Mark a 3/4-inch melamine board with the desired mantle measurements to serve as a form for pouring your mantle. Mark the length and width of your mantle onto the board with a pencil, adding 3/4 inches of space to each side of the board to allow space for the walls of the form. Use a straightedge to keep your lines straight and a carpenter’s square to create 90-degree angles for your corners. Mark a second board to create side panels for the mantle, penciling off two length boards and two width boards, all of the same height, using the depth of your desired mantle as the height for the panels.
Cut the melamine boards with a table saw along the marked lines. Assemble the boards using wood screws on the joints to create an open-topped box with the interior sized the exact size for your mantle. Place a screw every 4 inches along the lengths of joints between side panels and the large baseboard, and at both the bottom and top of the side panels where they join with one another.
Place strips of masking tape along the interior of the panels lining all joints. Lay a bead of black caulk along all of the interior joints to seal the joints and avoid leakage of the cement. Smooth the beads along the surface of the tape with a wet finger. Allow the caulk to dry overnight and then remove the tape, leaving only the joints filled with the caulk and none on the melamine panel surfaces.
Use tin snips to cut a steel mesh sheet to the size of your mantle measurements less 1 inch on each side. Clean the mesh using a piece of #0000 steel wool dampened with acetone.
Cut edge mold strips to fit the length and width of the mantle with a utility knife, and adhere them to the inside of the edge panels using a bead of caulk on the rear of the strips. The base of the melamine board will create the top of your mantle, so invert the mold strips to place them so that the top of the edge is against the bottom of the panel.
Use an electric drill and a paddle bit to create low-slump Portland cement-based concrete, adding just enough water to the mix to make the concrete hold together and not slump when placed in a cup and inverted onto a surface as a test. Add any colorants you wish to include in the mantle to the concrete while mixing.
Place columns of the low slump concrete onto the bottom of the melamine board spaced every 5 inches and rising half the height of the form. Lay the steel mesh onto the columns, centered in the mantle form so that it sits at the halfway height with a full inch of clearance on all sides to prevent it from being exposed to air after placement.
Mix a second batch of concrete, adding water until it’s the consistency of a thick batter. Add colorants to the mix if used with the low-slump concrete so that the batch matches the color of the low-slump mixture. Pour the mix into the form, filling it completely around the low-slump columns and over the mesh.
Level and smooth the concrete within the form using a concrete float run across the top of the form, along the surface of the concrete. Vibrate the form by placing a hand-held sander with no sandpaper attached onto the sides of the form, turn the sander on and then run it around the edge of the form. This will force all air bubbles to escape, removing possible voids within.
Drag a screed across the mantle form top with a back and forth sawing motion to level it again after removing the possible air pockets. Cover the form with plastic sheeting secured with masking tape against the side of the melamine for 10 days as the concrete cures. Remove the sheeting after the 10 days and cure uncovered for an additional 10 days. Unscrew the form and remove from the mantle for installation.