Making a concrete countertop is easy enough that you can do it yourself. However, since it should be two inches thick, it will be heavy and you will need help moving it during a couple steps. It is best if you have a garage or out of the way place to cure the concrete for a few days. The grinding and polishing step is best done outside since it is a wet messy job. It is important to wear safety glasses, gloves and a mask while mixing the concrete and using solvents or adhesives.
If the kitchen is over a basement or crawlspace, add steel supports under the floor to reinforce the floor for the added weight of the countertop. The cabinets may also need to be reinforced with 3/4 inch plywood and a plywood top under the concrete. Check with the cabinet manufacturer where possible.
Make a accurate templates for a perfect fit and label them as you go. Cardboard can be recycled for this, but the cardboard's factory cut edges should be used around the edges and inside a sink cutout. Walls may be out of square. Allow for overhangs such as 1 1/2 inches from the cabinet face and 1/8 inch around the sink cutout.
Build the mold carefully paying attention to details cutting 3/4 inch melamine with a fine tooth blade on a tablesaw. Use concrete wire mesh unless you have large overhangs. Cut strips of melanine 2 inches wide for the concrete depth. Then cut the lengths of the strips with a miter saw. Lay the template upside down on the mold. The bottom of the mold will be the top of the countertop. Predrill holes every 3 inches in the sides of the mold. Countersink drywall screws flush with the top being careful not to drill so hard that the melamine splits. Corners must endure a lot of pressure so reinforce them with blocks of scrap melamine.
If the counter is large, split it in sections so it is easier to manage and future stress fractures will occur in a seam where repair is much easier. The seam can be made on an angle so it looks less like a fracture. Make a cutout hole for the sink faucet with PVC pipe with the outside diameter large enough to fit hoses through for 1.5 inches and then a circle of 1/2 inch plywood to give the counter's 2 inch depth for the faucet and a place to get wrenches in to screw the faucet down.
Seal the mold with silicone caulk so that it is watertight and no wet concrete will leak out. Use painters tape on each seam side to keep the edges clean and remove the tape before the caulking dries overnight. Lightly grind off any rough edges or they will be transferred to the countertop. Seal cut sides of the melamine with spray adhesive so they will not swell when the wet concrete is against them. Cover with clear packing tape neatly trimmed. Cover any screw heads not under the tape with plumbers putty to keep them free of concrete so they can be unscrewed when it is time to break up the mold.
Concrete begins to set in 30 minutes so the mold must be ready before mixing the concrete. It must be level, well supported, and screwed to its support. Use heavy duty sawhorses with 1 1/2 inches of plywood on top and drilled into the sawhorses. Screw the edges of the mold into the plywood and screw scrap wood pieces around the mold edges into the plywood.
Cut 2 flat sheets of concrete wire mesh to size leaving 1 inch short of all mold edges and tie them with wire. Use pieces of styrofoam to space the wire 1 inch off the bottom of the mold. Vacuum the mold, then clean with rubbing alcohol. Put screws around the mold's outside edges and tie the wire cage supports around the screw heads. Remove the styrofoam spacers and the cage should now remain suspended away from the mold's bottom. You can spray spray adhesive on the bottom of the mold and sprinkle it with crushed colored glass pieces for an extra touch.
For beginners, a kit like Cheng™ Pro-Formula Mix contains color tinting, water reducers and support fibers plus instructions for perfect concrete for countertops. Bags of Quickrete 500 can be purchased at stores like Lowes or Home Depot. Or check out Glass Fiber Reinforced Concrete (GFRC) if you are concerned about weight. Have help ready to clean up the rented or borrowed concrete mixer and tools while others help pour the mixed concrete. Use glove covered hands to gently put concrete in the mold on top of crushed glass or shovel it in if there is no glass to be careful with. When the mold is about half full, vibrate the concrete by rapping rubber mallets along the bottom and sides until trapped air bubbles are released to the surface or use an electric orbital sander turned on edge.
Finish filling the mold adding a little more than it can hold. Screed the concrete with a straight board long enough to go all the way across the mold. Start on one side and push the screed back and forth along the sides all the way until the finish is level. Fill any low spots and re-screed them. Vibrate the concrete again for at least ten minutes until the air bubbles stop coming out to the surface.
The temperature for curing should be between 70 and 90 degrees Fahrenheit and it should be done in the shade so it will not dry too quickly and crack. Wait a minimum of 4 days before breaking the mold but less than 10 days if the countertop is to be ground and polished. After the 4th day, start unscrewing all the mold screws. Do not apply pressure to the green concrete or it will chip and crack. Have another person help lift it off the mold bottom and lay the countertop right side up on a piece of styrofoam.
For a solid color, use a wet grinder with diamond coated grinding pads plugged into a ground-fault circuit interrupter (GFCI) outlet on a grounded electrical circuit to polish the counter. Grinders like the Hellcat Wet Grinder have a GFCI on them for added protection from shocks and are double insulated from water. Wear rubber gloves and rubber boots for added protection from shock and do this messy wet part of the project outside.
To expose the crushed glass more, grind keeping the grinder level and starting with a 50 grit pad moving forward and backward, side to side until the desired aggregate is exposed. Then move up through the 100 grit, 200, and 400. Fill in holes or gaps with concrete slurry that comes with the kit and go back over it with the 400 grit pad when the slurry has dried. Finish off with the 800 and 1500 pads.
Get 3 or more people to help you with the installation and agree on the plan. Apply lots of caulk on the plywood top to keep the countertop in place. For added support drill underneath into the concrete and secure with tapcons. Strips of galvanized steel can be cut and glued with Liquid Nails onto plywood edges to cover them or wood trim matching the cabinets can be attached.
Follow the kit directions on applying the penetrating sealer. Polish with carnauba wax for added shine and protection. It resists heat so hot pans can be set on it and it protects against stains from citrus foods and wine. Reapply the wax monthly for best protection.
This process can be used to make a concrete topped table or desk as well. Concrete is not great for the planet but is probably better than granite, much cheaper, you can make it yourself and be more creative and it does not offgas so is healthier. To see some samples of concrete countertops or to have a Greenville company make them for you, visit Solid Designs, 526 W. Washington Street, Greenville, SC 29601, 864.877.7751, firstname.lastname@example.org.