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.
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.
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