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Experiments on electromagnetic radiation (EMR) from coal and rock under different loading modes have been conducted in laboratories and in the field, but there are few reports about EMR from concrete after it has been exposed to high temperatures. To determine the feasibility of using EMR to evaluate the condition of concrete after high temperature events, like structure fires, three groups of concrete specimens were tested by uniaxial compression after being heated to elevated temperatures; the EMR signals were recorded and analyzed. The results show that the compressive strength decreases with exposure temperature higher than 100 °C and the modulus of elastic is generally inversely proportional to exposure temperature. Low frequency EMR (∼10.9–131.5 kHz) was observed during loading. The EMR impulses respond well to the stress, so they should reflect the stress state in the specimens. In our experiments, a large EM signal arrives that nearly coincides with the main fracture and then the signal rapidly attenuates; a series of low-amplitude oscillations follow. For specimens after being heated to different temperatures, the EMR waveforms are similar in shape but differ in intensity. The principal frequency and the corresponding maximum amplitude in the EMR spectra increase with the exposure temperature. Based on the exclusion method, piezoelectric effects and motion from the variable velocity of charged particles are believed to be the probable mechanisms causing EMR to be generated from the specimens.

Coal and rock dynamic disaster in deep coal mining is a typical phenomenon of coal and rock failure under the coupled effects of stress and gas. Gas and stress have increasingly contributed to the occurrence of dynamic disasters. A more thorough understanding of the characteristics and mechanism of infrared thermal images of coal and rock under the coupled effects of stress and gas is needed. In this paper, infrared radiation experiments of coal fracture under the coupled effects of stress and gas were conducted with an air sealed cylinder. The variation law of infrared radiation during coal failure under the effect of gas was studied. Three main findings were obtained from the experiments. (1) The infrared radiation temperature is strongly affected by gas during the failure process of coal samples. The infrared radiation temperature of coal fracturing without gas show a decreasing—increasing steadily—increasing abruptly trend. In contrast, in the fracture of coal with gas pressure via the exchange of heat between gas and the coal surface, the infrared radiation temperature at the coal surface has a decreasing—decreasing rapidly—rising trend. (2) The effects of different confining gas pressures on the fracture of coal induce various trends in the infrared radiation temperature. In the early stage of loading, a higher confining pressure of gas results in a smaller decrease in the infrared radiation temperature. Moreover, at the end of loading, the infrared radiation temperature at the surface of the coal body increases with an increasing confining gas pressure. The infrared radiation temperature has a minimum value during the loading process. When the confining pressure of gas is 0.2, 0.4 and 0.6 MPa, the infrared radiation temperature is a minimum at 74.1%, 67.1% and 58.3% of the load ratio, respectively. (3) Cloud maps of the infrared thermal image of the failure of gas-containing coal and contours of the infrared radiation temperature (which reflect the occurrence, expansion, rupture position and evolution of the crack temporally and spatially) can be used to predict the form and strength of the deformation and failure of gas-containing coal and locate precisely the position of a coal and rock dynamic disaster.