Issue 50

Q. Hu et alii, Frattura ed Integrità Strutturale, 50 (2019) 638-648; DOI: 10.3221/IGF-ESIS.50.54 640 M ATERIALS AND METHODS Granite description he candidate granite was collected from Danfeng in Shanxi province, China. Petrographic thin section was prepared and observed by optical polarizing microscopy (Leitz Laborlux 12 pol, Germany), as shown in Fig. 1. The mineralogical composition for Danfeng granite is: 64 % feldspar (54 % K-feldspar and 10 % plagioclase), 30 % quartz and 6 % biotite. Auxiliary minerals including magnetite and apatite are also found. The grain size of single mineral ranges from about 0.3 mm to 3.5 mm. Besides, partial graphic and vermicular textures can be also observed. X-ray fluorescence (XRF, PANalytical, B.V. Axios) was also employed to obtain the composition of granite, as listed in Tab. 1. The mean value of UCS for the granite at room temperature is 101.84 MPa. Figure 1 : Petrographic thin microscopy image of the granite (Kfs-potash feldspar; Pl-plagioclase; Qz-quartz; Bt-biotite). Oxide SiO 2 Al 2 O 3 K 2 O CaO Fe 2 O 3 Na 2 O Content / wt.% 75.90 14.87 4.05 2.57 1.34 0.48 Table 1 : Composition of Danfeng granite. Heating treatments The two heating conditions were performed with a 1.5 kW microwave oven with a frequency of 2.45 GHz (HAMiLab- M1500, SYNOTHERM) and a 12 kW muffle furnace (KSS-1700, Xiangtan Sanxing Instrument Co., Ltd), respectively. For a simplified description, microwave irradiation group was named group A and the resistance-heating group was named group B. Specimens were heated up to set temperatures (400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 900 °C and 1000 °C) with a heating rate of 7 °C/min and then kept for 15 min. Real-time surface temperatures were measured by the infrared thermometer pre-installed in apparatuses, i.e. following temperatures represented the surface temperatures except for special description. For the cooling treatment, the power was turn off and the specimen was allowed with a slow cooling rate to simulate the cooling of surrounding rocks. Specimen preparation Considering the complete penetration of microwaves and the cavity dimension of microwave oven, the granite was cut and polished as a cube of 20 mm by using a disk sawing machine. The internal structure variability from one specimen to next due to the grain size was attenuated by multiple tests. Specimens with distinct cracks were discarded to avoid the influence of initial macro-cracking. Furthermore, all specimens were cleaned and dried at 110 °C for 12 h in a conventional drying oven in order to eliminate the influence of moisture. Uniaxial compressive strength testing Uniaxial compressive strength (UCS) testing was performed on heated specimens using a compression testing machine (TYE-300) with a maximum load of 300 kN. For all testing, a load rate of 0.3 kN/s was adopted to fail the specimen and T