Introduction: Concrete Christmas Tree

This concrete christmas tree is made using the same foam forming approach as the Concrete STOP.

Materials + Supplies:
  • Foam
  • Plastic Liners
  • Base Board
  • Spray Adhesive
  • Rubber Gloves
  • Something to Cut Foam
  • Sandpaper
  • 100% Silicone Caulk
  • CHENG Corner and Seam Shaping Tools
Concrete Mix:
  • Sakrete 5000+ Concrete Mix (or similar)
  • CHENG Evergreen SmartColor Pigment
  • Decorative Aggregate

Step 1: Preparation

First thing to do is get some foam and figure out what shape to cut.

Any type of EPS (expanded polystyrene) foam will work, from styrofoam that you find in the trash, to insulating foam that you buy at a building supply store. Foam that is more dense will sand to a smoother finish. For this project, two sheets of 1" thick pink insulating foam were glued together with an adhesive made for foam. This adhesive won't melt the foam and gives a strong bond.

  • Spray both sides liberally with foam-lok adhesive.
  • Wait 4+ minutes and then press together.
  • Check for gaps around the edge and press them together.
Using a 3D modeling program like sketchup is a quick way to make a model and see how it looks. From the model you can print out a 1:1 template to trace onto the foam.

The main concern in this project is that the walls aren't too thin and that the piece will have room for a tea light candle at the base.

Step 2: Trace Shape Onto Foam

  • Print and cut out a 1:1 template.
  • Trace the shape onto the foam.

Step 3: Cut Foam

  • Cut the foam out with a hot wire, CNC router, jig saw, utility knife, scroll saw, band saw, laser cutter, whatever you have available.
  • Leave some room when cutting the foam so you can sand it to the exact size.
In this project, the foam will be lined with plastic, so the quality of the cut isn't that important.

Step 4: Shape and Wrap Inner Knockout

  • Glue the template onto the foam.
  • Sand the sides down until it's the right size.
  • Wrap the knockout with tape.

To wrap the knockout, I used our Glossy Form Tape, which comes in 2" or 2.5" thicknesses and is made to wrap sink knockouts for concrete countertops. Wrapping the knockout isn't necessary in this case but it will hide any imperfections in the foam and give the concrete a glossy finish.

Step 5: Build Outer Form

The original plan was to cut the outer form out of the pink foam. That idea was scrapped because the foam wasn't cut very well and sanding would have taken too long. Instead I used a foam rail and 2" wall liner from our foam countertop making system.

Foam Outer Form:
  • Trace shape onto waterproof base board (3/4" melamine).
  • Cut a section of foam with a utility knife.
  • Stick to baseboard with double sided carpet tape or spray adhesive.

Form Liner:
  • Bend the form liner until it fits inside the form.
  • Apply double sided carpet tape.
  • Stick liner in place.
Using a thick plastic form liner like this will give the concrete a uniform texture on the outside, and it will help maintain straight sides on the finished piece. The bends in the liner are also going to give the edges a nice radius, so there won't be any sharp corners.

Step 6: Seal Form

This step is kind of optional, but I think it's really important for a quick and dirty form like this. It will help keep the form walls secure during the pour, and also keep the concrete from bleeding out.
  • Run a bead of caulk around the inside edge of the form, and up the vertical edge to cover the gap where the two ends of the form liner meet.
  • Tool the silicone bead to create a round-over with something like the Corner and Seam Shaping Tools.
  • Scrape away the excess with a razor blade, or wait for the silicone to cure and pull up the excess.
  • Clean the walls and base board with denatured alcohol.

Step 7: Glue Inner Knockout in Place

With the outer form complete, the next thing to do is glue the inner knockout to the base board.
  • Place the knockout in the form and trace around it with a pencil so you have some idea of there it needs to go.
  • Spray the back side of the knockout with adhesive.
  • Clean any adhesive off the sides of the knockout with denatured alcohol (it will leave a texture in the finished concrete if you don't remove it).
  • Wait for the adhesive to get tacky, and then press it down to the base board.
If the glue doesn't seem like it's holding, you can drive a screw down through the knockout and into the base board to keep it from moving.

Step 8: Add Decorative Aggregate

Decorative aggregate can be crushed glass, precious stones, mirrored glass, or anything that is hard enough to be polished. Instead of adding decorative aggregate throughout the mix, it is placed only where you need it, so nothing is wasted. When you use decorative aggregate, you commit to polishing the surface.
  • Lightly mist spray adhesive in the bottom of the form.
  • Sprinkle decorative aggregate in.
  • Clean adhesive residue from the walls with denatured alcohol.
This piece has a combination of crushed mirrored glass (mirrored on all sides), amazonite, glacier blue agate, moonstone, and carnelian.

Step 9: Mix Concrete

A quick volume calculation told me the project would weigh between 2-3 lbs. So I mixed up 5 lbs. of 5000 psi. concrete and added Evergreen pigment.
  • Mix dry material together first (concrete mix, admixtures, pigment, etc.)
  • Blend to a uniform consistency.
  • Add 1/2 of recommended water and mix.
  • Add more water until it reaches the right consistency (not dry, has body, not runny)

Step 10: Fill the Form

The main thing when filling a narrow form like this is to be careful around the liners. If the liner separates from the foam walls, concrete can get down there and it will mess up the finished piece.

Vibrate the form as much as possible to help air bubbles escape. Press the concrete down into the form walls with your fingers. Use a pencil or a stick and use it to help break up big air pockets and consolidate the concrete.

Use a small putty knife or your hands to smooth the top flat.

Step 11: Trowel in Decorative Aggregate

Troweling in decorative aggregate on the top surface means both sides of the piece have to be polished to expose the aggregate.
  • Press the aggregate down into the concrete with your fingers or use a putty knife to smooth it in just below the surface.
  • When the piece is finished, cover it with plastic and let it cure for 3-5 days.

Step 12: Demold

After 4-5 days the piece can be removed from the form. If you don't wait long enough, there's a good chance that you'll crack it when demolding, especially with a piece this thin.
  • Gently pry away the foam walls.
  • Move the piece sideways against the base to break the adhesive seal.
  • Avoid prying on the concrete.
  • Remove the plastic form liner
This piece wasn't vibrated enough and there were huge voids where the concrete didn't fill in around the decorative aggregate at the bottom. This can be fixed by filling the voids with slurry.

Step 13: Slurry Fill

You can create your own slurry by mixing cement, pigment (if needed), and water. Acrylic slurry is will shrink less than non-acrylic slurry, and it's also stickier and better for filling vertical surfaces.
  • Mix the Acrylic Slurry with water to a toothpaste-like consistency.
  • Wetting the surface of the concrete will help the slurry fill deeper holes.
  • Over-fill any holes or voids with slurry so they can be sanded down flat later.
  • Let the slurry cure overnight.

Step 14: Polishing

Use a variable speed concrete polisher to expose the decorative aggregate on both sides.

The foam knockout is left in the center to give some protection against accidentally breaking this thin piece.
  • Polishing with 100 grit pads will quickly expose the aggregate.
  • Keep the surface flat by polishing each section equally.
  • Progress through polishing pads until you reach 800 or 1500.
  • Use diamond hand pads to clean up any sharp edges.

Step 15: Remove Inner Foam

The best way to remove the foam is by drilling or cutting out the center so it can collapse and slide out.
  • Drill or cut the foam in the center until it can collapse.
  • Pull out the knockout.
  • Clean up any sharp edges with diamond hand pads.

Step 16: Finished Piece

When the form is removed, the finished piece can be sealed, which will help bring out some of the color and keep it looking nice for a long time.

Happy Holidays from everyone at CHENG Concrete!