Effects on Hardened Concrete and Long-term Durability

Today we will discuss segregation in fresh concrete. Can anyone tell me what segregation means?

Isn’t it when the components of concrete separate?

Exactly! Segregation is the separation of coarse aggregates from the cement paste. This can lead to a structure with non-uniform strength and increased likelihood of cracking.

What causes segregation, and how can we prevent it?

Good question! Segregation can occur due to poorly graded aggregates or excessive vibration. We can prevent it by using well-graded aggregates and optimizing our water-cement ratio. Remember, we can use the mnemonic 'Graham's Quick Solution' to remember: 'Grade aggregates, QUench Vibrations'.

What are the effects of segregation on hardened concrete?

Effects include non-uniform strength, honeycombing, and poor surface finish. Thus, controlling segregation is essential for durability.

Next, let’s dive into bleeding. Who knows what bleeding in concrete refers to?

It’s when water rises to the surface after the concrete is poured, right?

Correct! Bleeding can lead to weak surfaces and poor bond strength. If we don’t manage it, it could cause problems with cracking or reduced durability.

How can we control bleeding?

We can use low bleeding cements and add mineral admixtures which help reduce water separation. Remember the acronym 'BLAST': Bleeding Low Admixtures Strong Texture!

What happens to the surface if bleeding occurs?

Bleeding leads to a laitance layer, which can weaken the durability and physical properties of the concrete. Therefore, it's crucial to measure and manage bleeding.

Let’s chat about slump loss. Who can define it?

It’s when the workability or slump of concrete decreases over time after mixing?

Exactly! Slump loss can cause challenges during placement. It can start as early as 20 minutes post-mixing. What causes it?

High temperatures and quick hydration?

Right! We can control slump loss by using retarders or chilled water in the mix. Remember our mnemonic 'Tigers Try Cold Retarders'—to help remember we can use chillers to keep our concrete workable!

So how does slump loss affect the hardened concrete?

It affects compaction, leading to weak spots called cold joints. Ensuring proper timing with placement is key to preserving strength.

Lastly, let’s talk about re-tempering. Can anyone explain what this is?

Is it when we add water after the concrete starts to set?

That’s right! While it may seem like a solution for workability, it compromises hydration, leading to reduced strength and permeability. So what should we avoid?

We should avoid re-tempering after 30 minutes, correct?

Exactly! Instead, we should focus on planning and scheduling efficiently to reduce delays. Remember, 'No Water Past the Half-Hour' helps us recall that re-tempering is risky!

Why does adding water later worsen the concrete?

Adding water disrupts the already begun hydration process and leads to inconsistent strength. Keep in mind: early quality control is vital for ensuring durability in structures.