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Help! My concrete is cracking! Answered

I constructed 4" triangular molds out of cardboard (I am making coasters).
I poured quikrete mortar mix into a cup, mixed with some water and poured into the molds until the depth was roughly .5".

About 1 hour into drying I realized that huge cracks were forming!

How do I eliminate or reduce cracking?

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seandogue (author)2018-02-27

add fiber (shredded fiber glass insulation will work) to the concrete mix and make sure to keep the concrete damp at a ll times during the curing process, which ideally will take several days..

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gmoon (author)2018-02-26

It's usually a good idea to keep concrete damp during the curing phase. Otherwise it's curing and drying unevenly. DEFINITELY don't try to speed the process (with heat).

I've done a lot of work with mixes over the last year. Quikrete patching mix (where a new electrical service enters the foundation), thinset for tile, self-leveling mix (for subflooring), grout for repointing brick. None of them exhibited cracking.

I'd recommend running some test batches of the same size/thickness with different consistencies. You'll find the right water:mix.

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steveastrouk (author)2018-02-25

You need to mix with the absolute minimum of water, so that the mortar just binds together if you squeeze it in your hand.

Also some fibre component - even paper - will help a lot.

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Toga_Dan (author)2018-02-25

I suggest making several at once with different methods+ additives. Thus, all at once you can see what works.

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caitlinsdad (author)2018-02-25

There's a whole chemical science of concete/cement but you might want to try one of the latex additives. I've heard a shot of white glue which is PVA sometimes works. The added reinforcement of fibers as suggested by rickharris helps too. You are working with such a small slab and thickness that different problems will occur, you need to use finer particles in your mix and vibrate well to get all the air out. Good luck.

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rickharris (author)2018-02-25

Concrete is a mix of cement and aggregate . or another binder.

Without the binder the cement shrinks when setting (a chemical process that will still happen underwater) try mixing in a binder such as horse hair, fine thread, wire, small aggregate ir similiar.

You will need to exoperiment.

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Toga_Dan (author)rickharris2018-02-25

fiberglass is used in concrete, too

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g-one (author)2018-02-25

While binding the concrete shrinks. It will shrink even more if you mixed it with too much of water. Make the mixture in a way that you still can form it with your hand.

The binding of concrete is not just drying. It's rather a crytallization of the cement. If your concrete dries too quick then this crytallization is interrupted. So keep you concrete humid and slowly let it dry at a cool place (without frost). It might be a good idea to cover your mold with a damp cloth or with foil.

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