Modification of Equation for Calculating the Bond Stress Between Steel Bars and Concrete

The bond between steel bars and concrete is a fundamental aspect of reinforced concrete, ensuring the efficient transfer of forces between the two materials and guaranteeing their compatibility and composite action. The conventional equation for calculating bond stress does not account for slip effects. However, significant slip can lead to considerable errors, which are disadvantageous when analyzing bond performance using finite element methods. To address this issue, this study proposed a modified bond stress calculation model, whose reliability was systematically validated through pull-out tests and comparative analysis with a CEB-FIP model. The experimental program employed a universal testing machine for load application, with a static strain measurement system continuously monitoring stress–strain data throughout the loading process. The experimental results indicated that the failure process of the specimens could be categorized into the following, four distinct stages: micro-slip stage, slip stage, descent stage, and residual stage. The discrepancy between bond stresses calculated by the modified and conventional equations increased with the increasing relative slip, reaching its maximum value in the residual stage. A comparative analysis of 23 groups of experimental data from the literature was performed, revealing that the difference in bonding stress reached a maximum of 20.45% in the residual stage, with a standard deviation (S) of 0.006 and a coefficient of variation (Cv) of 0.321, reflecting small fluctuations but large dispersion. In contrast, the S of the residual bonding stress was 0.016, and the Cv was 0.080, showing lower dispersion but greater volatility. Further comparison with the CEB-FIP model confirmed the superior applicability and computational accuracy of the proposed modified model. Therefore, the modified equation should be utilized for calculating bond stress when significant slip occurs, particularly during the residual stage.