Issue 50

Q. Hu et alii, Frattura ed Integrità Strutturale, 50 (2019) 638-648; DOI: 10.3221/IGF-ESIS.50.54 643 UCS Fig. 5 shows the UCS of 42 specimens after two heating treatments. A similar variation pattern can be defined with temperature ranging from 400 °C to 900 °C, which is the initial strengthening, subsequent weakening and final stabilizing. In this range of temperature, the maximum decline in group A is about 20 % around 700 °C, whereas about 10 % is displayed in group B. Over 900 °C, the group A presents a second decrease by approximately 70 % than untreated specimen, whereas the group B exhibits almost unchanged. The growth observed for the temperature less than 500 °C, so called the thermal-induced strengthening, may be due to the strain hardening by localized transition plasticity development [27]. It is accommodated by changes in mineral decomposition/growth or monomineralic phase transitions. Here, the strengthening less than 500 °C is related to the iron mineral transition, which is reflected as the wide endothermic peak at 300 °C on the DSC curve. Noted that the uniaxial compressive strength testing was conducted after the cooling treatment, i.e. the strengthening on rock by localized transition plasticity was irreversible. Moreover, the strengthening is also related to mineral expansion which narrowed the cracks or pores to improve the integrity of granite. When temperature came to the next threshold, the transition of α-β quartz at 573 °C, numerous micro-cracks generated due to the compression of expanded quartz grain, inducing the weakening of granite structure. The quartz is considered as significantly affecting the mechanical performance of granite, however, the average decrease between 573 °C and 900 °C is only about 10 MPa in two groups, perhaps due to the not much content of quartz in the granite [28]. In third stage, combined with the TG/DSC results, the initiation of stabilizing stage corresponds to the ending of mass loss, which proves the importance of water loss on damaging the rock structure exposed to high temperature. The plateau also illustrates that if the further reduction of mechanical strength is expected, the much more heat energy will be required, as given in the group A at 1000 °C. See the partial melting of the specimen in group A at 1000 °C, the internal maximum temperature definitely exceeded the measured surface temperature. The distinct deterioration is most likely attributed to that melting destroyed the original crystal-mineral framework of granite. The contrast in two groups was also discussed in detail. In strengthening and weakening stage, the UCS of group A is lower than group B by approximately 20 MPa and 10 MPa respectively. It means that temperature-controlled microwave irradiation can reduce the strengthening due to localized transition plasticity and further promote the rock structure to deteriorate in weakening stage. It is beneficial to ensure the stability of surrounding rocks that the strength of directly irradiated zone is lower than surrounding rocks at the same temperature. Next, the standard deviation (SD) in the two groups show the difference of granite deterioration at the same temperature under the two heating treatments. Considering the short time of thermal preservation, the biggish deviation for temperature less than 600 °C is firstly related to uneven heating. For the temperature in excess of 600 °C, contrary to the small variability of group A, the group B still shows greater dispersion. This variation not only illustrates uniform heating of microwave irradiation on the rock at high temperatures, but also reveals the mechanical performance of rock exposed to high temperature induced by microwave irradiation is more accurately to forecast. Certainly, the variability of mineral composition/distribution from the one specimen to the next should not be ignored. All of these variabilities can influence the manner by which the damaging conditions of rock exposed to microwave irradiation in a practical engineering. Before excavation, a comprehensive mineralogical investigation for the target rock mass and the irradiation testing in a broad range of temperature on samples should not be absence. Ideally, microwaves can be employed to irradiate the vulnerable positions (e.g. cleavage plane and surface edge) and kept the whole rock mass being heated up to 600 °C, instead of irradiating the whole rock mass or heating it up to very high temperatures (seeing the plateau at T = 600-900 °C), so as to preserve the original intention of reducing energy consumption. Figure 5 : The UCS of specimens after heating treatments: (a) microwave irradiation group and (b) resistance-heating group.