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The effects of elevated temperatures (Tm), related to the exposure period (th) and the curing age (ta), on the residual fracture properties of normal-strength concrete (NSC) and high-strength concrete (HSC) were investigated by conducting three-point bending tests on 87 notched preheated beams. Most beams were exposed to temperatures between 100°C and 600°C for 12 h at 14 days, while some NSC beams were heated either for various exposure periods up to 168 h at 14 days or for 12 h at 7, 28 and 90 days. The weight loss (ω) was also monitored. The measured residual properties included the energy parameter (fracture energy GF), a number of strength parameters (compressive strength fcu, tensile strength f′t and modulus of rupture fr), stiffness parameters (Young's modulus Ec and Poisson's ratio νc) and the brittleness parameter (the characteristic length lch). ω increased with Tm and th but decreased with ta. There existed a transition point for ω at 200°C which could mark a distinction between physical and chemical processes. GF increased with Tm and ω up to 300°C and then decreased. GF also increased with th at lower temperatures but decreased at higher temperatures, and increased with ta as well; fcu, f′t and fr did not change very much with Tm up to 200°C and decreased thereafter. A longer th had an intensifying effect on all strengths at lower temperatures but a damaging effect at higher temperatures. All strength parameters increased with ta. Ec and νc decreased continuously with Tm, th and ω. Ec increased whereas νc decreased with ta. The concrete became less brittle with increasing Tm, th and ω or with decreasing ta. In this study, all fracture parameters tended to become stable at 90 days. Finally, a linear relationship between GF and fcu existed not only for room temperature but also for higher temperatures.

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