Thermo-mechanical behaviour of granite during high-speed heating

Abstract

Fire accident is a common and serious damage phenomenon affecting tunnels, historical sites, buildings, monuments, etc. However, the understanding of the thermo-mechanical behaviour of rocks during fire and the corresponding damage on the rocks is still limited. In this study, the thermo-mechanical behaviour of Eibenstock granite under the temperature of 400 °C and 800 °C was studied applying heating rates of 5 °C/min, 200 °C/min, 300 °C/min, and according to ISO 834 standard fire curve. A supersonic frequency induction heating system was used to conduct the required heating scenarios and to perform uniaxial compression tests under the desired temperatures. A threshold modulus Ec obtained from dividing peak axial stress by the peak axial strain is defined to characterize the stiffness of the sample at various temperatures. Both tangent Young's modulus and threshold modulus are greatly influenced by maximum temperature, while the influence of heating rate is relatively small. The maximum temperature has the dominant impact on the uniaxial strength of granite sample, while the influence of heating rates is again negligible. The peak axial strain increased obviously with rising temperature. It first increases at 200 °C/min heating rate and then reduces slightly in scenarios with heating rate of 300 °C/min or heating according to ISO 834. The observed failure modes after uniaxial compression tests are influenced by peak temperature and heating rates. The granite specimen can be crushed in the shape of blocks, fragments or powder. The deduced friction angle for various heating scenarios is more or less consistent, while the cohesion shows a strong temperature-dependent characteristic. Although the cooling process can lead to an increase in crack density, UCS, peak axial strain and cohesion are enhanced at lower temperatures. The presented results help to understand the damage mechanisms of granite caused by fire, and can be used to develop guidelines for repair and maintenance as well as assessment of risks of tunnels and historical buildings after fire accidents.