Piezopermittivity of cement mortar with various water contents and its application to capacitance-based structural self-sensing of stress

The piezopermittivity of cement mortar without functional admixture relates to capacitance-based structural self-sensing of stress. The capacitance increases with increasing water content, so the effect of the water content on the self-sensing is of concern and is thus addressed in this work. Upon compression, the transverse capacitance measured using coplanar electrodes decreases reversibly for any stress/water-content combination. The water content ˂50% influences the capacitance little. The capacitance is an ineffective stress indicator when the water content (˃20%) is not fixed. For a given water content, the stress is effectively indicated by the fractional capacitance decrease, which increases with increasing stress, regardless of the water content. The fractional change in capacitance due to stress is independent of the water content when the stress exceeds ∼30 kPa. With limitations on the stress/water-content combination, the stress sensing based on the fractional change in capacitance is effective. The capacitance decrease is mainly due to piezopermittivity, with the transverse piezopermittivity coefficient ∼+5000; the dimensional change contributes negligibly. The fractional decrease in permittivity is greater when the strain is smaller.