19.1. Fresh Concrete Behavior

Let's talk about how recycled aggregates behave in fresh concrete. One important factor is their increased water absorption. Can anyone tell me what this might mean for our concrete mix?

Does it mean we might need more water in the mix?

Exactly! However, if we add too much water, we risk reducing the strength of the concrete. This is a lesson in finding the right balance. We can remember this with the acronym 'WAC' for 'Water Absorption Challenge.'

So, how do we manage that?

Good question! One method is to pre-soak the recycled aggregates before mixing them in to reduce their absorption. What do you think would happen if we forgot to do that?

It could make the concrete too dry and less workable, right?

Correct! Managing water absorption is crucial for maintaining workability.

What is workability exactly?

Workability refers to how easily we can mix, place, compact, and finish the concrete. It’s key to ensuring good performance.

To recap, when using recycled aggregates, monitor their water absorption closely to maintain the proper mix. The 'WAC' acronym can help remind you of the water absorption challenge.

Let’s shift our focus to another aspect of recycled aggregates: their surface texture. How do you think the texture of an aggregate can influence concrete’s behavior?

A rough texture might need more paste to make everything stick together, right?

That's exactly correct! Rough surfaces create higher friction and require more paste for good bonding. Let’s remember this with the mnemonic 'Rough Equals More!' This helps us remember that rough textured aggregates require more paste.

So, if we have smooth aggregates, we'd use less paste?

Exactly! Smooth aggregates can lead to reduced paste requirements. However, we always need to ensure a good balance between workability and strength.

What could happen if we don’t adjust for surface texture?

If adjustments aren’t made for surface texture, we might end up with weak points or areas that are not well adhered in the concrete, potentially leading to failures.

In summary, remember 'Rough Equals More' for surface texture adjustments as they directly impact the paste requirement.

Now let's discuss the modifications required when using recycled aggregates in fresh concrete. What modifications do you think are important?

We might need to add extra water or admixtures to help the mix?

Yes, that's very true! Adjusting the water content is critical due to the absorption properties of recycled aggregates. Think of it as 'Adjust Always' to help remember the need for constant evaluation of your mix.

What about grading the aggregates? Does that matter?

Absolutely! Careful grading of recycled aggregates helps us maintain consistency and improves the overall performance and quality of the concrete. If we don’t grade properly, we could face issues with segregation.

What is segregation?

Segregation is when the different components of the concrete mixture separate, leading to weak points in the hardened product. Proper grading minimizes this risk.

To wrap it up, always remember 'Adjust Always' and focus on consistent grading to effectively modify mixes when using recycled aggregates.