4.1 - For Creep
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Understanding Creep
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Today we're going to discuss creep in concrete. Can anyone tell me what creep means in this context?
I think it means the concrete bends over time under constant stress?
That's partly correct! Creep is the gradual deformation of concrete under constant stress over a long duration. It doesn't recover like elastic deformation. Creep occurs because concrete has a viscoelastic nature.
So, does that mean it keeps deforming as long as the stress is applied?
Exactly! This time-dependent aspect of creep is crucial for understanding how concrete will behave in real structures.
Mechanism of Creep
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Now, let’s examine the mechanisms of creep. What do you think causes this deformation over time?
Is it related to how water moves in the concrete?
Correct! Creep involves moisture movement in gel pores, viscous flow in the cement paste, and microcracking. All these factors affect how concrete behaves under stress.
What about the structure of the concrete? Does that play a role?
Yes, internal structural rearrangements in the C-S-H gel also contribute significantly to creep. This complexity makes predicting creep challenging.
Types of Creep
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Let’s differentiate the types of creep. Who can explain what basic creep is?
Isn't basic creep the one that happens in sealed conditions?
Exactly! Basic creep occurs without moisture exchange. Now what about drying creep?
That happens when the concrete loses moisture while under load, right?
Yes! And let's not forget autogenous creep, which occurs even without moisture loss. It’s especially significant in high-strength concrete.
Factors Affecting Creep
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Now that we know the types of creep, let’s talk about the factors affecting it. What can you think influences creep?
I think the water-cement ratio must matter. More water means more porosity, right?
Great point! Higher water-cement ratios lead to more creep. The age at loading also plays a significant role; younger concrete is more susceptible to creep.
And temperature can affect it, too, right?
Exactly! Higher temperatures accelerate the creep rate. You’re all grasping these concepts well!
Measuring Creep
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Let’s wrap up by discussing how we measure creep. What do you think we need for a creep test?
Do we use special equipment? Like for measuring strain?
Exactly! We use dial gauges or LVDTs to record the strain during the test, along with a creep frame. The Creep Coefficient is also a vital parameter to understand!
What does it represent?
The Creep Coefficient indicates the ratio of creep strain to elastic strain, providing insight into deformation behavior under sustained load.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
Creep is caused by the viscoelastic nature of cement paste in concrete, resulting in time-dependent strain. This section discusses the mechanism, types, factors affecting creep, measurement methods, and control measures for mitigating this phenomenon.
Detailed
Detailed Summary
Creep is a critical phenomenon that describes how concrete deforms gradually when subjected to constant stress over a sustained period. Unlike elastic deformation, which occurs instantly and recovers when the load is removed, creep continues as long as the stress is applied.
Mechanism
Creep primarily occurs due to the viscoelastic nature of the cement paste, and several mechanisms contribute:
- Moisture movement in gel pores
- Viscous flow of the hydrated cement paste
- Microcracking within transition zones
- Rearrangements in the calcium silicate hydrate (C-S-H) gel.
Types
There are three principal types of creep:
1. Basic Creep: Happens in sealed environments without moisture exchange.
2. Drying Creep (Pickett Effect): Occurs when moisture is lost under load.
3. Autogenous Creep: Takes place even without external moisture in high-strength concrete.
Factors Affecting Creep
Key factors influencing creep include:
- Stress Level
- Water-Cement Ratio (w/c)
- Age at Loading
- Moisture Conditions
- Aggregate Content
- Type of Cement
- Temperature.
Measurement
Creep is assessed through specific tests using specialized equipment and parameters like the Creep Coefficient, with typical values varying depending on concrete type. This understanding is important for designing structures, especially in critical infrastructure, to ensure stability and longevity.
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Mitigation Strategies for Creep
Chapter 1 of 2
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Chapter Content
Use low w/c ratio and high-strength concrete.
Increase aggregate content (preferably with low creep aggregates like basalt).
Use supplementary cementitious materials (like fly ash, silica fume).
Provide adequate curing before loading.
Design with creep coefficients in mind using IS:456 or ACI guidelines.
Detailed Explanation
This chunk focuses on several strategic measures that can minimize creep in concrete structures. The first point emphasizes the use of a lower water-to-cement (w/c) ratio, which reduces the amount of water in the mix, leading to denser concrete with lower creep. The use of high-strength concrete enhances the structure’s ability to withstand loads without excessive deformation. Increasing the content of aggregates, especially those that exhibit low creep properties like basalt, provides a more stable matrix in the concrete, further mitigating creep effects. Supplementary materials like fly ash and silica fume enhance the overall properties of the concrete, reducing creep via pozzolanic reactions. Adequate curing is essential before applying loads to the concrete, allowing it to develop strength properly. Lastly, designing with acknowledged creep coefficients based on relevant standards like IS:456 or ACI ensures that the long-term deformations are anticipated and accounted for in the design phase.
Examples & Analogies
Think of concrete like a sponge. If the sponge (concrete) is filled with too much water (high w/c ratio), it becomes softer and more likely to deform under weight over time, similar to how a soggy sponge changes shape easily when pressure is applied. Using less water and adding strong, dense materials makes the sponge firmer and less responsive to deformation, allowing it to withstand pressure much better.
Implications of Creep in Design
Chapter 2 of 2
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Chapter Content
Design with creep coefficients in mind using IS:456 or ACI guidelines.
Detailed Explanation
This chunk highlights the importance of considering creep coefficients in the design process for concrete structures. Creep coefficients are numerical factors that help engineers predict how much deformation will occur over time under sustained loads. When using established guidelines like IS:456 and ACI, engineers can calculate potential creep-related deformations, allowing them to design structures that accommodate these changes. This proactive approach ensures that long-term serviceability and structural integrity are maintained, preventing issues such as excessive deflections and potential cracking.
Examples & Analogies
Imagine planning to stack a pile of books on a shelf. If you know that the shelf will bend a little over time due to the weight (creep), you can either use a sturdier shelf or stack the books in a way that suits the shelf's strength. Similarly, engineers must plan ahead by considering how much the concrete will 'sag' or deform over the years and design accordingly to keep everything level and functional.
Key Concepts
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Creep: The gradual deformation of concrete under sustained stress.
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Viscoelasticity: The property of concrete that leads to time-dependent strain and deformation.
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Creep Coefficient: A measure to quantify creep in terms of elastic deformation.
Examples & Applications
A high-rise building designed without accounting for creep may face excessive deflection over time, leading to structural issues.
Bridges constructed with segmental segments must consider creep to avoid displacement of bearings over time.
Memory Aids
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Rhymes
When concrete bends and starts to seep, it's called creep, by stress we keep.
Stories
Imagine a heavy rock sitting on a soft sponge. Over time, the sponge slowly flattens under the weight; that's like creep in concrete!
Memory Tools
CREEP: Constant Relaxation and Elasticity Even Pressing.
Acronyms
Creep
- Constant stress
- Rate increases
- Elasticity changes
- Effects seen over time
- Predictability.
Flash Cards
Glossary
- Creep
Gradual increase in strain in concrete under constant stress over time.
- Basic Creep
Creep that occurs when concrete is loaded in a sealed environment without moisture exchange.
- Drying Creep
Creep that occurs when concrete loses moisture under load.
- Autogenous Creep
Creep that happens even in the absence of external moisture movement, especially in high-strength concrete.
- Creep Coefficient
A parameter representing the ratio of creep strain to elastic strain.
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