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There are two principle mechanisms in transferring shear forces across a reinforced concrete crack, namely, aggregate interlock and dowel action.
The aggregate interlock refers to the interaction between rough surfaces of the crack. The shear stiffness of aggregate interlock is influenced by the axial tensile stiffness of the reinforcement.
When shear displacement occurs, there is a tendency for the crack to widen. In the meantime, the reinforcement restrains the crack widening, which subsequently increases in axial stress of the reinforcement. On the other hand, dowel action refers to the shear resisted by the reinforcement.