Creep and Shrinkage Law for Concrete at Variable Humidity

The drying creep effect is modeled by a nonlinear coupling between two types of hidden stresses, those in solids and those in (hindered adsorbed) water. Material parameters, as functions of pore humidity, of equivalent hydration period, and of relaxation time, are identified from the existing test data, using an optimization algorithm based on a least-square criterion and coupled with a finite element program for strain and stress history in a drying concrete cylinder. The ratios in which pore humidity modifies the creep parameters are found to be nearly the same for all concretes. Realistic estimates of stresses induced by shrinkage and by drying creep are also reached. The formulation is also extended to variable temperature. The success in fitting the data corroborates the underlying irreversible therodynamic theory of the creep mechanism, and especially confirms that creep at working stress levels is caused primarily by the diffusion of solids which, in turn, is facilitated by the presence of water and is nonlinearly accelerated by its migration.