Creep occurs over time and at very low stresses. It's rate does increase at at stresses over about .4 - .5 Fc but it is still a time based effect.
Plastic deformation is a result of the change in the stress strain relationship of concrete (Concrete Modulus which is the slope of the stress/strain curve) at high stress. It is relatively linear up to about .4 - .45 Fc and then drops significantly as stress increases. It is an instantaneous effect unlike creep.
