A true prediction model for the 28-day compressive strength of concrete via effective conductivity at microwave frequency is presented for the first time in this paper. Prediction of compressive strength of concrete at early ages has attracted a lot of attention, but several defects still remain in previous researches using nondestructive techniques such as the use of electrical surface resistivity. The results demonstrated that the dynamic ranges, the initial values, and the initial slopes of measurement curves can be used for cure-state monitoring, hydration rates prediction, and initial w/c ratio determination. Statistical analysis and exponential curve fitting have been performed on the temporal measurements of early-age (a few first days) CBMs. The comparative measurements are conducted using a microwave near-field noninvasive testing technique at two frequency bands, namely, R-band (1.7 − 2.6 GHz) and X-band (8.2 − 12.4 GHz). Mortar specimens with different w/c ratios and constituent ratios are presented. In this paper, measurement results of monitoring of reflection coefficients, electrical conductance and susceptance of early-age concrete, and In the construction industry, the determination of initial water-to-cement (w/c) ratio and cure-state monitoring of early-age CBMs are still challenging tasks. Microwave noninvasive and evaluation techniques have shown to be promising for characterization of cement-based materials (CBMs).
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