A multi-analytical approach to assess potential damaged areas on the building materials of monumental structures.

The integrated studies within the field of non-invasive diagnostics for the characterization of the state of conservation of stone building materials of monuments have the common target to meet the current and future needs of society in the field of cultural heritage. The conservation of the built heritage requires many interventions aimed at analyzing the state of health of the monuments, their preventive preservation, reconstruction, and restoration. All actions and diagnostic analyses should respect the historical significance of the investigated monuments and the physical properties of the materials that make up the cultural heritage structures. In this context, the use of non-invasive diagnostic techniques of various nature (e.g. terrestrial laser scanner (TLS), digital photogrammetry, acoustic, electrical) plays a role of fundamental importance both in the preventive preservation and in the monitoring of monumental structures over time. In fact, the use of such techniques is also particularly effective in controlling the effectiveness of restoration interventions.

In this study we have examined different architectural elements (walls and semi-columns) of the Basilica of San Saturnino relevant monument in the historical centre of the town of Cagliari (Italy). The analysis has been carried out by different geomatic (TLS and digital photogrammetry) and geophysical (acoustics and electrical) techniques. The combined application of digital photogrammetry and terrestrial laser scanning can provide high-resolution 3D models calibrated and textured with both reflectance and natural colours useful for evaluating the state of conservation of surface materials and for rationally planning further geophysical analyses, particularly the acoustic ones carried out with tomographic methods. In this study the acoustic techniques applied in the ultrasonic range have been used essentially in two modes, namely: surface and transmission. The obtained 2D models adequately describe the longitudinal velocity distribution both on the shallow parts of the investigated old walls and on the internal section of different architectural elements (semi-columns) showing the influence of the variation of intrinsic rock properties on the ultrasonic longitudinal wave propagation. The electrical resistivity data acquired on the surface of the old walls have allowed to depict the resistivity distribution on the wall surface. Electrical resistivity is a physical property of a rock that characterizes its conductive properties. For porous media such as the building carbonate rocks under study, the electrical resistivity depends on many physical properties (i.e., internal structure, water content, fluid composition and porosity) and together with the elastic properties can be useful to interpret the various properties of the investigated materials and their conservation state. In conclusion, the non-invasive techniques applied in this study can effectively aid the restoration of the building materials at Basilica di San Saturnino, Cagliari, Italy. This study confirms how each technique gave a range of different information to the restorers, and the interventions that can be undertaken in light of the acquired knowledge of the investigated monument.

 

Acknowledgements: The authors would like to thank the Ministero della Cultura- DIREZIONE GENERALE MUSEI - DIREZIONE REGIONALE MUSEI SARDEGNA (ITALY) for their kind permission to work on the San Saturnino Basilica (Cagliari – Italy).