Dielectric Properties of Class 2 UGM Under Varying Moisture and Temperature at 2.45 GHz for Microwave Dry-Back Modelling

Unbound granular materials (UGMs) are essential components in road construction, particularly in the base and sub-base layers of pavements. After compaction, these layers undergo a dry-back process to reduce excess moisture before sealing, thereby minimising sealing aggregate embedment and rutting under traffic loading. Conventional dry-back methods for compacted UGMs rely on solar radiation, which is time-consuming and dependent on climatic conditions, often delaying construction schedules. Microwave technology could offer a rapid alternative, but its implementation in road construction requires a better understanding of the complex permittivity (dielectric properties) of UGMs to optimise energy absorption and heat generation during microwave dry-back. This study characterises the complex permittivity of Class 2 UGM as a function of volumetric moisture content (VMC) and temperature using a Modified Free-space Transmission technique. Measurements were conducted across frequencies from 300 kHz to 4.5 GHz at controlled temperatures between 20 and 90 °C. The observations were fitted using exponential regression models to predict dielectric behaviour at only 2.45 GHz, a frequency commonly used in industrial microwave applications. Regression models developed incorporating both VMC and temperature achieved good predictive accuracy (R² > 0.8). Results reveal the dielectric constant (εr′) increased exponentially with VMC. In contrast, dielectric loss (εr″) decreased, an atypical trend likely reflecting the transition from bound to free water. Temperature effects were also notable, with higher temperatures reducing dielectric loss by decreasing dipolar relaxation. Sensitivity analysis indicated that VMC had a greater influence on εr′ as compared to temperature, while both parameters contributed comparably to εr″. The proposed empirical model provides a practical tool for predicting energy absorption in compacted UGMs, offering key insight needed to optimise microwave-based dry-back methods in pavement construction.