Long-term Monitoring of Salt Movement in Masonry Materials

The recrystallization of salts due to changes in moisture content is one of the major damage functions in historic materials like natural stones, tiles, plasters, and mortars. This paper presents results of a long-term wireless monitoring of salt and moisture movement at a test wall. Within the EU-project “Smart monitoring of Historic Structures – SMooHS (www.smoohs.eu), a potential difference measurement node was developed in combination with ion-selective electrodes based on silver/silver-chloride for the detection of chloride salts in porous masonry materials. Masonry walls placed in outdoor climate conditions in Bologna were preloaded by capillary suction with diluted NaCl solutions for several months. The salt migration and content were monitored with the developed wireless sensor system over a period of more than three years. Different regimes of potential difference signals were identified dependent on sensor position, moisture content and climate conditions. A comparison with a model for hygrothermal analysis shows comparable results.