Thermal stress and weathering of carrara, pentelic and ekeberg marble

Abstract

This chapter describes laboratory studies for the formation and growth of micro-cracks in marble. It focuses on findings of the Swedish group involved in the EU funded HERMES project that investigates the deterioration of marble because of many natural causes. Swedish Ekeberg, Italian Carrara, and Greek Pentelic marbles have been studied using four-point bending in both air and water environments, at different loading rates. The single edge notched beams (SENB) were prepared for samples testing. It has been shown that a water environment surrounding the tested marble bars led to the decrease in the apparent fracture toughness. Also, the apparent fracture toughness decreases with decreasing loading rate. The existence of a large process zone full of micro-cracks and crack branching from primary crack, starting from a precut notch, is thought to reduce the fracture toughness value. The laboratory results give evidence of environmentally assisted slow crack growth along the grain boundaries. Thermal gradients and temperature fluctuations cause high stress, particularly in calcitic marbles like Carrara marble. Conservation practices should be directed towards reducing temperature fluctuations in a marble object. The sheltering of sculptures in the winter with wooden constructions is beneficial in reducing the number of freeze-thaw cycles that the object encounters and reduces weathering damage.