Author Options:

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?


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..


4 months ago

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.

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.

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

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.

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.


4 months ago

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.