7. Materials with Aging

We call aging the change in the mechanical properties of a given material with age which is the time period between some origin more or less arbitrarily established and the time of observation. Concrete is a material that may be used as an example: from the moment of casting (taken usually as age zero) it begins to increase its strength and to decrease its deformability. In the case of polymers both physical (reversible) and chemical (irreversible) aging are observed. In the present chapter we introduce the equations for viscoelasticity with aging for situations in which compliance diminishes (“hardening”) and for situations in which compliance increases (“softening”) with age in integral form and through rheological models and state variables equations. The time-age equivalence model applied to the physical aging of polymers is also discussed.