CHC:Natural Stone in Facade Cladding
Since ancient times natural stones like marble and limestone have been used for construction and artistic purposes. In recent decades, these limestones increasingly gained importance, especially in the design of thin-plate facade cladding. Facades, which are exposed to the weather, experience biological, chemical and physical processes causing a structural loosening, accompanied by a material softening, which in the long term results in a need to replace the facade at great cost. In recent years, quite a few papers were published dealing with weathering and deformation of different marbles. Limestones, on the other hand, were very rarely mentioned. Therefore, very little is known about the weathering-related softening of Untersberger marble, one of the most important limestones in the Salzburg area. The focus of this work is on the softening behavior of limestone caused by the action of external parameters such as temperature changes and freeze - thaw - cycles, taking into account humidity as an essential climate variable. Basis for the characterization of damage dynamics is the measuring of petrographic and petrophysical properties, taking into account the structural characteristics of freshly fractured and artificially weathered limestone samples. For this purpose, 6 different varieties of Untersberger marble were examined. For comparative purposes, 2 varieties of Sankt Margarethen limestone and Danube limestone were studied.
The weathering process is driven by the formation of thermal and hydric microcracks. With respect to the expansion behavior, the anisotropic properties of calcite in the investigated limestones play only a subordinate role. Important internal influencing factors, however, are grain size, grain size distribution, grain boundary geometry and the intensity of the thermal preliminary damage. An irreversible residual strain as a measure of the accumulated opening of microcracks happens only in a moist state and progresses during continuing thermal cycling. If the limestone is dry, temperature changes cause only small changes but no significant residual strain. For a characterization of the damage dynamics, different material-specific parameters were compared. Using the static and the dynamic elastic modules, the dynamic shear modules and measuring the p-wave-velocity provided the most reliable results.