Thin High-Strength Concrete Garden Bed Molds

Blending the Concrete Ingredients

Now I’ll use my drill with a mixing attachment to blend the dry ingredients, starting with the pea gravel and cement. I’ll add some sand then blend again.

a man mixing concrete in two pails — With my mixer I blend the dry ingredients

I mixed the dry ingredients for about 30 seconds each time here.

Then I’ll dump that mix into the now empty white pail, to turn it over, and blend again.

pouring dry concrete mix into a pail — Adding dry ingredients on top of the water mix

Mix the High-Strength Concrete

I’ll add about half of my water and plasticizer to an empty pail and pour some of my dry mix on top. I’ll mix that some — then go in and hand stir it to make sure there’s no dry clumps in the bottom.

adding water and plasticizer to a 5 gallon pail — Adding water and plasticizer to a 5 gallon pail

pouring dry concrete mix into a pail — Adding dry ingredients on top of the water mix

man mixing concrete in a pail using a drill — Mixing the concrete in a pail

mixing concrete in a pail — The concrete is now fully wet and smooth

Then add some more dry mix and water and blend it again. Then the last bit of water and dry ingredients.

And finally some hand stirring to be sure it’s all well mixed and smooth.

wet concrete in a 5 gallon pail — The concrete is fully mixed

Fill the Concrete Molds

Then into the shop, one more stir. The gravel has a tendency to settle so you don’t want to leave it alone for long. I’ll pour enough to fill a 36″ mold half way. I can tap my bench with a rubber mallet to bring any bubbles to the surface.

a man stirs concrete in a pail — Hand stirring the wet concrete in the pail before pouring it into the mold

A man pours wet concrete from a pail into a plastic mold — Filling a 36″ mold half way

I can then lay in the wire grid, or ladder, if you like.

Another hand stir, then top up the mold, just an eighth inch or so shy of full. Then more vibrations from a mallet to bring bubbles up.

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A man lays a wire grid into wet concrete in a form — laying a 6×6 wire grid onto the wet concrete in the mold

A man pours wet concrete from a pail into a plastic mold — Pouring another batch of concrete into two 24″ molds

A man pours wet concrete from a pail into a plastic mold — Pouring another batch of concrete into two 24″ molds

A man pours wet concrete from a pail into a plastic mold — Pouring another batch of concrete into two 24″ molds

I made a slightly bigger batch to fill two of the 24″ thin molds.

The recipe can be scaled up or down.

Recipe for Two 24″ Molds

And here’s the recipe to fill two 24″ thin molds:

16 lbs Pea Gravel
16 lbs Masonry Sand
10.6 lbs Portland Cement
3.55 lbs Water
1.4 fl oz Plasticizer

When the concrete began to solidify I covered the molds with plastic.

Remove the Pipes from the Molds

The next morning I uncovered the molds and removed the pipes. These pipes have a hole that accepts a nail to give a better grip while pulling and twisting.

a man inserting a nail into a hole in a pipe — After 20 hours I remove the plastic pipes from the mold

gloved hands pulling a plastic pipe from a concrete mold — Pulling the pipes from the casting once it has solidified

a man spraying water on new concrete under plastic sheet — I spray the fresh concrete with water before covering it with plastic sheet

gloved hands pulling a plastic pipe from a concrete mold — Pulling the pipes from the casting once it has solidified

And the pipes came out easily. These pipes are a smaller diameter than my original molds and they also are more flexible.

I’ll give the 36″ casting a spray of water before covering it again with plastic. Then I’ll pull the pipes from the 24″ molds.

And these pipes also came out well. The petroleum jelly works well for this feature of these concrete panels.

And these molds were also sprayed with water and covered with plastic.

Unmold the Thin Concrete Castings

Two days later I’ll unmold the castings. And as I do with the thicker original molds, I grip the mold underneath from the ends and rotate it over against the bench. Then gently work the mold free by lifting from one end and pressing with my other hand in toward the middle.

a man removing concrete from a plastic mold — After two days I can remove (or unmold) the casting from the mold

a man holding a concrete mold against a workbench — I lift the mold then push it against the end of the workbench

A man putting a concrete mold upside down — Rotating the mold upside down without putting any pressure on the flange edge

Hands on the back of a plastic concrete mold — Lifting with my right hand while pushing with my left to free the casting from the mold

a man holding a plastic mold over a concrete casting — These thin panels almost fall out of the mold

a thin concrete casting being held by a man — A sleek, thin concrete casting right out of the mold

And being a shallower casting, they are easy to remove, and the wax applied to the mold also gives the panels a beautiful sleek finish that’s very smooth.

I set these new cast panels on the floor of my shop and wrap them in plastic.

a man setting a concrete casting on a workshop floor — Setting the new casting on the shop floor

unmolding two concrete castings — Unmolding the second 24″ casting

two concrete castings on a workbench — The two 24″ thin castings look really good

man wrapping concrete castings in plastic — The fresh castings are wrapped in plastic, kept damp, and left to cure

And the 24″ casting dropped out as well. These fresh castings I’d say are a medium grey colour. And as they cure and slowly dry they will lighten quite a bit. Maybe in the future I’ll try adding some cement dye to see how that looks with this mix.

I spray the castings with water to keep them damp while they cure for a few weeks.

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