The scratching test, an alternative technique for determining strength properties of historical mortars

The knowledge of mechanical properties of materials is fundamental for solving short and long term mechanical stability problems. As the main problem consists in collecting the data, many investigators have been encouraged in finding alternative simple techniques to assess mechanical properties. A simple scratching method to determine the strength of rock samples has been developped at University of Minnesota around ten years ago and have been until now widely used for petroleum engineering applications. The original testing apparatus was referred to as the Rock Strength Device and several papers have been presented in the literature to describe the methodology allowing to assess strength of petroleum rocks with scratching tests. The initial scratching methodology has been studied and improved in the framework of researches conducted in the Department of Mining Engineering at the Polytechnic Faculty of Mons. The new optimized methodology appears to become an interesting alternative technique which allows assessing the strength of geomaterials, stones and artificial rocks (concrete, ...) This paper describes first the basic principles associated with the scratching test and the interpretation of collected data in term of strength. Then, it summarizes results obtained by the scratching methodology on different types of natural stones commonly used in heritage buildings in Belgium. Finally, the paper discusses the results of a preliminary test programme carried out on cement and hydraulic lime mortars and outlines the importance to take complementary parameters into account when considering these kind of materials. Finally, some practical aspects of the scratching method applied in a recent research programme concerning the mechanical behaviour of historical lime mortars are detailled