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A short class with Sarah and her students. The other ways are under More.
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Today, we'll start with segregation. Can anyone tell me what is meant by this term in the context of concrete?
Is it when the different parts of the concrete mix, like the aggregates and cement, separate?
Exactly! Segregation refers to the separation of coarse aggregates from the finer components, leading to a non-uniform mix. Why do you think this could be problematic?
It could weaken the construction because the strength would vary!
That's right! This can result in issues like honeycombing and voids, which decrease durability. Remember: S.E.C. - Segregation Equals Compromise.
Can we see this happening on site?
Great question! You'll often see segregation when concrete is dropped from heights or poorly graded aggregates are used. Understanding these factors is crucial.
Now, let's move on to bleeding, a specific type of segregation. Who can explain what happens during bleeding?
It's when water rises to the surface of the concrete mix, right?
Correct! This can lead to weak top layers and other issues. What factors could cause this?
I think having too much water in the mix and poor quality aggregates could contribute.
Exactly! And excessive bleeding can create laitance, which weakens the bond with reinforcement. An easy way to remember is: B.W.A. - Bleeding Weakens Adhesion!
What do we do to control bleeding?
Great question! Using proper aggregates, optimizing the mix design, and adding trusted admixtures can help control bleeding.
Next, let's discuss slump loss. Who can tell me what that term means?
It’s when the workability or slump of concrete decreases after mixing, making it harder to work with.
Exactly! Slump loss can start as soon as 20 to 60 minutes after mixing. What causes it?
High temperatures and delays during transportation can contribute to slump loss, right?
Spot on! It can lead to cold joints and poor compaction. Remember the mnemonic C.A.R.E. – Compaction Always Requires Efficiency.
How can we manage this slump loss?
Using retarders and controlling temperature can help. Efficient site practices are also essential!
Lastly, let's cover re-tempering. What is it, and why might it be used?
It’s when water is added to concrete after it starts setting to regain workability.
Yes! But what problems can this lead to?
It can weaken the concrete by disrupting the hydration process.
Exactly! The key here is to avoid re-tempering if possible, and if necessary, do so very cautiously. Keep in mind: R.E.X. – Re-tempering Equals X-cessive Weakness.
Are there alternatives to re-tempering?
Yes! Using slump-retaining admixtures and planning effectively can help avoid the need for re-tempering.
The separation of coarse aggregates from finer components within the concrete mix, affecting its strength.
The process where water rises to the surface of freshly placed concrete, leading to potential structural weakness.
The decrease in workability of concrete over time, crucial for placement efficiency.
Adding water to concrete after it has begun to set, often leading to reduced strength.
For concrete not to be a waste, keep segregation out of the paste.
Imagine a chef blending cake ingredients; if the flour sits on top, the cake won't rise evenly, just like concrete with segregation.
Use B.A.R.S. - Bleeding Affects Resistance and Strength.
S.E.C. for Segregation Equals Compromise.
Term
What is fresh concrete?
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What is segregation?
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What does bleeding mean?
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What is slump loss?
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What is re-tempering?
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A plastic, workable mixture of cement, water, aggregates, and sometimes admixtures before it hardens.
The separation of concrete components, primarily due to differences in particle size and density.
A specific type of segregation where water rises to the surface due to particle settlement.
The reduction in workability of fresh concrete over time.
The practice of adding water to concrete after initial setting has begun to regain workability.
Fresh concrete's properties are essential for quality and strength, influenced by phenomena such as segregation and bleeding.
This section discusses the definition of fresh concrete, its essential properties, and the significant phenomena affecting it, including segregation, bleeding, slump loss, and re-tempering, which can alter the durability and strength of hardened concrete.
Fresh concrete is defined as a plastic and workable mixture combining cement, water, aggregates, and sometimes admixtures. Its characteristics are critical for ensuring the strength, durability, and quality of the hardened concrete. Key phenomena that can adversely affect fresh concrete include segregation—where the components of the mixture separate, resulting in non-uniform composition—and bleeding, which refers to the rise of water within the concrete that can lead to weakened surfaces. Other issues discussed include slump loss and the technique of re-tempering, which entails adding water to regain workability. Proper management and understanding of these factors are essential for successful concrete handling, placement, and finishing on construction sites.