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WATER CONTENT ESTIMATION USING NON-DESTRUCTIVE TOOLS APPLIED TO ARCHAEOLOGICAL MATERIALS
Oriol Sánchez Rovira1, 2, 3, 4, David Giovannacci1, 2, Jean-Didier Mertz1, 2, Jérôme Wassermann3, Béatrice Ledésert4, Ronan Hébert4, Yannick Mélinge3
IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.
– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –
VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.
1 LRMH, Laboratoire de Recherche des Monuments Historiques, Ministère de la Culture et de la Communication, 29 rue de Paris, 77420 Champs-sur-Marne, France
2 Centre de Recherche sur la Conservation (CRC), Muséum national d’Histoire naturelle, CNRS, Ministère de la Culture, 36 rue Geoffroy Saint Hilaire, 75005 Paris, France
3 L2MGC ; Laboratoire de Mécanique et Matériaux du Génie Civil-EA4114, CY Cergy Paris Université, 5 mail Gay Lussac, Neuville sur Oise 95031 Cergy-Pontoise, France
4 GEC, Laboratoire Géosciences et Environnement Cergy, EA 4506, CY Cergy Paris Université, 1 rue Descartes, Neuville sur Oise 95031 Cergy-Pontoise, France
Abstract
Water content in stone is of primary relevance for the preservation of cultural heritage. High water content promotes the development of microorganisms and causes mechanical or physico-chemical alterations by swelling/shrinkage or dissolution/recrystallization of salt. The identification and then the control of the water transfer remain important to assess the risk of damage. A good prediction of the water content helps to develop some preventive action dedicated to the conservation of Heritage.
The most accurate methods to measure water content require sampling and can only exceptionally be used. Few methods are non-destructive, but their accuracies are often limited.
The aim of this study concerns the application of Non-Destructive Technics (NDT) to determine the water distribution within the building masonry, in particular the infrared imaging and electrical method.
The electrical method enables to image the spatial and/or temporal variation of the electrical properties related to the water content distribution in the materials while infrared thermography provides the boundary limit conditions by the means of surface thermograms. Those methods are performed for several samples used as building materials in the archeological site of Vaux de la Celle (Genainville, France). The site concentrates various structures built between the 2nd and 4th century AD. The temple structure is constructed at the lowest part of the valley with its foundations in direct contact with the vadose zone where the water table fluctuates. Reproducibility and reliability are also provided through several experimental configurations.
Introduction
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
