New classification system for fire-damaged concrete based on the strain energy dissipated in a hysteresis loop

This paper reports on experimental research into further developing the stiffness damage test (SDT) as a quantitative tool to be used in assessing fire-damaged concrete. The research programme reported here aimed at establishing the correlation between the strain energy dissipated in a hysteresis loop, performed on fire-damaged concrete core, and the post-firing residual stiffness characteristics. The dependency of such correlation on the level and mechanism of damage was investigated by involving five test variables, namely: maximum temperature of exposure, method of cooling, duration of thermal exposure, grade of concrete and type of aggregate used. The investigation involved approximately 180 concrete cores (75 mm in diameter × 175 mm long) which were extracted from river gravel and limestone aggregate concrete blocks. The cores were subjected to various heating and cooling regimes in the temperature range 217-470°C. The extent of fire damage was assessed using the SDT for determining the characteristics of quasi-static low-stress load-unload cycles. The strain energy dissipated in a hysteresis loop was adopted as a dimensionless damage index (DI) by dividing it by the stress range. Both the DI and the residual plastic strain were found to correlate well with the percentage residual initial and chord moduli. Such strong correlation is used to introduce a new classification system for the level of fire damage in concrete. A DI of 5 is associated with a 24% reduction of the chord modulus while a DI of 20 is associated with a 66% loss of the chord modulus. The coefficient of variation, which is used as a repeatability indicator, of the SDT parameters was found to be within acceptable limits. The correlation between the maximum temperature of exposure and the residual stiffness characteristics was also established. It is evident that the residual stiffness of fire-damaged concrete is not a sole function of the maximum temperature of exposure. The method of cooling, duration of exposure, and type and grade of the fire-affected concrete materials are major determinants in the extent of damage.