Prediction of interface shear strength between ultra-high-performance concrete and concrete using machine learning method

Ultra-high-performance concrete (UHPC) bonded to normal concrete (NC) can significantly enhance the mechanical performance of UHPC–NC composite structures, and the interface shear strength is a crucial indicator for assessing the bonding performance. In this study, interpretable machine learning (ML) methods were used to analyse the effects of different parameters on interface shear strength. A database consisting of 305 UHPC–NC shear tests was created, and the isolation forest algorithm was applied to filter outliers. Subsequently, four ML models were trained to predict the interface shear strength of UHPC–NC composite structures. Among them, the extreme gradient boosting (XGBoost) model demonstrated the highest prediction accuracy, achieving an R² value of 0.95. Shapley additive explanations (SHAP), partial dependence plots (PDP) and individual conditional expectation (ICE) were used for feature importance analysis, aiding in the interpretation of the ‘black box’ nature of the ML models. The results demonstrate that the normal compressive stress at the interface is the most influential factor affecting interfacial shear strength. Finally, a physically meaningful predictive equation for the interface shear strength of UHPC–NC composite structures was proposed based on the XGBoost model combined with curve fitting. This equation enhances the prediction accuracy of interface shear strength for UHPC–NC structures and offers deeper insights into the model’s decision making process.