Non-destructive testing of hardened cement specimens at microwave frequencies using a simple free-space method

In this research paper, we show microwave reflection and transmission properties measured from various sides of hardened mortar and concrete specimens with different water-to-cement ratios. These properties are important in predicting/measuring accurate electrical properties of cement-based materials which can eventually be utilized in structural health monitoring, public safety, and propagation-related research. Measurements are carried out by a simple and relatively inexpensive non-destructive free-space set-up with its newly developed calibration technique at X-band (8–12 GHz) during 3–36 months after samples’ preparation. A new approach (relative difference approach) is introduced for transmission properties to predict the state and degree of hydration inside specimens. It is shown that concrete specimens will complete the hydration sooner than mortar specimens with the same water-to-cement ratio (w/c) ratio due to heavy aggregates. In addition, among the specimens, while the concrete specimen with a higher w/c ratio will exhibit faster hydration during approximately 3–6.1 months, the mortar specimen with a lower w/c ratio will display rapid hydration during approximately 6.1–30 months. Besides, it is demonstrated that the magnitude of transmission properties for the top (or bottom) is less than those for the right (or left) side due to the effect of gravels and sand particles in specimens. Measurement results indicate that a varying electrical property depending on height should be used for cement-based materials in the predicting the channel properties in propagation-related researches.