Why is the Initial Trend of Deflections of Box Girder Bridges Deceptive ?

Large-span prestressed concrete box girders typically exhibit during the initial years deceptively small deflections. or the deflections may even go upwards. However, optimistic expectations are premature. After about three to ten years. the deflections may begin to increase rapidly, become excessive. It is shown that, in order to predict the deflectiOns correctly, one must use a creep and shrinkage model that realistically describes the moisture diffusion process, which causes that the shrinkage and drying creep of the thick bottom slab of the box cross section are greatly delayed compared to the thin top slab. This delay cannot be predicted with the classical approach. in which either the shrinkage strain and the creep coefficient (or compliance/unction) are considered as uniform throughout the cross section, or the thickness effect is simply described by a multiplicative factor on shrinkage strain.