Mechanical stress induced electrical emissions in cement based materials

Abstract

This work deals with the underlying physical mechanisms and processes that dominate the fracture of cement based materials and their electrical properties. Electric current emissions were recorded when hardened cement pastes and cement mortars suffered mechanical loading in various modes. Such electric current emissions are known as Pressure Stimulated Currents (PSC) when the applied loading is compressional while they are mentioned as Bending Stimulated Currents (BSC) when the material suffers bending loadings. The physical mechanism responsible for the PSC and BSC emissions can be interpreted in terms of the Moving Charged Dislocations model that correlates mechanical deformation and electric charge distortions in the sample bulk. Laboratory experiments were designed based on the mechanical and physical properties of cement. To conduct the experiments all the background material concerning cement fracture mechanics, the microstructure of the hardened cement paste and the Interfacial Transition Zone of cement mortar were taken into consideration. Additionally, the experience of the PSC technique when it was applied on marble samples was used to guide the experimental procedures and compare qualitatively and quantitatively the experimental results. The relationship between the emitted PSC and the strain was established for the very first time for cement based materials in the present work. When the material was stressed within the range where stress and strain are linearly related a linear relation between PSC and stress rate (d/dt) was observed. Deviation from this linearity appeared when the applied stress was in the range where the applied stress and the yielded strain were not linearly related. Slightly before fracture, intense, non-linear PSC emissions were detected. The damage of the sample structure due to excessive loading in the plastic region significantly affected the recorded phenomena. Bending tests proved that similar electric current emissions are detected when a sample beam suffers 3 Point Bending Tests. The dependence of the emitted electric current on the way of fracture (i.e. compressional or tensional) was proved. It was also shown that the magnitude of the emitted electric current is directly related to the magnitude of damage due to the external loading. Thus, as it was expected, the electric current emitted from the tensed zone is significantly greater than the corresponding emitted from the compressed zone.