To establish a working system in field to evaluate the degree of fire-damage readily for both brittle and ductile tunnel supporting materials, this research focused on the failure characteristics and evolution on both macro-scale (stiffness, strength and toughness) as well as micro-scale (localization, crack initiation and propagation). After applying various heat-driven-damage treatments, both brittle material such as concrete (rock-like material) and ductile steel were conducted uniaxial compressive and tensile tests respectively. Furthermore, this study propose a velocity ratio, defined as VS/VP, to be an index to identify the degree of thermos-induced damage in field.
By applying maximum temperature varied from 25 to 1200 °C with constant rate of heating (5°C /min), exposure time (300 mins), and cooling condition (cooling in furnace), the uniaxial compressive and tensile tests with post-peak behavior were obtained by controlling the lateral COD (crack opening displacement) as a feedback signal to conduct the closed-loop experiences. Associated with ultrasonic pulse (UP) and synchronized acousto-optic nonintrusive technique (AE & ESPI), macro- and micro-scale failure of materials were examined corresponding to entire loading histories of uniaxial compressive tests.
The results show that VS/VP has a positive correlation with degree of thermos-induced damage on concrete, the correlation coefficient of VS/VP between stiffness, strength and toughness are -0.90, -0.87 and -0.77 respectively. It proves that the VS/VP can be used to identify the degree of thermos-induced damage such as fire disaster. In macro-scale, concrete suffer heat-driven damage cause the post-peak stability from snap back (Class II) convert to snap through (Class I), and the transition temperature is between 200 to 400°C. In micro-scale, as the maximum temperature increase, the more severe the degree of thermos-induced damage, in addition, it leads to more insignificant "localization" phenomenon gradually. On the other hand, ductile material such as steel during heat-up and cool-down treatment shows insignificant response of V P, V S.
