Creep of concrete is a kind of deformation which occurs because of sustained loading. The gradual increase in strain, without increase in stress, with the time is due to creep. From this explanation it can also be defined as the increase in strain under sustained stress. It takes place only under stress. Under sustained stress, with time, the gel, the adsorbed water layer, the water held in the gel pores and capillary pores yields, flows and readjust themselves, which behaviour is termed as creeping in concrete.
Deformation due to creep occurs in the direction of applied force. For example if a column is getting more compressed or a beam is under bending stresses. Due to creep there is very less chance of structural failure. When a load is applied to concrete, it experiences an instantaneous elastic strain which develops into creep strain if the load is sustained for a long period of time.
All materials undergo creep under some conditions of loading to a greater or smaller extent. But concrete creeps significantly at all stresses and for a long time. Furthermore, creep of concrete is approximately linear function of stress upto 30 to 40 per cent of its strength.
Measurement of creep
Creep is usually determined by measuring the change with time in the strain of specimen subjected to constant stress and stored under appropriate condition. A typical testing device is shown in Figure. The spring ensures that the load is sensibly constant in spite of the fact that the specimen contracts with time. Under such conditions, creep continues for a very long time, but the rate of creep decreases with time.
Under compressive stress, the creep measurement is associated with shrinkage of concrete. It is generally assumed that the creep continues to assume a limiting value after an infinite time under load. It is estimated that
- 26 per cent of the 20 year creep occurs in 2 weeks.
- 55 per cent of 20 year creep occurs in 3 months and
- 76 per cent of 20 year creep occurs in one year.
Factors affecting creep
There can be different factors that affect the creep of concrete, some of them are:
- Water cement ratio
- Influence of mix proportions
- Type of cement
- Age of concrete
- Aggregates type
- Type of curing
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