Differential Stone Decay of the Spanish Tower Façade in Bizerte, Tunisia

AbstractThe Spanish Fortress of Bizerte in Tunisia shows differential erosion patterns on the rock ashlars used in the construction of its main façade (sixteenth century) exposed to marine aerosol action and several restoration works. In order to determine the origin of this erosion and the degree of stone decay, a combination of microdestructive and nondestructive techniques have been used on-site and in the laboratory. Moisture measurements, ultrasonic velocity propagation, and water absorption by a Karsten pipe test, together with polarized light and fluorescence optical microscopy, mercury intrusion porosimetry, and ion chromatography analyses, were carried out to perform petrophysical characterization of stone samples and determination of soluble salts. Results show that the differential stone weathering is caused by small variations in the petrographic characteristics of the construction’s geomaterials, such as the type and degree of cementation, porous network configuration, and presence or absence of soluble salts. These variations are also detected by the portable nondestructive techniques, showing their analytical sensitivity to small petrophysical changes even in the same type of rock and their performance in predicting future degradations not currently visible on the surface of the rocks.