Influence of rift and bedding plane on the physico-mechanical properties of granitic rocks. Implications for the deterioration of granitic monuments

The influence of the natural planes in granitic rocks (rift and bedding plane) on some of their physico-mechanical properties was evaluated with a view to establishing how the disposition of these planes in granitic construction materials affects the deterioration of granitic monuments. Blocks were cut from three granites that have been widely used in construction and restoration in Santiago de Compostela (Galicia, NW Spain), and their capillarity, permeability to water vapour, thermal expansion and expansion upon immersion in water, and compressive and bending strength were determined in three different orientations with respect to a reference plane. The results showed that the granitic rocks were strongly anisotropic and that this anisotropy was reflected in the values of most of the measured properties, especially thermal expansion, expansion upon immersion in water, and mechanical resistance. The disposition of the natural planes was clearly identified for only one of the granites; for the other two, the relation between these planes was less straightforward and its identification would require the use of a more exhaustive sampling strategy than that used here. The variation in the physical properties with the orientation of the granite blocks suggested that blocks positioned with the rift vertically oriented would be particularly prone to deterioration.