Materials + Supplies:
- 3/4" Melamine-coated particle board or other water-proof baseboard (plastic sheet, plywood sealed with packing tape, etc.)
- Insulating Foam / Styrofoam 2" thick
- Spray Adhesive (3M 77)
- CHENG Outdoor Pro-Formula Mix
- Sakrete 5000+ Concrete Mix (or similar)
- Jig Saw
- Sandpaper (120, 220 grit)
- Bucket / Wheelbarrow
- Shovel / Trowel
- Particle Mask
- Rubber Gloves
- Diamond Hand Sanding Pads (optional)
Step 1: Print out the Template
- Print out the template.
- Cut out the template with scissors.
- Trace around the template on the foam with a permanent marker.
The font you use should be bold and thick, like Impact in this example. The concrete won't be very strong if it's less than 1" thick, and even then it will be easy to break, so take that into consideration when determining the size of the letters.
Step 2: Cut out the Letters
- Gently clamp the foam over the edge of a work table.
- Drill a hole so the blade of the jig saw has a place to start.
- Cut out the negative space first, like the middle part of the letter O and the P. Cutting them out first will be easier and safer than trying to do it later.
- Then cut out the rest of the letters, following the traced lines. Leave some room so you can sand it to the exact size.
- After the shape is cut out, sand the inside of the foam by hand with 120 or 220 grit sandpaper. The smoother you can sand it, the smoother the sides of the finished concrete piece will be.
DOW makes a rigid polystyrene insulating foam that works well for projects like this and is available in 2" thickness from most hardware stores or places that sell building supplies. Owens Corning makes a pink insulating foam has the same qualities and will work as well, but you don't usually see it over 1" thick. White styrofoam will also work, but it's not as dense and doesn't sand as well as blue foam. Experiment with what you can find in the dumpster. Anything more rigid is unnecessary, and anything with an open core won't really work.
Step 3: Glue to the Base Board
- Place the foam on the base board and trace around the inside and outside with a pencil.
- Put the negative spaces in place, like the center of the O and P. Trace around them.
- Spray one side of the foam with spray adhesive, hold the can at least 12" away from the foam because the aerosol can melt the foam. You can also use silicone caulk, which will provide a better seal against bleed out, but takes a few hours to cure.
- Use the pencil lines as a guide and press the foam down firmly to the base board. To get the best seal, follow the instructions on the spray adhesive and spray a little bit of foam on the base board too.
- The spray adhesive and silicone will leave some texture on the concrete unless you clean it off the base board with denatured alcohol.
Pour the form soon after it has been glued down with spray adhesive. The spray adhesive won't stick as well the next day and this can cause problems if the concrete bleeds through and forces the foam up. If you're using silicone, wait for it to fully cure before pouring (3+ hours). The silicone will bond just as well over time, so you could pour the silicone-sealed form a week later and still get good results.
Step 4: Mix + Pour Concrete
For more tips on mixing concrete, check out the How-To-Mix Concrete Instructable.
- Add the dry materials to the bucket (Concrete Mix and any Pigment or Admixtures like CHENG Pro-Formula).
- Blend the dry materials together until the color is consistent. Break apart any clumps of concrete or throw them out.
- Add 3/4 of the recommended water to the mix.
- Scrape around the walls and the bottom of the bucket with a trowel.
- Gradually add the remaining water.
- Take a handful of concrete and drop it into the form, being careful about damaging the foam.
- Work the concrete into the corners and fill it to the top.
- Vibrate the form using drop compaction (pick up one side of the base board a few inches and repeatedly drop it to help the air bubbles rise to the surface).
- Top off the form with concrete and smooth the top flat with a trowel, always being careful about the foam.
- Cover the concrete with plastic to keep the humidity inside while the piece cures. It should cure somewhere in the shade, never in freezing weather or direct sunlight.
Step 5: Remove the Foam
- Slide the piece sideways to break the seal to the base board.
- Break the foam away with your hands and a plastic putty knife.
- Try not to gouge the concrete or bend the piece because it can break at the thinner points.
- Scrub away the foam left on the surface with a bristle brush or an old tooth brush. You could dissolve any residue with denatured alcohol or acetone but it's not necessary.
- Sand any rough edges with diamond hand pads.