What Is A Concrete Creep? Factors Affecting Creep Of Concrete
What Is A Concrete Creep?
Creep in concrete is a type of deformation that happens due to prolonged loading. It is characterized by a gradual increase in strain over time without an increase in stress.
It only occurs when the concrete is under stress and is caused by the movement and readjustment of water within the concrete’s pores. Creep in concrete occurs in the direction of the applied force, such as compression in a column or bending stress in a beam.
It usually does not result in structural failure, but it is a significant factor in concrete’s behavior over time. All materials experience creep to some degree under certain loading conditions, but concrete is particularly affected by it at all stresses for extended periods. Additionally, creep in concrete is roughly a linear function of stress up to 30-40% of its strength.
Creep can cause significant displacement in a structure depending on the material used, design and the conditions it is subjected to. It can also lead to functional issues, changes in stress distribution, loss of prestress, and even structural failure.
Research has shown that the creep of concrete is directly proportional to stress levels up to 30-40% of its strength. With the exception of metals in their final stages before failure, concrete creep is significantly greater than in other types of crystalline materials.
As a result, due to its magnitude being several times higher than initial loading strains, it is a crucial factor in the design and construction of concrete structures.
When a sustained load is removed, the strain quickly decreases by the same amount as the initial elastic strain. Since the elasticity modulus increases with age this strain is typically less than the original elastic strain.
The three phases of creep deformation in concrete are primary, secondary, and tertiary. During primary creep, the rate of deformation begins quickly but slows over time. Secondary creep involves a steady rate of deformation, while in tertiary creep, the rate of deformation increases until the structure fails.
Factors Affecting Creep Of Concrete
The creep of concrete is affected by several factors, including the quality of aggregate, ratio of the concrete mix, properties of cement, temperature, stress level, humidity, and age at loading. The aggregate’s strength, elasticity modulus, and volumetric content all play a role in restraining the creep of the concrete, with higher strength and modulus leading to less creep.
The water-cement ratio, type of cement, and fineness of cement also have an impact on concrete creep. Temperature can have both positive and negative effects on creep, with high temperatures during curing decreasing creep and high temperatures during loading increasing creep.
The stress level and humidity also have an impact on creep, with higher stress and lower humidity leading to more creep. Finally, the age at which a solid part is loaded also plays a role in the magnitude of creep.
Effects Of Creep In Concrete
The effects of creep in concrete include:
- Increased deflection of reinforced concrete beams over time, which can be a significant design factor.
- Increased deflection and potential buckling in eccentrically loaded columns.
- Relief of stress concentrations caused by shrinkage, temperature changes, or support movement in statically indeterminate constructions and column and beam junctions.
- Damaging effects in mass concrete structures such as dams due to temperature differences at the interior and surface, which can lead to cracking.
- Loss of prestress in prestressed concrete structures due to creep.
- Adverse impact on structural integrity and economic impact if not properly projected, due to early-age deflections and cracking caused by quick construction practices.
Is Creep Good For Concrete
Creep can result in significant loss of prestress and lead to excessive deflections and cracking in large-span prestressed segmental box girder bridges, cable-stayed bridges, arch bridges, roof shells and other structures. This is often caused by underestimating the effects of multi-decade creep.
Non-uniformity in creep due to variations in temperature, humidity, age, and concrete type can also cause cracking. Interactions with masonry or steel components can also contribute to this problem.
In tall buildings, differences in column shortenings can be particularly problematic, and in slender structures, creep may lead to long-term instability and collapse.