Shrinkage and creep of concrete

Shrinkage

Shrinkage is the time-dependent reduction in volume of concrete, without any applied load, caused by loss of moisture and by chemical reactions (hydration and carbonation). Typical long-term values are 0.0003 to 0.0008 (300 to 800 microstrain).

Types of shrinkage

Effects: surface and through cracks, warping of slabs, loss of prestress, and extra deflection. It is reduced by low W/C, low paste content, large well-graded aggregate, proper curing and joints. When no test data are available, IS 456 (Cl. 6.2.4.1) allows an approximate design shrinkage strain of 0.0003.

Creep

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.

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.

 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.