CREEP
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 the decreased strain is equal to the elastic strain at the given age.This quick recovery is then followed by a continuous decrease in strain,known as creep recovery that is a part of total creep strain suffered by the concrete.
TYPES OF CREEP
The following definitions are used for creep.
- Creep is long term deformation due to loading.
- Total creep is the strain due to loading and drying.
- Basic creep is the strain due to loading with no loss of moisture.
- Specific creep is the creep per unit stress.
Basic creep is almost impossible to measure because it involves keeping a test specimen under load for a long time (often up to 20–30 years), while sealing it to prevent any loss of moisture.Therefore, experimental data generally gives the total creep, but this will depend on the extent and timing of the drying. For most structural purposes, creep is assumed to be proportional to stress, so the specific creep is used.
Effects of Creep In Construction
Creep causes:
- Deflection in structures under continuous loading.This may cause bridges to sag,or cladding systems on buildings to buckle. A tall building may get 50–100 mm shorter during its design life.
- Stress relief that reduces cracking.
- Loss of prestress due to creep of both the concrete and the prestressing steel.
FACTORS AFFECTING CREEP
The factors affecting creep are:
Water content of the concrete mix.High water contents give high creep.
Age at load transfer. If structures are permitted to cure for longer before loads are applied, creep will be reduced.
Section thickness.Thick sections will creep less because moisture movement is reduced.
Humidity. Creep is higher in humid environments.
Temperature. Creep increases with temperature.
Most design codes will include a method of calculating specific creep that will take account of many of these factors.
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