Acoustic emission monitoring of frescos degradation in a XVIIth Century chapel of the "Sacred Mountain of Varallo" (Italy)

The present paper describes the results of the Acoustic Emission monitoring, performed for five months in order to assess the structural behaviour and the frescos degradation of a XVIIth Century Chapel belonging to the Sacred Mountain of Varallo (Italy). The Sacred Mountain of Varallo is composed of 45 Chapels, some of which are isolated, while others are part of monumental groups, containing over 800 life-size wooden and multicoloured terracotta statues, which represent the Life, the Passion and the Death of Christ. The site is considered the most notable example in the group of Sacred Mountains of Piedmonts, a complex that has been included in the UNESCO World Heritage List since 2003. The Chapel number 17, dedicated to the Mount Tabor episode of Christ Transfiguration, shows some structural concern due to an existing large crack and some degradation of the high valuable frescos, which tend to detach from the masonry support. In order to assess the evolution of the phenomena, an Acoustic Emission monitoring apparatus was used, with six sensors that enable the recording of the signals and localization of the events. In addition, a detailed survey of the Chapel was provided, together with a structural finite element simulation. The results of the monitoring show that the large crack is stable, while the process of detachment of the frescos is evolving cyclically. It seems that the frescos degradation could be mainly related to the diffusion of moisture in the mortar substrate. Some preliminary laboratory tests also confirm that Acoustic Emissions are recorded in mortar samples subjected to moisture diffusion.