Abstract: The energy pile is a new type of ground source heat pump technology that places the ground heat exchanger in the building pile foundation to achieve underground heat exchange. However, the expansion and shrinkage of plie materials threat the long-term running and the building safety because that the energy piles often operate in cooling and heating thermal loads at different seasons. Grouting materials with proper thermal-mechanical characteristics are crucial for the safe use and widespread of the energy piles. This paper aims to investigate thermal-mechanical behaviours of concrete piles mixed with different contents of polypropylene fibre and steel fibre. The results show that thermal conductivity increased when adding steel fibre into concrete but decreased when adding polypropylene fibre. The maximum thermal conductivity was measured to be 2.44 W/(m·K) for the concrete mixed with 1.3% steel fibre. The thermodynamic step heating test showed the strain of the concrete reinforced both by steel fibre and polypropylene fibre was effectively reduced. The reductions of concrete reinforced by steel fibre and polypropylene fibre were about 62.43% and 61.11%, respectively. Furthermore, heating expansion and cooling shrinkage strain of concrete reinforced by steel fibre were both reduced during the heating and cooling load testing cycles, and the strain was the minimum in the whole process. Compared with three different types of concrete, steel fibre reinforced concrete is suggested as a suitable material for the grouting of energy piles.

Key words: energy piles, fibre reinforced concrete, deformation properties, strain