# Monitoring and Calibrating Building Materials Drying Kinetics with Capacitance Sensors
## 1 Introduction  
Water in all forms is considered as the major decay factor of building materials and components [1]. The presence of moisture in porous building materials is associated with almost all decay phenomena [2], including electrochemical corrosion of metallic components, chemical deterioration and dissolution of materials, chemical processes, freeze-thaw deterioration, discoloration, volume changes, and biological decay, such as mold [3]. UNI EN 772-1 describes that if moist stones are forced to dry, compressive strength may be reduced by approximately 20% [4, 5]. Furthermore, moisture alters the thermal properties of materials impacting heat transfer flows, influencing materials' insulation properties and human comfort and health. To sustain the building envelopes' performance, it is important to maintain proper temperature and humidity levels [6]. Thermal conductivity measured for traditional bricks from dry to semi-saturated state can be two times higher, escalating to over 100% in fully saturated conditions. In the case of a more porous material, moisture content between 20-30% may result in a severe loss of thermal resistance [5]. Worldwide, 19-80% of buildings exhibit moisture-, and mold-related damage [7]. Therefore, quantifying moisture content in building materials is critical to accurately diagnose building pathologies for the adoption of appropriate intervention measures and for evaluating the effectiveness of the applied treatment solutions [8]. For measuring moisture content of materials, both direct and indirect methods are used. The gravimetric method is a direct method which requires actual samples, ensuring reliability but limiting its applicability, particularly in historic materials/structures. The gravimetric method is straightforward despite the variety of different testing standards; this method requires weighing a sample in both dry and saturated states [9; 10]. Alternatively, indirect (non-destructive) testing methods can be applied. Non-destructive testing for measuring moisture content uses physical properties as proxies for moisture, such as electrical, visual, or thermal measurements [11]. Undoubtedly, the direct gravimetric method is the most accurate method, since factors such as the homogeneity, density, porosity, surface treatments, and degree of material decay may negatively affect the accuracy of indirect methods [12]. Regarding the indirect methods applied in recent years, electrical-based sensors are a very promising technique for obtaining moisture content values of building materials and components. The word \"sensor\" originates from the Latin word \"sensorius\", meaning \"to perceive, to feel\". Their function is primarily based on measuring the resistivity or capacitance of building materials. The presence of water changes the conductivity of building materials by allowing the estimation of moisture content by measuring resistivity. Additionally, moisture modifies a material's dielectric property, allowing the use of capacitance methods to detect these dielectric variations due to the presence of water [13]. The use of capacitance sensors is not new [14]. This method is based on the high dielectric constant (\(\epsilon=80\)) of water, in comparison to the low dielectric constant of building materials. Consequently, changes of a material's dielectric properties can be attributed to its varying moisture content. A European standard has been established for this technique regarding wood products [15], while the European standard EN 16682 [16] has proposed specifications for wood and masonry materials. According to this standard, \"the capacitance methods respond to the total amount of water molecules, but the output depends on other factors too, including how the molecules are distributed inside the material, the signal being higher for molecules close to the sensor\". There is a lot of criticism of the application of this method, because the presence of electrolytes in masonry constitutes a limitation for the application of the capacitive method [12].