MATERIALI COMPOSITI PER APPLICAZIONI DI INGEGNERIA CIVILE: ESPERIMENTI, MODELLAZIONE E PROBLEMI DI PROGETTAZIONE

One of the challenging tasks in the field of the applied structural mechanics is related to the conception, design and use of advanced microstructured materials. In this context, and referring to applications in Civil Engineering, a great research effort has been focused in the last decades on the employment of composite materials for strengthening and retrofitting of existing structures as well as for conceiving and designing new structural paradigms, such as those related to the so-called all-composite structures. In particular, a well-established type of composite material adopted for civil applications is represented by the Fiber-Reinforced Polymers (FRP). In this view, analytical and experimental assessment procedures are essential for allowing the identification of dominant mechanical features of both materials and struc tures, as well as for defining proper technical standards and design guidelines. On the other hand, the need to satisfy specific requirements and/or to enhance the structural performance (in terms for instance of stiffness, strength, and durability) push towards the conception and design of advanced and innovative materials. To reach such a goal, a conscious knowledge of dominant multiscale effects and mechanisms occurring through the microstructural arrangement should be properly identified. Motivated by the above observations, the main purpose of present thesis focuses on the modelling and the analysis of the mechanical response of some traditional and novel composite materials for civil engineering applications. In particular, composite materials comprised of basalt-based reinforcement are analyzed, by critically discussing with a perspective view on the feasibility of structural applications. As a specific topic, FRP-based strengthening of existing concrete structures has been addressed via analyt ical models, experimental investigations, and design issues. In detail, flexural behavior of FRP-strengthened reinforced concrete beams has been evaluated with an analytical mesoscale approach that takes into account the non-linearity of the different constituents. In this context, the influence of the constitutive features of the strengthening composite on the overall structural performance has been specifically addressed. Moreover, the debonding failure that may occur at the FRP-concrete interface, penalising the FRP ex i ploitation, has been experimentally investigated for FRP-concrete systems by addressing basalt-based FRP, and deriving measures of debonding load and effective bond length. Starting from these experimental data and by referring to similar tests available in lit erature -also associated to different FRP types-, a large database has been identified. It has been used to verify soundness and effectiveness of available technical indications and design guidelines against debonding failure. As a result, an effective refinement of technical relationships, based on a novel calibration of the corresponding model param eters and able to successfully describe the experimental evidence, are proposed taking into a careful consideration the influence of both FRP stiffness (mainly related to the fiber type) and FRP implementation procedure. From an engineering point of view, the mechanical response of composite materials is usually described by considering simplified approaches in micromechanical modelling. For instance, possible asymmetries in composite constitutive response under tension and compression states are generally neglected. Aiming to furnish a contribution in this con text, the non-linear mechanical response of bimodular materials (i.e., materials exhibit ing different moduli in tension and in compression) has been investigated, by referring to the so-called Curnier-type materials. Finally, the effective constitutive response of heterogeneous materials comprising bimodular phases has been studied via a computa tional strategy based on an iterative finite-element scheme. As a result, clear numerical indications on the influence of the local non-linearity induced by heterogeneous strain fields on the macroscale response of the material are obtained and discussed. Accord ingly, proposed results, obtained by addressing both composites comprised of bimodular phases and bimodular porous materials, highlight how bimodularity features could be exploited for designing of novel engineered materials for advanced applications. Keywords: composite materials, fiber-reinforced polymers, basalt fibers, basalt-based composites, FRP-concrete systems, flexural strengthening of RC elements, experimen tal testing, debonding failure, technical standards, heterogeneous materials, bimodular materials, non-linear constitutive response, computational homogenization.