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. #aircrete

Here is the weight of 3 cured panels made from regular concrete: . 48″ – 69lbs, 36″ – 50lbs, 24″ – 33lbs.

See the Youtube Video and download the Plans.

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.

Background for the Cast Concrete Garden Box Project – PART 3 #aircrete

In Part 1 I made the forms from plywood. In Part 2 I simplified the design to make the forms easier to build. And for this video, I’ll cast the panels with lightweight concrete.

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.

Main Formulations

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.

If you haven’t seen Part 1 and Part 2 of this series then you might get more from this video if you watch them first.

I’ll be using the forms I built in Part 1 and Part 2.

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

Lava Rock – 1st test

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.

Lava Rock – 2nd test

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.

Lava Rock – 3rd test

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.