In this brief tutorial, we cover the process of casting concrete from start to finish. By the end, you'll know how to make your own planters, candles, and other creations. We also get nerdy by calculating our time and material costs, to prove how this part-time hobby can also be a lucrative business.
Hi guys, my name is Jen, and this is my first time ever casting concrete. I don’t usually think of myself as a maker or a creative, but I do love all things DIY. I was excited to learn how to cast concrete, but nervous. That whole “I’ve never done it, so it’s impossible,” mentality crept in.
So why did I get out of my office chair? My boss asked me to. Now imagine my shock when I enjoyed the entire process. It was actually very fun and easy to learn. Ready? Here we go.
Step 1: Clean the Mold
Our first step is checking the mold for cleanliness. It’s always a good idea to wipe down the mold and clean out any debris, especially if you are re-casting a mold used previously.
Step 2: Assemble the Shell
The mold we are using for this demonstration is our XXL Flat Icosahedron Silicone Mold. Like many of our larger molds, it requires a 3D printed shell to hold the sides perfectly rigid.
Assembling the shell, as it turns out is pretty fool-proof – just line up the edges of the shell with the visible seam on the silicone mold. Next you just need to clamp the shells together. Don't be afraid of the clamps breaking as you slide them into place. They're quite durable and designed fit snug around the shell.
Step 3: Mix Concrete
We use a 1:5 ratio typically when mixing water and concrete. In this case we used 400g of water, and 2000g of concrete. We also add a bit of water reducer into the mix, which makes the concrete easier to work with.
Add your 400g of water to the bucket first. If you’re making colored concrete, add any pigments directly to the water now and swish until mixed. Add the 2,000g of concrete and drill mix on medium-high speed working all around the bucket until completely smooth and free of lumps.
Then sprinkle in about a tablespoon of Flow Control, mix again for approx. 30 seconds or until fully worked in and fluid. Overmixing, undermixing, letting it sit too long, adding too much water – All these things that could go wrong worried me, but this all turned out to be super easy.
Pouring the concrete was pretty simple – Pour a small amount into the mold until concrete had reached all the corners in the bottom, and then pick up the mold and tap it against the table a few times, up and down rapidly just to make sure it settled in and minimize air pockets or bubbles.
Then pour the rest of the concrete into the mold, gently tapping against the sides to level the top and see how much concrete will fill in. Ideally you want the concrete to sit flat just at the top of the mold, but if you have a little extra it’s easy to scoop out until it’s just right. Shake and vibrate until the bottom is flat and smooth.
Pro Tip: Keep smaller molds around, so when you have excess mix you can always put it to use by filling up a small mold! BOOM, eliminating waste.
Step 5: Slap That
We always shake and “slap” the molds once the concrete has been fully poured – again, helping the air escape and making sure the top layer of concrete is as even as possible. This step is great for getting out some extra aggression. Hello, concrete therapy!
Step 6: Cure
I was surprised at how fast the curing process is. With this concrete mix, 30 minutes is all it takes. The concrete will get thirsty during this curing process, as it forms all those strong bonds! We water cure large concrete pieces like this to eliminate any issues like cracking or shrinkage. Keep a spray bottle filled with room temp water handy.
Once the top surface loses it’s shine, you can tell it is starting to get thirsty. Spray down the top surface to keep it wet for 1 hour after pouring. If cracking or shrinking issues persist, try water curing for longer than 1 hour. You can also try soaking a hand towel in cold water and draping it over the top as it cures so you don’t have to spray!
Pro Tip: Water-curing your concrete will also hugely improve the strength and physical properties of the concrete.
Step 7: Demold
Once the concrete has cured, you get to de-mold your new planter, which is absolutely my favorite part of the process. Pulling back the edges of the mold alongside the concrete is oddly satisfying.
From there just invert the mold, get a good handle on it until you can pull out the middle – definitely the hardest part strength-wise, but the most rewarding once it comes loose!
Peel away from the corner, fold inwards, and pull. Great, it’s out!
Step 8: Clean Up
Before admiring the fruits of your labor, remember to wipe down the used mold and tidy up the workspace. To get a perfectly finished concrete piece you'll want to sand the bottom to remove any uneven or rough surfaces, and seal the concrete to protect it from outside contaminants and preserve the finish – but we’ll get to that in another blog post!
Optional Step: Make a Profit
Here’s a quick cost breakdown for what it took to make one of these planters. Your costs may be different, but this will give you a general idea. This was calculated using Cement-All quick setting cement mix (made by RapidSet) and Carbon Black pigment by Buddy Rhodes.
2kg Concrete: $1.73
15g Black Oxide Pigment: $0.30
1/5 Mixing Bucket: $0.30
1/500 Mold: $0.50
1/200 Sanding Pad: $0.07
Total Costs Per Planter: $3.25
It only took about 9 minutes to set up & pour the mold - demolding the concrete vessel and cleaning it up took less than 7 minutes. For material costs, we estimate a mere $3.25 per planter. Sell each piece for a fair price, and that's a pretty penny in your pocket!