Study of the aging hereditariness of concrete through a novel viscoelastic formulation

This thesis focuses on the study of the creep deformations exhibited by concrete structures, with a particular attention to long-span prestressed box girders. During their service life, such structures can experience excessive multidecade deflections mainly due to the creep phenomenon and the large difference in shrinkage between the top and bottom slabs, sometimes causing damages of structural elements and huge economic losses. In order to prevent such consequences, the multidecade deflections of this class of structures need to be carefully predicted; therefore, very refined creep constitutive laws are required for relevant creep analyses. The most widely used creep model for the prediction of the time-dependent behavior of highly creep-sensitive structures is Model B3, which was calibrated through a data bank comprising results coming from different laboratories spread throughout the world. In this thesis, an already existing viscoelastic formulation, conceived for any viscous kernel, is integrated with Model B3 and the resulting finite element scheme is successfully applied to study the long-term behavior of a realistic structure, the Colle Isarco viaduct in Italy. Another contribution to this research work concerns the prediction of multidecade deflections exhibited by concrete structures through a novel creep constitutive law based on variable-order fractional calculus, resulting in an excellent feature with respect to classical creep models. Indeed, the creep deformations obtained through the proposed model are very close to the deformations evaluated by means of Model B3. Moreover, the suggested creep law is characterized by less aging terms than Model B3, with the consequent advantage to exactly derive the relevant relaxation function from the fundamental relationship of linear viscoelasticity. In order to perform creep analyses with the suggested fractional-order law, a numerical integration scheme characterized by a fractional-order viscous kernel is also developed and verified on realistic concrete structures subjected to multiple load histories. To the best of the author's knowledge, this research work presents the first creep constitutive lawavailable in literature that, through fractional operators, explores the time-dependent behavior of aging materials. Furthermore, a suitable numerical integration scheme is introduced and successfully applied to representative concrete structures.