On-site determination of compressive strenght for historical mortars with the scratching test: method and preliminary results

Since the civil authorities have understood the necessity to preserve old heritage buildings, important budgets are allocated to finance heavy restoration campaigns. For historical masonry buildings affected by structural pathologies, the restoration campaigns have to be preceded by stabilisation works. In order to precise the structural behaviour of such constructions and to propose effective solutions for the future, the engineers need a correct knowledge of the constitutive materials (stone, mortar, ...) Several on-site determination techniques exist, showing their own advantages and disadvantages. This paper presents a scratching device, daily used by the petroleum industry, for the measurement of mechanical properties of rock materials. The scratching test has already shown its suitability in that field and the paper proposes to extend it for the on-site determination, from a small piece of mortar, of the uniaxial compressive strength - UCS. First, it presents the method : the data recorded during a scratching test are interpreted thanks to a phenomenological model in order to compute the intrinsic specific energy - ? - of the material, which is correlated with the UCS. Afterwards, the paper presents the results of a preliminary calibration campaign performed on lab mortars in order to determine the relation between?? and the UCS. Finally, it depicts the application of the technique in the framework of stabilisation works on the Our-Lady cathedral of Tournai (UNESCO World Heritage). The preliminary results are encouraging and further researches are in progress.