CREEP OF CONCRETE

Creep of Concrete

Creep can be defined as the elastic and long term deformation of concrete under a continuous load.Generally, a long term pressure changes the shape of concrete structure and the deformation occurs along the direction of the applied load.When the continuous load is removed  the strain is decreased immediately.The amount of decreased stain is equal to the elastic stain at the given age.The quick recovery is then followed by a continuous decrease in stain known as creep recovery that is a part of total creep stain suffered by the concrete.

Creep Coefficient

The ratio of the ultimate creep stain to the elastic stain at the age of loading is termed as creep coefficient.

Factors Affecting the Creep of Concrete

Following are the factors affecting the creep of concrete:

1. Water-Cement Ratio

When the water-cement ratio is increased, the higher water-cement ratio will result in more porous and weaker cement paste which will deform more under a given load.

2. Humidity

The rate and ultimate magnitude of creep increased as the humidity of atmosphere decreases.The relation between relative humidity and creep is not linear concrete under sustained load in air at 70% relative humidity will have an ultimate about twice as large as concrete in air at 100% relative humidity.The ultimate creep in air at 50% relative humidity will be about three times as large.

3. Aggregate

When aggregate volume is increased, creep will be less as the aggregate is more rigid than the cement paste.Aggregate with moisture movement and low modulus cause a large amount of creep.

4. Admixtures

Some admixtures (mainly accelerators) are also responsible for causing creep in concrete.

5. Age of Concrete

The age at which a concrete member is loaded will have a predominant effect on the magnitude of creep.This can be easily understood from the fact the quality of gel improves with time.Such gel creeps less, whereas volume gel under load being not so stronger creeps more.The rate of creep rapidly decreases with time.The time taken by a concrete structure to attained creep is 5 years.