Effetti di scala sulla capacità di rotazione plastica di travi in calcestruzzo armato

The effect of the structural dimension is often neglected in most of the models provided by the international codes for the design of reinforced concrete structures. Although a lot of studies put in to evidence the important influence of the structural dimension on the mechanical behaviour of concrete structures, only in few situations this aspect is taken into account, usually according to empirical relationships based on experimental results. The problem of size-scale effects is not easy to analyse and, both due to computational aspects and to the requirement of establishing simple design formulae, most of the models in the available codes focus on simplified limit configurations where the mechanical response is predicted by imposing the equilibrium equations. In this context, nonlinear Fracture Mechanics models can profitably be used to obtain more accurate descriptions of the structural behaviour, with the possibility to give an insight onto the problem of size-scale effects. In the instance of quasi-brittle materials like concrete, in fact, Fracture Mechanics theories have been demonstrated to be an effective analysis tool, as, e.g, for the evaluation of minimum reinforcement in reinforced concrete beams in bending. This Thesis aims at addressing in a rational and exhaustive way some important size effects characterizing the mechanical behaviour of concrete and reinforced concrete structures. Among which, we mention the analysis of the tensile and compressive behaviours of plane concrete and the evaluation of the rotational capacity of reinforced concrete beams in bending. Both these aspects are very important in the modelling and designing fields and they are only qualitatively included in the provisions of the design codes. After a wide review of the state-of-the-art literature, original features of this Thesis include: (1) the definition of a new nonlinear model, referred to as Overlapping Crack Model, for the description of the behaviour of concrete in compression, based on the concept of strain localization; (2) the development of a new unified model, obtained by merging the two elementary models in tension and compression, the cohesive and overlapping ones, able to fully analyse the behaviour of reinforced concrete beams in bending by varying the main design parameters; (3) a wide validation of the proposed model by means of a comparison with the results of several experimental programmes; (4) the analysis of the main lacks in the design codes concerning the ductility of reinforced concrete beams and a proposal of new design diagrams that are easy to use.