A non-destructive multimethod approach to the diagnostic evaluation of masonry pillars

The paper presents the results of a multi-method test conducted on a pillar of the portico of the basilica of San Giorgio in Velabro in Rome, for the evaluation of conservation and structural conditions. The portico of San Giorgio in Velabro has a conservation history made of construction events, periods of abandonment and traumatic events. Added to the façade of the basilica in the mid-thirteenth century, it underwent transformation works in the seventeenth century and punctual restoration works in the nineteenth and twentieth centuries. In 1993, a terrorist attack, carried out by the explosion of a car bomb, caused the almost total collapse of the portico. The pillar chosen as case study for the experimentation has not collapsed, but nowadays it has a passing crack visible on both sides. The experimentation conducted is part of the research activities of the national project CHANGES, Cultural Heritage Active Innovation for Nex-Gen Sustainable Society. In this case, it is included in the activities of Spoke 5, Science and technologies for sustainable diagnostics of Cultural Heritage, which includes, among its activities, the design of multimodal and multi-scale diagnostics. The diagnostic design developed for this case study is based exclusively on the use of non-destructive methods: ultrasonic tests, GPR and thermography. The aim of the experimentation is to verify the data obtainable from each investigation method and their overlapping in order to have the most complete interpretative dataset on the internal conditions of a masonry pillar. The instrumentation used in the experimentation is supplied to the AStRe-LabMat DSDRA laboratory of Sapienza University of Rome and consists of: Boviar All in One System for sonic and ultrasonic tests, Proceq GP800 Ground Penetration Radar, FLIR T540 Thermal camera. The ultrasonic tests were conducted in transparency and tomographic mode at four different heights. The GPR scans, on a vertical surface, were conducted in "area scan" mode on the surfaces corresponding to the greatest extent of the crack. Thermographic images were conducted on all surfaces of the pillar under examination. The results of the investigations highlight three different types of information that can be obtained from the individual non-destructive diagnostic methods, which integrated together provide a broader framework of knowledge of conservation conditions. Ultrasonic investigations provide precise locations of the internal detachments between the portions of masonry. The GPR survey allows the geometry of the detachment to be identified and visualized by software processing. The thermographic images show thermal phenomena due to surface deformations related to the inner dislocations. These results allow to elaborate a diagnostic procedure based on a non-destructive multi-method approach for the on-site study and evaluation of masonry pillars.