Creep

Tue 2 Apr, 16:21

Creep is the continuous deformation of a member under sustained load. It is a phenomenon associated with many materials, but it is particularly evident with concrete. The precise behaviour of a particular concrete depends on the aggregates and the mix design as well as the ambient humidity, member cross-section, and age at first loading, but the general pattern is illustrated by considering a member subjected to axial compression. For such a member, a typical variation of deformation with time is shown by the curve in figure.

The characteristics of creep are:

1. The final deformation of the member can be three to four times the short-term elastic deformation.
2. The deformation is roughly proportional to the intensity of loading and to the inverse of the concrete strength.
3. If the load is removed, only the instantaneous elastic deformation will recover - the plastic deformation will not.
4. There is a redistribution of load between the concrete and any steel present.

The redistribution of load is caused by the changes in compressive strains being transferred to the reinforcing steel. Thus the compressive stresses in the steel are increased so that the steel takes a larger proportion of the load.

The effects of creep are particularly important in beams, where the increased deflections may cause the opening of cracks, damage to finishes, and the non-alignment of mechanical equipment. Redistribution of stress between concrete and steel occurs primarily in the uncracked compressive zone of a flexural member, however, often helps to restrain the deflections due to creep.

Reinforced Concrete

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Creep

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