Ductile-to-brittle transition in high-performance prestressed concrete beams: new standards for a safe and effective design

The structural behaviour of prestressed concrete beams is considerably affected by different nonlinear phenomena occurring in the post-cracking and crushing regimes, such as snapback or snap-through instabilities.Design procedures included in current technical Standards are not able to take into account the actual flexural crushing regime, since the adopted constitutive laws overlook the strain-softening and strain-localization behaviour of the concrete matrix.Moreover, design provisions are usually based on Plasticity Theory, leading to completely disregard size-scale effects and ductile-to-brittle transitions as functions of the beam depth.The present work intends to outline a comprehensive theoretical framework for prestressed concrete structural behavior by means of a Fracture Mechanics approach.The Cohesive/Overlapping Crack Model (COCM) is able to simulate the strain-softening and strain-loaclization behaviour of concrete both in tension and compression, predicting the nonlinear crushing behaviour of prestressed concrete beams.As a matter of fact, the correct estimation of scale effects on maximum reinforcement percentage requires a thorough knowledge of the complex phenomena characterizing the compression crushing failure, leading to define the field in which prestressed concrete structures can develop a safe ductile behaviour.New standard requirements for an effective structural design are formulated.