Influence of sustained loading on resistance and deformation capacity of reinforced concrete members in compression

As acknowledged explicitly or implicitly in most of current design codes, a high level of sustained loading has a detrimental effect on the concrete compressive strength. However, its beneficial effect in terms of deformation capacity is neglected in most cases. This latter topic and its practical consequences are addressed in the present paper on the basis of an experimental and theoretical investigation. In particular, the development of nonlinear creep strains in structural concrete members is investigated, clarifying the internal redistribution of forces between concrete and the reinforcement. The results of an experimental program with refined measurements are presented to better understand the phenomenon and to verify the predictions of a mechanical model developed by the authors on the time-dependent response of concrete. The experimental program consists of 14 prismatic specimens with different reinforcement ratios tested under a wide range of uniaxial stress rates. The results allow clarifying the material response and validating the mechanical model, both in terms of strength reduction and enhancement of deformation capacity. Finally, the results of parametric analyses, performed considering different concrete ages, reinforcement ratios, and materials properties are presented to evaluate the practical implications of the findings.