Cast Concrete Raised Garden Beds – Precast Mold Build PART 2

By Kent Gravelle

Make the ENDS

animation of parts for the concrete raised garden beds forms — End assemblies for the Concrete Form

To make the ends, I cut a length of 2×6 for the three pieces that make up the end assembly. These are ripped on the table saw to width then back to the mitre saw to cut down for each component.

2x4 end parts cut on a miter saw — The End components are first ripped to width then cut down on the miter saw

One of these blocks I take to the drill press for the dowel hole.

drill press — One of the End blocks is drilled for the peg dowel and assembly holes drilled with a 1/8″ bit

I’ll then drill the pilot holes for assembly with an eighth bit.

cutting wooden dowel on a saw — The dowels for the pegs need to have their diameter reduced a bit so the plastic pipes fit over easily

The dowels need to be sanded down to reduce their diameter to fit inside the pipes. I use a disc sander for this. You could also use a small hobby sander or a belt sander instead.

gluing a wooden dowel in the end assembly — Dowels are cut to length and glued applied. Then tapped into the holes in the Ends and Sides

These are then cut to length and glued and tapped into the holes.

assembling the end parts of the form — The End components are glued and screwed together

I assemble the ends with glue and screws checking that everything lines up well and is square.

assembling the end parts of the form — Quick clamps are just awesome

A clamp helps to hold the pieces while I run in the screws.

Test Fit and Assembly

With all the parts of the form ready, I assemble it and test the fit.

final assembly of the mould — Assemble the form now that the main parts are made

I number each form and all the parts so I can reassemble it again the same way.

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labelling the form parts — If everything fits well then label the main parts with a Sharpie

assemble the forms to cast concrete raised garden beds — Self drilling cabinet screws are a good option

I used particle board screws for this but there’s some self drilling cabinet screws, for a few more dollars, that would save time and pre-drilling.

I used some wood filler to fill screw holes — A bit of wood filler used on some screw heads and knot holes

I filled any knot holes or screw heads with a bit of wood filler. Some of the insets had holes predrilled from the other plywood forms. They were the right size so I attached them with screws from the top.
No worries. Wood filler filled the screw heads and only took a minute to apply.

Apply Finish

I remove the screws and disassemble the form and apply two coats of Food Grade Mineral Oil.

use mineral oil to coat the concrete forms — Butchers Block oil

This oil is sold as Butcher’s Block or Cutting Board oil. You can also buy it in Feed Stores for about half the price.
I liberally apply it with a cloth. It doesn’t take long and each coat took about 10 minutes per form.

Pipes and Wire

animation of parts for the concrete raised garden beds forms — Pipes and Wire are cast into the panels

While this is drying (or soaking in) I’ll cut the plastic pipe to length.

cutting plastic pipe with a miter saw — Plastic conduit cut to length. A hacksaw would work just fine for this

I use wire wrapped around the pipes to add strength to the ends where the concrete is the thinnest.

galvanized wire wrapped around plastic pipes — Galvanized wire “springs” are made by wrapping them loosely around a pipe

I wrap wire around a pipe like a spring. Then open it up a bit.

wire grid added to wet concrete for reinforcement — Concrete reinforcement grid is cut with bolt cutters

With small bolt cutters I cut a section of stiff wire mesh to reinforce the center section of the panel. This will be laid in the concrete as it’s poured.

Final Assembly

Now assemble the forms with the wire and pipes. I add some latex calking to make the forms water tight and to add a small fillet in the corners.

adding latex caulking to make molds water tight — Latex caulking added to gaps and corners

This takes only a few minutes per form. This also fills any small gaps where the sides, ends, and base meet.
Latex works well as it’s not overly strong and will allow you to disassemble the mold easily once the concrete sets up. Don’t use regular silicone for this. It’s too strong and will be difficult to remove later. Latex caulk is all that’s needed.

apply latex caulk to seal the concrete forms — Add caulking when Ends are first attached to the base

I realized later that it’s easier to add the caulking to the ends before the pipe is in place.

cast the CONCRETE RAISED GARDEN BEDS

I set the forms over sawhorses and level them in both directions.

levelling forms with a 24" spirit level — Forms are levelled

I cover the wire and pipes with a cloth and spray the form with vegetable non-stick cooking spray.

adding latex caulking to make molds water tight — Cloth covers pipe and wire

spray the concrete raised garden bed forms with cooking oil — Cooking Spray works as a non-petroleum release agent

I’m going to use a crack resistant concrete mix that has fibres added for more strength. It’s only a few dollars more per bag so why not?

bags of quickrete concrete mix with added fiber — Concrete mix with fibres

Some have suggested that fibre reinforced concrete would be strong enough so wire would not be needed. But adding the wire gives me an extra bit of insurance that doesn’t cost much in time or materials. So I’m sticking with it for these castings.

mixing concrete by hand with shovel and wheelbarrow — Mix concrete in a wheel barrow

I mix the concrete in a wheel barrow and shovel it into the form filling it half way.

filling the form with wet concrete mix — Shovel wet concrete into form

I use a wooden mallet to settle the concrete and bring bubbles to the surface.

using a reciprocating saw without the blade to vibrate the concrete — Vibrate the forms with a power tool

You can use the edge of an orbital sander or a reciprocating saw without a blade to vibrate the forms and settle the concrete.

filling the form with wet concrete mix — Wire mesh piece added then topping up form with more concrete

I lay in the wire mesh then top up the form with more concrete. I’ll let this set up for a bit then come back with a corner tool to round over the edges on the sides.

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