Thin Concrete Mold Preparation
As I’ve shown in my previous video, I now prefer to use a paste finishing wax in the cavity of the mold, and brushed on petroleum jelly on the pipes.
You simply wipe on a thin coat of wax paste in the mold. I make sure I get wax on all the inner surfaces. Then let it dry to a light haze. And this takes 10 or 15 minutes. Then with a clean cloth I wipe it off and bring up a shine.
I’ve been getting a much better surface finish on the concrete castings by applying a wax instead of the mineral oil I was using.
And I’ve found it’s easier to remove the pipes once the concrete hardens when using petroleum jelly. It goes on thicker than mineral oil and I can get away with leaving the pipes in longer than the usual 6 hours or so.
I use a small brush to apply the petroleum jelly to the pipes to get a thicker coat than just wiping some on with a cloth.
And you always want to be filling the molds with concrete on a sturdy and level surface.
Concrete Reinforcement
I like to add some 9 gauge wire grid to reinforce the thin concrete panels. And I’ll use some 6″ x 6″ grid. It’s available from most building supply stores. I cut it down to size with small bolt cutters.
This 9 gauge wire grid is 0.15 inches in diameter. Any wire grid that’s less than 3/16″ diameter will be okay. And I’ll want to cut it two inches or so shorter than the space I have inside the mold.
Thin Concrete Ingredients and Recipe
The ingredients for this mix are from building and landscaping supply stores. For this concrete blend, I’ll be using pea gravel, masonry sand, portland cement, and a plasticizer (or water reducer). And of course, cold clean water.
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For my first experiments I used a powdered plasticizer called Supercizer 7. It worked okay but, had a very sour smelling odour. And, it causes some water separation after I poured the mix into the molds. Not a big concern but thought I’d mention that.
The water reducer I prefer is popular and available from most concrete casting suppliers or websites. It’s called ADVA Cast 555.
The water reducer allows you to make a pourable mix with much less water. By reducing the water you end up with dense and very strong concrete. Making these thin panels is very similar to pouring concrete countertops.
It’s pretty amazing to see the effects of the plasticizer. The mix at the beginning looks very dry. Then within a few minutes of blending it smooths and liquifies right in front of your eyes.
Additional Tools I Use
Along with the tools I already mentioned, you will need gloves, a stir stick to hand mix the concrete. Something like a 1 by 2 or wooden broom handle, two 5 gallon pails, a can or measuring cup, and a respirator or mask when you are exposed to any concrete dust.
High-Strength Concrete Recipe
So here is the weights of all the ingredients including the water:
To fill one 36″ thin mold:
12 lbs Pea Gravel
12 lbs Masonry Sand
8 lbs Portland Cement
2.6 lbs Water
1.2 fl oz Plasticizer
I weight out all the ingredients before I start adding any water. And I find using two 5 gallon pails helps to blend the dry ingredients.
Into my first pail, the beige one, I’ll weigh out the pea gravel. Then into my second pail, the white one, I’ll weigh the masonry sand.
I’ll get my respirator on with P100 filters and add portland cement to the gravel. Next I’ll weigh my water into another pail. And for this, one of my earlier experiments, I’ll add the powdered plasticizer to the water and dissolve it.
