Elsevier

Engineering Geology

Volume 185, 5 February 2015, Pages 71-80
Engineering Geology

Critical degree of saturation: A control factor of freeze–thaw damage of porous limestones at Castle of Chambord, France

https://doi.org/10.1016/j.enggeo.2014.11.018Get rights and content

Highlights

  • Microclimatic and frost data of limestones of Chambord castle, France were analyzed.

  • In laboratory, limestone samples were saturated at various degrees of saturation.

  • Petrophysical and mechanical properties were studied after 50 freeze–thaw cycles.

  • Freeze–thaw cycles applied has lower influence on frost damage than supposed.

  • Critical degree of saturation of limestone controls the frost damage.

Abstract

The paper analyzes the petrophysical and mechanical properties of two porous limestones that were used in the construction and restoration works at the castle of Chambord in France, a UNESCO World Heritage site. The original construction material, the tuffeau limestone with a total porosity of 45 ± 0.6%, and the replacement stone of later restorations, the Richemont limestone with a total porosity of 29 ± 0.7% were subjected to freeze–thaw tests under laboratory conditions to evaluate the role of critical degree of saturation and pore-size distribution in frost damage. Laboratory tests were coupled with in situ measurements of temperature and relative humidity at stone surface at the castle of Chambord. In situ data show that the stones in the castle experienced several freezing–thawing cycles annually. The limestone samples under laboratory conditions were subjected to up to 50 freeze–thaw cycles under eight different degrees of saturations. The total porosity, tensile strength, ultrasonic pulse velocity, the mercury intrusion porosimetry and scanning electron microscopy techniques were employed to analyze the conditions of samples during the cycles. The experimental results show that when the degree of saturation of the two studied limestones exceeds 80–85%, the freeze–thaw, damage occurs even after a few freeze–thaw cycles. The effect of freezing is very fast if the water saturation is sufficient. Moreover, results indicate that these stones have the same critical degree of saturation of about 85%, despite the differences in porosity. Finally, the results indicate that the increase in the number of freezing–thawing cycles has no effect on the critical degree of saturation, but the frost damage is mostly controlled by pore-size distribution rather than by total porosity. Accordingly, critical degree of saturation can be defined as an intrinsic stone property.

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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