Concrete is quickly joining marble, tile, and laminate as a fashionable yet durable material for countertops. While you might envision an ugly grey slab, concrete can provide any colour you wish to your bathroom or kitchen--even the blackest black imaginable. Simply frame a mould for your countertop and determine the amount of concrete needed. Mix the concrete, add colourant, pour into the mould and finish the counter. You will soon enjoy the stylish and elegant surface that you created.
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Things you need
- Tape measure
- Melamine boards
- Circular saw
- 4 cleats
- One-pound box of 16d-galvanized nails
- 2-by-4-inch boards
- Sheet of foam board
- One-pound box of 3-inch Phillips screws
- Concrete mixer
- Quick drying concrete mix
- Carbon black pigment
- Power sander
- Concrete wax
- Epoxy concrete sealer
Measure the countertop width and length; add 1 inch to the length and cut two of the 2-by-4s to size, using a circular saw. Cut two boards to exact width. (The extra inch on length allows the boards to form a square edge when placed around the counter, creating an overhang for the concrete countertop.) Place boards, flush with counter, around the counter base and nail every 12 inches to attach. Cover the top of the wood with tape to prevent the concrete from later sticking to it.
Remove the sink from the counter. Measure the sink opening and cut a piece of melamine board to fit flush in the sink hole. Nail cleats to the inside of the sink opening, just low enough to allow the melamine board to sit flush with the countertop. Place melamine on top of cleats.
Place the removed sink on top of a piece of 2-inch rigid foam board. Trace the outline of the sink on the board and cut out the outline. Spread caulk adhesive on the bottom of the foam cutout and place this form on top of the melamine exactly where the sink goes; this form saves the place for the sink when reinstalled. Screw to the melamine underneath with a screw in each corner.
Create foam plugs for the sink faucets if they are separate from the sink unit. Stick these plugs in the holes remaining to block concrete from filling up the openings.
Cut four strips of 2-inch melamine boards to the same dimensions as the 2-by-4 boards already attached. Secure these strips to the outside of the 2-by-4 boards, screwing in place every 12 inches to create the outside concrete form.
Mix the amount of concrete you need with a rented concrete mixer. (See Resources for concrete calculations and bags of concrete mix needed.) Add water and concrete mix slowly, alternating to aid mixing. Use as little water as possible--the finished mix should be similar to stiff oatmeal or cookie dough. Pour in liquid or powder carbon black as directed--generally around 0.907 to 2.27 Kilogram of pigment per bag of concrete. Mix thoroughly.
Spread the concrete mixture across the prepared countertop. Fill your forms about halfway, then smooth the surface with a float. Lay metal lath across the exposed concrete to create strength, then continue to add the rest of the concrete over top of the lath.
Hit the melamine form edges with a hammer to eliminate air bubbles. Pound the concrete surface with a flat board or float to pack the concrete even more. Screed across the surface with a 2-by-4 or use a float to smooth completely.
Allow the concrete to set until the water disappears; this can take several hours. Touch the concrete to determine if it is ready -- if it leaves a peak like egg whites, let it sit longer; if it shows your fingerprint, it is ready.
Work the surface with a trowel, smoothing until water appears. Wait a little while longer, then trowel again. Repeat until concrete begins to look polished and silky. Cover with plastic and let the concrete set four or five days to cure.
Remove all forms from the edges and the forms covering the sink and faucets. Carefully pry them away from the concrete, taking care not to damage the concrete. Take the melamine insert and cleats out of the sink cavity as well. Leave the concrete to set another 3 or 4 days.
Sand the countertop and edges with a power sander, working from coarse to fine grit. Pour a small amount of water on the surface to wet sand, clearing away grit as it forms by wiping with more water. Allow it to dry 24 hours when finished.
Wax the exposed concrete with a small amount of concrete wax. Follow with an epoxy sealer to prevent staining of the concrete surfaces (an acrylic sealer is not considered "food-grade"). Pour a small amount on and wipe across with a paint brush to spread. Allow this to dry for a day.
Wash across the sealed surface to remove any sticky feeling. Sand again and reapply sealer, repeating the procedure. Let the sealer dry again before installing sink and faucet fixtures.
Tips and warnings
- While iron-oxide black pigment and surface colourants may be used, carbon black will produce the blackest black. However, make sure to seal it well, as carbon black will also weather out of concrete easier.
- Remember that concrete is caustic; avoid contact with skin and wear gloves to protect yourself.
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