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Poured Concrete Raised Garden Beds – Complete Precast Form Build PART 2

By April 12, 2019November 24th, 20202 Comments

Make your own Concrete Raised Garden Beds! This is PART 2, showing a simpler method that uses standard framing lumber. You can build these forms and precast your own 48″, 36″, or 24″ reinforced concrete panels that lock together to make long lasting and durable concrete garden boxes. See the complete build video that shows step-by-step how to make the wooden molds for the precast garden boxes. An alternative to concrete garden edging.

Here is the weight of 3 panels that are cured: . 48″ – 69lbs, 36″ – 50lbs, 24″ – 33lbs.

See the Build Video and download the Plans.

How much concrete does it take to pour each size of panel? 
Here’s the approximate square foot volume of each size of the 2.5” thick panels:
24” panel  = 0.23 cu ft
36” panel  = 0.36 cu ft
48” panel = 0.49 cu ft
Bags of ready mix concrete are sold in various sizes. Here is how much a bag of Quikrete will make:
40lb bag = 0.30 cu ft
50lb bag = 0.375 cu ft
60lb bag = 0.45 cu ft
80lb bag = 0.60 cu ft
Check the instructions on the bag to see how much it makes.

Background for the Cast Concrete Raised Garden Bed Project – PART 2

This is a follow-up to my blog post and video on casting your own concrete garden box panels. A year has passed so I think it’s time to see what improvements I can make.

There was a ton of interest in that video, so I wanted to shoot a sequel, AND find a faster, easier, and cheaper way to build the forms and cast the panels. These reinforced concrete panels link together to create long lasting, rot-proof garden boxes.

If you haven’t watched my first video then please have a look to see my original plywood form design.

I got a lot of comments and suggestions from the first video and I was overwhelmed by the positive reaction it received. So, thank you for your support and contribution!

I’ll address some of the most common comments and questions I received from that first video in the Q & A section below.

I first made these plywood forms and cast panels about 8 years ago, long before I planned a YouTube Channel. I was casting these for myself so at the time I wasn’t concerned that the forms be simple. I just wanted them to be reusable and for them to work.

the concrete panels are pinned together with rebar

the concrete panels are pinned together with rebar

I’m not a professional form builder, so I’m sure there’s other ways to cast these panels, but this is what works for me.

Poured concrete raised garden beds

Many commented that the forms were too complicated to build. So I decided to go back and look at my original design to see if I could simplify and improve upon it. I still want to cast the concrete panels in the same size and shape so the new ones would connect and fit to the original ones.

My original plywood forms work very well so, if you’re already building yours that way then keep at it.

The NEW form design uses standard framing lumber instead of plywood. There’s fewer parts, and I’ve simplified the finishing. Instead of multiple coats of polyurethane, I’m using Food Grade Mineral oil to coat the forms. It’s cheaper, faster, and easier to apply. I like it as its non-toxic and won’t go rancid like vegetable oil.

You don’t need a lot of tools to make these forms and I’ll show some alternatives as I build them.

As before, I have a full set of plans available for download.

So here’s the new form design I came up with.

Precast concrete raised garden beds

These panels link together to create long lasting, rot proof concrete garden boxes.

making a tall bed by staking more panels

Concrete Garden Box Panels Stacked

DIY concrete raised beds.

I have plans available for download and I also have a list of all the tools I use for each project. So please check those out.

  1. The PARTS
  2. the BASE
  3. Make the INSET
  4. Make the SIDES
  5. make the ENDS
  6. TEST FIT and ASSEMBLY
  7. apply FINISH
  8. PIPES and WIRE
  9. final ASSEMBLY
  10. pour CONCRETE
  11. make the GARDEN BOX
  12. Q & A – from the first video

Concrete Garden Box Sides – the Build

The Parts

There’s three main parts to the form. The base, two identical ends, and two identical sides.

animation of parts for the concrete raised garden beds forms

Main Parts of the Form

The base is made from a 2×10 with a beveled 1×4 as an inset, the ends and sides are made from a 2×6. The ends and sides have a peg made from wooden dowels to hold a pipe in place. The pipes are plastic electrical conduit with galvanized wire wrapped around them for reinforcement. When filled with concrete half way, a stiff wire mesh is laid in for additional strength.

the BASE

animation of parts for the concrete raised garden beds forms

Concrete Garden Box Form Base

I start by cutting a 2×10 to length, then ripping it to width on my table saw. This one will make a 48″ long panel. I cut these to length with a mitre saw.

Concrete Garden Box Forms

Cutting a 2×10 to length

But, a square and circular saw will work just as well.

circular saw and large speed square for cutting lumber to length

Circular saw an square can be used instead of miter saw

To make the new forms the same inside dimensions as the previous plywood versions, I rip the 2×10 down to 8″ wide. I use my portable table saw for this.

rip lumber on a cheap portable table saw

2×10 is ripped on table saw

A guide and a circular saw will also work.

ripping lumber with circular saw and guide

Circular saw with a guide is an option for ripping lumber without a table saw

I rip both edges on the table saw to give me a square corner on both sides. Framing lumber comes with a small rounded corner and I want to remove these.

I’m making three different lengths of forms this time. To cast a 48″, a 36″, and a 24″ panel. The drawings have dimensions for each. And I’ll show you some garden box variations you can assemble with these different sizes at the end of this build section.

After the base is ripped and trimmed to the final length, I’ll lay out the end cuts and inset location.

ruler and sharpe used to lay out the concrete form base

Laying out base corner cuts and inset location

I’ll remove the corner pieces with a jigsaw. You could use a handsaw for these cuts instead.

jig saw cutting the form base

Using a jigsaw to cut corners

I check the cut with a square.

 

Make the INSET

animation of parts for the concrete raised garden beds forms

The inset is a decorative element in the casting of the panels

The inset is an angled piece attached to the base mainly for decorative purposes. It’s not functionally required but I like the look. You could personalize your panels here by adding an oval, or circles, or a logo, or anything you want.

Just be sure that whatever you use has a bevelled edge so you can remove the panels from the base after casting. A 1×4 (which is actually 3/4 x 3 1/2) can be used to make this. I had some slightly thinner 5/8 inch boards so I used them. Either would work fine.

concrete raised garden beds - ripping the form inset at an angle

Ripping at 30 degrees on the table saw

I rip these at 30 degrees on my table saw and cut to length on the miter saw.

angle cuts on the miter saw

Cutting the inset to length at 30 degrees on the miter saw

I attach them with glue and screws from underneath. If you don’t use glue then you can change the inset design in future castings.

gluing the plywood inset

Inset is attached to the base with glue and screws

Make the SIDES

animation of parts for the concrete raised garden beds forms

Make two identical sides that attach to the Base and the Ends

The sides are made from a 2×6. Like the base, they are cut to length on the miter saw.

chopping the sides on a miter saw

Cutting the side pieces to length on the miter saw

Then ripped down to width on the table saw. (Depending on the tools you have you might decide to rip just the one side down, and that’s okay.)

ripping the form side walls on a table saw

Ripping sides on the table saw

After ripping I do the final trim to the exact length back on the miter saw.

drilling parts on a press

Drill 5/8″ hole in sides with Forstner bit on the drill press

The sides are laid out and the hole for the dowels drilled. I use a forstner bit for this but any 5/8″ drill bit would do.

a drill guide is a good alternative to a drill press

Drill guide is a cheaper alternative to a drill press

I use a small bench top drill press but you could carefully freehand this or use a drill guide. You want the pipe holder pegs to be at right angles (or 90 degrees) to the sides.

I like to pre-drill the screw holes as well. It makes assembly a bit easier later on.

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.

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.

The concrete is covered with plastic and left for a few days.

striping the form from the concrete casting

Removing screws from form after concrete is set

stripping the form from the concrete casting

Removing the Ends

Now the screws can be removed and the sides and ends can be gently pried free.

a 24 inch concrete garden panel

Completed 24″ panel

The newly cast panels should be kept wet and allowed to cure for about 4 weeks. Your concrete raised garden beds will be that much stronger and durable the longer you cure them.

The latex caulk can be easily scraped off with a putty knife and the form parts wiped clean with a rag.

Make the Concrete Raised Garden Beds

I cut rebar into one foot lengths with a cutoff blade in my angle grinder. I clamp the long length of rebar across sawhorses.

cutting rebar with an angle grinder

Cutting rebar with angle grinder

A hacksaw will work but it takes longer. Some building supply stores sell rebar pre-cut in shorter lengths as well.

framing square works great for setting up the beds

Checking for square

I level a spot in my garden and overlap two panels. I check the corners for square then drive in the rebar thru the corner and into the ground to secure them.

panels are made level and square during assembly

Levelling the panels

I repeat this on each corner to complete the Garden Box.

making a tall bed by staking more panels

Making a double height concrete garden box

For deeper concrete raised garden beds you can stack and alternate the panels if you like. You will need longer rebar for this.

Here’s a few different size garden boxes you can make using these three panel lengths.

making a long thinner garden bed for flowers or vegetables

Concrete Garden Box – 36″x96″

making a long thinner garden bed for flowers or vegetables

Concrete Garden Box  – 24″x96″

assembling the panels to make a garden bed

Concrete Garden Box half double height

I would normally lay out and level the ground before setting the panels in place. But, for this demo, I used some wood blocks instead.

We put a layer of cardboard down over the grass then fill the box with compost and soil before planting.

Q & A – from the first video

So, in this section I’d like to answer some questions from the previous video and add some thoughts on casting these concrete panels.

What about making the forms from melamine?

I happen to like working with plywood so that’s the material I chose. But instead of plywood, these forms could be made using melamine coated board. That’s a really good choice for small form work and melamine doesn’t require any finishing. I don’t know about the longevity of particle board near moisture, but if you were only doing a few castings then it should work just fine.

Can you make the panels lighter?

Many people asked about using light weight concrete and other alternative materials to make the panels lighter. For me, the weight isn’t a big concern but, enough people have asked about it that I’ve decided to shoot another video covering just that. So look for that coming up soon. It seems that you have to sacrifice strength when you go to lightweight concrete, but it’s worth having a look at, so I’ll see what I can come up with. Be sure to hit the subscribe button and the bell to be notified when I post that.

Can you make the panels taller for deeper beds?

I decided on an 8 inch height for the panel as it seems that you can grow just about anything in 8 inches of soil. A taller panel might get a bit too heavy to handle easily. However, these panels can be stacked to create deeper beds. These concrete raised garden beds are deep enough for our needs.

stacked for taller concrete raised garden beds

Concrete Garden Box panels can be stacked for deeper beds

How will these panels do in the winter?

Another concern was the winter durability of the panels and how they might survive freeze and thaw cycles. The rebar that pins the corners together allows for a bit of movement for expansion and contraction. You could also use a thiner rod than this rebar for even more play at the connection. The pipe holes could also be filled or capped to prevent water from pooling inside.

a foam gasket can be inserted in the joint to add some flexibility

Foam sill gasket may work to help ice from forming in the joints over winter

A thin flexible gasket could also be added between panels to keep out moisture and add some flexibility at the joint.

Can you move the concrete raised garden beds after the panels are set and linked together?

A washer could be welded to the top of the rebar to make it easier to remove if you ever wanted to disassemble the panels and move the bed.

There’s pro’s and con’s to both these designs.

The new framing lumber version has fewer parts and is easier, faster, and cheaper to build than my original plywood forms. And I have also found that it’s easier to remove the ends by gently rocking them. Plus, you can remove the base from the finished panel easier by tilting it up on its side.

The plywood forms give a better finish, should last longer, and have fewer (if any) knots. And plywood is not susceptible to the distortions of framing lumber. Framing lumber will have a greater tendency for warping, cupping, or twisting from moisture penetration, during the casting process. And, I’m finding it’s harder to source good quality, straight and true lumber these days. I try to be as accurate as possible when I build these forms. Doing so ensures I get a good fit in the final castings.

So I hope you have enjoyed this post, and thank you so much for reading. If I make any further improvements or changes I’ll post them here.

And, while you are here, please hit the Support Link to help us make more projects and videos like this. We really appreciate anything you can do to help us out!

 

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