1.1 - Definition
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Understanding Creep
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Today, we're discussing creep in concrete. Can anyone tell me what creep means?
Is it how concrete can change shape over time?
Exactly! Creep is the gradual increase in strain under constant stress over time. Remember, it's time-dependent unlike elastic deformation.
So, it keeps deforming as long as the stress is there?
That's right! And if we think of total strain, it’s the sum of elastic strain and creep strain. This is crucial for structural stability!
What causes this creep to happen?
Great question! Creep happens mainly due to the viscoelastic nature of cement paste and involves moisture movement and internal rearrangements.
To help remember this, think of 'Moist Movement' as an acronym, MMM, for Moisture, Microcracking, and Movement.
So what types of creep are there, then?
We have basic creep, drying creep, and autogenous creep. Each type has its own conditions that trigger it.
In summary, creep is key for long-term durability of structures—it’s vital for us to consider it in designs.
Life Cycle of Creep
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Let's dive into the factors affecting creep. Can anyone name a few?
I remember stress level being one of them!
Correct! The stress level affects creep, particularly if it’s below 30-40% of compressive strength. What else?
Water-cement ratio?
Yes! A higher w/c ratio can increase creep by creating a more porous structure. This is why mix design is critical.
And what about age at loading?
Absolutely! Early loaded concrete generally shows more creep due to its weaker structure at that stage. Let’s remember 'AGE' for Age, Aggregate, and Environment to factor in!
How about moisture conditions?
Spot on! High humidity can reduce creep but drying conditions can enhance it. Always consider the environment when designing.
To wrap up, understanding these factors helps ensure durable and safe structures!
Measuring Creep
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Now that we understand creep, how do we measure it?
With a test, right?
Yes, a creep test. We typically use a creep frame with a dead load and dial gauges.
And how do we isolate shrinkage too?
Good point! We use companion specimens without load to see only the total deformation due to creep.
What's the creep coefficient?
Excellent query! The creep coefficient, φ, is the ratio of creep strain to the elastic strain at loading. Typical values vary based on the concrete type. You can recall it as 'C' for Creep in φ!
Why are those values different between normal and high-strength concrete?
Higher strength concrete usually exhibits less creep due to denser structure—very important for bridge and high-rise designs!
In conclusion, accurate measurement is vital for predicting and managing structural performance over time.
Introduction & Overview
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Quick Overview
Standard
Creep refers to the time-dependent deformation of concrete when subjected to constant stress. It is a critical factor in structural design as it affects the long-term performance and stability of concrete structures.
Detailed
Detailed Summary
Creep is defined as the gradual increase in strain or deformation of concrete when exposed to a constant stress over a prolonged period. Unlike elastic deformation, which is instantaneous and reversible, creep is time-dependent and increases as long as the stress is applied, irrespective of the load's constancy. The total strain in concrete can be described mathematically as:
Total Strain = Elastic Strain + Creep Strain
Key Mechanisms Involved in Creep:
Creep occurs mainly due to the viscoelastic behavior of cement paste, including:
- Moisture movement in gel pores
- Viscous flow of hydrated cement paste
- Microcracking within transition zones
- Rearrangement of internal structures within the calcium silicate hydrate (C-S-H) gel
Types of Creep:
1. Basic Creep: Occurs in sealed environments without moisture exchange.
2. Drying Creep: Associated with the loss of moisture, significantly increasing strain under load.
3. Autogenous Creep: Manifesting in high-strength concrete due to internal chemical reactions, even without external moisture changes.
Factors Influencing Creep:
Several factors affect the degree of creep, including:
- Stress Level
- Water-Cement Ratio (w/c)
- Age at Loading
- Moisture Conditions
- Aggregate Content
- Type of Cement
- Temperature
Understanding creep is vital for civil engineers as it impacts the serviceability, durability, and safety of structures such as high-rise buildings and bridges.
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What is Creep?
Chapter 1 of 3
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Chapter Content
Creep is the gradual increase in strain or deformation in concrete when it is subjected to a constant stress over a long period.
Detailed Explanation
Creep refers to how concrete changes over time when it is under a constant load. Unlike when you first apply a load, where the concrete might just bend a little (this is called elastic deformation), creep involves a slow, continuous change in shape or size. This happens because the concrete adjusts to the load over extended periods, which can be a huge factor in how buildings and structures are designed and maintained.
Examples & Analogies
Think of a wet sponge. When you press on it, it might flatten immediately but over time, as the water inside moves around, it continues to change shape slightly under the pressure. Similarly, concrete changes shape slowly when a heavy weight stays on it for a long time.
The Nature of Creep
Chapter 2 of 3
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Chapter Content
Unlike elastic deformation, creep is time-dependent and continues as long as the stress is applied, even if the load remains unchanged.
Detailed Explanation
Creep is unlike normal elastic deformation, which happens instantly and can revert if the load is taken away. Creep keeps progressing as long as the load is there, which can mean that over time, even if nothing changes in the load itself, the concrete continues to deform more and more. This time-dependent aspect means engineers must be careful in designing structures that will bear loads for extended periods.
Examples & Analogies
Imagine a person standing on a soft carpet. Initially, the carpet will compress under their weight, but if they stand there long enough, the carpet will leave an imprint. Even if they stay still, the compression takes time to resolve when they step off. Likewise, concrete under load will behave continuously as time passes.
Understanding Strain Components
Chapter 3 of 3
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Chapter Content
Mathematically: Total Strain = Elastic Strain + Creep Strain
Detailed Explanation
This formula shows that the total strain (the overall deformation of the concrete) is made up of two parts: the elastic strain, which is the initial bending or stretching from the load applied, and the creep strain, which represents the additional changes that occur over time due to creep. Understanding this equation is crucial for engineers to predict how much a concrete structure will change under load over time.
Examples & Analogies
Think of it like pulling a piece of stretchy fabric. When you first pull it, it stretches quickly (elastic strain). But if you keep pulling it, it continues to stretch more over time, resulting in both the initial stretch and the long-term change in size (creep strain).
Key Concepts
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Creep: The time-dependent deformation of concrete under sustained stress.
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Basic Creep: Creep occurring in sealed environments.
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Drying Creep: Creep caused by moisture loss from concrete.
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Autogenous Creep: Creep occurring due to internal shrinkage of concrete.
Examples & Applications
A prestressed concrete beam experiences creep over time, causing deflection if not accounted for in design.
In high-rise buildings, failure to estimate creep can lead to issues with alignment and structural integrity.
Memory Aids
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Rhymes
Creep will keep, in stress it weep, concrete bends, but never ends.
Stories
Once there was a bridge that slowly bent under the weight of time because it forgot to account for time's creep, which made it sag without warning.
Memory Tools
Remember 'MELT' for Creep's Mechanisms: Moisture, Elasticity, Load, Time.
Acronyms
CREEP can stand for Constantly Rearranging Elastic Energy under Pressure.
Flash Cards
Glossary
- Creep
The gradual increase in strain of concrete under constant stress over time.
- Basic Creep
Creep occurring in a sealed environment without moisture exchange.
- Drying Creep
Creep resulting from moisture loss in concrete under load.
- Autogenous Creep
Creep that occurs due to internal chemical shrinkage without moisture movement.
- Creep Coefficient
A ratio to measure creep strain compared to elastic strain at loading.
- WaterCement Ratio (w/c)
The ratio of water to cement in a concrete mix, affecting creep and strength.
- Viscoelastic
Material behavior that exhibits both viscous and elastic characteristics when deformed.
Reference links
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