Bond behaviour of ribbed steel bar and concrete at 24-h elevated temperatures: Experimental study and theoretical analysis

This study attempts to remedy the lack of understanding on the bond behaviour of steel bar in concrete at elevated temperatures in current research. Pull-out tests were conducted using a unique test setup, and the effects of temperature (120 °C – 350 °C), heating duration (3 h – 24 h), and anchorage length (105 mm – 175 mm) were given attention. In addition, calculation theories for bond behaviour at elevated temperatures were derived. All specimens eventually showed failure characterised by pull-out-splitting. The increase in temperature induces a continuous decrease in bond strength and a continuous increase in peak slip. Moreover, the bonding performance deteriorates gradually with the heating duration, but could be stabilised at 24 h. This can be interpreted as a sufficiently long heating duration allowing full development of concrete thermal damage. Under the same heating regime, the specimens with longer anchorage lengths show lower bond strength, however, the degree of change in bond strength with temperature is not significantly related to anchorage length. The specimens with anchorage lengths of 105 mm, 140 mm, and 175 mm respectively exhibit 15.4 %, 16.9 %, and 16.4 % reduction in bond strength, as the temperature increases from 120 °C to 350 °C. Increasing the temperature helps the bond stress to distribute more uniformly in the bonding zone. However, increasing the anchorage length intensifies the non-uniform distribution of bond stress and the stress peak becomes larger. The bond-slip constitutive model developed considering anchorage length is proved to accurately predict the bond behaviour of steel bar in concrete at elevated temperatures. For reinforced concrete structures working in thermal environments, 0.08 is suggested as ribbed steel bar shape factor.