Dielectric deamplification of multiphase cementitious composites in the frequency range of 0.5GHz to 4.5GHz

Dielectric properties (dielectric constant and loss factor) of hydrated cementitious composites (e.g., Portland cement paste and concrete) are the basis for the use of microwave and radar nondestructive testing/inspection techniques (such as GPR) in civil infrastructure systems (e.g., buildings and bridges). Among different types of cementitious composites, hydrated cement paste (hcp) is the major element providing the bonding strength in cementitious composites. Since hcp is porous and multiphase, its measured effective dielectric constant and loss factor are the function of not only frequency, temperature, and humidity but also its design parameters such as the water-to-cement (w/c) ratio. The removal of moisture content (liquid phase) in hcp materials is known as the dielectric deamplification in hcp and other cementitious composites. This phenomenon affects the performance of microwave/radar sensors when applied to concrete structures. It is also known that interactions among liquid phase (moisture), gaseous phase (voids), and solid phase (hcp) provide insightful information regarding material composition and structural integrity (e.g., cracking in hcp) of the composite. In this paper, the dependence of dielectric deamplification in hcp on the w/c ratio of hcp in the frequency range of 0.5∼4.5 GHz at room temperature (25 C) is investigated. Panel specimens made of hcp designed with different w/c ratios (0.35, 0.40, 0.42, 0.45, 0.50, 0.55) and cured for 7 days were prepared. Some hcp panels were oven-dried at 105 C in order to remove non-chemically bound moisture in hcp. Dielectric constant and loss factor of hcp panels were measured using an open-ended coaxial probe system and a network analyzer. From the measurements, it is found that the dielectric deamplification in the measured dielectric constant of hcp exhibits a pattern different from the one in the loss factor of hcp, while both patterns are related to the w/c ratio of hcp. This finding can be useful in predicting the material composition of hcp and other cementitious composites using microwave/radar sensors.