Issue 41

S. Zhao et alii, Frattura ed Integrità Strutturale, 41 (2017) 412-423; DOI: 10.3221/IGF-ESIS.41.53 413 R ESEARCH ON FIBER - REINFORCED CONCRETE OF ENERGY PILES Principles of material selection ig. 1 shows the load-strain curve of the non-tube pile and Fig. 2 shows the load-strain curve of the buried pipes in the pile. It can be seen from the figures that if a heat transfer tube is buried in the pile, the concrete pile in the vicinity of the heat transfer tube is damaged more seriously, and the distribution of cracks is also very uneven, which indicates that after the heat transfer tube is buried, the compressive bearing capacity of the concrete pile foundation decreases greatly. After the heat transfer tube is buried in the pile foundation, the distribution of the strain curve of the pile is greatly changed, and the strain variation of the steel and the concrete is increased, which is more likely to cause the concrete pile to break [1-6]. Figure 1: Load-strain curve of the non-tube pile. Figure 2: Load-strain curve of the buried tubes in the pile. In addition, since the heat transfer tubes embedded in the concrete piles are to conduct the heat storage, a large amount of heat is stored in the energy piles, causing heat accumulation, which in turn causes the temperature of the center of the concrete piles to rise. When the concrete temperature rises to 100 ℃ , the internal free water begins to evaporate, and the chemical composition and physical structure of the concrete will change to about 500 ℃ [7-10]. Due to the decomposition of Ca (OH) 2 leading to the concrete cracking, the concrete pile needs to remain at a high temperature to have favorable mechanical stability. Meanwhile, the expansion coefficient of the concrete materials is required to be as close to the expansion coefficient of the material of the heat transfer tube, so that it can effectively reduce the presence of an air film to avoid the unfavorable thermal conductivity of the pile. Based on the analysis above, it is required that the selection of the concrete material of energy piles should meet the following conditions: firstly, the pile foundation of the building can still have stable mechanical properties at high temperature; secondly, the material of the pile foundation should have good thermal conductivity, so that the heat exchange can be conducted completely between the energy pile and soil; thirdly, the thermal expansion coefficient values of the heat transfer tubes and the concrete should be the same if possible to ensure that the heat transfer tubes and the concrete are tightly integrated without an air film, which would affect the thermal effect [11-15]. Choosing raw material The main materials referred to in connection with the energy piles contain cement binders, aggregate, fiber, admixtures, and so on. Kind of cement: composite Portland cement. Type of coarse aggregate: basalt gravel with a density of 2.6×10 3 Kg/m 3 . Type of fine aggregate: natural sand; its modular and density are 2.6 and 2.55×10 3 Kg/m 3 respectively, its moisture content is 5%. Type of fiber: the fiber-reinforced material mentioned here refers to steel fiber, whose length is 30-40 mm and whose tensile strength is 4500 MPa. Type of material for storing heat: graphite and copper slag have a better thermal conductivity and the density of the graphite and copper slag in this experiment is 1.8×10 3 Kg/m 3 and 3.3×10 3 Kg/m 3 respectively. F