The problem of mechanical compatibility of natural building stones in the restoration of monuments. Part II: Specimens with modified boundaries

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 configurations of the boundaries of the specimens are examined in this Part II as a possible means of modifying the mechanical behaviour 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 advantage of the experimental results an alternative compatibility criterion 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 indeterminate 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.