Making your own concrete countertop is one of the more challenging—and rewarding—DIY projects. If you’re a rookie, keep your goals simple. Plan for basic color treatments and plain edges, and also plan to spend a few weekends on the project.
This project is a simple, 2' x 6', 2" thick countertop island with a smooth surface and a cutout for a square cooktop.
To determine the size of your countertop, measure the base cabinets or your existing countertop. If measuring the base cabinets, add an extra 3/4" for an overhang. Our island countertop will be 3’ x 4’.
The key to a great countertop is a well-built mold to pour the concrete into. Start with a 4' x 6' piece of 1" thick melamine particleboard. Set firmly on a pair of sawhorses. Measure and mark the exact dimensions on the mold base, then cut using a circular saw (image 2).
Cut the sides of the mold. Measure and mark four 2-3/4-inch strips. Use a table saw to make the cuts. Each strip should be 4-feet long.
Then attach the sides by drilling 2-inch pilot holes every 6 inches. Join two of these 4-foot strips to the 4-foot sides of the base, using two-inch wood screws. Trim the ends strips and attach them in the same way to the remaining sides of the mold. Use a square to check corners, and the outer mold is complete.
If you need to have a cutout in your countertop for a cooktop or sink, measure and mark it on the mold's base. To start the cut, drill pilot holes on the inside corners of the portion that will be removed. Then insert a jigsaw into the holes and cut from hole to hole along the edge marks.
Then measure and cut the sides for the cutout, attaching them by butting them against the edges of the base inside the cutout.
Carefully clean the mold of sawdust and other materials. The bottom of the mold will be the top of the countertop, so it’s important the concrete sets on a debris-free surface.
Run a small uniform bead of 100-percent silicone caulk in all the inside corners and seams of the mold. Smooth the bead with a caulk tool and let dry thoroughly for 24 hours. You can also use the tip of your finger to smooth out the caulk. The silicone will seal the joints of the mold and prevent the wet concrete from leaking.
Once the outer mold is done, you’ll need to build a support frame to surround it. The concrete you’ll be pouring is heavy – about 10 to 15 pounds per-square foot and you don’t want the edges of the mold to bend with the weight.
Measure the mold and cut a series of 2x4s for a support frame around it. Lay three boards underneath the mold - lengthwise, each a bit longer than the actual mold.
You’ll use this under hang to support the end 2x4s. Attach them with 2-inch screws. Then add the remaining two side pieces and attach them to the first two, completing the frame.
The frame should be tight against the mold, to prevent the heavy concrete from pushing the mold out of shape. But do NOT attach the frame to the mold itself. Rather, the 2x4s the frame should be attached to each other and the mold should lie within the frame.
The last step in preparing the mold for the concrete is to cut a section of galvanized structural stucco wire. This will be added to the concrete during the pour to add strength and prevent cracking. Use metal snips to cut the wire to the shape of the mold so that it comes about 1 inch off the edges all around.
You can find galvanized structural stucco wire at most concrete supply stores. Once cut, set the wire aside until you’re ready to pour the concrete.
For our 12-square-foot countertop, we needed three 60-pound bags of ready-mix concrete. Add water to the concrete and mix with a shovel per the manufacturer's instructions.
If you want to add color to the countertop, now’s the time to add pigment to the mix. Pigment additives come in powder or liquid. Liquid pigments are easy to measure and mix, especially with small concrete batches like this one. But don’t forget to account for the amount of water in the pigment when measuring the water for the concrete. Controlling the amount of water added to the concrete mix is critical to producing consistent color. Refer to the manufacturer's guidelines.
Mixing the concrete correctly is critical to its strength and durability. When it achieves the texture of peanut butter it’s time to add it to the mold. Remember that the concrete at the bottom of mold will become the top of the concrete slab.
Using a small spade or bucket, pour the concrete into the mold, pressing and compacting it as you fill the mold to a depth of about 1 inch or halfway full.
Set the galvanized wire into the concrete, taking care that it does not touch the edges of the mold. The wire will keep the concrete from cracking as it dries and it will also add strength.
Continue to fill the mold on top of the wire, tamping the concrete with a trowel, as you go along to ensure it is well-packed. Your objective is to slightly overfill the mold. The level of concrete will drop slightly in the mold as it settles.
Smooth the concrete surface with a hand trowel. This will draw the aggregates to the top.
To settle the concrete, use an orbital sander without sandpaper against the sides of the mold. The vibrations will help bring air bubbles in the concrete up to the surface.
When finished, gently cover the countertop with a sheet of plastic or damp burlap to protect it from dust and dirt.
Let the concrete cure at least a week—the more it cures, the stronger it gets.
Remove the 2x4 support frame from the sides and ends of the mold.
Carefully drill two 2-inch screws equal distance apart, halfway into each of the melamine sides. Be careful not to drill all the way through – you don’t want to disturb the mold edge.
Remove the screws holding the mold sides to the base. Then use a hammer and the new screws to pry each side away from the concrete slab. Take your time with this step. You don’t want a misstep that will cause a chip or any breakage.
Get help from another person or two to flip the slab over. Remove the cutout sides using the same technique. The weight of the slab usually makes removing the melamine base an easier task.
When the slab comes out of the mold, it’s going to have imperfections. Use an orbital sander to remove any imperfections along the surface and edges.
Wearing a respirator, start with 100-grit sandpaper. You want a standard, smooth edge. Keep one hand on the top of the sander as you polish the edges. Be prepared; you’ll go through plenty of sandpaper and this is a dusty process. Work to progressively finer grits, finishing with 220-grit.
Sand evenly, checking the smoothness with your hand. Keep sanding and testing until each edge and surface feels smooth to the touch. When done, wipe the slab with a damp rag to remove any loose grit and concrete dust.
Prepare the surface for finishing by etching it with a solution made from 1 ounce of muriatic acid mixed in 1 gallon of water. Wipe the surface thoroughly with a sponge dipped in the acid solution. Wear eye protection, a respirator and acid-proof gloves. If you’re working inside, open doors and windows for better ventilation.
Rinse the slab with fresh water to remove the acid mixture, and let it dry completely.
Apply a concrete sealer using a sponge or brush. Work in long strokes from one edge to the other. Let the sealer dry then apply a second coat, working at right angles to the first. Keep applying coats until the concrete won’t absorb any more. Let it dry thoroughly, about thirty minutes.
Prepare the installation by running a thick bead of silicone caulk around the upper edge of the cabinet.
Bring the countertop in, set it in place, and press down gently to seal the caulk.