Microwave dry-back of compacted unbound granular materials: an experimental and numerical study

After compaction, wet granular pavement layers undergo a dry-back process to remove excess moisture and attain optimal strength and stiffness, and increased performance against cyclic loading. However, the current dry-back method relies on solar radiation, which is time-consuming and weather-dependent, often delaying construction timelines. In this study, the feasibility of microwave drying was investigated as an alternative method to accelerate the dry-back process of compacted unbound granular materials (UGMs). Experimental and numerical analyses were conducted to evaluate the drying behaviour under microwave exposure. The study considered two conditions: direct contact and a 1 cm gap between the microwave applicator and the UGM surface. A three-dimensional finite-element model was developed using COMSOL Multiphysics to simulate the coupled heat and mass transfer, with model validation performed using experimental data. A comparative analysis with solar drying was also conducted using HYDRUS-1D under realistic climatic conditions. The findings demonstrated that microwave drying substantially reduced drying time compared to current dry-back methods while minimising weather-related delays. Despite its efficiency, microwave drying presented challenges such as localised heating and limited energy penetration in deeper layers. The study highlights the potential of microwave technology for pavement dry-back and provides recommendations for optimising microwave drying to enhance energy efficiency in field implementation.