The role of surface micro-cracks in cementitious materials responsible for the Pickett effect

The Pickett theory for the development of drying creep has been discussed by many authors to explain creep-induced shrinkage strains and the development of microcracking caused by creep loads. This paper suggests a micromechanical modeling approach of drying creep in cement pastes by taking into consideration the heterogeneous distribution of cement phases in constructed representative virtual microstructures. Damage was considered in a viscoelastic behavior law for the development of a desiccation model that takes into account the diffusion of water in the capillary porosity. Tensile and compressive tests were simulated to determine the strengths of the cement paste, and were compared with experimental data, and then drying and creep were simultaneously simulated. The results of the simulations show the presence of the Pickett effect in both tension and compression. The analysis of damage evolution at different depths of a macroscopic beam illustrates an apparent surface microcracking higher than the core of the volume when typical creep loads are applied. The proposed model can therefore be used to predict creep phenomena in such a material.