Forms for Concrete Countertops

Concrete countertops for the residential kitchen have been taking a bit more market share lately. They are not that expensive to make in terms of material cost. The real expenditure is in labor. It is a good DIY project to undertake but it is NOT simple for the first timer. Building one takes cement, gravel, coloring pigment, sealant, and the proper forms for concrete countertops.

The Best Material for Forms

The countertop can either be built at a remote location and then installed on-site or can be built right on the counter. Building it on the counter saves the trouble of transporting the finished countertop. The form is essentially a shallow box with the knockouts properly positioned for the concrete to form around.

The part of the form that gives it its horizontal strength are 2” X 4” boards that form the outer edge of the countertop. But pine is porous and rough, so melamine (a form of vinyl) strips are placed inside the boards to provide a slick, smooth surface for the concrete to contact.

The bottom of the form is a solid sheet of melamine which can be removed after the concrete has cured.

Any bare wood can be covered with electrical tape to prevent the concrete from adhering to it. A bead of silicone caulk is applied to the inside corners of the form so that the resulting countertop will have smooth, rounded edges.

Using Knockouts for the Sink and Faucets

Knockout forms for the sink and faucets are easy to use and are reusable. Although they add to the cost of the concrete countertop project, they do make the job easier and make for a more professional looking finish.

The critical factor when using these knockouts is proper placement. They have to be in exactly the proper location. Cured concrete is not very forgiving when trying to make adjustments.

Once the knockouts are in place, reinforcing steel mesh should be placed inside the form where the concrete will be poured (not within an inch of the edges), similar to the way a concrete road uses rebar. Of course, the gauge of the form steel is not the same as road rebar!

The reason for using the steel reinforcing mesh is not only to give the concrete countertop strength, but to minimize the cracking that occurs as the concrete dries and cures.

Pouring the Concrete

The concrete is done in two pours. The first pour is just standard concrete. The second pour will contain the additional ingredients that give the countertop its color. Since the concrete is mixed drier than concrete for outside use, it is not going to be self-leveling; it must be well tamped down by hand.

Care must be taken that no air pockets remain in the mixture.

After the concrete has been curing for about two hours, begin to trowel it using a steel trowel. Be careful not to be too aggressive. Such action has the potential of pulling aggregate upwards which will weaken the entire countertop.

Removing the Form from the Finished Countertop

The concrete should cure for at least 24 hours before removing the form. If the countertop is being cast in a dry climate, keep moist burlap on the surface. The longer allowed for curing, the stronger the countertop will be. Finally, remove the form carefully. If it is not built in place, transport it to the cabinet with care. Concrete weighs approximately twenty pounds per square foot.