If the load is maintained for a considerable period of time, concrete undergoes additional deformations even without any increase in the load. This time-dependent strain is termed as creep.
The reversal of creep is not fully complete, continuous use of load over period of time results in a residual deformation or permanent deformation or irreversible creep.
In creep, loss is due to sustained applied stress. They increase with higher cement content, higher water content, lower aggregate content, low relative humidity, high temperature, small thickness of the member, etc.
If the load is removed, only the instantaneous elastic deformation will recover- the plastic deformation will not.
Creep is factored in when concrete structures are designed.