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Deformation of a structure due to a sustained load is known as concrete creep. Concrete can alter shape if it is subjected to prolonged strain or stress. This deformation occurs most often in the direction of the applied force. A concrete column compressing or a beam bending are examples of this. Concrete does not always fail or break down as a result of creep. When a load is applied to concrete, an instantaneous elastic strain occurs, which develops into creep strain if the load is sustained.
Concrete’s stress-strain curve is not a straight line, and at a certain value, the strain will continue to increase without a substantial increase in stress. The time-dependent component of the tension that arises as a result of stress is also known as concrete creep.
The slow transfer of stress from concrete to steel is advantageous in RCC columns because of the creep feature of concrete. Due to creep, internal and surface temperature conditions induce fissures in the interior of a mass concrete construction, such as a dam.
Concrete creep has been discovered to be a linear function of stress up to 30 to 40% of its strength. Except for metals in the last stages of yielding prior to collapse, the order of magnitude of creep in concrete is substantially greater than in other crystalline materials. As a result, because creep is several times greater than the strain on loading, it is a critical consideration in the construction of concrete buildings. When the sustained load is removed after a period of time, the strain quickly falls by the same amount as the elastic strain. Because the modulus of elasticity increases with age, this strain is usually less than the original elastic strain.
Factors affecting Creep of Concrete:
1) Quality of aggregate:
Aggregate has an extremely low creep rate. The aggregate has a restraining impact on the magnitude of creep, which influences the creep of concrete. The paste that creeps under load is restrained by non-creeping aggregate. The greater the aggregate strength, the greater the restraining effect and, as a result, the less the amount of creep. The creep will be reduced by 10% if the volumetric content of the aggregate is increased from 65 to 75 percent.
One of the most critical parameters determining creep is the aggregate modulus of elasticity. It’s easy to imagine that the higher the elasticity modulus, the less creep there is. The creep rate of lightweight aggregate is significantly higher than that of normal weight aggregate.
2) Ratio of the concrete mix:
The single most critical factor determining creep is the quantity and quality of paste material. A poor concrete mix will result in excessive creeps, which are inversely proportional to the strength of the concrete as the water-cement ratio rises. All additional parameters that influence the water/cement ratio have an impact on concrete creep.
3) Properties of cement:
The type of cement effect has an impact on the concrete’s strength when the load is applied. The fineness of cement has an early impact on strength development and, as a result, creep. The finer the cement, the more gypsum is required because regrinding cement in the laboratory without gypsum results in an improperly retarded cement with significant creep.
The temperature of the concrete might have two anti-creep effects. During the curing process before being loaded, when a concrete member is subjected to a greater than the normal temperature the strength increases and the creep strain decreases. On the other hand, high temperatures during the load period can cause creep. The effect of temperature on creep in nuclear reactor vessels is of great importance.
5) Stress level:
Creep and applied stress have a proportional relationship. Because solid stresses creep even at low-stress levels, there is no lower limit of proportionality. Concrete creeps more when it is subjected to greater tension.
The amount of moisture that seeps from the concrete is affected by the humidity in the air. As the humidity level rises, the rate of moisture loss decreases. It results in less seepage. As a result, the lower the relative humidity, the higher is the creep for a particular concrete. Concrete creep is increased when it is allowed to dry while under stress.
7) Age at loading:
The creep magnitude is heavily influenced by the age at which a solid part is loaded. The values of creep coefficient at different ages of loading:
|Age of loading||Creep coefficient|
Effects of Creep in Concrete:
1) Creep increases the deflection of reinforced concrete beams over time and maybe a key design factor.
2) Creep increases deflection and can cause buckling in eccentrically loaded columns.
3) Creep may relieve stress concentration caused by shrinkage, temperature changes, or support movement in statically indeterminate constructions and column and beam junctions. All concrete constructions will benefit from the creep property of concrete to reduce internal stresses caused by non-uniform loads or constrained shrinkage.
4) Because of the difference in temperature conditions at the interior and surface of mass concrete structures such as dams, creep is damaging and may be a cause of cracking in the interior of dams. As a result, all precautions and actions must be taken to ensure that the interior of a mass concrete construction does not experience an increase in temperature.
5)Prestress loss in a prestressed concrete structure is due to concrete creep.
6) Concrete members will be subjected to loads as great as the design loads at an early age as a result of quick construction practices; this can result in deflections owing to cracking and low elastic modulus at an early age. As a result, creep has a substantial impact on structural integrity as well as the economic impact it will have if projected incorrectly.
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