  The solidification theory has been accepted as a thermodynamically sound way
to describe the 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 marked multi-decade decline of creep
viscosity after the hydration effect became too feeble, and for increase of
creep viscosity after any sudden change of pore humidity or temperature.
Recently, though, 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 (XMPS) that overcomes both problems and also improves a few other
features of the model response. To this end, different nano- and macro-scale
viscosities are distinguished. The aforementioned incorrect predictions are
overcome by a dependence of the macro-scale viscosity on the rate of pore
humidity change, which is a new feature inspired by previous molecular dynamics
(MD) simulations of a molecular layer of water moving between two parallel
sliding C-S-H sheets. The aging is based on calculating the hydration degree,
and the temperature change effect on pore relative humidity is taken into
account. Empirical formula 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.
