Contribution of clayey–calcareous silicite to the mechanical properties of structural mortared rubble masonry of the medieval Charles Bridge in Prague (Czech Republic)

The Gothic fill masonry of the arches and pillars of the Charles Bridge in Prague (Czech Republic) can be characterised as structural mortared rubble masonry (MRM). The mortar, a mixture of hydraulic lime binder and fine-grained filler (river sand), fastens together larger pieces of natural stone, which are a porous fine-grained clayey–calcareous silicite. A set of specimens were subjected to the uniaxial compressive load. The specimen set was composed of some of the mortars alone, the natural stone alone, or variable mixtures of both (i.e. mortared rubble masonry materials). Along with the ultimate strength, the sampleʹs stress–strain behaviour was recorded. The compressive strength, as well as the modulus of elasticity of the mortared rubble masonry, increased with the proportion of natural stone in the specimens (having a compressive strength of 12.55 MPa for a specimen having almost 4 vol.% of coarse aggregate, and 61.49 MPa for a specimen having almost 82 vol.% of coarse aggregate; with a modulus of elasticity of 6 GPa for a specimen having almost 4 vol.% of coarse aggregate, and over 14 GPa for a specimen having 23 vol.% of coarse aggregate). The compressive strength of the mortar (6–11 MPa) is lower than that of MRM; however, the modulus of elasticity (7–16 GPa) can reach the same values as that for MRM. The natural stone, used as the coarse aggregate, showed the highest compressive strength (80–140 MPa) and the highest modulus of elasticity (25–28 GPa) from among the materials studied. This observation suggests that it is not only the hydraulic lime-based mortar, but also the coarse aggregate (clayey–calcareous silicite), as well as favourable bonding on the natural stone–mortar interface contribute to the superlative mechanical properties of the studied MRM.