Finite element modelling of a Capacitive Stress Sensor

Safety, functionality, durability and robustness are the main features that buildings and infrastructures must guarantee over time, despite being progressively subjected to aggressive environmental conditions and the aging of materials. One of the most effective methods to check the efficiency and safety of structures is Structural Health Monitoring (SHM), which allowsthe measurement of the stress state inside solid bodies made with building materials like concrete or masonry. In most cases, the strains on the external surfaces of the structure are evaluated using strain gauges or optical fibres while internal stresses are commonly measured using flat jacks. Two new types of stress (force) sensors to be embedded inside concrete or masonry have been conceived and patented in recent years: a ceramic sensor, based on piezoelectric principles, and a capacitive one. This work aims at studying the capacitive sensor through finite element simulations, with the scope of evaluating the best internal geometry able to guarantee the reliability of the measurements. In fact, such sensor is subjected to sustained loads over a long time, taking into account slow phenomena, like concrete (or mortar) shrinkage and creep. The simulations highlighted the capability of the sensor to withstand typical loads of buildings under serviceability conditions and the almost neglectable effect of the rheological phenomena of the concrete or mortar on the sensor measurements. The optimal internal configurations for the reliability of the measurements have been identified.