Creep of Hardened Concrete

Creep of Hardened Concrete

Definition

• Creep: The gradual deformation of hardened concrete under sustained stress over time.

Key Points

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• Time-Dependent Strain: Unlike elastic deformation, creep occurs progressively with time, even under constant load.
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• Impacts: Affects the structural integrity and longevity of concrete structures by altering their dimensions and potentially causing serviceability issues.
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• Factors Influencing Creep:
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  • Stress Level: Higher sustained stress typically results in greater creep strain.
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  • Age of Concrete: Older concrete experiences less creep compared to newly hardened concrete.
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  • Humidity and Temperature: Higher humidity and temperature can increase the creep rate.
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  • Concrete Mix Composition: Properties such as water-cement ratio, aggregate type, and admixtures can influence creep.
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  • Curing Conditions: Proper curing reduces the rate and amount of creep by ensuring adequate hydration.

Mechanism

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• Microstructural Changes: Creep involves microstructural adjustments within the concrete, including microcracking and internal movement of water and cement gel particles.
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• Viscoelastic Behavior: Concrete exhibits a combination of elastic (instantaneous deformation) and viscous (time-dependent deformation) behavior.

Measurement

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• Creep Coefficient: The ratio of creep strain to initial elastic strain, used to quantify the creep potential.
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• Creep Testing: Typically involves applying a constant load to a concrete specimen and measuring deformation over time.

Mitigation

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• High-Performance Concrete (HPC): Use of HPC with low water-cement ratios and high-strength aggregates can minimize creep.
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• Reinforcement Detailing: Adequate reinforcement can limit the deformations due to creep.
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• Pre-stressing: Pre-stressed concrete can counteract the effects of creep by applying an initial compressive force.

Practical Considerations

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• Design Allowance: Engineers must account for potential creep in their design, particularly for long-span bridges, tall buildings, and other critical infrastructure.
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• Monitoring: Regular inspection and monitoring of concrete structures help detect and address creep-related issues early.

Consequences of Neglecting Creep

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• Structural Deformation: Significant and possibly uneven deformations.
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• Cracking: Increased tendency for cracking, which may compromise strength and durability.
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• Maintenance Costs: Higher long-term maintenance and repair costs due to unexpected deformations.

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These study notes cover the essential aspects of creep in hardened concrete, including its definition, factors influencing it, its mechanism, and practical considerations.