Читать книгу Monument Future - Siegfried Siegesmund - Страница 381

The damage of the stone building materials is closely related to change in the equilibrium between the stone and the atmosphere. Thus, instabilities introduced by the environmental variations are the driving force of stone damage. In such cases, the biggest threats to the stone are related to those 252cyclic factors (Benavente et al., 2008), which are related to water and heat transfer.

The patterns of stone deterioration (ICOMOS ISCS, 2008) depend on the nature of the material and the weathering processes and are mainly linked with water content in its different aspects as its spatial and temporal distribution.

The most common methods used to characterize water content and distribution within the porous materials need sampling in order to determine it by gravimetry (EN 16682, 2017). However, for heritage building, due to the invasive nature of the direct measurements approach it is better to use indirect non-destructive methods to characterize the water content.

Indirect methods analyze the variation of a physical property and/or quantity of the materials which can be exploited to characterize moisture content. Nuclear magnetic resonance (NMR), evanescent-field dielectrometry (EFD) (V. Di Tullio et al., 2010), infrared thermography and electrical resistivity surveys are the most common indirect non-destructive methods of sensing the water content of porous media.

Infrared thermography allows to image the temperature map of the surface of the materials. Depending on endogenic or exogenic conditions, the temperature of the damped areas may vary from the dried parts. These thermal behaviors make the infrared thermography a powerful imaging method to make qualitative measurements of water content distribution. However, because the relationship between temperature and water content is highly affected by the material properties and the environmental conditions, quantitative measurements need calibration curves generated through controlled laboratory conditions (Grinzato et al., 2011).

Resistivity methods are mostly used in geophysical survey for geological and archaeological applications. However, since they monitor the resistance of the material to the passage of an electric current, it can be applied to characterize water content in porous building materials. Indeed, this resistance is directly influenced by water content, its salinity, its temperature, as well as by its distribution within the pore network (Hassine et al., 2018). As for the infrared imaging method, due to the complex relationship between the different parameters affecting the resistivity measurements, quantitative analysis needs prior calibration data.

Thanks to resistivity imaging methods providing volumetric information, and infrared thermography providing information from surface, the combination of the two methods can provide complementary information to characterize water content and its distribution.

The work presented in this paper is dedicated to highlight the complementarity of the infrared and electric imaging methods. Such methods are used to characterize water content variations in limestone from an archeological site. The final goal is to establish the basis of a non-destructive method-based water content characterization protocol in situ.