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Interactive Audio Lesson
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Definition and Mechanism of Creep
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Today, we'll dive into the concept of creep in hardened concrete. Creep is defined as the time-dependent increase in strain when load is sustained over time. It’s crucial to understand how this affects our structures.
What exactly causes this creep in concrete?
Great question! Creep occurs due to viscous flow within the cement paste and can be influenced by factors like humidity, water content, temperature, and loading history. This means that the conditions around your concrete can change how much it creeps!
So, is it that higher stress leads to higher creep? How does that work?
Exactly! Higher stress increases the rate of creep due to the material’s response to sustained load. Think of it like a sponge: the more you press down, the more it deforms!
What about the age of the concrete? Does it really matter?
Yes, the age does matter! Younger concrete tends to creep more because it has not achieved full strength. The cement particles are still reacting, and the structure is more malleable.
To summarize, creep is a gradual strain increase under constant load and is affected by stress level, age, temperature, and humidity.
Stages of Creep
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Let’s explore the stages of creep. There are four stages: instantaneous strain, primary creep, secondary creep, and tertiary creep. Can anyone explain what happens in these stages?
I think the instantaneous strain is the immediate deformation when a load is applied?
Exactly! That’s the first stage. After that, we have primary creep where there's rapid strain increase.
And what follows that?
Next is secondary creep, which develops at a slower rate over time, leading to gradual and steady change. Tertiary creep is where the strain begins to accelerate, which can ultimately lead to failure, although we hope to design structures to avoid this stage.
Can you give an example of tertiary creep in real structures?
In well-designed structures, this stage is rare. However, it might occur in poorly maintained structures under constant heavy loads. It’s something we always try to mitigate!
To summarize, we discussed the stages of creep, highlighting instantaneous strain, primary creep, secondary creep, and the rarely occurring tertiary creep.
Effects of Creep
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Now, let's focus on the effects of creep. Creep can have significant implications for prestressed concrete, especially. Can anyone tell me how?
I believe it can cause a loss of prestress, right?
Exactly! As creep progresses, it can lead to loss of prestress, causing potential performance issues in structures intended to be robust!
What about deflection? Does creep affect that?
Yes, indeed! It can lead to increased deflection over time, which is crucial in considering the serviceability and safety of beams and slabs.
So, how do engineers usually manage these issues?
Engineers factor in creep by designing with sufficient safety margins and considering long-term effects during analysis and design stages.
In conclusion, the effects of creep include loss of prestress, deflection in structural members over time, and a redistribution of internal stresses that must be managed in design.
Introduction & Overview
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Quick Overview
Standard
Creep refers to the gradual increase in strain in concrete when subjected to a constant load over time. Its effects include loss of prestress in prestressed concrete, deflection in structural elements, redistribution of internal stresses, and stress relaxation in complex structures. Understanding creep is crucial for ensuring the longevity and structural integrity of concrete elements.
Detailed
Effects of Creep
Creep in hard concrete is a crucial phenomenon that has significant implications for structural behavior. It is defined as the time-dependent increase in strain resulting from a constant load, especially in compression. The mechanism behind creep involves viscous flow and microcrack development within the cement paste, influenced by factors like water content, temperature, humidity, and loading history.
The stages of creep include:
1. Instantaneous Strain: The immediate deformation that occurs upon applying the load.
2. Primary Creep: A rapid increase in strain shortly after loading.
3. Secondary Creep: A slower and steady rate of strain development over time.
4. Tertiary Creep: An acceleration in creep leading potentially to failure, although this stage is uncommon in well-designed structures.
Several factors influence the extent of creep, including stress levels (greater stress leads to higher creep), the age of the concrete (younger concrete experiences more creep), as well as environmental factors like humidity and temperature. Creep significantly affects prestressed concrete by causing a loss of prestress and altering internal stresses. Additionally, it results in deflections of beams and slabs over time, necessitating careful consideration in design practices.
Audio Book
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Loss of Prestress in Prestressed Concrete
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Creep leads to loss of prestress in prestressed concrete.
Detailed Explanation
Creep occurs as concrete gradually deforms under a constant load over time. In prestressed concrete, where tensioned steel strands are used to counteract tensile forces, this deformation means that the initial prestressing force can decrease. As the concrete creeps, the strands may lose part of their effectiveness, resulting in less compressive strength at the structure’s service level.
Examples & Analogies
Consider a rubber band that you stretch and hold in place for a long time. Over time, the band may lose its tightness and no longer hold its original shape as effectively. Similarly, over time, the prestressing force in concrete diminishes due to creep.
Deflection in Beams and Slabs Over Time
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Creep causes deflection in beams and slabs over time.
Detailed Explanation
Deflection refers to the bending or sagging of a beam or slab under load. Creep contributes to this deflection because the continuous load on the concrete results in gradual deformation. Over time, as the concrete creeps, the deflection increases, which can affect the structural integrity and usability of the beam or slab.
Examples & Analogies
Imagine a long wooden plank placed between two tables and weighted down. Over time, as it holds that weight, it slowly bends downward more and more. This gradual bending is similar to the creep observed in concrete structures, where prolonged load leads to increasing deflection.
Redistribution of Internal Stresses
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Creep can lead to redistribution of internal stresses.
Detailed Explanation
As concrete creeps, the distribution of internal stresses within the concrete structure changes. Initially, stresses may be concentrated at certain points. However, as creep alters the geometry or dimensions of the structure, these stresses redistribute to maintain equilibrium. This redistribution can lead to unexpected stress concentrations that may cause further damage or failure.
Examples & Analogies
Think of a sponge being squeezed. Initially, the pressure is concentrated where you apply force, but as the sponge deforms and redistributes the pressure internally, other areas experience changes in pressure. This mirrors how internal stresses are redistributed in a creeping concrete structure.
Stress Relaxation in Statically Indeterminate Structures
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Creep can result in stress relaxation in statically indeterminate structures.
Detailed Explanation
In statically indeterminate structures, the internal forces are more complex because the structure has more supports than necessary to maintain equilibrium independently. Over time, as creep occurs, the stress in the various components of the structure may relax, which can change the load paths and overall behavior of the structure. This relaxation can lead to alterations in deflection and the potential for unexpected failure modes.
Examples & Analogies
Imagine a team of people holding a rope tightly during a game. Initially, everyone is exerting force equally, but as they tire, they may loosen their grip. This change can impact how the rope behaves, similar to how internal forces change in a structure experiencing creep.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Creep: Gradual strain increase under sustained load over time.
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Instantaneous Strain: Immediate deformation occurring when load is applied.
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Primary Creep: Rapid increase in strain immediately after loading.
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Secondary Creep: Long-term, steady increase in strain.
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Tertiary Creep: Accelerated creep strain potentially leading to failure.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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When a heavy beam is subjected to continuous load, it will gradually bend or deflect more over time due to creep.
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In prestressed concrete bridges, excessive creep can lead to a noticeable reduction in prestress force.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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Creep, creep, slow and deep, in concrete, it likes to keep. Over time strains will grow, watch the beams, the changes show.
📖 Fascinating Stories
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Imagine a busy construction site where concrete is poured. As days go by, the heavy beams start to settle. They feel tired under their load, which grows heavier each day. The workers must monitor these beams to ensure they don’t collapse under the weight. This gradual settling is called creep.
🧠 Other Memory Gems
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To remember the stages of creep: Instantaneous > Primary > Steady > Tertiary (I Please Stop Tearing).
🎯 Super Acronyms
To recall the factors influencing creep, think 'SAGE' - Stress, Age, Gage (temperature), and Environment (humidity).
Flash Cards
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Glossary of Terms
Review the Definitions for terms.
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Term: Creep
Definition:
Time-dependent increase in strain in concrete under constant loading.
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Term: Instantaneous Strain
Definition:
Immediate deformation that occurs upon applying load.
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Term: Primary Creep
Definition:
Rapid increase in strain that occurs immediately after loading.
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Term: Secondary Creep
Definition:
Steady and slower strain increase over a prolonged period.
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Term: Tertiary Creep
Definition:
Accelerated plastic strain leading to potential failure, rarely seen in well-designed structures.