Thermal conductivity of controlled low strength material (CLSM) under various degrees of saturation using a modified pressure plate extractor apparatus – A case study for geothermal systems

Abstract

Conventionally, controlled low strength material (CLSM) is defined as a self-compacting and self-leveling material. High thermal conductivity CLSM can be, however, used as the heat transfer medium (thermal grout) in geothermal systems due to its good flowability and low shrinkage. Similar to general soils or earlier developed thermal grouts, the degree of saturation of CLSM is strongly related to its thermal conductivity, which is an input parameter in the design of geothermal systems. Hence, this relationship should be explored for the optimum design in these systems. This paper addresses the thermal conductivity of CLSM made with coal ash and excavated soil as a function of the degree. In order to do this, the pressure plate extractor apparatus (SWC-150) was modified by coupling with a single thermal needle probe. In the experimental program, all general properties (e.g. flowability, bleeding, fresh density, initial setting time, and unconfined compressive strength) were performed first. Afterward, thermal conductivity, the degree of saturation, and matric suction were measured simultaneously by the modified apparatus. As a result, all general properties of the prepared CLSM satisfied the specifications of CLSM. In addition, linear relationships between thermal conductivity and degree of saturation of the proposed CLSM mixtures were observed. It is interesting to note that thermal conductivity of CLSM incorporating excavated soil, which works as filler the CLSM mixture, was relatively improved when compared to that of CLSM without excavated soil. Finally, new predictive models for thermal conductivity estimation of CLSM were proposed based on both normalization and regression methods.