10.3
Creep in Concrete
Creep refers to the time-dependent increase in strain under a sustained load, excluding other time-dependent deformations associated with shrinkage, s…
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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Q1: What causes creep in concrete under sustained loading?
Creep occurs when concrete experiences time-dependent strain increase under constant stress. The primary cause is loss of physically adsorbed water from calcium silicate hydrate in the hydrated cement paste. Additional factors include concrete's non-linear stress-strain characteristics, microcrack propagation in the interfacial transition zone, and elastic deformation in aggregates as stresses transfer from paste to aggregates over time.
Q2: How does stress relaxation differ from creep in concrete?
Stress relaxation occurs when a restrained concrete specimen is subjected to constant strain, resulting in gradual stress reduction over time. Unlike creep, which involves strain increase under constant stress, stress relaxation maintains fixed strain while stress decreases. Both phenomena reflect concrete's time-dependent behavior, but stress relaxation is observed in constrained conditions where deformation cannot occur freely.
Q3: What happens to concrete when a sustained load is removed?
Upon unloading, concrete undergoes immediate elastic recovery that is less than the initial elastic strain experienced during loading. This is followed by gradual strain reduction called creep recovery. However, creep is not entirely reversible; residual deformation always remains in the member, indicating permanent plastic deformation has occurred.
Q4: Why is creep irreversible in concrete structures?
Creep is irreversible because the loss of physically adsorbed water from calcium silicate hydrate is a permanent process that cannot be fully reversed upon unloading. Although some elastic recovery occurs, residual deformation persists. This irreversibility is critical in prestressed concrete design, where stress estimations during relaxation are vital for long-term structural performance and safety.
Q5: How does microcrack propagation contribute to concrete creep?
Microcrack propagation in the interfacial transition zone between aggregate and cement paste contributes to creep by allowing stress redistribution and internal deformation. Combined with elastic deformation in aggregates as stresses transfer from paste over time, these microcracks facilitate the gradual strain increase characteristic of creep, alongside water loss from the cement paste.
Q6: What role does the interfacial transition zone play in concrete deformation?
The interfacial transition zone is the region between aggregate and cement paste where microcrack propagation occurs under sustained loading. This zone is particularly susceptible to cracking due to stress concentration and weaker bonding. Microcrack development in this zone contributes significantly to types of non structural cracks in concrete and facilitates the time-dependent strain increase observed during creep.
Q7: How does creep affect hysteresis in cyclically loaded concrete?
Creep's irreversibility plays a pivotal role in hysteresis observed during cyclic loading of concrete. The residual deformation that remains after unloading creates energy dissipation loops in stress-strain cycles. This effect is particularly important in prestressed concrete structures, where understanding creep-induced hysteresis helps engineers accurately estimate stress changes and ensure long-term structural reliability.