Bond performance in reinforced concrete exposed to moderately-high sustained temperatures

Many reinforced-concrete structures often found in nuclear power plants, waste-treatment plants, and metallurgical and chemical factories, are exposed to moderately-high temperatures (generally not exceeding 350 °C) for long periods. Bar-concrete bond in such conditions, however, have received so far limited attention mostly because of the complexity of the tests. In this work, the bond performance in concrete-embedded ribbed bars under moderately-high sustained temperatures was experimentally and theoretically studied. The specimens tested have a bonded length-to-bar diameter ratio of 8.75 and a cover-to-bar diameter ratio of 5.75. The results show that especially above 200 °C bond strength and stiffness markedly decrease, while bar slip at the peak of the bond stress and bond energy-absorption capacity increase. As an example, a 24-h exposure to 350 °C causes a 18 % and 85 % decrease in bond strength and stiffness, respectively, while bar slip at the peak stress and bond energy-absorption capacity increase by 114 % and 41 %, respectively. At relatively small values of bar slip, the bond stress is mainly provided by chemical adhesion, and the bond stiffness is more sensitive to temperature changes. Similarly, higher temperatures (up to 350°C) primarily affect bond performance due to the mechanical degradation of the concrete, that changes also the profile of the bond stress and of the slip along the bonded length. According to theoretical analysis, the higher the temperature and the longer the heating process, the more pronounced the shift of the maximum bond stress away from the loaded end of the anchored bar. A formula for the calculation of bond strength and a bond-slip constitutive model under moderately-high sustained temperatures are proposed as well.