Capacitance-based stress self-sensing in cement paste without requiring any admixture

This work reports capacitance-based stress self-sensing in cement paste without requiring any particular admixture (whatever type). The in-plane capacitance is measured between two coplanar electrodes (aluminum foil) adhered to the cement slab by double-sided adhesive tape. The capacitance decreases with increasing normal stress, whether the stress is purely compressive or flexural compressive, due to piezoelectricity. The capacitance decrease is completely reversible in the low-stress regime (normal stress up to 19 kPa), and partially reversible above this stress. The minimum normal stress change detected is 0.2 kPa, which, in case of flexure, corresponds to a flexural stress change of 8.5 kPa. The change in capacitance per unit normal stress change is up to 0.061 and 0.101 pF/kPa for purely compressive and flexural compressive loading, respectively; the value is higher in the low-stress regime, which gives superior linearity and reversibility. Low-stress sensing is relevant to pedestrian monitoring and room occupancy monitoring.