Make a concrete countertop
DIY concrete experts show how to design, build and install a beautiful concrete countertop for a newly remodelled kitchen.
It’s a good idea to build the moulds 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.
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1. Make the mould templates
Before building the countertops, ensure that you cabinets can support the weight of concrete countertops. The process of pouring concrete countertops begins by creating accurate templates of the counter space. The moulds for the counter-tops will be made from the templates.
There are two ways to create a template:
A. Trace the countertop base on an oversized piece of wood; or
B. Create a template by connecting strips of wood veneer. This project uses the wood veneer method (image above).
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 a staple gun or Bosch Tacker. On corners, use a diagonal piece for extra support secured using the hot glue gun (image above). 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 2 metres, 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 countertop – are usually no more than 20mm. Mark seam locations and overhanging edges on your template with a permanent marker (image above).
Remove the template from the countertop and move it to where the countertop mould is being made.
2. Transfer the templates to the mould material
The moulds are built with 18mm BisonLam, a particle board with a smooth finish that the concrete won't stick to and which will produce a smooth, even surface for the countertop. Although this type of board is expensive, it will mean far less work to smooth and finish the countertop.
To create the moulds flip over the templates and trace them onto the surface of the melamine with a pencil. It is important to flip the template because the concrete is poured face down – the bottom of the mould will be the top of your countertop. Transfer your seam locations and overhangs onto the mould.
This project has two seams requiring three moulds for three sections of countertop. Trace the overhang on the edge of the mould 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.
3. Construct the moulds
Using a table or circular saw, cut out the sides and bottom of the mould. The bottom of the mould is the area that was traced out; the sides should measure the length of the corresponding side of the mould. The width of the sides equals the thickness of the countertop – 60mm – added to the thickness of the 18mm BisonLam. Use a new, sharp blade to make your cuts to prevent the melamine from chipping.
Construct moulds by attaching sides to bottom of the mould (image above).The interior of the mold should be a laminate surface. Attach the sides to the bottom with screws 200mm apart. 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.
4. Seal the mould - create the edges
The next step is to seal the mould and create the edges of the countertop using silicone sealer. Lay masking tape on either side of your seams at the bottom and edges of your mould to create a uniform bevelled edge. Lay a bead of silicone sealer along all seams. Run a finger along the joints to move the silicone into the joints and create a uniform bevelled 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 planks across three saw horses and covering those boards with a 19mm sheet of shutterply.
Once the table is prepared, and the silicone is dry, prep the mould for the concrete. Remove the blue tape, and run your thumb along the silicone to remove any imperfections. Clean the mould with clean rags and either denatured alcohol or acetone. These liquids will remove dirt and any excess water from the mould. Wear protective gloves while cleaning the mould.
5. Mix the concrete - cut the steel mesh
If you are adding colour, talk to the distributor or retailer where you purchase your cement about how to get the right colour for your mixture. Coloured pigments are tricky to work with and advice from an experience hand is well worth it. To determine the amount of concrete needed for your project, it is best to have the supplier calculate this for you. You will also find information on the Cement and Concrete Institute (www.cnci.org.za) website.
Adding steel mesh to the countertops adds strength and prevents cracking. 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 colour of the concrete. Lay it over the mould, and, using bolt cutters, cut it to shape, leaving ends about an 2cm from the edge of the mould.
If you can’t find steel mesh, you can use 18mm rebar set in a cross pattern and secured with steel wire to the mold. Drill screws every 10 to 15cm around the top of your mould; you will suspend the steel mesh from these screws after you've poured in half of the concrete.
When mixing the concrete wear safety glasses, a dust mask and old or protective clothing – this is an extremely dusty and dirty process. Pour the concrete into waiting 5 litre buckets.
6. Pour the concrete - set the mesh
Pour the concrete in the buckets into the waiting moulds. Add enough concrete to fill a little over half of the mould. Evenly spread the concrete in the mould and near the corners. When the mould is half full, lay the steel mesh or rebar into the mould so it is suspended about 10mm from the bottom of the mould. Fill the rest of the mould with concrete.
Vibrate the concrete by tapping the bottom and sides of the mould with a rubber mallet and running a 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 mould.
Create a smooth surface by using a trowel or screeding with a straight plank of timber. Finish working the wet concrete by running a metal spatula along the edges of the mould to clean off excess concrete.
Allow the concrete to set up. It can take as long as 10 days to cure.
7. Remove the mould
Remove the sides of the mould first, being careful not to press against the concrete. Cut the wire that was suspending the steel mesh as close to the surface of the concrete as possible. Remove the screws attaching the sides to the bottom and use a pry bar and a hammer to gently pry the mould apart.
When the sides of the muold are off, flip the concrete countertop over to expose the surface – this is a two-person job. 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.
8. Clean, sand and seal the countertop
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.
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. 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.
9. Install the countertop
Dry fit your countertop pieces to the cabinet tops. If the countertops are not perfectly level, add thin strips of board to create level surfaces. Once level, secure the countertops to the cabinet top with dollops of silicone sealer placed every 10cm. 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, you can use a coloured grout to match your countertop colour (add grout sealer to the grout mix). Tape either side of your seam with masking tape, run grout 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.