Effect of sustained thermal exposure on the bonding characteristics of plain bars in concrete

Abstract

Some reinforced concrete structures, such as nuclear power plant containments, chimneys, and factory thermal workshops, usually serve in thermal environments (generally not exceeding 350 °C). The purpose of this study is to explore the bonding characteristics of plain bars (PB) in concrete under sustained thermal exposure, which has received little attention in the past. Pull-out tests in thermal environments, with temperature (150 °C – 350 °C), thermal exposure duration (3 h – 24 h), concrete strength (C40 – C80), and reinforcement type (PB and ribbed bar (RB)) as variables, were conducted. The results show that bond failure for PB-concrete specimens under thermal exposure is characterised by reinforcement pull-out. The increase in temperature induces a continuous deterioration of bonding properties, as evidenced by the decrease in bond strength and bond stiffness, as well as the increase in peak slip and energy absorption. The bond stress of PB comes from chemical adhesion and friction. At 150 °C, chemical adhesion decreases slightly and friction increases slightly, resulting in a small reduction in bond strength. With increasing temperature, the hydration products dehydration becomes more prominent, the damage of chemical adhesion and friction intensifies, and the reduction in bond strength is increasingly evident. The deterioration of bonding properties does not stop when the specimen cross-section temperature distribution reaches uniformity, but continues to evolve with thermal exposure duration and can be essentially stabilized within 24 h. Furthermore, a bond strength formula and a bond-slip constitutive model for PB in concrete under sustained thermal exposure were established.