Grouting can significantly improve the mechanical properties of a fractured rock mass. A set of indoor grouting test systems is designed, and 3D scanning and direct shear tests are carried out for 10 groups of grouted joint specimens. In this study, grouted joints are simplified to unfilled joints and cement stone (including the bulk paste and transition zone). The impact of cement stone on unfilled joints is defined as the filling effect and bonding effect. First, 3D morphological characteristics of unfilled joints are analyzed. Then, a weak-filling joint shear model is derived based on the existing shear strength model for unfilled joints. Finally, the shear model for grouted joints is derived from the shear model for filled joints. Reinforcement mechanisms, failure modes and causes of error in grouted joints are discussed. The effects of roughness, filling degree, rock to filling material strength ratio, normal stress and cohesion on peak shear strength are considered comprehensively in the new model for grouted joints. The new model is consistent with the basic form of the Mohr-Coulomb criterion. The good agreement between test data and calculated results from the new model indicates the new model is suitable for predicting the shear strength of grouted joints. [ABSTRACT FROM AUTHOR]