Insights into the piezoceramic electromechanical impedance response for monitoring cement mortars during water saturation curing

Lead Zirconate Titanate (PZT) based electromechanical impedance (EMI) sensors were used to monitor the mechanical properties development of different water to cement ratios (w/c) cementitious mortar mixes, during the first 28 days of curing under water. Through using the analytical procedure proposed in this study to analyse the EMI data, the different mixes mechanical properties development through the curing period were detected, and the EMI response was able to provide a more detailed interpretation regarding the difference between the surface and the bulk material mechanical properties development. Both the peaks from the impedance signature (Z) and the first difference of the impedance signature (dZ) showed shifts to higher frequency ranges as the age of the samples increased, indicating an increase in the material stiffness. Furthermore, the compressive and the flexural stresses showed an R2 > 0.8 and > 0.9, respectively in relation to the frequency shifts. The relationship between the PZT-EMI response through the curing period and the sample’s mechanical properties was shown to be frequency-dependent; hence a numerical analysis using ANSYS Workbench 18.1 was undertaken to understand this frequency-dependence phenomenon. From the numerical model, the impedance signature response at higher frequency ranges was shown to be dominated by the response from the surface of the hosting material, whereas the response from the specimen's interior dominated the lower frequencies EMI response. The analytical approach proposed in this study is expected to assist in differentiating between internal cementitious materials processes, such as internal curing, and those originating at the surface, such as aggressive chemical agents’ penetration