Concrete Creep

When a concrete member is subjected to a constant load, id deforms instantly based on what you have learned in mechanics of material. But amazingly there is another deformation that happens over a long period of time. In other words, a sustained load causes a degree of deformation a long time after the initiation of loading. This secondary deformation is referred as concrete creep. Creep increases during the time even if the loads remains constant.
However creep does not necessarily cause the structural failure of the member, but it can be problematic in terms of building authentic. It can also undermine the safety feeling of the occupants as nobody wants to live under a deformed roof.
What factors are influential on the amount of creep deformation?
Generally there are 3 factors affecting on creep behavior of concrete members. Aggregate, the quality of concrete mixture and concrete age. Briefly, the paste is more responsible than aggregates for creep behavior. However, stronger aggregates, or a their higher proportion can reduce the creep deformation of concrete members. Increase in water/cement ratio can sharply increase the creep behavior of the members. Finally, as the paste is going to be strengthen during the time, the age at which the member is loaded plays a crucial role in the creep behavior. Younger concrete members show a larger creep deformation.
What are the creep consequences?
Reduce the authenticity of the building
Undermining the safety feeling of the occupants
Cracking in plaster coating
Causing problems for delicate machines and installations attached to the structure
How to estimate the amount of creep deformation
To estimate the amount of creep, first you need to calculate the elastic deformation of the member (equation 1). Table and figure below provides a coefficient based on time. By multiplying the amount of λ in the amount of elastic deformation, the approximate creep deformation is estimated.

)3

1. is the ratio of compression reinforcement/ effective cross section (As’/bd)
   Age of concrete
   2 +5 years
   1.4 12 months
   1.2 6 months
   1  3 month

is reduced as time pass away. And after 5 years the rate of creep is going to become constant.