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

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.