Modeling of Creep and Hygrothermal
Deformations of Concrete: Intriguing
Consequences of Nano-Porosity
Zdeněk P. Bažant1 and Gianluca Cusatis2
2Rensselaer
Polytechnic Institute,
Abstract: Portland cement concrete is a rather unusual porous material. It exhibits both capillary porosity and sub-capillary nano-porosity. The physical and chemical processes in the nano-pores (or gel pores) are believed to be the cause of intriguing creep properties, very different from those of other viscoelastic materials. The creep amplitude changes by an order of magnitude due to multi-year aging. This is explained only partly by chemical hardening because the creep amplitude shows a several-fold decrease for many years even after the chemical hydration process, which fills the pores with calcium silicate hydrates (C-S-H, or tobermorite), has stopped and the elastic stiffness ceased to grow. The Pickett effect, also called the drying creep, has been a perplexing phenomenon studied for over 60 years without reaching complete understanding, especially from the materials science viewpoint. This effect, which represents a strong transient increase of creep due to a change of environmental humidity, is only partly explained by the difference in microcracking between the companion creep and shrinkage specimens, and is certain to have a major source in the nano-structure and nano-pores. A related effect is the transitional thermal creep, which is a transient creep increase caused by any change of temperature. The lecture reviews these special consequences of nano-porosity and then focuses on the mathematical modeling. Outlined is the microprestress-solidification theory, which a theory that has achieved a unified description of the known creep properties of concrete, including the Pickett effect and the transitional thermal creep, and also the decline of creep amplitude after the hydration process has terminated. However, identification of the physical nature of microprestress build-up and relaxation in the nano-structure is still lacking. A possible clarification of this question that can result from the recent advances of F.-J. Ulm et al. at MIT and H. Jennings et al. at Northwestern is discussed. Computer simulations and their comparisons with various typical experiments are reviewed. In conclusion, some promising research directions are suggested.
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