Critical degree of saturation: A control factor of freeze–thaw damage of porous limestones at Castle of Chambord, France
Graphical abstract
Introduction
Frost is one of the main causes of the damage in cultural built heritage in cold regions. The durability of stone structures against frost strongly depends on hydro–physico-mechanical parameters. It was demonstrated that intrinsic properties of the stones like total porosity, pore connectivity, pore size distribution, mechanical strength, mineralogy, grain-size, and environmental conditions affect both the stone's durability and mechanism of frost weathering (Mutlutürk et al., 2004, Yavuz et al., 2006, Takarli et al., 2008, Tan et al., 2011, Bayram, 2012, Jamshidi et al., 2013). Most of the previous experimental works dealing with the deterioration of stone under freezing–thawing conditions were performed on fully water saturated samples. However, natural stones are almost never fully saturated. Consequently, in order to understand the mechanisms of stone damage and simulating the real field problems, experiments with stones having various water contents are necessary (Matsuoka, 2001). It was also pointed out that effective microgelivation requires an initial degree of saturation in excess of 80% and it is followed by rapid freezing. However, Matsuoka (2001) also emphasized that rocks can uptake water during slow freezing, and thus, for a frost damage, a high initial water content is unnecessary. The role of porosity in the durability of porous stones have been studied in details taking into account the salt weathering susceptibility (Benavente et al., 2004, Yu and Oguchi, 2010) and pore structure (Benavente et al., 2001). The frost damage of porous materials was also explained by the critical degree of water saturation (Fagerlund, 1977a, Fagerlund, 1977b). Sulfate attack is also considered as one of the main causes of damage observed on limestone buildings (Török, 2003, Siegesmund et al., 2007, Kloppmann et al., 2011), however this research focuses on other aspects of limestone decay. This paper provides information on the mechanism of freezing–thawing related to stone deterioration by using the example of the castle of Chambord in France. It uses two approaches: i) the in situ monitoring of stone surface temperature and meteorological data in order to identify the risk of damage by freezing to two limestones, the tuffeau and Richemont, that were used in the construction and restoration of the castle, and ii) laboratory experiments aiming to determine the critical degree of saturation and pore-size distribution that triggers the freezing damage of these two stones. Stones used in the construction of monuments such as Chambord castle can gradually deteriorate over a long period of time in response to the action of water and local environmental conditions. The deterioration in the castle of Chambord belongs to three main categories: biological colonizations (mosses and lichens), spalling (centimeter-thick) and flaking (millimeter-thick). The factors leading to these deteriorations have never been defined precisely.
Section snippets
The castle of Chambord and its building stones
The Royal Castle of Chambord at Loire Valley, in France, a UNESCO World Heritage site since 1981 is located in a rural area at a distance about 150 km to SW of Paris, and at latitude of 47°36ʹ N, and longitude of 1°31ʹ E. Its average elevation is about 84 m above sea level (Figure 1). The area experiences a mild humid temperate climate with warm summers and no dry seasons.
The castle of Chambord is the largest castle in Loire Valley (155 m × 115 m) built between 1519 and 1547; and the main building
The studied stones
Two French stones were presented in this study: tuffeau and Richemont stones. Tuffeau is a soft-porous stone and dates from the Turonian age, the upper Cretaceous period, approximately 88–92 million years ago. It comes from the quarries at Tuorain/Anjou close to Loire river (NW France). It is used in many numbers of the castles in Loire Valley-France because of its light weight, special esthetics with shine white and easy to form. Richemont is a fine-grained limestone that has the same
Climatic data
In this study, the freezing events on the stone surface and in the atmosphere were identified by analyzing the data measured in stones by using the sensors, and the meteorological data recorded at Bricy air-Base station for the periods 2009–2012. The statistical analysis of the meteorological data recorded at Bricy air-Base station during 1973–2012 and 2009–2012 suggests that the trends during the two different periods are similar (Figure 5). Therefore, the data acquired for an annual period
Discussion
The results of freeze–thaw tests of the two limestones show that the signs of frost damage are not visible on samples having water saturation of less than 85%. However, indirect tensile strength values show minor changes when air dry samples and moderately saturated (up to 80%) freeze–thaw subjected samples are compared (Figure 14). Indirect tensile strength results presented in the figure provide data for 4 to 6 samples for each degree of saturation. The minor changes in the tensile strength
Conclusions
This study of porous limestones suggests that field observations of both ambient air temperatures and stone surface temperatures in combination with relative humidity data are needed to identify dew temperature at stone surfaces and to determine the availability of moisture within the pores. The moisture content has a crucial importance with regard to freeze–thaw damage of stones. The moisture in the pores of the stone is also related to precipitation events or elevated ground water, however
Acknowledgments
The authors acknowledge the financial support provided by Université d'Orléans for the visiting research period (June 2013) to Ákos Török to Centre de Recherche sur la Matière Divisée (CNRS-CRMD).
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