Make your own Lightweight Concrete Garden Boxes! This is PART 3, showing options to make your garden boxes from lightweight concrete. You can build these forms and precast your own 48″, 36″, or 24″ reinforced lightweight concrete panels that link 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.
Here is the weight of 3 cured panels made from regular concrete: . 48″ – 69lbs, 36″ – 50lbs, 24″ – 33lbs.
Background for the Cast Concrete Garden Box Project – PART 3
This is part three of my series on casting your own reinforced concrete garden box panels. These panels link together to make long lasting rot proof garden boxes.
I had many viewer requests to make the panels lighter so I did some research into this. It turns out there are MANY ways to make lightweight concrete so, to simplify things, I decided to focus on just three formulations.
I wanted to use readily available materials that you could pick up at any big box, building, or garden supply store. Or that you could easily order online.
For the first mix I substituted the gravel aggregate for lava rock, in the second mix I used fine vermiculite. And in the third I’ll replace the sand AND gravel aggregate for foam and try an aircrete version of the garden panel.
I’ll show the mixing, pouring, and unmolding, then look at the weight and durability results.
I let all the test panels fully cure for a month, keeping them damp and covered. Then I let them dry for a week or so before weighing.
So here’s the first formulation.
Lava Rock Lightweight Concrete
So this will be my first attempt at making some lightweight concrete. This blend uses portland cement, lava rock, sand and some glass fiber for reinforcement.
You add about one pound of this fiber per cubic yard of concrete.
So when I calculated how much I needed per batch it came down to a third of an ounce per 48″ panel. So I just used a pinch. So here’s the proportions I used for the first attempt at Lava concrete:
one part Portland Cement
three parts Lava Rock
two parts Sand
a pinch of Glass Fiber
These proportions are by volume.
I put the sand, lava rock, and fiber in the wheelbarrow first, and blended that before I added the Portland cement. Once that was well mixed, I slowly added water. I realized as I was doing this that it wasn’t looking as smooth as I wanted and it looked quite lumpy. I thought well, I’ll go ahead with it and put it in the form.
I used a reciprocating saw, without the blade, to vibrate the form and settle the concrete mix. Then I laid in a piece of reinforcing wire mesh.
This galvanized wire mesh is cut from a large hog panel fence I bought from my local farm supply store. Then I topped up the rest of this and vibrated it some more. I tried to smooth it out with a trowel and it wasn’t coming out very well. But in the bottom of the wheelbarrow I had lots of sand mixture left over, so I just did the best I could with that.
For my second attempt I changed the ratio a bit. I went with one part portland cement, two parts lava rock, and three parts sand and again a pinch of glass fiber.
I figured that I needed to add more sand to get a smoother mix. So I started the same. Mixed it all up in the wheelbarrow and it came out a little bit wetter than I liked so I added a bit more sand to absorb some of that water.
I could tell right away this consistency was closer to regular concrete. So same thing, fill the mold halfway, lay in the reinforcing wire grid, and then vibrate and smooth it out.
I used a little bit of the extra mortar type mix in the bottom of the wheelbarrow to improve the surface finish on the first form. And this seemed to work pretty well.
I covered the forms with plastic and left them to set up. The next day I removed the form sides, and ends, and these castings came out fairly easily.
I was pretty happy with how the panels looked but, I could tell right away that they were a little bit heavier than I expected.
I went back to our landscape supplier and got some lava rock that closely approximated gravel.
This red lava rock has a lot of smaller pieces, instead of the same three quarter inch size. I compared the weight of gravel to lava rock for an equal volume. The lava rock weighed approximately half that of the gravel.
For my third attempt with the more variable size lava rock the mix I used is: one part Portland, three parts lava rock, two parts sand and again a pinch of glass fiber.
I measured all the ingredients and mixed up a slightly larger batch. Again starting with lava rock, the sand with fiber, and then adding the Portland cement and then the water. And into the forms as before. It settled nicely with vibration and all looked good.
The next day, I remove the plastic and disassembled the forms. These panels came out of the forms quite nicely. And this was my final version of the lava rock mixture.
I substituted the gravel for lava rock in this first lightweight concrete attempt. The lavacrete, if you will, worked very well once I used a variety of rock sizes that approximated gravel in my mix. The finish is very good and the panel feels solid and durable.
This panel was approximately 10% lighter than regular concrete. Not a lot lighter but I’d say its almost equivalent in durability and strength. I had hoped that this panel would be much lighter given that the lava rock was about half the weight of the gravel.
I suspect that the voids in the lava rock were filled with sand, cement, and water during the mixing so I lost most of those air spaces that would have made it lighter.
Now let’s look at the vermiculite mix.
Vermiculite Lightweight Concrete
So for this blend, I picked up a bag of number three vermiculite. It’s a mineral that’s expanded by heating in a furnace. It looks a bit like mica.
The ingredients are: portland cement, sand, vermiculite and glass fiber. I wetted the vermiculite slightly first before adding it to the sand in the wheelbarrow to make it easier to blend.
For this first test it’s one part portland cement, two parts vermiculite, and one part sand, and as always, a pinch of glass fiber. This mixed easily in the wheelbarrow.
Once it’s well blended, I add the Portland cement. It felt quite dry and it took a bit more water than I expected.
And like before I filled the forms half away, settled it a bit with the reciprocating saw, add the mesh, and then top it up and repeated on the other form. And smooth the bubbles that formed with a trowel, and repeated that for the other form.
And these are the vermiculite 36 inch panels coming out of the forms. Removing the sides and ends and then just gently prying the panel free with a paint scraper.
This blend mixed very well and was noticeably lighter, even in the wheelbarrow. And this made it easy to fill the forms.
It settled well with vibration and had a nice texture while troweling and edging. It came out of the forms easily and has a nice smooth finish. It felt noticeably lighter than regular concrete and I think it would be quite durable.
It weighed, on average, 38% lighter than regular concrete!
Now let’s look at the aircrete.
The aircrete uses fewer ingredients: Portland cement, shampoo to create a foam, and some glass fiber for extra strength.
So I begin by diluting the shampoo in water. 15 fluid ounces of shampoo to 2.5 gallons of water.
This will be the dilution that I’ll use to create the foam. I stir this with a paint mixer attachment for my drill on a low setting — just to dissolve the shampoo in the water.
I use Suave Daily Clarifying Shampoo as my foaming agent.
I’m weighing the cement and to that I’ll add a little handful of the fiber. I then weigh the water.
And then I also weigh the shampoo dilution and it’s 3 pounds to make my foam. It’s easier to make twice the foam needed. I find it just mixes better.
I experimented with a some different shape mixers to make my foam.
With wire, I sewed some window screen onto this paddle thin-set or grout mixer. When I tried to make foam, it spun the entire mixture a little too much. So this one was a little too aggressive.
The next time I sewed the screen to a smaller, egg beater type mixer. I tried adding a little bit of stainless steel scrubbing pad to the center of it. I don’t think that really made a difference either way.
So this one worked better because it didn’t spin the entire mixture and didn’t cause it to overflow from the pail so I got a lot better result this way. I’m using a 5 gallon pail with 3 lbs of shampoo dilution. So when the pail is full I get a foam density of 3 lbs per 5 gallon volume.
Now with a paint mixer, I’ll add my Portland cement and fiber to the water, a bit at a time. I want to create a smooth portland cement slurry. So I add a bit, spin it, add a bit, spun it until its smooth.
I also stirred the mix by hand to make sure that there was no dry cement stuck to the edge, or clumps in the bottom of the pail. And this worked pretty well. Now the slurry is ready for the foam.
I add half the pail of foam to the cement slurry.
I’ll use my mixer to blend it all evenly. I hand stirred with an improvised stir stick to make sure that everything was blending right to the bottom of the pail, and I got a nice even aircrete mixture. And this looked really good.
I was really happy with how this turned out.
Then I added the aircrete to the form — giving it a bit of a jiggle to help settle it into the corners.
And then I added a bit more to top it up.
Try to get as close as I could to being full, without over filling too much. A little bit over is okay cause the mix is going to settle and some of the bubbles are going to collapse.
Then I can smooth it with a trowel. And then lay in the pipe wrap wires, and once it sets up a bit I gently lay in a galvanized wire reinforcing grid, so it settles in right in the center. And I made a couple more batches and filled a few more forms. I was happy with how this was blending and how the grid was sitting in well.
The aircrew had enough density that I felt comfortable that the wire mesh was not going to sink all the way to the bottom of the form. I need to push it into place so I was happy with the aircrete density.
It was a hot day so I think that also helped the aircrete to set up faster — and I think that’s quite desirable because then there’s not as much time for the foam bubbles to pop. When the aircrete set up I covered the forms with plastic and left them for a couple of days.
I remove all the screws from the form, gently pry off the sides, remove it from the base — and then remove the ends. And I was really happy with how they looked. It felt quite solid.
Now this panel was the most fun to make, and certainly the most unique of the options I tried.
I was able to get a very dense foam without a foam generator. For my testing this worked well but, if I was going to get into making a lot of aircrete, I’d build or buy a foam generator like the ones that Harry or Darwin use.
I followed Aircrete Harry’s formulation for what he referred to as a “standard” mix. My aircrete held its shape and there was little if any bubble collapse as it was setting up.
The aircrete panels came out of the forms well. The surface finish looked good and the bubbles looked uniform. The panels were very light, on average 80% lighter than regular concrete. Wow! That’s pretty impressive!
But, even after curing though, I found the panels were not very durable. There’s no sand or gravel to add that impact or abrasion resistance. I did a little test of the durability of all the panels by running a weed eater or line trimmer against them.
Not a scientific test, by any means, but something the panels may have to endure.
There was noticeable damage to the aircrete panel but the others fared okay.
Another bigger issue is what was happening to the aircrete panels as they cured. They were bending and cracking.
I suspect that with a lower strength in general, the internal stresses caused by shrinking and drying led to this bowing along the length, and this lead to these cracks forming. This is what I think is happening anyway.
Perhaps a mix with a greater ratio of cement to foam would prevent this. It would bring the weight up a bit. And there’s still the durability issue. If you have made these panels with aircrete, or you know how to prevent this warping and cracking then leave me a comment below.
So, I think for me, of these three, the best formulation for ease of mixing, weight reduction, AND durability is the vermiculite blend. If weight was an issue for my concrete panels I’d opt for the vermiculite mix.
I’m not saying that the lava rock mix or aircrete are not worth trying, because you might get better results than I did.
There’s also lightweight concrete ready-mix bags made by Sakrete but they’re not available in my area so I was unable to test it. So that might be a good option too.
One last thing – as I was doing these tests I came up with another way of making the panels that’s faster and lighter without sacrificing ANY strength OR durability. And that is coming right up in part 4.
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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- Dewalt Compact Job-Site Table Saw (DW745)
- Dewalt 12″ Sliding Compound Miter Saw (DWS779)
- 10″ Bench Drill Press with Laser
- Dewalt 20V MAX Compact Drill/Driver set
- Dewalt Miter-Saw Workstation Tool Mounting Brackets (DW7231)
- Dewalt 20V Battery Charger
- Excellent Hammer
- Screwdriver Set
- Sanding Blocks
- Wood Filler
- Adjustable Combination Square
- Framing Square
- Johnson Speed Square
- Small Bolt Cutters
- Irwin Tool Quick-Grip Clamp Set
- Aluminum Ruler
- Drill Guide
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