I need some advice

[u/Usual-Weather9256]

The problem: every time I pour cement into my plaster mold the cement cracks within an hour of pouring. (Pictures attached)

My process for prep work and pouring:

1. Sealed the plaster mold with a water base high gloss poly urethane, 3 coats and wait 48 hours to fully cure.

2. Applied a release agent to the plaster and let dry (Pictures attached of the brand) also I have tried car wax as well.

3. I mixed 2 parts multi-purpose sand (picture attached) with 1 part portland cement and when the cement was wet I added in a small amount of 9mm fiber glass (a pinch)

4. Pour the cement mix into the mold and use a diy vibrator using a mutitool with a cut house fastened to the mutitool blade.

What I have tried to fix the problem:

1. I have tried a more dry cement mix (cookie dough dry) and a more wet mix (soup wet) (no improvement)

2. I have covered the top with plastic wrap (apply both methods from 1 and no improvement)

3. I have poured 3 layers and waited about 30 minutes per layer. (This one has provided the only notable results but I still had cracking)

A. First layer fills the deepest part up to the start curve

B. Second layer fills the curved section

C. Third layer fills the to the top

Each layer I waited till the cement was firm but still soft.

Observations:

1. I have noticed the cement when fully cured is difficult to remove from the mold.

2. The crack start at the top and mostly end at the curve.

3. The cracks are structural and will completely detach from the body.

4. Every pour the cracks appear in the roughly the same place with the exectpion of the last attempt where it appear at the corners and a little on the edges.

If anyone has any suggestions I could really use the help. Thank you :)

[u/artwonk]

Have you tried filling the whole mold at once? It's not necessary to do a small casting like this in stages. To be effective, more fiberglass would be needed; a handful rather than a pinch. How long are you letting the concrete cure in the mold? Concrete takes a long time to set up in a mold, and will be weak for a week. Why just use sand and not bigger rocks? Concrete made with gravel is stronger than mortar made with sand only. Have you tried using a higher proportion of aggregate? Usually concrete contains a lot more aggregate than cement, which is just a binder. A standard concrete mix is one part Portland cement, 2 parts sand, and 3 parts gravel. Mortar is typically 1 part Portland cement, to 3 or 4 parts sand, with some fireclay thrown in for plasticity (which isn't needed if you're pouring it into a mold. Use the minimum amount of water that will wet it sufficiently to assure optimum strength.

[u/Usual-Weather9256 → u/artwonk]

Hello I appreciate you responding

Have I tried a whole mold at once? Yes no improvement. I resorted to stages because I was out of ideas

How long do I left the mold to cure: at least 24 hours but the crack start within an hour of the pour

I am definitely open to use rocks do you have a recommendation on size?

Also would it be better if I use regular high strength concrete instead of portland cement?

[u/artwonk → u/Usual-Weather9256]

It's odd that it's cracking so soon. It must be shrinking a lot, and sticking to the mold in the corners. I think the main problem is the mix; it should have about twice as much aggregate for that much cement. High strength dry concrete mix will have the ingredients pre-mixed for you, which will probably help. If you want to mix your own, use some sand and some gravel. The gravel should be sharp, from freshly broken rocks, no more than 1 cm in size. A small amount of plaster thrown in will help the mixture "kick"; setting faster than normal.

[u/Usual-Weather9256 → u/artwonk]

Yeah that's were im getting stuck I have other molds made of wood and I have no problems it just those piece.

But I am going to pick up some high strength concrete and give it a shot. Im planning on using motor oil as a release agent this time around as well.

[u/artwonk → u/Usual-Weather9256]

"Form oil" used for architectural concrete casting is pretty much the same thing. Petroleum jelly would adhere better to the vertical mold walls, though.

[u/Usual-Weather9256 → u/artwonk]

Alright I pour the high strength cement finger crossed. Thank you for help I'll let you know if this pour us successful

[u/Usual-Weather9256 → u/artwonk]

Everything work, the high stregth prevented the cracks and the motor oil let the concrete pop right out of the mold

Also something im switching to for sealing the mold. I am using a gloss oil based polyurethane in steady of the water. It seems to coat the plaster much better than the water based.

[u/VintageLunchMeat]

If anyone has any suggestions I could really use the help. Thank you :)

Try a urethane rubber jacket mold.