Assessing the risk of salt weathering in sandstone by instrumented monitoring of the electrical impedance

Salt weathering of natural stone is one of the most severe and most costly deterioration at historic buildings. The existence of salts and salt mixtures within the material requires adequate measures to avoid further deterioration. An inevitable prerequisite before conservation activities can be conducted is the preliminary assessment by laboratory investigations, but also by increasing the knowledge of the environmental influences that affect the salt weathering rate and the actual processes in the stone. The instrumented monitoring of a material's electrical impedance in combination with the ambient conditions allows the acquisition of information on its moisture content, both with and without salts. A joint analysis including the supplemental measured parameters may be used to draw conclusions on the dynamic interaction between moisture and salt transport, as well as salt phase changes, and hence provides the possibility to evaluate the risk of salt weathering to the structure.