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Make Lightweight Concrete Garden Boxes – PART 4 using CSA Cement and Thinner Panels

By July 8, 2019 3 Comments

Make your own Thin, Lightweight CSA Concrete Garden Boxes! Build the forms and cast your own reinforced high strength concrete panels that join together to make long lasting and durable garden boxes. I’ll make a thinner, stronger lightweight concrete panel for my garden panel from CSA Concrete. This is Part 4 of the series.

See the Youtube Video and download the Plans.

Background for the Cast Concrete Garden Box Project – PART 4

Hi it’s Kent from MAN about TOOLS and today I’m making new concrete garden box panels that are thinner, lighter, stronger, and faster to cast.

CSA Thin Concrete Garden box panels

CSA Thin Concrete Garden box panels

This is Part 4 of my series on making these reinforced concrete garden box panels.

In Part 1 I made the forms from plywood. In Part 2 I simplified the design to make the forms easier to build. In Part 3 I cast the panels with lightweight concrete.

These panels have pipe cast in the corner so they can be joined or pinned together to make long lasting rot proof garden boxes.

Pinned at the corners with metal bar

Pinned at the corners with metal bar

In my last episode I looked at making the panels lighter using a few lightweight concrete formulations. The panels I cast were designed for traditional portland-based cement and it has some limitations. The panels needed to be a certain thickness to have enough strength around the pipes.

Portland cement based panels need to be thicker due to pipes cast in

Portland cement based panels need to be thicker due to pipes cast in

I started looking for ways to make the panels lighter by altering the concrete but, I think it makes just as much sense to find a way to make them thinner. So just use less concrete. There’s another type of concrete thats based on a different cement called CSA. It’s super high strength and you can cast it much thinner. It sets up very fast and it gets stronger, faster than portland based cement.

And this, I think, is yet another way to make panels lighter.

I still like the basic look and shape of the panels, so I made some modifications to the form design so these thinner panels still link together in the same fashion as the previous ones. I also will be using a smaller diameter pipe in the corners, and that will require a smaller diameter steel rod to join them. Rebar is too thick but there’s some alternatives. And I’ll get to that later.

So here’s the modified design and how I made these new forms.

the BASE

The base is made from a 2×10 with a thin beveled inset, the ends and sides are made from a 2×4. The ends and sides have a peg made from wooden dowel to hold a piece of plastic water pipe in place.
There’s three main parts to the form. The base, two identical ends, and two identical sides. When filled with CSA concrete, a galvanized wire mesh is laid in for additional strength.

Model of assembled form showing grid placed in concrete

Model of assembled form showing grid placed in concrete

I start by cutting a 2×10’s to length, then ripping it to width on my table saw. This one will make a 48″ long panel. I’m making three different lengths of forms time. To cast a 48″, a 36″, and a 24″ panel. The drawings have dimensions for each. This offers some variation depending on the size garden box you want. I cut these to length with a mitre saw. But, a square and circular saw will also work well.

Cutting 2x10's for form bases

Cutting 2×10’s for form bases

These thinner panel forms are constructed in a very similar manner as the ones I built in Part 2 of this series.

Using a miter saw but a square and circular saw would work fine

Using a miter saw but a square and circular saw would work fine

To make these new panels the same width and height as the previous portland cement versions, I rip the 2×10 down to 8″ wide. I use a portable table saw for this. A guide and a circular saw will also work.

Ripping the bases to width. A circular saw and guide would also work

Ripping the bases to width. A circular saw and guide would also work

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 this. (Depending on the tools you have you might decide to rip just the one side down, and that’s okay.)

Cutting base with jigsaw

Cutting base with jigsaw

After the base is ripped and trimmed to its final length, I’ll lay out the end cuts and inset location. I’ll remove the corner pieces with a jigsaw. You could use a handsaw, or a bandsaw, for these cuts instead. A jigsaw blade can wander so I check the cut with a square.

Checking cuts with small square

Checking cuts with small square

the INSET

The inset is an angled piece attached to the base mainly for decorative purposes. It’s not functionally required but I like the look. Thin plywood can be used to make this. I rip these at 30 degrees on my table saw and cut to length on the miter saw.

CAD model of base and inset

CAD model of base and inset

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.

Ripping plywood at 30 degrees on table saw

Ripping plywood at 30 degrees on table saw

I attach the inset with glue, and weigh it down until the glue dries. If you use short screws instead of glue you can change the inset design in future castings.

Inset is glued to base and held in place with weights

Inset is glued to base and held in place with weights

the SIDES

The sides are made from a 2×4. Like the base, they are cut to length on the miter saw. Then ripped down to width on the table saw.

Sides are ripped on table saw

Sides are ripped on table saw

The sides are laid out and the hole for the dowels drilled. 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.

Sides and end block with dowel glued in

Sides and end block with dowel glued in

I like to drill pilot holes for the assembly screws at this time. I find it makes assembly easier.

the ENDS

Three blocks for the end assemblies

Three blocks for the end assemblies

To make the ends, I use a length of 2×4 (that’s already ripped to width) for the three pieces that make up the end assembly. These are cut on the mitre saw.

End assembly fits end of base

End assembly fits end of base

One of these blocks I take to the drill press for the dowel hole. I’ll then drill the pilot holes for assembly with an eighth bit. Dowels are cut to length and glued and tapped into the holes. I assemble the ends with glue and screws checking that everything lines up well and is square.

Drilling dowel hole on drill press

Drilling dowel hole on drill press

The ends can also be made with two pieces instead of three if you’re okay drilling the dowel hole into end grain. Either method works.

TEST FIT

With all the parts of the form ready, I assemble it and test the fit. I use self drilling cabinet screws for this.

Test fit all parts by assembling the form

Test fit all parts by assembling the form

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

Parts labelled so can be reassembled the same each time

CSA Thin Concrete Garden Boxes – Parts labelled so can be reassembled the same each time

apply MINERAL OIL

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

Butcher's Block Oil

Butcher’s Block Oil

FINAL ASSEMBLY

While this is drying (or soaking in) I’ll cut the plastic pipe to length. And I cut a section of galvanized wire fence to reinforce the center section of the panel. This will be laid in the concrete once it’s poured.

Cutting plastic pipes with miter saw

Cutting plastic pipes with miter saw

Now I reassembled the forms with the pipes on the dowel pins. I add some latex calking to make the forms water tight and to add a small fillet in the corners. This takes only a few minutes per form. This also fills any small gaps where the sides, ends, and base meet.

Forms ready for concrete

Forms ready for concrete

Latex works well as it’s not overly strong and will allow you to disassemble the mold easily once the concrete sets up. I don’t use regular silicone for this. I find it too strong and will be difficult to remove later.

Latex caulking in corners

Latex caulking in corners

I set the forms over sawhorses and level them in both directions. I cover the pipes with a cloth and spray the form with vegetable non-stick cooking spray.

pour CONCRETE

I’m using CAS mortar mix for these thinner forms. In North America, the Rapid Set brand can be found at Home Depot.

CSA Mortar mix and plasticizer

CSA Mortar mix and plasticizer

To make the concrete flow and pour like water I’ll be using a plasticizer. One small bag per batch. This mortar mix sets up very fast, so to give me more time, and to slow this down,

Glass fibers to reinforce concrete

Glass fibers to reinforce concrete

I’m going to add a small amount of Citric Acid powder. This powder can be found in most health food stores or a grocery store in the canning section. In addition I’ll add a third of an ounce of glass fibre. This is the manufacturers recommended amount.

Here’s the proportions for the mix: A 55 pound bag of CSA Mortar Mix, 5 quarts of water, a bag of plasticizer, and about 2 ounces of citric acid powder.

CSA Concrete Garden Box formula

CSA Concrete Garden Box formula

I add the water to my mixing pail and dissolve the citric acid powder. Then I add the plasticizer and dissolve it too. Then I add about a third of the bag of mortar mix and some glass fibre. I’ll blend this until smooth, adding more dry mortar mix slowly.

Adding dry CSA mortar mix to pail

Adding dry CSA mortar mix to pail

One batch will fill two 36″ forms or a 48″ and a 24″. I fill each form with the concrete mix. And vibrate the forms with a recip saw (without a blade) to bring any bubbles to the surface. When the concrete begins to thicken I lay in the galvanized wire grid and push it into the centre with a trowel.

Pouring CSA concrete into garden box panel forms

Pouring CSA concrete into garden box panel forms

I mixed one final batch to fill the last 48 and 24 inch forms. Laying in wire grid as it solidifies.

Pushing wire grid into thickening concrete with trowel

Pushing wire grid into thickening concrete with trowel

I wet the forms when the concrete hardens and begins to dry on the surface, and a white spotty haze forms. I do this water curing for about an hour.

Water curing concrete

Water curing concrete

After an hour, the concrete can be removed from the forms.

Form side removed showing pipe embedded in concrete panel

Form side removed showing pipe embedded in concrete panel

Now the screws can be removed and I can gently pull off the sides. And then each end. And I can free the panel from the base with a little help from a paint scraper. And here’s a 48″ panel coming out of the form.

CSA concrete garden box panel and form base

CSA concrete garden box panel and form base

The newly cast panels should now be at half their full strength. They will reach near maximum strength in about 4 weeks if kept wet. The latex caulk can be easily scraped off with a putty knife and the form parts wiped clean with a rag.

Thin CSA panel is 40% lighter than portland version

Thin CSA panel is 40% lighter than portland version

So I really like how these concrete panels came out of the forms. It’s the nicest concrete I think I’ve ever poured. The original portland based concrete forms are about 50 pounds for a 36″ panel. These new panels are only 30 pounds. So pretty significant savings in weight and concrete

Cutting electrical fence post for panel pins

Cutting electrical fence post for panel pins

I cut electrical fence posts into one foot lengths with my reciprocating saw. A hacksaw or a cutoff blade in an angle grinder would work too. And I painted these bars with some rust paint.

Spraying pins with rust paint

Spraying pins with rust paint

I level a spot in my garden and set the panels in place. I check the corners for square then drive in the bars thru the corners and into the ground to secure them. I repeat this on each corner to complete the garden box.

Tapping in pins to assemble garden box

Tapping in pins to assemble garden box

For deeper boxes you can stack and alternate the panels if you like. You will need longer pins for this.

CSA Thin Concrete Garden Box - Oh she's a beauty huh??

CSA Thin Concrete Garden Box – Oh she’s a beauty huh??

So a couple of things as I wrap up this episode in the series. I didn’t know that this was going to be a series, but this is how it goes. The two previous form designs work well so, keep making panels with those if you have them. If you want the panels lighter then there’s quite a few alternative concrete formulations available that work in those first two forms. I only covered three in the last episode, and I’m sure there’s many more. So? Maybe part 3.5 in the future? We’ll see.

Talking, talking, talking

Talking, talking, talking

With these new panels I not sure if any wire reinforcing grid is required at all. This CSA concrete might be strong enough without it. So it might be just habit that I add a grid. But, if a panel was ever to break the wire would at least hold it together so it still functions.

Pushing wire grid into center - 36" form

Pushing wire grid into center – 36″ form

Please leave a comment with your thoughts or suggestions. I am amazed by the wealth of knowledge out there and I so appreciate everyone who has shared it.

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.

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