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

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 degrees C and 600 degrees C for 12 h at 14 days, while some NSC beams were heated either for various exposure periods up to 168 h at 14 clays or for 12 h at 7, 28 and 90 days. The weight loss (omega) was also monitored. The measured residual proper ties 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 v(c)) and the brittleness parameter (the characteristic length l(ch)). omega increased with T-m and t(h) but decreased with t(a). There existed a transition point for omega at 200 degrees C which could mark a distinction between physical and chemical processes. G(F) increased with T-m and omega up to 300 degrees 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 degrees 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 v(c) decreased continuously with T-m, t(h) and omega. E-c increased whereas v(c) decreased with t(a). The concrete became less brittle with increasing T-m, t(h) and omega 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.