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When a loaded concrete member is subjected to constant stress that is sustained over time, the strain in the member increases, resulting in its deformation, which is known as creep.

This is primarily because, under continuous stress, the hydrated cement paste loses the physically adsorbed water from the calcium silicate hydrate in it.

Creep can also result from the concrete's non-linear stress-strain characteristics, microcrack propagation in the concrete’s interfacial transition zone, and elastic deformation in aggregates due to the transfer of stresses from the paste to the aggregates over time.

When a loaded concrete specimen is restrained, subjecting it to constant strain, creep is observed as a gradual reduction in stress over time, known as stress relaxation.

Consider a plain concrete specimen continuously subjected to uniaxial compression. Upon unloading, an immediate elastic recovery occurs that is less than the initial elastic strain experienced by the specimen when it was loaded.

This is followed by a gradual strain reduction, known as creep recovery. However, this is not entirely reversible; a residual deformation always exists in the member.