Modeling and numerical investigation of the viscoelastic behavior of laminated concrete beams strengthened by CFRP strips and carbon nanotubes

The paper aims to prove that the application of innovative constituents and materials can noticeably affect the mechanical behavior of structures. The research is focused on the long-time behavior of concrete beams reinforced by Carbon Fiber Reinforced Polymer (CFRP) strips applied on their external surfaces. The concrete and the CFRP are both characterized by time-dependent mechanical features according to the theoretical framework provided by the linear viscoelasticity. Their properties are described through the introduction of the proper creep functions. The reinforcing strips are made of a polymer matrix reinforced by straight long Carbon fibers and randomly oriented Carbon nanotubes (CNTs). Due to the presence of two reinforcing phases at different levels (nano- and micro-scale), a multiscale model is introduced to compute the global mechanical properties of these innovative composites. Their characterization at the nano-scale is accomplished through the Eshelby-Mori-Tanaka scheme, whereas the Hanh approach provides the overall engineering constants of the CFRP strips. The Timoshenko beam theory for laminated beams is employed to describe the mechanical behavior of the structures. A numerical solution is developed to achieve the time dependency of central deflections and the redistribution of stresses along the thickness of the layered structures. Several responses are investigated to show also the effect of the mass fraction of CNTs, the thickness and the number of CFRP strips. The results presented in this paper could be taken into account to improve the structural response of concrete beams in contrasting the creep phenomenon due to the intrinsic nature of the material