Thermo-hygric weathering of Carrara Gioia marble monitored with Nonlinear Resonant Ultrasound Spectroscopy

Carrara marble is widely used, in buildings and sculptures, in ancient and recent works. Yet, it can develop deteriorations over time, such as bowing, cracks, expansion, reduction of mechanical strength, when it is exposed to environmental conditions. Previous studies have shown that these deteriorations can result from exposure to temperature variations, and that they can be enhanced by additional humidity variations. However, the mechanisms at stake at the microstructure level are still not well-understood.

This work is therefore focused on the understanding of the degradation mechanisms induced on marble by temperature and humidity fluctuations. Laboratory experiments are carried out on Carrara marble samples first heated at different temperatures and then undergoing thermal, hygric, and thermo-hygric cycles. The temperature investigated during cycling belongs to the mild temperature range (40 – 105 °C) in order to simulate outdoor exposure conditions.

The mechanical state of Carrara marble samples is non-destructively monitored during the cycles with Nonlinear Resonant Ultrasound Spectroscopy (NRUS), through the evolution of resonant frequency and of nonclassical nonlinearity. The first parameter is related to sample stiffness, and the latter is highly sensitive to any change occurring at the microstructure level (micro cracks, friction, capillary effects, etc.). Additionally, microstructural characterization (mercury intrusion porosimetry, optical microscope and SEM observations) is made on marble samples to link the evolution of the NRUS parameters to the changes occurring in marble microstructure.

The impact of heating on marble is first studied for temperatures between 40 and 250 °C, and the progressive granular decohesion of the material is monitored with NRUS. Marble state is also characterized during adsorption-desorption cycles, which shows that relative humidity fluctuations alone do not induce permanent damage. Finally, the influence of thermal cycling at mild temperatures and the impact of combined temperature and relative humidity cycling are studied with NRUS.