Elastic deformation, shrinkage and creep

Elastic deformation

Immediate strain that appears on loading and disappears on unloading. For concrete it follows a curved stress-strain path, but within working stress it is treated as linear with $E_c = 5000\sqrt{f_{ck}}$ N/mm$^2$ (IS 456, Cl. 6.2.3.1). Elastic strain $\varepsilon_e = \sigma/E_c$.

Shrinkage

Shrinkage is the time-dependent reduction in volume of concrete, without any applied load, caused by loss of moisture and by chemical reaction (hydration and carbonation). Typical long-term values are 0.0003 to 0.0008 (300 to 800 microstrain). Main types are plastic, drying, autogenous and carbonation shrinkage. It is reduced by low W/C, large aggregate, good curing and joints. IS 456 (Cl. 6.2.4.1) gives design 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 values are 2.2, 1.6 and 1.1 for loading at 7 days, 28 days and 1 year (IS 456, Cl. 6.2.5.1). Creep raises deflections and causes loss of prestress; part of it is recoverable on unloading.