Creep in concrete is when under sustained load, strain increases without an increase in the load. Excessive creep can lead to unacceptable deformation, redistribution of the stresses, even collapse of the structure. Creep is generally recoverable after removing the load.
It has to do with the structural deformation of the Concrete matrix under constant long term load.
In pre-stressed concrete creep is anticipated and works to loosen the pre-stress tension or complete negation of the pre-stress.
Dynamic Viscous Damping is an empirical damping factor obtained by testing the structures dynamic response under applied seismic vibration as opposed to just randomly assuming a 2-5 % damping ratio.
Concrete structures show damping of vibration for a variety of reasons, not completely understood beyond plastic damping.
All these influences are summed up together under dynamic equivalent viscous damping ratio.
this ratio is calculated WRT the speed of vibration S.
Here is a figure of a model vibration result graph.
Some of the deformation under this damping is reversible.