Microprestress - solidification theory for aging and drying creep of concrete

A new physical theory for the effects of long-term aging and drying on concrete creep is proposed. The previously proposed solidification theory, in which aging is explained and modeled by the volume growth (into the pores of hardened Portland cement paste) of a nonaging viscoelastic constituent (cement gel), cannot explain long-term aging because the volume growth of the hydration products is too short-lived. The paper presents a revision of solidification theory in which the viscosity of the flow term of the compliance function is treated as a tangential viscosity of a nonlinear viscous power law govering very large and highly localized microstress in the hardened cement paste. The microstress is called the microprestress because it is produced during hydration by very large and highly localized volume changes, independent of loading. An essential aspect of the theory is that the applied external load or the macroscopic continuum deformation of concrete can cause only very small changes of the microprestress, such that the response to load is determined by tangential linearization. Relaxation of the microprestress causes the tangential viscosity to increase, which reduces long-term creep. A decrease of relative humidity in the pores causes (due to changes of capillary tension, surface tension and disjoining pressure) a large increase in the microprestress, which in turn reduces tangential viscosity and thus increases the creep rate. This explains the drying effect (Pickett effect). An improved constitutive law is formulated, verified and calibrated by comparisons of finite element solutions with test data.