Contribution of Linear and Nonlinear Acoustic Methods to the Evaluation of Consolidation Treatments on Thermally Decayed Carrara Marble

Cultural objects made of marble are highly sensitive to environmental alteration processes. Specifically, thermal fluctuations cause a microstructural decohesion phenomenon that progressively leads to the granular disintegration of exposed surfaces. The conservation of such decayed objects can only be improved through the application of consolidation treatments. Yet, the evaluation of their effectiveness and durability remains limited to surface level or requires destructive testing. To date, only ultrasonic pulse velocity (UPV) is used to nondestructively measure macroscopic cohesion gain. However, this technique cannot detect microstructural changes in the granular assembly. To overcome this limitation, a combination of linear (UPV) and nonlinear (nonlinear resonant ultrasound spectroscopy, NRUS) acoustic methods was applied to assess the impact of thermal decay and further consolidation with ethyl silicate, diammonium hydrogen phosphate, and biomineralization of Carrara marble. The durability of those treatments was then followed along artificial cycles of thermo-hygric conditions. Overall, significant cohesion gains (>50%) were observed in the consolidated samples, suggesting the effectiveness of all the considered treatments, even though their performance (DAP > TEOS > biomineralization) varied significantly. However, the definition of the nonlinear parameter from NRUS highlighted their contrasting capacity to bridge microcracks crossing the granular assembly and sustain those bridges durably. This study thus highlights the importance of combining classical acoustic techniques with nonlinear ones to decouple macroscopic cohesive gains from microcracks bridging. In this respect, consolidation treatments must satisfy both criteria to meet the requirements of durability expected for the preservation of cultural objects.