Optimizing Backfill Materials for Ground Heat Exchangers: A Study on Recycled Concrete Aggregate and Fly Ash for Enhanced Thermal Conductivity.

This study investigates the potential use of recycled concrete aggregate (RCA), fly ash (FA), and their mixture (RCA+FA) as backfill materials for shallow vertical ground heat exchangers (GHEs). Granulometric, aerometric, and Proctor compaction tests were conducted to determine soil gradation, the void ratio, and the optimal moisture content (OMC) for maximum dry density. RCA demonstrated efficient compaction at lower moisture levels, while FA required higher moisture to reach maximum density. A 10% FA addition was optimized to fill voids in the RCA soil skeleton without compromising structural stability. Thermal conductivity tests were performed using a TP08 probe in both dry and wet states. The results showed that the RCA+FA mix exhibited a notable increase in thermal conductivity at around 6% moisture content due to the formation of water bridges between particle contacts. FA, in contrast, displayed a more linear relationship between conductivity and moisture. The RCA+FA mix achieved higher thermal conductivity than either material alone, particularly near full saturation, making it a promising option for efficient heat exchange. Thermal conductivity modeling, based on the Woodside and Messmer model, confirmed the RCA+FA mix's high conductivity and estimated full saturation conductivity values with a small error. The Kersten number () was employed to predict conductivity across varying moisture levels, with results showing a strong correlation with saturation ratio ().