Cast a bench slab for the outdoor kitchen in fine concrete. Can scrape the surface with my hand?

[linnarsson]

Hello.

I've built a form in formplyfa and filled it with fine concrete and reinforcement, of course. It's now been 3 days and I've been continuously moistening the visible surface. Now it's dry, but it seems like I can scrape off cement with my hand. That is, it turns into very fine sand, as if it has been sanded with fine sandpaper. Is this normal and perhaps just the outermost layer? I haven't experienced this before.

Thoughts?

Best regards,
Johan

 

[Janus82]

Have you mixed the concrete according to the instructions? After three days the concrete should be hard and not crumble apart.

 

[linnarsson]

Did you mix the concrete according to the instructions? After three days, the concrete should be hard and not crumble.

I mixed the cement like I always have. There was a description on the package, but I couldn’t understand the amount of water because it didn't even remotely look like cement (far too little water). So, I mixed as I always have (the same with fine concrete) and have never noticed a problem.

Would you say something went wrong here?

 

[Janus82]

If you have too much water, the strength of the concrete decreases. If it's just a little too much water, it doesn't make a practical difference for home use, but if you take way too much, then as you described, the concrete will just become crumbly.

How much more water than recommended did you use?

 

[linnarsson]

If you have too much water, the strength of the concrete decreases. If it's just a little too much water, it doesn't make a practical difference for home use, but if you add way too much, as you described, the concrete will just become crumbly.

How much more water than recommended did you use?

I can't say how much water I used, unfortunately. I relied on my usual sense, which has never been an issue before.
The instructions for Finja fine concrete say a maximum of 3 liters of water per 25 kg. But with that amount of water (i.e., only 3 liters), it's still just "concrete dust." It doesn't become workable cement by any means with just 3 liters of water for a bag..

 

[linnarsson]

Here is the product sheet for the finbetong: https://www.finja.se/storage/ma/eed...831A95D26B6CFC2FB46ED8/Finbetong_74_PB_sv.pdf

 

[APR]

For my part, I have experienced several times that the building supply store (in my case Bauhaus) has had products with expired shelf life on the shelves. As a result, the cement in the product may have absorbed moisture and not work as intended, perhaps this has happened? However, Finja's bags are now made of plastic, which has significantly extended the storage time from when they used paper bags (I think it was 6 months then).

 

[linnarsson]

In my experience, I've often encountered that the hardware store (in my case Bauhaus) has construction products with expired shelf life on the shelf. As a result, the cement in the product may have absorbed moisture and not work as intended, could this be what's happened? However, Finja's bags are now made of plastic, which has significantly extended the shelf life from when they used paper bags (I think it was 6 months back then).

Thanks for the response. Unclear about these specific bags. But I scraped a little more and it seems like it's only the outermost layer, just a few mm, that I can remove. Then it's hard. So maybe it's possible that it was too wet on the exposed side. I've followed instructions online about keeping the surface moist, etc. Perhaps the cement became too diluted right there at the outermost surface. I'll open the mold on Tuesday when I'm home again and see how the other sides look. 😊

 

[linnarsson]

Have now opened two of the pages and everything looks good. It's just the top layer that's "bösar" by a few mm. Thanks everyone for the responses.