Materials + Supplies:
- Plastic Liners
- Base Board
- Spray Adhesive
- Rubber Gloves
- Something to Cut Foam
- 100% Silicone Caulk
- CHENG Corner and Seam Shaping Tools
- Sakrete 5000+ Concrete Mix (or similar)
- CHENG Evergreen SmartColor Pigment
- Decorative Aggregate
Step 1: Preparation
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.
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.
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
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.
- Bend the form liner until it fits inside the form.
- Apply double sided carpet tape.
- Stick liner in place.
Step 6: Seal Form
- 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
- 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.
Step 8: Add Decorative Aggregate
- Lightly mist spray adhesive in the bottom of the form.
- Sprinkle decorative aggregate in.
- Clean adhesive residue from the walls with denatured alcohol.
Step 9: Mix Concrete
- 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
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
- 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
- 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
Step 13: Slurry Fill
- 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
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
- 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
Happy Holidays from everyone at CHENG Concrete!