Casting a thin concrete slab

[Helioz]

Good afternoon,
I think I will try with a pre-mixed concrete in a bag, I cast in sturdy molds that are anchored to a table, but still the slab bends about 1-2 mm so it becomes like a stretched U.
To me, it seems like there is shrinkage in the top side of the casting when it sets, even though I water it!
If anyone knows why, it would be interesting to get an answer

Do you cover the mold with plastic right after casting, or do you start watering the next day?

 

[AXS]

1-2 mm maybe you can live with? Probably the concrete cures faster at the top due to too little moisture.

 

[Ogguls26]

Hi,
partly it's fun to succeed so it turns out perfect and partly to understand why it happens and how you can adjust so it turns out right.
Wrap directly, water in the morning on day 2, on day 2-3 the board starts to become concave.
1-2 mm against the next board that joins doesn't look nice and underlying drawers reveal that the board is not straight which doesn't look nice.

 

[pelle242]

Ordinary concrete shrinks when it cures, so if it bends, it doesn't cure evenly. I would have tried using expansion concrete instead, which doesn't shrink.

 

[Ogguls26]

Gonna try expanders, also think the problem lies there. I'm a bit stingy too, so when I bought material last time expanders were more expensive, but now it gets more expensive anyway:x