(2) Cardboard Tubes (best with foil or plastic linings for water resistance)
(4) 1" Screws
Clear Packing Tape
Thin Foam Wrap (a packing / shipping material)
Sacked Concrete / Rapid Set (if you're in a hurry, although it tends to wash out colors)
CHENG Pro-Formula Mix
Tin Snips / Wire cutters
Long Needle Nose Pliers
Phillips Screw Driver ( + )
Weights / Rocks / Sand
Diamond Sanding Pad / Sandpaper
Step 1: Prepare Tubes
The largest tube will become the outside form. In this case it's about 5" diameter and 9" tall (it contained coffee grounds). The inside tube is about 4" x 7" (shredded potato chip container). This means the wall thickness is only about 1/2" - do not go any thinner, this is already pushing the limits.
1. Starting with the Interior form, trace the diameter on a piece of thin foam padding (this is usually used for packing / shipping). The idea is to tape a circle of foam to the base so when we go to de-mold that metal ring isn't trapped down inside.
2. After the foam circle is taped to the base, cut a ~2" foam strip to wrap and tape around the bottom edge (to keep that ring from getting trapped).
3. Screw the (4) 1" screws into the top of the Interior form. The screws should rest on the lip of the Outer cylinder, helping to keep it lined up during the pour.
4. Clean up the forms, Inside and Out and remove any oils or residue with denatured alcohol. If you don't clean the containers out, you'll see it on the surface of the finished concrete for sure.
Step 2: Mix, Pour, Vibrate Concrete
5. In a clean bucket, mix up some concrete. If you're using CHENG Pro-Formula, follow the directions in this link. Pro-Formula comes in a range of colors, is easy to use, and gives good results. The mix in these photos looks a bit too wet. A mix that's too wet won't be as strong, but for a small project like this it won't make much difference.
6. Fill the Outer cylinder with concrete first, smearing concrete around the inside surface as you go along. This will help the concrete fill the edges and should minimize air bubbles.
7. With the Outer cylinder about half full, wipe the exterior of the Inside cylinder with concrete and gradually push it down into the form. The concrete will be displaced and spill up out of the edges.
8. Take some weights (lead, solid metal, stones, sand, etc.) and put them into the Interior form. You want to offset the buoyant force of the fluid concrete. Too much weight and the Inside form will sink (and bend the locating screws). Not enough weight and the Inside cylinder will want to float up.
9. Vibrate the form by lifting it and dropping it repeatedly on the work surface (this is called drop compaction). Do this for a few minutes to help coax the air bubbles into rising to the surface.
10. With your vibrating finished, make sure the Inner cylinder is centered within the Outer one. Then move it gently to a quit place where it will be undisturbed.
11. Cover the piece with plastic and let it cure for 1-2 days.
Step 3: De-mold
13. Hopefully the metal ring from the Inner cylinder isn't trapped. Use some long needle nose pliers if it is, although sometimes things just get stuck.
There's not much to this, just be careful not to cut yourself on the metal rings, and remember not to pry against the concrete because you'll just scratch it.
Step 4: Grind, Polish, Seal
15. The top surface will be a bit rough, and if you don't have a 5" orbital polisher, you might try sanding it with 60 grit sandpaper on a rubber pad or fixed to a block of wood. It will take forever but you can get it looking decent if you're patient. You can also grind and polish concrete using the same equipment you would use to grind and polish glass. At this step you'll get the best results if you have the right tool for the job (5" orbital polisher with pads for concrete).
16. (Optional) Seal the concrete with something like the CHENG Acrylic Sealer. Sealing concrete is a whole other thing, but there's some step by step information at that link. Sealing isn't necessary but it will protect the concrete and keep it looking good over the long term.
17. (Optional) If the cylinder is going to be a planter, drill a 1/2" hole in the bottom with a masonry bit.
Step 5: Variations
This piece hasn't been ground, and the top surface is how it came out of the form. If the outside tube has a metal lip, the concrete will fill to that edge and the top surface will be nice and flat.
Step 6: More Advanced: Use a Wine Cork to Cast a Drain Hole
1. Determine how thick the bottom of the piece will be, and then cut the cork to that thickness. In this case it's a little bit over 1/2".
2. Mark the cut line around the cork and use a utility knife or a fine-toothed hand saw to cut it.
3. Mark the center of the tube, spray both faces of the cork, and press it in place making sure it stays centered. Let the adhesive dry before starting the pour.
4. The interior tube should now rest on top of the cork (it needs to make complete contact with the bottom of the tube to create the hole).
5. Mix, Pour, and Vibrate the concrete. To eliminate the most air bubbles, use a small vibrating table or a table-mounted vibrator.
6. Remove lead weights and de-mold the finished concrete using tin snips and pliers. Be careful not to cut yourself on any metal. Wiping off the concrete while it's wet would have made this de-molding process a lot less messy.
7. Completely remove the form tubes and then pry out the cork knockout with a small slotted screwdriver. Try not to use the edge of the hole for leverage, it will damage the concrete. Instead, drive the screw driver into the cork and force it up without prying on the concrete.
8. In this piece, the knockout didn't make complete contact with the bottom of the tube, and 1/8" of concrete was filling the hole. This was simply tapped away with a rubber mallet and a slotted screwdriver.
9. The edges can be sanded with diamond hand pads and then the piece is ready to be finished or potted with a plant as is.
Step 7: Final Photos
Although they might not make very good containers for food, with a cork lid or a wooden top you've got a nice little vessel for holding whatever.