How to Create and Install Concrete Countertops
Concrete countertops are becoming more and more popular, mainly because natural stone is durable.
Before building the countertops, talk to your carpenter or cabinetmaker to ensure that your cabinets can support the weight of concrete countertops, which can weigh (at 2-inch thick) between 12 and 22 pounds per square foot.
If necessary, add extra support to the cabinets. Often, 3/4” plywood sheets are attached to the top of the cabinets to give the concrete a flat, level surface to sit upon. In this case a trim piece matching the cabinets will be needed to cover the plywood edge.
It’s a good idea to build the molds and pour the countertops off-site, in a garage or workroom. It ensures a clean kitchen during this very dusty and dirty process. Avoid places that are exposed to the weather and have drastic temperature change.
The process of pouring concrete countertops begins by creating very accurate templates of the existing counter space. The molds for the countertops will be made from the templates.
There are two ways to create a template: 1). Trace the countertop base on an oversized piece of wood; or 2). Create a template by connecting strips of wood veneer. This project uses the wood veneer method (Image 1).
The template should be the exact size of the countertop base or cabinet top – not the size of the countertop that will go on it. Using wood veneer strips, secure the strips to the surface of the cabinet top with the hand stapler. On corners, use a diagonal piece for extra support secured using the hot glue gun (Image 2). If the template is particularly long or wide, add strips for cross supports. Score and snap off ends with the utility knife and square.
The template-making process is the time to decide on design features like control seams and countertop overhangs. If countertops are made in sections larger than about 8 feet, they are prone to cracking and more difficult to move. Reduce the chances for cracking by incorporating joints or seams in the countertop.
Countertop overhangs – the countertop lip that extends beyond the face of the countertop – are usually 1/2” for standard cabinets and 3/4” for flush cabinets.
Mark seam locations and overhanging edges on your template with a permanent marker (Image 3).
Measure the countertop base again and write these measurements on the template to ensure accuracy (Image 4).
Remove the template from the countertop and move it to where the countertop mold is being made.
The molds are built with 3/4”- thick melamine (Image 1), a medium-density fiberboard with a smooth laminate finish that the concrete won't stick to and which will produce a smooth, even surface for the countertop. When buying melamine, it is important to buy pieces with perfectly smooth surfaces; cracks or dents will distort the surface of the countertop.
To create the molds flip over the templates and trace them onto the surface of the melamine with a pencil (Image 2). It is important to flip the template because the concrete is poured face down – the bottom of the mold will be the top of your countertop.
Transfer your seam locations and overhangs onto the mold (Image 3).
This project has two seams requiring three molds for three sections of countertop. Trace the overhang on the edge of the mold that will have the overhang. An easy way to do this is to get a small piece of solid material the width of your overhang – such as a wooden block – and run it along the edge of the template to create a uniform overhang (Image 4).
Using a table saw, cut out the sides and bottom of the mold. The bottom of the mold is the area that was traced out; the sides should measure the length of the corresponding side of the mold plus a width of 2-3/4 inches. The width of the sides equals the thickness of the countertop – 2 inches – added to the thickness of the 3/4” melamine). Use a new, sharp blade on the table saw to make your cuts to prevent the melamine from chipping.
Construct your molds by attaching the sides to the bottom of the mold (Image 1).
The interior of the mold should be a uniform laminate surface with no exposed particle board. Attach the sides to the bottom of the mold with screws placed at each corner 6 to 8 inches apart (Image 2).
Pre-drill all holes. Drill at a downward angle to preserve the integrity of the melamine surface; any breaks in the surface of the melamine will distort your countertop when the concrete hits it.
The next step is to seal the mold and create the edges of the countertop using silicone caulk. Lay blue painter's tape on either side of your seams at the bottom and edges of your mold to create a uniform beveled edge (Image 1).
Lay a bead of 100-percent silicone caulk along all seams (Image 2).
Run a finger along the joints to move the silicone into the joints and create a uniform beveled edge. Use the same finger around your edges and don't stop or you will create a stop-and-start mark in the silicone that will transfer to the countertop edge. Allow the silicone to dry completely.
While the silicone is drying, prepare the worktable. You want to pour the concrete on a raised surface (just below elbow height is ideal) that is perfectly level in all directions. If you do not have a table, you can create one by running wooden 2x4s across three saw horses and covering those boards with a 3/4” sheet of plywood.
Once the table is prepared, and the silicone is dry, prep the mold for the concrete. Remove the blue tape, and run your thumb along the silicone to remove any imperfections (Image 3).
Clean the mold with clean rags and either denatured alcohol or acetone (Image 4). These liquids will remove dirt and any excess water from the mold. Wear protective gloves while cleaning the mold.
This project uses a non-shrinking concrete mixture with a high-early strength of 3000 psi; it is premixed so you only need to add water (Image 1).
If you are adding color, talk to the distributor or retailer where you purchase your cement about how to get the right color for your mixture. Colored pigments are tricky to work with and advice from an experience hand is well worth it (Image 2).
To determine the amount of concrete needed for your project, it is best to have the supplier calculate this for you. However, a rough rule of thumb is the following: Calculate the square feet of the project. At 2 inches deep, you can pour 8 to 10 square feet with 100 pounds of cement/sand mixture. Thus for 8 for 10 square feet of countertop with a depth of 2 inches, you would need two 50-pound bags of mixture.
Adding steel mesh to the countertops adds strength and prevents cracking (Image 3).
Thoroughly clean the steel mesh using steel wool and acetone or denatured alcohol. If you don’t clean it, rust may work its way into the color of the concrete. Lay it over the mold, and, using bolt cutters, cut it to shape, leaving ends about an inch from the edge of the mold. If you can’t find steel mesh, you can use 3/4” rebar set in a cross pattern and secured with steel wire to the mold. Drill screws every 4 to 6 inches around the top of your mold; you will suspend the steel mesh from these screws after you've poured in half of the concrete.
To mix the concrete in the cement mixer, wear safety glasses, a dust mask and old or protective clothing – this is an extremely dusty and dirty process.
Add water according to the instructions on the bag. Never add more water than recommended. The desired consistency is one that rains or “sheets” off of the cement mixer barrel as it turns over.
When the batch is complete, pour the concrete into waiting 5-gallon buckets and remove any excess concrete from the mixer using a spatula. Wash the cement mixer thoroughly.
Pour the concrete in the buckets into the waiting molds. Add enough concrete to fill a little over half of the mold. Evenly spread the concrete in the mold and near the corners (Image 1).
When the mold is half cull, lay the steel mesh or rebar into the mold so it is suspended about 1 inch from the bottom of the mold (Image 2).
Fill the rest of the mold with concrete.
Vibrate the concrete by tapping the bottom and sides of the mold with a rubber mallet (Image 3) and running a concrete vibrator or a vibrating sander with the sandpaper removed. Vibrating the concrete liquefies it, getting rid of air bubbles and drawing it into the corners to ensure a smooth finished product.
Vibrate and add concrete until the concrete is flush with the edge of the mold. Create a smooth surface by using a trowel or screeding with a straight 2x4. Finish working the wet concrete by running a 2-inch metal spatula along the edges of the mold to clean off excess concrete.
Allow the concrete to set up. With a fast-setting mixture, you can unmold the concrete after 24 hours; with a normal mixture, it can take as long as 10 days to cure.
Remove the sides of the mold from the concrete first, being careful not to press against the concrete. Cut the wire that was suspending the rebar or steel mesh as close to the surface of the concrete as possible. Remove the screws attaching the sides of the mold to the bottom and use a pry bar and a hammer to gently pry the mold apart. Again, be careful not to press the pry bar or hammer against the concrete.
When the sides of the mold are off, flip the concrete countertop over to expose the surface – this is a two-person job (Image 1). Have one person push the concrete to one end of the table while the other person flips it over and the first person gently lowers it into place.
Clean the concrete with a mixture of 10 parts water to 1 part muriatic acid. Wear protective gloves and work in a well-ventilated area when using muriatic acid. Cleaning the concrete will not only prepare it for sealing but will also remove any white residue or efflorescence that appears on the surface of your countertop (Image 1).
Put the water/muriatic acid mixture in a bucket, thoroughly dampen a sponge and clean off the concrete with the sponge, removing any excess with a hand squeegee. Repeat this process until the white residue is completely removed.
To smooth the surface of the countertop, sand the surface starting with 120-grit sandpaper, then 180-grit, and finally 220-grit. Remove dust from the sanding with clean water.
Seal your countertop with a penetrating sealer. With gloves on, fold a clean white rag without any wrinkles. Fully saturate the rag with the sealer and run the rag along the surface of the concrete in long strokes (Image 2).
Overlap each stroke with the previous one. Allow the sealer to dry – it can take up to 2 hours -- and follow with a coat of sealer in the opposite direction.
Repeat this process, crosshatching coats, until the concrete does not absorb any additional sealer.
Apply a coat of beeswax to the concrete before installing it. The wax goes on with a clean soft cloth before being buffed off (Image 3).
First, dry fit your countertop pieces to the cabinet tops (Image 1).
If the countertops are not perfectly level, add small wooden or plastic shims to create level surfaces. Once they are level, secure the countertops to the cabinet top (3/4” plywood) with cowpie-sized dollops of 100-percent silicone caulk placed every 4 to 6 inches. The concrete should be secured using a lot of caulk because it is still in the process of drying and could move or curl if not properly secured.
Once the countertops are set, caulk the seams using silicone latex caulk – (Image 2). You can use a colored caulk to match your countertop color. Tape either side of your seam with blue painter's tape, run a bead of caulk along the seam, run your finger along the seam, and when it is dry, remove the tape.
The countertop is done and in place. Maintain the countertop by cleaning it with mild detergent and water. Avoid products with ammonia.
Beeswax can be applied as often as needed.
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