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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.