Thermal conductivities under unsaturated condition and mechanical properties of cement-based grout for vertical ground-heat exchangers in Korea—A case study

• The effects of varying degree of saturation on the thermal conductivities of cement-based grout for vertical ground-heat exchangers were investigated.
• Empirical equations representing the relationship between the degree of saturation and the thermal conductivity of the cement-based grouts were derived.
• Changes in the mechanical properties with respect to the mix proportions of cement-based grouts were also investigated.

In this study, the thermal conductivities of cement-based grouts for backfilling of vertical ground-heat exchangers were investigated under the saturated, air-dried and unsaturated conditions. The mechanical properties including the p-wave velocity, elastic modulus and Poisson’s ratio also were measured. The mix proportions applied were quartzite sand/cement ratios of 1, 1.5, 2, 2.5 and 3, with a fixed water/cement ratio of 0.7. Mixed-grout specimens were prepared in both rectangular parallelepiped shape (11 cm × 6 cm × 4 cm) and cylindrical shape (5 cm dia. × 10 cm height). Their thermal conductivities decreased linearly with decreasing degree of saturation. Also, as the sand/cement ratio was increased, the thermal conductivity at the same degree of saturation was higher, and the thermal conductivity decreased more steeply as the degree of saturation decreased. Based on these results, an empirical equation representing the relationship between the degree of saturation and the thermal conductivity of the cement-based grouts was derived. The results obtained in this study are expected to be utilized as input data for thermal and mechanical behavior analyses of ground-heat exchangers and their installations.