Study On The Thermo-mechanical Properties Of Concrete And Heat Transfer Behaviors Of Energy Piles

Shallow geothermal energy as a clean, efficient and environmentally friendly renewable energy, has gain more and more attention from society. Ground source heat pump system (GSHP) is mainly using shallow geothermal resources for heating and cooling buildings by the buried heat exchangers. Energy pile is a new application type of GSHP, which buries the pipes within the building pile foundation. It makes use of the good heat storage capacity and heat conductivity of concrete, increase the contact area between the heat exchangers and soil, thermal conductivity higher than the traditional borehole heat exchangers. Moreover, it can save a large amount of underground space resources as well as drilling costs. However, it also has brought some problems. Energy piles have to bear the overlying building load as well as affected by the temperature changes. So the characteristic of energy pile under the conditions of thermo-mechanical coupled should be paid more attention, which relates to the security, stability and efficiency.In this paper, the spiral coil type energy pile is the main research object. Firstly, studied the physical and mechanical properties of different mix proportion concrete, finding that reduce the water-cement ratio, increase the coarse aggregate volume fraction, and choose the optimal fine aggregate volume fraction can make concrete to the large thermal conductivity and strong uniaxial compressive strength.Secondly, in order to reveal the principle of concrete thermal stress changing with the temperature, experiments were carried out with MTS. The mathematical relationship between the thermal stress and temperature was established based on the experimental results, putting forward the three main factors for influencing the stress-temperature curve:1) the initial stress state; 2) boundary constrain conditions; 3) elasticity modulus, E and linear thermal expansion coefficient, a of concrete. So the suggestion is to select small linear expansion coefficient of concrete on the premise of guarantee the elasticity modulus under the same conditions, for reducing the thermal stress. In terms of the physical experiments, appropriately increase water-cement ratio can effectively reduce the linear expansion coefficient, thus there need to determine the optimal water-cement ratio combining thermal conductivity, compressive strength and linear expansion coefficient.And then, analyzed the thermal conduction characteristic of energy piles and the main influence factors of temperature filed distribution by means of numerical simulation, finding that increase thermal conductivity of concrete, and increase the pile diameter can improve heat conduction of piles, but there will be more thermal stress caused by temperature gradient within piles.Finally, an in-situ test was implemented with the purpose of studying the characteristic of strain and stress at different positions of the pile body under the conditions of thermal-mechanical couplings. The testing results show that the thermal stress may be greater than the initial structure stress, affecting the distribution of stress field. The maximum of structure stress is distributed at the top of pile, but the maximum of the coupled thermal-structure stress is distributed at the low parts of the pile. Meanwhile, numerical simulation was carried out with ANSYS to analyze this phenomenon. The strain, stress and displacement contours were consistent with the experiment results. Therefore, a stress generalized model of energy pile under the condition of thermal-mechanical coupling was come up with, revealing the characteristic of stress changes.To sum up, this paper studied the properties of concrete and the behavior of energy pile with the coupling conditions by means of experiment, numerical simulation and theoretical analysis, revealing the main factors for the heat transfer and thermal stress of energy pile, and providing some supports or help for these kind of engineering.