Round Concrete Garden Bed Casting from Molds

Lay in Wire Grid

I’ll place the wire grid in the center of the space and gently press it to keep it in place. More concrete is shovelled in and spread around. This is settled with more vibration.

round garden bed molds 51 — Rotating the wire coils down into the mix

round garden bed molds 52 — Rotating the wire coils down into the mix

Ideally I want the mold very slightly under filled. I sometimes have to remove a bit or add a bit to achieve this. Myself, I usually end up with it filled to the top. That’s also okay. Overfilling can lead to it spilling over on the bench or having it too thick and then not fit as well with other panels in the garden.

round garden bed molds 54 — Using a recip saw (without blade) to vibrate the bench and further settle to wet concrete

I little troweling on top is done to smooth the surface. Now just leave it be for a few hours until it solidifies. At 70 degrees this should be around 6-8 hours. If your mix was too wet you may see water begin pooling on the surface. If so just leave this as it is. It should evaporate over time.

Leftover Mix

I find there is usually some wet concrete mix left over from the pour. I take this leftover and shovel it into a plastic pail or wastebasket. I then have a piece of galvanized wire bent to fashion a handle. I embed this in the concrete once it’s settled.

round garden bed molds 55 — Shovelling the leftover wet concrete into a plastic pail to make a weight with a wire handle

round garden bed molds 56 — Shovelling the leftover wet concrete into a plastic pail to make a weight with a wire handle

On the right you can see a weight from a previous pour.

After a few days I can pop this weight out of the pail. I use these around our property for weighing down sheet metal that’s covering firewood or milled lumber.

round garden bed molds 58 — Remove the weight casting from the plastic pail

8 Hours After Pouring Panels

Okay, back to the garden bed molds. Once solidified after 8 hours it’s time to remove the pipes.

round garden bed molds 59 — 8 hours after pouring

Pull Pipes

I insert a small nail into the hole and pull while rotating back and forth to remove the pipe. Try to pull the pipe straight out. I have a paper towel ready to wipe the petroleum off the pipe before I set them down. I’ll do the same for the rest of the pipes.

round garden bed molds 60 — Checking that the concrete has solidified after 8 hours

The petroleum jelly works well and all the pipes came out without any problems.

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round garden bed molds 61 — Pulling the pipes after 8 hours

Cover & Wait

If the surface of the concrete is dry I give it a spray with water before covering with plastic. I will keep the concrete wet and covered for three days ideally. The concrete will begin to cure and strengthen during this time.

round garden bed molds 62 — Spray with water before covering with plastic sheets

It’s important to keep the ambient temperature around 70 degrees for these three or so days. If cooler, it will take longer to cure. Too hot and it may dry too quickly and also not be as strong.

3 Days after Pouring Panels

After three days in ideal conditions it’s time to release the castings from the molds.

round garden bed molds 63 — Three days later I remove the plastic and get ready to demold the panels

Demold the Castings

I gently pull on the mold to release it from the side walls of the casting. I grip and lift from underneath then press the top of the mold against the bench while I rotate it over. You don’t want to stand a fully loaded mold on the thin plastic flange edge for fear it might crack.

Now I gently lift the end of the mold while applying some pressure downward with my other hand. I want to slowly coax the mold off the concrete. Sometimes you will need to go back and forth from end to end to release it. Once released have a look inside the cavity. It should look clean with no concrete bits or residue. If it’s clean then the concrete was left in the mold long enough.

round garden bed molds 64 — Pull slightly on edges before rotating the mold face down on the bench

round garden bed molds 65 — Pull slightly on edges before rotating the mold face down on the bench

If you find that you are having a hard time demolding the casting and you are worried that any more force may damage the plastic then there’s another method that might help.

Apply Gently Heat

With a hair dryer or heat gun you can gently warm the plastic evenly to allow it to expand a bit. Be careful to not apply so much heat that it permanently warps or melts the mold. I have my heat gun set on the medium setting and keep it moving over the mold. This should take 1-2 minutes. Now the mold should easily release from the concrete. I checked that it is warm to the tough and not hot.

round garden bed molds 68 — Warming the plastic mold with a heat gun to gently expand it to aid in demolding

round garden bed molds 69 — Warming the plastic mold with a heat gun to gently expand it to aid in demolding

Here’s the rest of the castings coming out of the molds. And for each pair of these I first demolded the casting just by hand, then used the warming technique on the second one.

Even though this is high strength concrete, It’s only three days old. So, I handle them gently when moving them aside. I also wear steel toed safety shoes in case I lose my grip while carrying one of the castings.

round garden bed molds 71 — Getting ready to demold a 24″ round end concrete casting

round garden bed molds 72 — Getting ready to demold a 24″ round end concrete casting

This is by far the best part of concrete casting. Seeing the final results of the prep work and mixing.

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