Postfiring stress-strain hysteresis of concrete subjected to various heating and cooling regimes

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Abstract

This paper reports an experimental investigation involving 90 siliceous aggregate concrete cores (75 mm dia. 175 mm long), which were subjected to 18 different heating and cooling regimes in the temperature range (217°C–470°C). The cores were heated to the point when the centre of the core reached the same temperature as the outer surface, the point of uniformity. Subsequently, the cores were either taken out of the furnace to cool or soaked at the test temperature for 2 h. Cooling was either by spraying the heated core with water for 5 min or by air-cooling in a controlled environment of 20°C and 65% RH. The extent of damage and internal fracture were assessed using the stiffness damage test (SDT) and the ultrasonic pulse velocity (UPV) as well as petrography methods. Both prolonged duration of thermal exposure and rapid cooling by water quenching resulted in a further loss of stiffness of fire damaged concrete cores. Extending the duration of thermal exposure by 2 h from the point of uniformity resulted in an approximately 10% further reduction in the residual chord modulus (Ec). However, spraying hot concrete with water after reaching the point of uniformity caused more reduction in stiffness than a 2 h extended duration of thermal exposure to temperatures below 320°C. The damage index (DI), which represents the dissipated strain energy in a hysteresis loop, showed a similar effect. The damage index of cores subjected to water quenching was significantly higher than the damage index of cores soaked at temperatures below 320°C. The residual plastic strain (PS), which is measured at the end of a hysteresis loop, showed very similar variation to that of the damage index. The ultrasonic pulse velocity (UPV) test method confirmed that spraying hot concrete with water from the point of uniformity was more damaging than a 2 h extended duration of thermal exposure to temperatures below 320°C. The extended duration of thermal exposure did not seem to affect the degree of non-linearity of the stress-strain response of fire affected concrete while water quenching resulted in a more concave response. The results are explained by identifying the various damage mechanisms associated with the various heating and cooling regimes. Copyright © 2002 John Wiley & Sons, Ltd.
Original languageEnglish
Pages (from-to)103-109
JournalFire and Materials
Volume26
Issue number3
DOIs
Publication statusPublished - May 2002

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