Bond and size effects on the shear capacity of RC beams without stirrups

• A new model based on NLFM concepts is introduced to analyze shear strength in RC beams without stirrups.
• The model proposed identifies the variables that govern shear failure, including the bond between concrete and steel.
• A failure criterion based on the crack depth observed on experimental results is proposed.
• Size effect in shear strength is reproduced by this approach, since it stems from non-linear FM concepts.
• Stronger steel-to-concrete interfaces lead to narrower openings and shorter cracks, so shear strength increases.

The paper presents a model which allows studying the influence of bond between the reinforcing bars and the concrete matrix and the size effect on the evaluation of shear strength in reinforced concrete beams without stirrups. The formulation assumes that shear failure is caused by the propagation of flexural cracks. When the crack length reaches a certain depth, the so-called critical depth, the section collapses. This depth depends on the position of the section being studied, the external load, the beam boundary conditions and geometry. Non-linear concepts of Fracture Mechanics are used to model concrete behavior in tension during the crack propagation. Size effect is reproduced through Bažant’s law (Bažant and Pfeiffer, 1987). Bond slip is considered by a rigid-plastic bond–slip curve. The results of the model can provide an understanding of the influence of steel-to-concrete bond on the shear strength and the size effect exhibited in test results and its asymptotic behavior. All these topics are of the utmost importance to concrete technology, yet they are not satisfactorily dealt with by construction codes.