Concrete

[Che69]

Hello,
I need to repair a 20x20 cm concrete area at the bottom of an opening please see picture, how should this be done, should I use primer to make the concrete adhere, will fine concrete work?

 

[neo11]

Can't see image.
Primer has nothing to do with adhesion, it prevents the concrete from drying too quickly.

 

[Jörgen Ö]

See no image.
Primer has nothing to do with adhesion, it prevents the concrete from drying too quickly.

Adhesion is absolutely one of the primer's properties, as well as binding dust, etc.

 

[neo11]

Concrete usually lies on horizontal surfaces and stays where it is placed. It does not move and does not require anchoring. However, there should obviously be no dust, sand, etc., present, but that can be vacuumed away. Dust can still appear even after priming. If one does not prime, the surrounding surfaces remain diffusion-open and will then draw out the water from the concrete too quickly, causing it to crack and weaken. Concrete needs to dry slowly, which is why priming is done. Additionally, one should apply membrane curing agents, water it afterward, or cover it with plastic to lock in moisture.

 

[Jörgen Ö]

Now it was about your claim that primer has nothing to do with adhesion......that it only prevents concrete from drying too quickly. In TS's case, it's probably not concrete he needs to repair the wall.....

 

[neo11]

Now, it was regarding your statement that primer has nothing to do with adhesion... that it only prevents concrete from drying too quickly. In TS's case, it's probably not concrete he needs to repair the wall...

No, that's what I'm saying. Primer has nothing to do with the adhesion of concrete. I haven't seen any picture from TS and thought it was a floor to be cast, not a wall. If it's a wall, then concrete is probably not relevant.

 

[Davion]

No, I’m saying it. Primer has nothing to do with the concrete adhesion.
I haven't seen any picture from the OP and thought it was a floor to be cast, not a wall.
If it's a wall then concrete isn't interesting.

But.
Come on.
Primer absolutely has to do with adhesion.
You apply a primer to increase the adhesion of the "real" concrete.
Primer ABSOLUTELY has to do with concrete adhesion.

Example: https://betong.se/2015/01/21/fraga-experten-gjuta-betong-ovanpa-betong/

Anyway.
Che69, First primer, then a concrete slurry (preferably with a splash of primer in it as well) and finally concrete.
Fine concrete (Finja's fine concrete is good stuff) is recommended, and primer works perfectly well with fine concrete.
I've just cast about 110 bags of concrete and plastered 20 bags of plaster with Finja so I dare say I have fresh experience.
Moreover, they were walls I cast up.

There is no picture, however, so your specific "case" cannot be commented on, so my answer will be more general.
If you're going to attach concrete to concrete, I recommend primer again, then a slurry and some kind of light reinforcement like chicken wire/plaster mesh (depending on thickness, of course, a reinforcing mesh might be better if you're making it thick), then the actual concrete. If it's a wall you just want to repair, I recommend Finja's "repair wall" instead. Easy to work with.

 

[witten]

I join those who say that it has to do with adherence - more specifically, it prevents the old concrete from sucking the water out of the new concrete.

An alternative to primer is usually to saturate the old concrete you're going to cast against.

 

[Che69]

Hi,
I need to repair a 20x20 cm concrete area at the bottom of an opening, please see the picture, how should it be done, should I use primer to make the concrete stick, is fine concrete suitable?

Thanks for all the tips