According to many code requirements for concrete structures, the stress in the compressed part of reinforced concrete (RC) beams in bending is usually computed by means of uniaxial stress-strain relationships. These approaches sometimes are not able to reproduce the structural response of the beam when crushing of concrete in compression occurs. As a consequence, both the bearing capacity of RC structures and their ductility are roughly estimated. In this paper, the postpeak behavior of compressed concrete is modeled through sliding planes up to the expulsion of v-shaped block. The concrete damage on the surface of these-planes is the cause of the softening branch in the moment-curvature diagrams of RC beams. The proposed mathematical description of the sliding phenomenon also shows how it is impossible to define an univocal stress-strain relationship for concrete in compression, whose softening behavior depends both on the dimension of the compressive zone and on strain gradient (i.e., the curvature).
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