3D internal reconstruction of the case study. Laboratory Investigations

Abstract

The main objective of the Heritage Within (HWITHIN) research project is to investigate what is hidden to our naked eye. One of the core actions of the project was to explore the use of Ground Penetrating Radar (GPR) and ultrasonic acoustic tomography to document and reconstruct precisely the interior of the constructive elements, almost on a stone-by-stone basis. However, precisely because it is not visible, the results obtained and the conclusions drawn cannot be verified with the reality. In the case of the Carmo Convent, the structural elements under investigations are the neogothic compound piers of the main nave, reconstructed after the 1755 earthquake. There is insufficient information on the layout of the interior morphology of the pillar’s cross-section. Additionally, finding relatable examples to compare to the compound pillars at the Carmo Convent are quite minimal, making it complicated for even initial assumptions to be established. Therefore, the team decided to construct 1:1 stone replicas of parts of the neogothic stone masonry pillars (the shaft and the pedestal). The specimens were used to assess the feasibility and efficiency of the non-destructive tests applied (GPR and acoustic tomography) to determine the layout of their internal morphology. Since the cross- section of the real pillars of the Carmo Convent is unknown, the replica only precisely depicts the exterior layout of the columns. The shapes of the stones were established on initial guesses based on literature review. Once the limestones were cut, several cross-sectional set-ups were prepared, mainly accounting for a wide range of possible interior core materials (from fully empty core to a solid limestone block filling). Acoustic tomography was carried out for each assembly so that accurate comparisons between the tomographic maps could be made. Moreover, given the known inner composition of the laboratory specimens, the results can help to understand the potential of the tomographic techniques by comparing the results with the expected visible interior. The present chapter shows some of the results obtained during the laboratory investigations, which were essential to interpret the ultrasonic tomography performed on-site at the old convent. The results confirm the potential of acoustic tomography techniques to convey an accurate idea of the cross-sectional elements within the pillar. The tomographic maps may not provide enough precision to determine exact dimensions of the materials and elements within the cross-section, but they can be complemented with GPR investigations and is always useful in a qualitative way, as it can provide information about the structural integrity of the materials and their arrangement.