UPV testing for decay and moisture detection in stone building materials: insights from lab measurements

Stone building materials have been widely employed in historical-architectural heritage worldwide, with local peculiarities, construction techniques and surrounding conditions, resulting nowadays in a great variety of decay patterns and pathologies, so that the reliable assessment of their state of conservation and residual performance is paramount in order to address effective and compatible conservation measures. In this regard, Ultrasonic Pulse Velocity (UPV) is acknowledged as a valuable tool for onsite investigation, in terms of overall quality and compactness, evidence of decay phenomena, and effectiveness of consolidation treatments. However, since the availability of laboratory data under controlled conditions is crucial for the interpretation of results from real cases, the paper is focused on a laboratory workflow for UPV testing on different types of sedimentary limestones from the main quarry basins of Apulia Region, South Italy. The main purposes were: (i) acquiring reference data that can support the interpretation of onsite measurements, particularly for detection of cracks, voids and discontinuities, that are expected to lower the velocity compared to undisturbed samples; (ii) identifying reliable relationships between the ultrasonic velocities and some physical/structural properties, including density, porosity and mechanical resistance, enabling the estimation of the expected ultrasonic response for further stone typologies of the same geographic area; (iii) verifying the correlation between the ultrasonic values and the variation in moisture content. The main results included the availability of reliable UPV values for all the eight investigated stone typologies, the identification of very good correlation trend between UPV and density and the good correlation trend between UPV and compression strength, as well as the evidence that the UPV variations from dry to saturated conditions are significantly different for low (UPV decrease) and high (UPV increase) porosity materials. However, the measurements did not lead to meaningful insights on the correlation between UPV velocity, water content, compression strenght and porosity. In the light of the above, the paper is meant to offer some useful insights in terms of data availability, as well as of methods and tools to be replicated in future studies, in order to make onsite UPV ultrasonic testing of architectural stone elements – especially monolithic ones, such as columns, pillars, architraves, cornices – more effective and trustworthy.