3 - Creep in Hardened Concrete
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Definition and Importance of Creep
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Today, we’ll discuss creep in hardened concrete. Who can tell me what creep means?
Isn't it about how concrete deforms over time under a constant load?
That's correct! Creep specifically refers to the time-dependent increase in strain, even when stress is constant. Why do you think understanding creep is crucial for engineers?
It helps us predict how structures will behave over time, right?
Exactly! Remember, the impacts of creep can affect deflection and internal stress redistribution in concrete elements.
So, what happens if we ignore creep?
Ignoring creep can lead to premature failures in structures, which could be disastrous.
Let’s summarize: Creep is time-dependent strain under sustained load, and understanding it helps ensure the integrity of concrete structures.
Stages and Mechanisms of Creep
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Now that we understand what creep is, let's discuss its stages. What are the stages of creep?
I remember there are four stages: instantaneous, primary, secondary, and tertiary creep.
Correct! Each stage has unique characteristics. Can anyone explain what happens during primary creep?
I think it’s when the strain increases rapidly after loading.
Exactly! And secondary creep develops a steadier rate. Tertiary creep, however, can lead to failure, which is rare in proper designs. Why do you think that might be?
Perhaps because adequate engineering typically mitigates risks!
Well put! Let's remember that understanding these stages helps design safer structures.
Factors Influencing Creep
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Next, let's consider what factors influence creep. Who can list some of these?
Stress level, age of concrete, humidity, temperature, and mix design!
Great job! For instance, how does temperature affect creep?
Higher temperatures can accelerate creep!
Exactly! And what about the age of the concrete?
Younger concrete tends to creep more.
Correct! Recognizing these factors allows us to create better concrete mixtures for longevity.
Effects of Creep on Concrete Structures
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Finally, let’s talk about the effects of creep. Can anyone mention some potential impacts?
Loss of prestress and increased deflection in beams!
Exactly! Those effects are especially critical in prestressed concrete. What else can creep lead to?
It can also redistribute internal stresses and potentially cause premature failures.
Right! Each of these effects highlights why engineers must consider creep when designing structures. Understanding these impacts can significantly improve serviceability.
Introduction & Overview
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Quick Overview
Standard
This section covers the definition of creep, its mechanisms, stages, influencing factors, and resultant effects in hardened concrete. Understanding creep is essential for predicting deflection, stress redistribution, and overall structural serviceability.
Detailed
Creep in Hardened Concrete
Creep is defined as the time-dependent increase in strain that occurs in hardened concrete under sustained load, even when the applied stress level remains constant. This phenomenon is essential to understand since it affects the long-term performance of concrete structures, particularly those subjected to continuous loads.
Key Mechanisms
Creep arises largely due to two factors: viscous flow and the development of microcracks in the cement paste. These processes are influenced by several variables, including water content, ambient conditions (temperature and humidity), and the concrete's loading history.
Stages of Creep
Creep occurs in distinct stages:
1. Instantaneous Strain: The immediate deformation caused by the application of load.
2. Primary Creep: Characterized by rapid strain increase following loading.
3. Secondary Creep: Involves a slower, consistent strain rate.
4. Tertiary Creep: An accelerated strain leading to failure, which is uncommon in well-designed structures.
Influencing Factors
Several factors can influence the extent of creep in concrete:
- Stress Level: Higher stress leads to greater creep.
- Age: Younger concrete generally exhibits more creep.
- Humidity: High humidity can mitigate creep.
- Temperature: Elevated temperatures can accelerate creep rates.
- Mix Design: A higher paste content tends to increase creep.
Effects of Creep
Creep can have various impacts on concrete structures, including:
- Loss of prestress in prestressed concrete, leading to efficiency and load-bearing capacity issues.
- Increased deflection in beams and slabs over time, impacting structural performance and safety.
- Redistribution of internal stresses, potentially leading to premature failures.
- Stress relaxation in statically indeterminate structures, affecting their long-term stability.
Audio Book
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Definition of Creep
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Chapter Content
Creep is the time-dependent increase in strain under sustained load. It occurs even when the stress level is constant, particularly in compression.
Detailed Explanation
Creep refers to how concrete deforms over time when a constant load is applied. Unlike immediate deformation that happens when something is loaded, creep happens gradually. For example, if a heavy weight is placed on concrete, it will instantly compress slightly. However, over days or even months, the concrete may continue to deform slowly. This effect is particularly significant when the concrete is under compression, where it tends to sag or settle.
Examples & Analogies
Think of a wet sponge being pressed down. Initially, when you press down, it squashes and absorbs the pressure. But if you keep pressing, it continues to deform gradually. Similarly, concrete under a constant load will keep deforming, leading to what we call creep.
Mechanism of Creep
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Chapter Content
Creep is caused by viscous flow and microcrack development in the cement paste. It is influenced by water content, temperature, humidity, and loading history.
Detailed Explanation
The mechanism of creep involves two key processes: viscous flow and the development of microcracks in the cement paste. Viscous flow refers to the gradual movement of materials that can deform under sustained stress. In addition, as concrete ages and is subject to load, tiny cracks, known as microcracks, can form within the material. These are often exacerbated by environmental factors such as temperature and humidity, where higher temperatures or lower humidity levels will lead to more pronounced creep effects.
Examples & Analogies
Imagine a plastic bag filled with honey. If you press down on it, at first, it won’t change much, but if you keep applying pressure, the honey will slowly ooze out. This is similar to how the internal structure of concrete can slightly deform over time when a load is continuously applied.
Stages of Creep
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Chapter Content
1. Instantaneous Strain: Immediate deformation upon loading.
2. Primary Creep: Rapid strain increase immediately after loading.
3. Secondary Creep: Slower, steady strain development.
4. Tertiary Creep: Accelerated strain leading to failure (rare in structures).
Detailed Explanation
The process of creep can be categorized into four distinct stages:
1. Instantaneous Strain is the immediate deformation that occurs as soon as the load is applied, such as when you place a book on a table.
2. Primary Creep refers to the quick increase in strain that occurs right after loading, where the material deforms significantly in a short period.
3. Secondary Creep happens over a longer duration, showing a slower, steady increase in strain that can persist for a long time under constant load.
4. Tertiary Creep is when the strain accelerates significantly and can lead to failure, although this is less common in practical structures.
Examples & Analogies
You can think of these stages like a person settling into a couch. When they first sit down, the couch compresses immediately (instantaneous strain). As they continue sitting, they start to sink a bit more into the cushions (primary creep). After a while, they may gradually sink into the couch further (secondary creep). If they continue relaxing too long without adjusting, the cushions might get overly compressed and lose support (tertiary creep).
Factors Influencing Creep
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Chapter Content
1. Stress level: Higher stress = higher creep.
2. Age of concrete: Younger concrete creeps more.
3. Humidity: Lower humidity = higher creep.
4. Temperature: Higher temperature accelerates creep.
5. Mix design: Higher paste content = more creep.
Detailed Explanation
Several important factors can influence the amount of creep in concrete:
1. Stress Level: The greater the load on the concrete, the more it will creep.
2. Age of Concrete: Freshly poured concrete will exhibit more creep compared to older, more established concrete because it has not fully cured.
3. Humidity: When environmental conditions are drier, concrete tends to creep more because moisture loss increases the material’s susceptibility to deformation.
4. Temperature: Higher temperatures can speed up the processes that lead to creep, causing more deformation to occur.
5. Mix Design: Concrete with a higher paste content (more cement in relation to aggregates) tends to creep more due to less rigid material properties.
Examples & Analogies
Consider baking bread. If you put it in a very warm oven (high temperature), it will rise (creep faster) compared to when it’s in a cooler spot. Similarly, if you use a lot of flour and less water in your dough mix (higher paste content), it becomes stiffer, just as a concrete mix with less water and more cement can creep more under pressure.
Effects of Creep
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Chapter Content
1. Loss of prestress in prestressed concrete.
2. Deflection in beams and slabs over time.
3. Redistribution of internal stresses.
4. Stress relaxation in statically indeterminate structures.
Detailed Explanation
The effects of creep can lead to several structural issues:
1. Loss of Prestress: In prestressed concrete, creep can reduce the effectiveness of the prestressing, which can compromise its structural integrity.
2. Deflection Over Time: Beams and slabs can sag or deflect due to sustained loads, especially noticeable over long periods.
3. Redistribution of Internal Stresses: Creep can alter the internal stress distribution within the concrete, leading to unexpected stress concentrations and potential failure points.
4. Stress Relaxation: In structurally complex items, such as those that are statically indeterminate, creep can lead to a relaxation of stress, causing further misalignment or instability.
Examples & Analogies
Imagine an elastic band stretched tightly. Over time, it may slacken, losing some tension (loss of prestress). Items hanging on it might droop lower as a result (deflection). If you pull that band too hard and for too long, the tension changes within the band, leading to points where it might unexpectedly snap (redistribution of internal stresses). This concept can be easily visualized in larger structures subjected to weight over extended periods.
Key Concepts
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Creep: Time-dependent strain under constant load.
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Stages of Creep: Instantaneous, primary, secondary, and tertiary creep.
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Influencing Factors: Stress level, age of concrete, humidity, temperature, and mix design.
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Effects of Creep: Loss of prestress, increased deflection, redistribution of stresses, and potential for failure.
Examples & Applications
A beam in a bridge experiencing gradual sagging due to creep under its sustained load over years.
Concrete structures subjected to high temperatures may experience more rapid creep, leading to misalignments in joints.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Creep in concrete takes its time, slowly growing like a climb.
Stories
Imagine a bridge that sagged each day, unnoticed at first, until it gave way.
Memory Tools
Remember 'I-P-S-T' for the stages of creep: Instantaneous, Primary, Secondary, Tertiary.
Acronyms
CREEP
**C**onstant **R**ate **E**xceeds **E**lastic **P**otential.
Flash Cards
Glossary
- Creep
The time-dependent increase in strain under constant load in hardened concrete.
- Instantaneous Strain
Immediate deformation upon the application of stress.
- Primary Creep
The rapid increase in strain immediately after loading.
- Secondary Creep
A slower, steady increase in strain following primary creep.
- Tertiary Creep
An accelerated strain stage that can lead to failure, although it is rare.
- Viscous Flow
A mechanism through which creep occurs, involving the slow movement of material under constant stress.
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