If you are in any doubt about the positioning of the sink and/or the holes for the faucets and accessories, replace the two pieces of the concrete countertop on top of the plywood and make sure that all of the holes and opening for the sink line up. Once you are confident that everything lines up remove the two pieces of the countertop from the plywood and proceed with this step.
Add additional drywall screws to hold the plywood in position to the base cabinets. The plywood should not have any wobble or spring to it. Ensure that all the screws are flush or slightly recessed with reference to the top of the plywood.
Take a small break. You have some thinking to do!
The next steps are to fix the concrete countertop to the plywood using construction adhesive. While that in itself is not overly difficult, you must consider how you will draw the two sections of concrete countertop together.
If the countertop is open on both ends, you can use pipe clamps, making sure that the clamp ends and body are protected from resting directly on the concrete with scrap pieces of wood.
If there is a wall on both sides of the countertop, you will not be able to use pipe clamps to draw the pieces together. In this case, having wood or plastic wedges or shims available that can be used between the wall and the concrete countertop to force them together is an option.
If there is a wall on one side of the countertop, but the other side is open you can look for a method to apply pressure to the open side, possibly wedging a couple of pieces of 2 x 4 between the end of the countertop and an available wall in order to close the gap between the two pieces of concrete countertop.
Whatever method you finally decide on, make sure that you have all the tools and/or materials available in the kitchen so that the exercise of forcing the two sections together can be accomplished efficiently once the pieces of countertop are placed in their final position.
Clean off the top of the plywood, using a vacuum cleaner, making sure that all saw dust has been completely removed.
Remove any tape and foam protection that you may have placed on the arms of the concrete countertop.
Apply a single layer of blue masking tape on the sides and top of the ends of each of the arms on both sides of the hole for the sink.
Using a caulk gun apply construction adhesive in a zigzag fashion over one half of the plywood.
Lift the applicable section of countertop, the one that will fit over the construction adhesive applied in Step 11 and place it on top of the plywood.
Note: Try to place the piece of concrete countertop into position with as little sliding motion as possible. In some cases some sliding action is unavoidable.
Apply a bead of silicone caulk to the ends of both of the concrete arms on the section of countertop that you have put in place.
Repeat steps 12 and 13 for the second section of concrete countertop.
Place a piece of scrap lumber on the outside and inside edge of the countertop where the joint is and using some bar or C clamps draw the front edges of each section of countertop so that they are aligned with one another. Do not tighten the C clamps or bar clamps, just allow them to be snug enough so that they remain in position.
Now is the time to draw the two pieces of concrete countertop together as per the discussion in Step 7.
Do not concern yourself if some of the silicone caulk and/or construction adhesive forces its way out of the joint it will come off with the masking tape that you applied over the ends of the arms of the hole for the sink.
Once you have clamped the unit tight lengthwise, tighten the C clamps or bar clamps on the front edge to ensure that both sides of the countertop are aligned with one another.
Allow the assembly to sit undisturbed for at least one full day.
After the 24 hour period you can remove the clamps and the masking tape from the ends of the arms for the hole for the sink. If there is any construction adhesive and/or silicone caulk visible after the tape has been removed from the arms, you can remove it with a razor blade.
Using tapcons screw through the plywood countertop base and up into the concrete countertop. One tapcon screw every 12 inches towards the front and back of the countertop is sufficient.
Mount the sink to the bottom of the concrete countertop, as shown in Figure 23, and install the faucets and other accessories if applicable.
Finishing the plywood edge
After the countertop is installed you will have a visible edge of the piece of 3/4 inch plywood between the cabinet frame and the concrete countertop. The edge of the plywood can be covered with numerous materials and it really depends on the overall décor of the kitchen on which material you choose. Some examples include, wood veneer, laminate, aluminum, stainless steel, or brass strips.
How to Make Concrete Countertops - Index