The indentation technique is tailored here to the needs of a portable tool for in situ diagnosis of mechanical properties and damage of natural building stones. Indentation tests were performed in twelve natural building stones (chalks, limestones, sandstones, marbles) and mortars used for restoration. A wide range of mechanical and petrophysical properties with different failure mechanisms in indentation is thus represented. Indicatively, the Unconfined Compressive Strength (UCS) ranges between 3.1 MPa and 116 MPa, and the tangent Young's modulus E50 at 50% of the maximum stress in uniaxial compression ranges between 0.9 GPa and 50 GPa. The tests were performed with three indenter diameters, 1-, 2- and 3-mm, to analyze scale effects in the results. Tests were performed at five different depths, as the technique will be used not only for surface measurements but also for measurements in the interior (tests at the bottom of a small drilled hole). Such measurements can provide information on stone damage with depth. The results are used to build correlation functions and databases between indentation parameters and stone stiffness and strength. The technique is applied to two marbles that had been artificially weathered with exposure to moisture and temperature cycles, and to a consolidated mortar, i.e. a mortar treated with a consolidant for improving its weathering characteristics.
Monumental structures represent our cultural and historical heritage. However, damage (weathering) of historical buildings, monuments, works of art and other cultural properties due to the aggressive urban environment of the last decades and the ambient climatic conditions, is reported from all over the world. This has resulted in a substantial effort from local and governmental authorities and the industry for quick and suitable measures for the preservation of national monuments. Several fundamental questions arise in the decision making of the optimum restoration and preservation strategy of old monumental building stones. Among others, these are questions on stone suitability, effectiveness of consolidation measures, and in situ quantitative measurements of the degree of damage of a part or element of a monument or building. Weathering of natural building stones is also of considerable practical interest to the quarrying and the construction industries.
An effort has therefore been undertaken to develop a portable tool that has been missing from the industry for the in situ, quasi non-destructive, reliable and accurate diagnosis of mechanical properties and damage of natural building stones in structures of cultural heritage for the improvement of preservation strategies.
The work builds on previous work by the authors and others on the determination of mechanical properties and damage through indentation testing. The indentation technique is a common method for measuring hardness in metals, glass and ceramics and also to measure the elastic parameters of surface coatings [e.g. 1-3]. In rocks, emphasis has been placed in the search for correlations between indentation measurements and rock parameters such as rock strength and stiffness [e.g. 4-9]. For our purpose the indentation technique is tailored to natural building stone applications.
2. STONE CHARACTERIZATION TESTS
A mechanical and acoustical characterization of 12 common building stones was performed to obtain their basic mechanical and acoustical properties.