Size effect on the contribution of the aggregate interlock mechanism in reinforced concrete beams without shear reinforcement

The shear strength of reinforced concrete beam remains an issue of great interest in structural engineering. For structural members without transverse reinforcements, the application of simple models (Strut and tie model, for instance) results an unsafe design. Also, most of the design codes are based on empirical formulas to design such members. These formulas are not adapted for some practical situations (Walls, Slabs, etc.). Different mechanisms are implicated in shear transfer. The contribution of the aggregates interlock mechanism has been considered in different approaches (MCFT theory, the critical shear theory, etc.). However, the crack-interface shear contribution is still difficult to assess. In the present paper, in order to clarify the contribution of this mechanism, three points bending tests were performed on geometrically similar beams. The Digital Image Correlation Technique is used to follow the kinematics of the cracking process. The fracture parameters obtained (Crack openings, crack sliding) by DIC are used to evaluate the contribution of the aggregates interlock mechanism using different analytical formulas. The results show that the crack-interface shear contribution is size-dependent and is dominant for large beams.