There are two ways to create a countertop using concrete; the first is to pour the concrete directly on top of the cabinets and then hand trowel the surface. The second is to build moulds for the countertops and pour the countertop offsite.
Making the template
If using the pouring method you need to create a template for which you trace the countertop base on an oversized piece of wood veneer or connect strips of wood veneer. The template should be the exact size of the countertop base and 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. Use a diagonal piece for extra support on the corners, using a hot glue gun.
While making the template decide on design features such as control seams and countertop overhangs. Using a permanent marker, mark the locations for the seams as well as the edge that will have an overhang added to the mould on your template and then remove the template from the countertop to where you will make your mould.
Creating Countertop Mould
Take the templates, flip them over and trace them onto the surface of the melamine with a pencil. Using a table saw, cut out the sides and bottom of the mould. The bottom is the area that was traced out and the sides should measure the length of the corresponding sides of the mould and a width of 2-3/4 inches.
Attach the sides to the bottom of the mould with screws placed at each corner, at approximately every six inches. Pre-drill all your holes, drilling at a downward angle so that there will be no breaks in the surface of the melamine which can distort your countertop, once the concrete hits it.
Seal the mould and create the edges of the countertop by using 100% silicone caulk. Put painter’s tape on either side of the seams and run a bead of silicone caulk along all the seams. Use your finger to move the silicone into the joints and around the edges and leave it to dry.
Prepare your worktable and once the silicone is dry, remove the tape and run your thumb along it to remove any imperfections. Clean the mould with rags and acetone to remove dirt and any excess water from the mould, but wear protective gloves while doing this.
Mix the concrete according to package instructions and pour into 5 gallon buckets and then into the moulds. Add enough concrete to fill a little over half of the mould and spread it evenly near the corners. Tap the bottom and sides with a rubber mallet and run a concrete vibrator with the sandpaper removed.
You can then vibrate and add concrete until it reaches the edge of the mould, after which you use a trowel and then finish your work on the wet concrete by running a 2 inch spatula along the edges of the mould, to clean off the excess. Allow the concrete to set and then remove when dry.
Now remove the sides of the mould and the screws attaching the sides to the bottom, then use a tool and hammer to gently pry the mould apart. Flip the concrete countertop over to expose the surface. Clean with a mixture of 10 part water to 1 part acid and remove any remaining white residue. Sand the surface of the concrete to smooth the surface and remove the collected dust with water.
Seal your countertop with a sealer, by folding a clean white rag, saturate with the sealer and run it along the surface of the concrete and overlap each stroke with the previous one. Allow to dry and follow with a coat in the opposite direction. Keep repeating till the concrete does not absorb any more sealer. Then apply a coat of beeswax before installing it.
Dry fit your countertop pieces and once they are in level secure the countertops to the cabinet top with a lot of silicone caulk, otherwise it will not be secured properly. Once the countertops are set use silicon latex caulk to caulk the seams. Tape either side of the seams with tape, run a bead of caulk along the seam and when dry, remove the tape. Your countertop is now ready to be used.