There are four steps in this Instructable - Preparing the Lid, Mixing and Pouring Concrete, Grinding and De-molding, Sealing and Finishing.
- Plastic Tupperware Lids
- CHENG Countertop Pro-Formula Mix
- Sakrete 5000+ Concrete or equivalent
- Decorative Aggregate - optional (semi-precious minerals, crushed glass, etc.)
- Particle Mask
- Rubber Gloves
Step 1: Preparing the Lid
- Coaster Size = 4+ inch
- Trivet Size = 6+ inch
- Flexible Plastic
- Plastic that is durable enough to be re-used
- At least 1/4 inch thick
- Not threaded inside (so concrete can be released from the lid without having to break it off)
- No logos
- Free / Cheap
- Paper / Cardboard
- Thinner than 1/4 inch
- Undercuts / Threads / Deep Logos
- Thin plastic is OK for one casting, but can't be reused easily.
- Clean the lids with soapy water using something that won't scratch, like a 100% cotton cloth.
- Use a light mist of spray adhesive in the lid if you want to include decorative aggregate, crushed glass, or any kind of inlays.
- Use crushed glass and decorative aggregate that complements the color of the concrete.
- Rubber stamps can be fixed to the form with spray adhesive and removed after the concrete has cured, leaving the rubber stamp pattern as a relief in the surface.
Step 2: Mixing + Pouring Concrete
- Figure out how much concrete you're going to mix. Each of these coasters weighs under a 1/2 pound.
- You can mix 10lbs of concrete by hand in a 5-gallon bucket with a trowel. For larger amounts, use a wheel barrow and a shovel.
- After the dry materials (concrete mix, admixtures, pigments etc.) are combined, begin adding water gradually.
- The best way to figure out how much water to add is by experience. If the mix is too wet, it will flow and pour easily, but there will be excess bleed water on the surface as the concrete starts to cure. If the mix is too dry, it will be gritty with sand.
- Excess water will weaken the concrete. If the mix gets too wet, add more concrete.
- Break apart or throw away any clumps of unmixed concrete before filling the form.
- Make sure the mold is clean and free of debris.
- Press a small handful of concrete into the mold and pat it down to work out any air bubbles
- Tapping the mold on the tabletop will further drive out any air
- Remove the rocks that protrude from the top surface. If you don't have an orbital polisher, grinding them down will be very time consuming (nearly impossible).
- It is important that the top surface is flat and smooth to the edge of the lid. Smooth the top of the concrete by screeding it with a straight-edge or a trowel.
- Place the concrete on a level surface and cover it with plastic to cure for 4 days. The plastic will help keep the humidity inside, if the concrete dries out too quickly it can cause small cracks.
- The curing process can be sped up by using a quick setting cement mix like Rapid Set, but this can also affect the color of the concrete.
- Curing should take place between 50 and 90 degrees Fahrenheit, never in direct sunlight, and concrete should never freeze while it's curing.
Step 3: Grinding + De-Molding
- While the concrete is still in the plastic lid, grind the top flat with an orbital polisher. Keeping it in the lid will give you an even edge to grind down to.
- Un-ground concrete will have a uniform color without any exposed aggregate.
- Concrete is extremely difficult to grind without the right diamond polishing tools. Even with a coarse diamond hand pad, it will take a very long time to flatten a 4" disc. This is why it's so important to smooth the back surface flat while the concrete is still work-able.
- Use a lapping wheel with diamond discs.
- Try a coarse sharpening stone.
- Try a coarse wet/dry sandpaper (45 or 60 grit).
- You will break the coasters in half during de-molding if you haven't waited long enough for them to cure (3-5 days).
- Pry the plastic lid off by pushing down with a flat screwdriver, working your way around the lid.
- Take your time and be careful not to scratch or chip the concrete during de-molding.
- If the concrete is really stuck, try blowing down between the edge of the plastic lid and the concrete with compressed air.
- If the concrete is still stuck, the last resort is to break away the plastic mold with wire cutters or scissors.
- These lids can be re-used dozens of times, just clean up the concrete residue with warm soapy water or denatured alcohol.
- Knock down any sharp edges with a diamond hand sanding pad or a coarse sharpening stone.
Step 4: Sealing + Finishing
- Concrete is vulnerable to etching and staining from anything acidic like lemon juice, vinegar, red wine, etc.
- Sealing isn't necessary but it will help protect the concrete from staining and keep them looking nice for a long time.
- Check out the Sealing Concrete Countertops + More Instructable for tips on applying CHENG Sealer.
- If your coasters don't sit flat, you can cut thin cork backs or little cork feet and attach them with spray adhesive.
Hopefully this little project will inspire you to make something concrete. Thanks for following.