Creep refers to the time-dependent increase in strain under a sustained load, excluding other time-dependent deformations associated with shrinkage, swelling, and thermal expansion in concrete. The primary mechanism behind creep involves the loss of physically adsorbed water from the calcium silicate hydrate within the hydrated cement paste. This process is further exacerbated by concrete's non-linear stress-strain relationship, microcrack development in the interfacial transition zone, and delayed elastic strain in aggregate.

Stress relaxation can be observed under conditions where a concrete specimen is restrained, resulting in a constant strain. This manifests as a progressive reduction in stress over time. When the load is removed, the specimen undergoes an immediate elastic recovery, less than the initial elastic strain, followed by a more gradual decrease in strain. This gradual decrease is known as creep recovery. Creep is irreversible, indicated by some residual deformation. This irreversibility plays a pivotal role in hysteresis observed during cyclic loading, pertinent to structures like prestressed concrete where stress estimations during relaxation are vital.