Experimental and numerical study on the reinforcement-concrete bond properties at sustained sub-high temperatures

Reinforced concrete structures serving in thermal environments, such as containment vessels of nuclear power plants and high-temperature workshops of plants, are subjected to prolonged sub-high temperatures (generally up to 350 °C). In this work, the mechanical properties of concrete and reinforcement-concrete bond properties at sustained sub-high temperatures (150 – 350 °C) were experimentally tested. The results indicate that when the temperature of the specimen cross-section reaches basic uniformity, the bond properties continue to degrade with increasing heating duration but eventually stabilise, remaining unchanged thereafter. Specifically, as the heating duration extends from 3 h to 24 h, the degradation of bond strength increases from 1.9 % to 3.4 % at 150 °C, from 5.9 % to 9.0 % at 250 °C, and from 12.4 % to 17.9 % at 350 °C. The deterioration in bond properties at high temperatures results from the continuous reduction in concrete strength and the ongoing development of thermal cracks with prolonged heating. After 24 h of exposure at 150 °C, 250 °C, and 350 °C, the compressive strength of concrete degrades by 7.0 %, 16.9 %, and 28.7 %, respectively, and the tensile strength degrades by 21.7 %, 31.8 %, and 49.1 %, respectively. Furthermore, the reinforcement pull-out process at high temperatures was numerically simulated, clearly illustrating the evolution of bond properties. The concrete stress exhibits a plough-shaped distribution during the ascending stage of bond stress, and the development of compressive damage in concrete significantly lags behind tensile damage. In addition, the bond stress distribution at high temperatures was also analysed based on the numerical model.