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This paper describes a laboratory investigation into the adoption of the stiffness damage test to assess fire-damaged concrete structures. Laboratory-prepared concrete cores (75 mm diameter, 175 mm long) were fired under different heating regimes and their uniaxial compression stress-strain response at low stress level was determined. The area of hysteresis of the load-unload loops and other characteristics of the response such as the degree of concavity, the loading chord modulus, the unloading modulus and the residual plastic strain provide a quantitative measure of the extent of structural damage caused by thermal exposure. Fire-damaged specimens were also monitored by measuring the ultrasonic pulse velocity. The microstructure of the damaged specimens was studied using scanning electron microscopy (SEM) and X-ray diffraction. 320°C marked the onset of significant modification in the characteristics of the stress-strain response loops, with a sudden increase in the damage index (area of hysteresis loops). The ultrasonic pulse velocity in the fired concrete showed similar variation with temperature to that of the elastic properties. At temperatures higher than 320°C, SEM photographs showed significant cracks in the cement paste, especially in the interfacial zone.

This paper reports the results of experimental research into assessing the fractured state of fire-damaged concrete under rapid cooling in an attempt to establish the effects of fire-fighting operations on the structural integrity of concrete buildings subject to a fire. The investigation was driven by the need to improve the material models used in predicting the fire resistance of concrete members or redesigning such members after a fire incident. The extent of damage was quantified using the stiffness damage test. The stiffness characteristics of 40 fired limestone concrete cores were determined from a series of quasi-static load—unload cycles. The strain energy dissipated in the specimen during such cycles provides a quantitative measure of the damage. The damage index is defined as the area of the hysteresis loop divided by the stress range. Other stiffness parameters, such as the initial and chord moduli of elasticity, were also determined. The extent of damage was also examined using ultrasonic pulse velocity (UPV) and dynamic modulus methods. Visual examination and scanning electron microscopy were also used to provide qualitative assessment. Quenching hot concrete with water was found to result in a reduction in the stiffness of fire-affected concrete over the entire range of temperatures investigated. The damage index for air-cooled concrete after firing to 320°C is at the same level as that for concrete heated to 220°C and cooled by quenching. The inference here is that the traditionally held view that 300°C marks the onset of significant damage in concrete is debatable. UPV measurements confirmed the damaging effect of rapid cooling on the stiffness properties of fire-affected concrete. A further 10% reduction of the UPVof quenched concrete compared to air-cooled concrete was observed. The dynamic modulus measurement also confirmed the adverse effect of quenching on the modulus of elasticity of fired concrete.