Continuous Monitoring of Hydric Deformation in Macigno Sandstone After Thermal Conditioning

Hydric deformation can contribute to progressive damage in building stones, particularly when moisture-induced strains interact with pre-existing fabric anisotropy and thermally induced microstructural changes. Six Macigno sandstone prisms (20 mm × 20 mm × 200 mm), three with the longitudinal axis orthogonal to bedding (MTO) and three parallel to bedding (MTP), were examined after drying at 60 °C and after sequential thermal conditioning at 350 and 500 °C. At each stage, hydric deformation was continuously monitored during 8400 min of water immersion, and at the end of each test, water absorption and ultrasonic pulse velocity were measured on the samples. Mean final hydric deformation in the 60 °C reference state was 0.54 mm m−1 for MTO and 0.44 mm m−1 for MTP. Along the sequential conditioning path, it decreased to 0.18 and 0.10 mm m−1, respectively, at 500 °C, whereas water absorption increased from about 0.42 to 0.58–0.59 wt.%. Ultrasonic pulse velocity decreased from 4347 to 3915 m s−1 for MTO and from 4770 to 4437 m s−1 for MTP. Bedding-related anisotropy persisted, while water absorption and hydric deformation followed divergent trends. Continuous acquisition further showed that the general temporal pattern of rapid initial deformation followed by a more gradual approach to a near-stable response was preserved after thermal conditioning, although the initial deformation rate decreased systematically.