Recent studies suggested that classic models based on stress-strain constitutive laws are not able to thoroughly predict the behaviour of reinforced concrete (RC) and prestressed concrete (PC) structures and the ductile-to-brittle transitions that might arise for large size-scale beams as well as high-performance concrete matrices. On the other hand, Fracture Mechanics has been demonstrated to be capable of describing in a unified approach the mechanical instability phenomena occurring for RC beams in bending, such as concrete cracking in tension, steel slippage and/or yielding as well as concrete crushing in compression. In this framework, numerical analyses recently carried out by means of the Cohesive/Overlapping Crack Model (COCM) have defined scale-dependent upper and lower reinforcement limits, in which RC and PC beams exhibit a stable and ductile behaviour.