applied Mathematical and Physical Sciences, Theocaris Building, Zografou Campus, GR 157-73 Athens, Greece, e-mail: stakkour@central.ntua.gr * Corresponding author Abstract It was pointed out in Part I of this short two-paper series, that the mechanical incompatibility between the authentic building stone of ancient monuments and the stones used as substitute ones during restoration projects, may be the reason of violation of basic restoration principles concerning the protection of the ancient material. In this context certain geometrical configura- tions of the boundaries of the specimens are examined in this Part II as a possible means of modifying the mechanical behav- iour of the substitute stones, in order to make them as compatible as possible with the authentic material. Modifications of both the contact surfaces (in order to change the friction conditions) of the specimens as well as of the free ones (in order to quantify the influence of transforming the smooth cylindrical surface to a fluted one) are examined experimentally. This approach is based on existing observations and numerical studies indicating that the behaviour of a stone specimen in the post-peak region is affected by the geometrical configuration of its boundaries. Taking advan- tage of the experimental results an alternative compatibility crite- rion is introduced for situations where the “required” quality of the building stone is its ability to withstand deformation without failing structurally, a characteristic pertinent to statically indeter- minate structures, whose design is based on deformation control. This criterion combines both peak stress and maximum failure strain providing a better insight into the problem of mechanical incompatibility of natural building stones. Keywords: compatibility; compression; friction; monuments; natural building stones; porous stones; restoration; soft rocks. 1. Introduction The problem of the mechanical compatibility of natural build- ing stones was studied experimentally in Part I [1] of the pres- ent short two-paper series. The study was focused on the stone of Kenchreae extensively used by ancient Greeks for the erec- tion of the monuments of the Asklepieion at Epidauros. The motive of the work was the need of the scientists working for the restoration of these monuments to assess the suitabil- ity of various natural building stones that could be used as substitutes of the authentic one either for the completion of damaged ancient structural members or even for the complete reconstruction of missing ones. Indeed for reasons beyond the will of the scientific committee responsible for the restora- tion project of the Epidaurean monuments, it has been proved impossible to obtain fresh authentic stone from the wider area of the ancient quarries [2] and therefore the use of substitute stones appears to be indispensable. The methodology adopted in Part I was based on the use of composite specimens, either cylindrical or prismatic, consisting of equidimensional parts of two different stones, subjected to unconfined uniaxial compression. It was proved that certain con- figurations exist for which the incompatibility of the constituent parts could lead the authentic stone to premature failure violat- ing basic restoration principles. In this context certain geometri- cal configurations of the boundaries of the specimens that could modify the mechanical behaviour of building stones (making them as compatible as possible with the authentic material) are examined in this Part II. This approach is based both on existing experimental results and in situ observations and also on numer- ical analyses according to which the mechanical behaviour of a cylindrical stone specimen under unconfined compressive load- ing is crucially affected (at least in the post-peak region of the axial stress-axial strain diagram) by the geometrical configura- tion of its contact – as well as its free – boundaries [3, 4]. 2. Experimental procedure 2.1. The experimental set-up Series of uniaxial compression tests were carried out with the aid of a stiff hydraulic Amsler loading frame of capacity 1000 kN. Taking into account that the maximum load did not exceed in any case 45 kN, it is concluded that the stiffness of the frame can be considered infinite, which is of vital importance if the post-peak behaviour of the material is to be studied. The load was applied statically at a rate not exceeding 10 -2 mm/min. It is to be emphasised here, that the loading rate is a very impor- tant parameter in testing natural building stones and geomateri- als in general. The results of a series of commercially tests for the same materials but at higher strain rates [5], listed in Table 1 , are very far from the results of the quasi-static tests described in previous paragraphs and listed in table 3 of Part I [1]. AUTHOR’S COPY | AUTORENEXEMPLAR AUTHOR’S COPY | AUTORENEXEMPLAR