Creep phenomenon with time-dependent graph

Creep is the gradual increase in strain of concrete with time under a constant sustained stress, over and above the instantaneous elastic strain. It is caused mainly by seepage of gel water out of the C-S-H layers and by slip of gel particles. Creep continues for years, with about 50% occurring in the first 3 months and 75 to 80% in the first year.

 strain
   |
   |                               _________
   |                        _..--''         |
   |                  _..--'     creep      |   instantaneous
   |            _..--''                     |   (elastic) recovery
   |        .-''                            |
   |        |                               '.
   |        |  instantaneous                  '-._
   |        |  elastic strain                     ''--.__   creep recovery
   |        |                                           ''--.__  (slow, partial)
   |        |                                                  ''-------- permanent
   |        |                                                             deformation
   +--------+------------------------------+---------------------------------- time
   0     load applied                  load removed

• On loading, an instantaneous elastic strain appears.
• Under sustained load, strain keeps rising with time (creep), fast at first, then slowly.
• On unloading, an instantaneous (elastic) recovery occurs, nearly equal to the elastic strain on loading (slightly less because $E$ rises with age).
• Then a slow creep recovery follows, which is only a part of the creep strain.
• The remaining strain is the permanent (irrecoverable) deformation.

Mechanism: (i) seepage of adsorbed and gel water from the C-S-H under sustained stress, (ii) viscous slip of gel particles, (iii) microcracking at high stress and (iv) delayed elastic effect of aggregates. Specific creep is the creep strain per unit stress.

Creep coefficient and factors. Creep coefficient $\theta = \varepsilon_{creep}/\varepsilon_{elastic}$. IS 456 (Cl. 6.2.5.1) gives $\theta$ = 2.2 (loaded at 7 days), 1.6 (28 days) and 1.1 (1 year). It increases with higher W/C and paste content, lower strength at loading, higher stress (up to about 40 to 50% of $f_c$), lower humidity, smaller members and lower-modulus aggregates. It decreases with later age of loading, good curing and stiff aggregates.

Effects: increases deflection of beams and slabs, loss of prestress, redistribution of stress from concrete to steel in columns, but also relieves stress concentrations and reduces cracking due to shrinkage.