AbstractMortar–concrete expanded piles are a new type of composite pile. Understanding the shear transfer characteristics of the mortar–concrete bonded interface plays an important role in the design and analysis of mortar–concrete expanded piles. The direct shear test study mainly focuses on the influence of different cement mortar strengths and normal stresses on the shear behaviors of the cement mortar–concrete interface. The experimental results showed that the peak shear stress at the mortar–concrete interface is strongly related to the bond strength of the interface. The interface residual shear stress is essentially the frictional stress, which is related to the normal stress and interface friction coefficient. Before the peak stress is reached, the influence of the normal stress on the interface behaviors is not substantial. In addition, sudden bond failure of the interface can occur. A calculation model of the cement mortar–concrete interface is proposed to simulate the experimental behaviors, including the trends of the variation in the shear stress with the change in the shear displacement and the behaviors of the shear stiffness, peak shear stress, and residual shear stress. The comparison of the estimated curve with the experimental curve indicated that the model can describe the load-transfer mechanism with sufficient accuracy. This study is of great significance to the wider application and development of mortar–concrete expanded piles.

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