Concrete Raised Garden Bed Molds- Plastic ABS

Ready Mix Concrete Blend

For this first pour I’ll just be using the standard ready mix concrete available at any building supply store.

breaking the paper bag with the edge of the shovel
breaking the paper bag with the edge of the shovel

As concrete mix is dusty, I like to mix it outside in a wheelbarrow then bring that into my shop to shovel into the molds. It’s also winter here so, I want the concrete to stay warm while it sets up.

I have 60 pound bags of concrete here and I need only 50 so I’ll leave some in the bag. Also, this is good practice in case you add too much water and need to add more dry mix.

adding most of the 60 pound bag of concrete to the wheelbarrow
adding most of the 60 pound bag of concrete to the wheelbarrow

So I slowly add water while mixing and turning with a shovel. I want the mix to be fully wet but not to settle like a wheelbarrow full of water.

adding clean cold water to dry concrete mix in a wheelbarrow
adding clean cold water to dry concrete mix in a wheelbarrow

When you shovel the mix it should still hold its peaks. It might take some practice if you are new to mixing concrete. Be patient.

adding water and mixing concrete in a wheelbarrow with a shovel
add a little water at a time
mixing concrete in a wheelbarrow with a shovel

Concrete as a Building Medium

I’m sure I’ve said this before in other videos but, I really like working with concrete. I think it’s such an amazing building medium. It’s very long lasting, durable and rot-proof. And it can be formed in so many different shapes and for so many applications. And for what you end up with, it’s still relatively cheap. Especially when you consider the potential lifespan of whatever your casting.

Filling the Molds with Concrete

In the shop I shovel some concrete into the form.

the first few shovelfuls of wet concrete mix going into the mold
the first few shovelfuls of wet concrete mix going into the mold
mounds of concrete added to the mold
mounds of concrete added to the mold

Then I give the form a shake to settle the concrete into the corners.

a gentle side to side shake settles the concrete
a gentle side to side shake settles the concrete

You can further settle the mix and get trapped air to come to the surface by tapping the workbench with a rubber mallet.

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a rubber mallet is used to vibrate the workbench and settle the concrete
a rubber mallet is used to vibrate the workbench and settle the concrete

I can then add some more wet mix to top up the form.

a small trowel is useful for pushing the concrete toward the edges
a small trowel is useful for pushing the concrete toward the edges

A small trowel is handy to spread the wet concrete around and toward the corners. Tap or vibrate it again, then smooth it level with the top of the form.

a small trowel is used to smoth the surface of the concrete
a small trowel is used to smoth the surface of the concrete

You only want the form filled to the top (or slightly below). Overfilling will make the panel too thick and may make the hole alignment off when you mate it to another panel.

Alternative Concrete Mixing Technique

Instead of wheelbarrow and shovel, another way is to mix the concrete in a pail with a high-torque heavy-duty drill and mixing attachment.

Some water is first added to the pail, then concrete, then mix it with the drill.

More water is added until you get a smooth wet consistency that’s not soupy.

Here’s the second form being filled from the pail.

wet concrete is poured from a pail into the mold
wet concrete is poured from a pail into the mold

This does work but, carrying the pail and pouring it into the form can be challenging for some. So, I prefer the wheelbarrow and shovel method myself.

the mold is gently shaken back and forth to settle the mix into the corners
the mold is gently shaken back and forth to settle the mix into the corners
the mold is gently shaken back and forth to settle the mix into the corners
the mold is gently shaken back and forth to settle the mix into the corners

Vibrating the Wet Concrete

I also like to use my reciprocating saw (without a blade) to vibrate the table to bring any trapped air to the surface.

a reciprocating saw (without the blade) is used to vibrate the table and settle the concrete
a reciprocating saw (without the blade) is used to vibrate the table and settle the concrete

A light troweling will give the inside surface of the panel a smoother finish and, I also do that to make sure it’s level to or slightly below the top of the mold cavity.

When filled and troweled smooth and level, I cover the forms with plastic. I don’t want the concrete to dry much. It undergoes a chemical reaction that generates its own heat as it uses the water in the mix to create strong bonds.

I like to keep my shop at around 70 degrees Fahrenheit. If it’s too cold and it will take the wet concrete longer to harden.

keeping my workshop at 70 degrees to aid in the concrete solidifying and curing
keeping my workshop at 70 degrees to aid in the concrete solidifying and curing

Wait Until Concrete Solidifies

In these ideal conditions the concrete should be solidified enough after 6 hours to remove the pipes. I slide a nail into the small hole in the pipe and with a twisting motion I slowly extract the pipes. And I’ll repeat that for the other three pipes.

the concrete was covered for 6 hours
the concrete was covered for 6 hours
checking that the freshly poured concrete has hardened
checking that the freshly poured concrete has hardened

I think there’s some resistance due to the plastic pipe rubbing against the hole in the plastic mold. Plus the pipe rubbing against the solidified concrete. I designed the holes in the mold with a tight tolerance and to be just a few thousands of an inch larger than the diameter of the pipe. So the pipe has an interference fit. It should stay in place but also be able to slide.

I wanted the pipe to make a seal when it’s inserted so no wet concrete would leak. So the tradeoff can be a little more effort when you go to remove them.

At this point the concrete is still moist and the pipes come out without too much effort.

If the pipes are really stuck you can give them a gentle tap with a rubber mallet to free them.

Followed by the twisting and pulling method. This can happen sometimes if I wait longer than six hours or so to pull the pipes.

Careful not to damage the end of the pipe hit with the rubber mallet. You don’t want to flair the end and increase its diameter.

With that done, I will cover the forms with the plastic sheets again.


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