Time-dependent behaviour of CFRP-strengthened reinforced concrete beams

Modern concrete structures, realized through complex sequential construction techniques and/or characterized by significant non-homogeneities, are in general very sensitive to the effects of time-dependent behaviour of concrete (creep and shrinkage). Guidelines for the evaluation of these effects were developed in the last decades by international pre-standard and standard institutions on the basis of a common, although progressively evolving, scientific background, and of a substantially worldwide harmonized format. The author discusses the development, with his large personal involvement, of this favourable scenario, evidencing areas of well established consensus and open problems. One pending problem is still represented by the uncertainties of prediction models, with particular regards to the multi-decade long-term prediction of creep. In what concerns the evaluation of the structural effects of creep, it is commonly accepted that a reliable analysis of the structural response in service conditions may be performed on the basis of the theory of ageing linear viscoelasticity, first established by Italian mathematician V. Volterra at the dawn of 20th century. The paper discusses the computational implications of this approach with reference on the one hand to the adoption of realistic advanced models for the prediction of the creep behaviour of concrete, and, on the other hand, to the complexity and sequential character of the constructions, and illustrates current updated guidelines developed at the international level for the evaluation of the effects of creep, both in the conceptual and preliminary design stages and in the subsequent detailed construction-stage and long-term reliability analyses of complex and sequential structures. These guidelines are intended to deal also with other phenomena, which are responsible of causing deviations from aging linear viscoelasticity, like tensile cracking, cyclic creep, and stress relaxation in prestressing tendons at variable strain, as well as the effects of humidity and temperature variations. The paper must be intended as a homage to the memory of CEB (Comité Euro-International du Béton, Euro-International Committee for Concrete) Honorary Member and member of the Academy of Construction and Architecture of the USSR Alexei A. Gvozdev, for long-time head of the laboratory of reinforced concrete of NIIZHB, the Institute for Concrete and Reinforced Concrete now named after him, for his crucial role in encouraging and assisting the author in the initial steps of transporting into CEB and FIP (Fédération Internationale de la Précontrainte, International Federation for Prestressing) ambient the fundaments of this new advanced format for creep analysis, to which the school of Soviet scientists and Gvozdev himself had given a fundamental contribution. The present edition of the paper incorporates some significant updates related to the advancement in the international debate, with respect to the previous edition published in the Journal “Industrial and Civil Engineering” (Promyshlennoe i grazhdanskoe stroitel’stvo) of December 2014.

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