Silvana FAIS(1,2) / Paola LIGAS(1) / Marcella PALOMBA(3)
(1University of Cagliari, Italy / 2Consorzio Interuniversitario Nazionale per l’Ingegneria delle Georisorse (CI.NI.Geo), Roma, Italy / 3Istituto di Geologia Ambientale e Geoingegneria del C.N.R. – Sezione di Cagliari, c/o Facoltà di Ingegneria, University di Cagliari, Italy)
Abstract:
Integrated in-situ Non-Destructive (ND) ultrasonic techniques and mineral analyses were carried out to assess the conservation state of masonry structures in a monumental building. In this experimental study, in situ measurements of ultrasonic longitudinal pulse velocity with different natural frequency (24-54 kHz) of the transducers were carried out to detect the elasto-mechanical conditions of the building materials. Ultrasonic measurements by surface or indirect transmission (transmitter and receiver along the same face of the investigated masonry wall) were performed using the “step by step” and refraction modalities. These different acquisition modalities were very effective both in detecting the shallow altered areas of the building materials (step by step modality) and in defining the thickness of the alteration (refraction modality). Based on the results of ultrasonic investigations, a sampling was planned in the critical sectors. Optical studies and X-Ray Diffraction (XRD) technique were applied on several microsamples, collected from both mortar and building stones, to determine the mineral composition and intensity of alteration of the materials. The mortar shows three different piled up layers. From the most shallow to deep layers of the mortar, the mineralogy consists of main and accessory primary minerals, and newly-formed minerals. The primary minerals are calcite, quartz, plagioclase, K-feldspar, which occur in different proportion in each mortar level; gypsum and illite occur in different proportion in the different layers, as newly-formed minerals. The building stones are generally limestones, mainly composed by calcite and minor K-feldspar and quartz; gypsum and illite occur in traces. The analysis of the available mineralogical data shows that the most altered zones, independently from type of material, are generally characterized by the presence of gypsum and illite. Mortar and limestone blocks, where lower acoustic velocities were detected, represent the most altered materials.
The integrated ultrasonic and mineral analyses show that the alteration phenomena mainly affect the mortar and the shallow part of the limestone blocks, generally waterlogged in the rainy seasons. These phenomena are enhanced not only by mineralogical composition and grain size, but also by the great difference in the elasto-mechanical characteristics among the different layers of the mortar and the shallow part of the stone building materials.
Keywords:
non-destructive ultrasonic techniques, mineral analyses, masonry structures, monumental buildings