Extended Microprestress-Solidification Theory for Long-Term Creep with Diffusion Size Effect in Concrete at Variable Environment

The solidification theory has been accepted as a thermodynamically sound way to describe creep reduction due to deposition of hydrated material in the pores of concrete. The concept of self-equilibrated nanoscale microprestress has been accepted as a viable model for the marked multidecade decline of creep viscosity after the hydration effect becomes too feeble, and for increases of creep viscosity after any sudden change of pore humidity or temperature. Recently, however, it appeared that the original microprestress-solidification theory (MPS) predicts incorrectly the diffusion size effect on drying creep and the delay of drying creep behind drying shrinkage. Presented here is an extension named XMPS that overcomes both problems and also improves a few other features of the model response. To this end, different nanoscale and macroscale viscosities are distinguished. The aforementioned incorrect predictions are overcome by dependence of the macroscale viscosity on the rate of pore humidity change, which is a new feature inspired by recent molecular dynamics (MD) simulations of a molecular layer of water moving between two parallel sliding calcium-silicate-hydrate (C-S-H) sheets. The part of aging that is not caused by microprestress relaxation is described as a function of the growth of hydration degree, and the temperature change effect on pore relative humidity is also taken into account. Empirical formulas for estimating the parameters of permeability dependence on pore humidity from concrete mix composition are also developed. Extensive validations by pertinent test data from the literature are demonstrated.