Instructables
Picture of DIY CONCRETE:: Coasters + Trivets
This instructable will show you a quick and easy way to make concrete coasters and trivets from the lids of plastic containers. The lids used in this Instructable are from plastic tupperware found in the top shelf of the dollar store. Look for lids that are not too shallow or you might have problems with the concrete breaking.

There are four steps in this Instructable - Preparing the Lid, Mixing and Pouring Concrete, Grinding and De-molding, Sealing and Finishing.

Materials: Concrete Mixing:
  • Bucket
  • Water
  • Particle Mask
  • Rubber Gloves
Tools / Supplies:
 
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Step 1: Preparing the Lid

A good lid for this project:
  • Coaster Size = 4+ inch
  • Trivet Size = 6+ inch
  • Flexible Plastic
  • Plastic that is durable enough to be re-used
  • Watertight 
  • 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
Less than Ideal:
  • Paper / Cardboard
  • Thinner than 1/4 inch
  • Undercuts / Threads / Deep Logos
  • Thin plastic is OK for one casting, but can't be reused easily.
Prepare the Lid for Casting:
  • Clean the lids with soapy water using something that won't scratch, like a 100% cotton cloth.
Decorating Concrete:
  • 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

Picture of Mixing + Pouring Concrete
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Mixing: For tips on mixing concrete, check out the How-To-Mix Concrete Instructable.
  • 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.
Casting:
  • 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.
Curing:
  • 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

Grinding with an orbital polisher:
  • 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.
Grinding without an orbital polisher:
  • 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).
De-Molding:
  • 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.
Sanding the edges:
  • Knock down any sharp edges with a diamond hand sanding pad or a coarse sharpening stone.

Step 4: Sealing + Finishing

Picture of Sealing + Finishing
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Sealing:
  • 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.
Finishing:
  • 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.
MacGyverG13 days ago

How do you hold the coaster in place during the orbital grinder phase? I haven't tried this yet, but in my mind I see this being flung off the table at some extremely high speeds.

CHENG Concrete (author)  MacGyverG13 days ago

Yes, this can be a problem. If you make a frame to go around the piece, from something like 1/4" wood, that help keep the piece from moving around. I usually just put the coaster on a piece of pink foam, and the coaster grabs into the foam enough that it doesn't fly away. When the piece is a bit thicker, it's pretty easy to hold in place with one hand while the other is holding the polisher. I've never had a piece fly away like you might think, they usually just spin in place.

ardnon1 year ago
These are great. I know I don't really need denatured alcohol for this project, but does anyone know where it is available in Canada? Having a really hard time finding any for other projects...
Denatured alcohol is just pure grain alcohol, made unpotable (toxic) by the addition of wood alcohol. In this way it can be sold in hardware stored without subjection to liquor tax.
CHENG Concrete (author)  ardnon1 year ago
Rubbing alcohol will work too!
shazni1 year ago
Please ...may I know the exact names for the discs I need to buy if I want to grind , polish and cut or what ever with concreat? I have an angle grinder with only a sanding pad and aluminum cutting disc. I would really like some help in this.
CHENG Concrete (author)  shazni1 year ago
There are a lot of pads out there for polishing concrete, some of them come from the granite industry, others are designed specifically for concrete, like the pads I've listed below. A rigid pad will be harder for a beginner to use, because you can accidentally dig into the concrete if you're not holding it flat. A softer pad is more forgiving and works well if the piece has contours. There's no industry standard for grit size, so you want to try and stick to one manufacturer rather than mixing and matching a set. For grinding deep into concrete to expose the aggregate, you'll start with a 50 grit pad, to just expose the fine sand you might start with a 400. The progression is usually like this: 50, 100, 200, 400, 800, 1500. This will produce a nice shine on concrete and there's usually no reason to go any higher. Even glass inlays will be pretty well polished at 1500.

Here's a link to a good 5" orbital polisher we use a LOT in our training: Alpha VSP-120
And a link to a soft pad set: 5" Diamond Wet Polishing Pads
And a link to a rigid pad set: 4 - Step Concrete Polishing System

The VSP-120 is a variable speed polisher, water-fed, up to 4000 RPM. It plugs into a standard wall socket and screws onto a garden hose. The pads are not designed to be used over about 4000 RPM. Above 4000 RPM the resin will start to melt and ruin the pad. Most of the pads out there designed to be used at a lower speed, not with a high speed angle grinder.

Alpha Tools recently came out with a wet / dry polishing pad system that is designed to be used up to 10000 RPM and will work on a high speed angle grinder. They sell a hub and a shroud that should fit most angle grinders out there. This is a real nice setup, and when used with a HEPA vacuum, there's no dust at all.

The important thing to know about diamond polishing pads is that if you buy cheap pads, they won't cut as well or last as long as the more expensive / quality pads. Diamonds are doing the cutting, and the cheaper pads simply use fewer diamonds.

If you're really doing a lot of concrete polishing, you're probably using a pneumatic polisher, hooked up to a huge air compressor.

For cutting and grinding, there are these cutting blades and grinding / shaping wheels that will work with an angle grinder.

I'd recommend a variable speed polisher like the VSP-120 and a set of soft pads. I like the soft pads because they're hook and loop backed and really quick to change out.

Hope this helps!
Thanks a lot!! yes it really helps :-)
they look like the lids from containers that you get from a deli ( for things like pasta salad).
thanks for they help...but what i ment was the disc pads that is used in the angle grinder to grind concrete. does anyone know exactly what is necessary??
inspiring!
nice
nice work
Amazing job!
Cool! So fancy looking :)