Piezoelectricity-based self-sensing of compressive and flexural stress in cement-based materials without admixture requirement and without poling

The self-sensing of compressive and flexural stress in cement-based materials (without embedded or attached sensors, without admixture requirement and without poling) by capacitance measurement has been demonstrated. The sensing involves two coplanar electrodes in the form of aluminum foil adhered to the cement-based material by using adhesive tape, which also serves as a dielectric film. The normal force associated with compressive or flexural loading is applied to the region between the electrodes. The stress regimes are the low-stress regime (up to 1500 Pa), the medium-stress regime (1500-7400 Pa) and the high-stress regime (7400-65 000 Pa). The lowest compressive/flexural stress demonstrated for effective sensing is 300 Pa. The fractional decrease in capacitance per unit stress is up to 2.2 × 10^-6 Pa^-1 and tends to decrease as the regime is changed from low to medium and to high-stress. The sensitivity tends to be greater for the compressive stress than flexural stress. However, when the normal stress (relevant to weighing) is considered, the sensitivity is higher for flexural loading than compressive loading. The strain contributes negligibly to the capacitance decrease, though the near-surface deformability of the cement-based material contributes. The change in capacitance upon stress application is attributed to the direct piezoelectric effect. The sensitivity is comparable for cement paste, mortar and concrete. Capacitance-based self-sensing is advantageous over resistance-based self-sensing in terms of the low-stress sensing effectiveness, absence of admixture or aggregate proportion requirements, essential absence of self-sensing effectiveness reduction by the presence of aggregates, wide applicability to existing concrete structures, and ease of applying electrodes.