Ultrasonic wave velocity measurements for detecting decay in carbonate rocks

Abstract

Ultrasonic testing is a non-destructive technique commonly employed for assessing indirectly some geomechanical parameters of natural building stones. Pulse wave propagation in rock materials depends not only on physical properties, e.g. density, elasticity, porosity and water content, but also on fabric elements and their arrangements, spatial distributions and preferred orientations. In contrast, ultrasonic velocity can reflect the state of conservation of stone by comparing data from fresh material with those obtained from tests made on decayed material. The ‘Avorio’ limestone is a valuable building and ornamental material, quarried in the Trani stone basin (Apulia, southern Italy), which is very popular in Italy and abroad where it is used in modern and historic buildings. The stone response to decay processes is not fully understood simply because this material has not yet been well studied. To better understand how durable this material is in terms of physical resistance to change with respect to salt and thermal deterioration mechanisms, selected samples of the ‘Avorio’ limestone were submitted to artificial ageing processes by thermal stresses and by stresses due to salt crystallization, performed independently. The artificial ageing processes involved thermal cycles at temperatures between 200 and 600°C, based on an experimental procedure, and a standardized salt crystallization test in sodium sulphate solution. Repeated measurements of pulse velocity, before, during and after every accelerated ageing cycle, were compared with mass loss and porosity changes, the latter by water imbibition and capillarity testing. Very statistically significant relationships between physical response and rates and patterns of the stone decay were illustrated. The experimental results showed that the ultrasonic pulse velocity decreases as deterioration of the stone increases under accelerated ageing: a strong negative correlation exists between P-wave velocity and porosity. This study confirmed that ultrasonic pulse velocity is a good indicator of damage for carbonate stones, particularly for varieties without clear anisotropies, in terms of structure and texture.