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.
- 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.