Residual fracture properties of normal- and high-strength concrete subject to elevated temperatures

Abstract

The effects of elevated temperatures (T m ), related to the exposure period (t h ) and the curing age (t a ), 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 G F ), a number of strength parameters (compressive strength f cu , tensile strength f′ t and modulus of rupture f r ), stiffness parameters (Young's modulus E c and Poisson's ratio ν c ) and the brittleness parameter (the characteristic length l ch ). ω increased with T m and t h but decreased with t a . There existed a transition point for ω at 200°C which could mark a distinction between physical and chemical processes. G F increased with T m and ω up to 300°C and then decreased. G F also increased with t h at lower temperatures but decreased at higher temperatures, and increased with t a as well; f cu , f′ t and f r did not change very much with T m up to 200°C and decreased thereafter. A longer t h had an intensifying effect on all strengths at lower temperatures but a damaging effect at higher temperatures. All strength parameters increased with t a . E c and ν c decreased continuously with T m , t h and ω. E c increased whereas ν c decreased with t a . The concrete became less brittle with increasing T m , t h and ω or with decreasing t a . In this study, all fracture parameters tended to become stable at 90 days. Finally, a linear relationship between G F and f cu existed not only for room temperature but also for higher temperatures.