Influence of initial moisture content on creep and shrinkage of concrete at constant and cyclic ambient humidity

Creep and shrinkage are strongly influenced by the moisture content of concrete and by the ambient humidity. The effect of the latter is currently considered in models and design codes such as the fib Model Code 2010 by time-average considerations, thus totally neglecting cyclic changes in ambient humidity normally observed in outdoor structures. However, the influence of the cyclic ambient humidity on the long-term deformations of concrete compared to the constant mean humidity has barely been investigated. Therefore, the developments of creep and shrinkage of a normal-strength concrete C30/37 with three initial (before testing) moisture contents at constant humidity as well as cyclic humidity were investigated. Results demonstrate that the drying shrinkage and drying creep, but also the basic creep, are higher with higher initial moisture contents at constant ambient humidity conditions. Additionally, creep strains sharply increased up to plus 60% due to cyclic humidity compared to those due to corresponding average constant humidity. The influence of cyclic ambient humidity is more pronounced for concretes with lower initial moisture contents. Those strong deviations in the creep behaviour (compared to existing design models) may cause severe safety risks in prestressed concrete constructions. Therefore, existing creep models should be modified considering the influence of cyclic changes in humidity and the combined influence of the initial moisture content.