Experimental measurement and microstructure-based simulation of thermal conductivity of unbound aggregates

Unbound aggregates are widely used for roadway and railway infrastructure. The thermal conductivity of unbound aggregates is important for predicting temperature field and thermal stress of structures. This paper developed laboratory measurement methods and microstructure-based simulation models to evaluate effective thermal conductivity of unbound aggregates. An image-aided method was used to randomly generate three-dimensional (3-D) geometry of aggregate. The aggregate particles were packed with discrete element method and then imported into finite element model for steady heat transfer analysis. Laboratory experiments were conducted to measure temperature profiles in the specimens of unbound aggregates and determine effective thermal conductivity through back-calculation. The effective thermal conductivity measured from the experiment was used to validate the results obtained from the simulation model. The analysis results show that smaller aggregate with the size of 2.36-4.75 mm has the smaller porosity and the greater thermal conductivity as compared to the aggregates with the size of 4.75-9.5 mm. However, the effective thermal conductivity of unbound aggregates is affected by volume fractions of the components and the contacts between aggregate particles. When the original rock is crushed into aggregates, the decrease in thermal conductivity is more significant for the rock with the higher thermal conductivity. The effective thermal conductivity could be underestimated if two-dimensional (2-D) cross sections of 3-D models were used in the simulation. The study results can be used for better consideration of thermal responses in the design and analysis of roadway and railway structure.