Best Forming Materials for Countertops

Question:
What’s the best forming material to use for concrete countertops? I’ve seen Melamine, Plexiglas, steel, foam and other materials used.

Answer:
The answer to this question depends on your mix and on the application, because they will dictate whether you need a form that is watertight, pliable, smooth, easy release, etc. Here are the main factors to consider.

Stiffness of the mix
If you use a stiff (zero-slump) mix, you do not need a watertight form. You can simply use reusable steel angle iron clamped down on a reusable steel casting table surface with form-release agent on it. (Note that angle iron won’t work, however, if you need to form curved edges.)

For very fluid mixes, watertight forms are essential, even if you are going to grind the surface. In fluid mixes, the cement paste will leak out of forms that aren’t watertight, which will affect both the appearance and soundness of the concrete at those leaky edges. When a form leaks, it is acting like a crude filter and the concrete matrix in that area loses cement paste, therefore becoming weaker. Even if the leak is small, water will be leached from the paste. Because of the change in the cement paste, the concrete color will be different (usually lighter). To make a form watertight, you need to caulk the joints. A 100% silicone caulk works well.

Countertop shape
If your countertops are to have curved edges, you will need a pliable forming material. For tight curves or irregular shapes, you can use foam that you cut with a band saw. For long, smooth curves, you can use Plexiglas or sealed Masonite that you bend and secure with wood blocks.

Finishing method
If you plan to grind the surface, you don’t need forms that will leave a defect-free surface when they are stripped. Therefore, there is no reason to use expensive Melamine and spend time caulking the joints (unless you’re using a fluid mix). If you are not going to grind the surface and instead are trying to create a perfect cement finish, it is essential to use smooth materials for all sides of the form. Melamine works, but it has a slight texture. For a smooth-as-glass finish, some people cast on Plexiglas or glossy Formica.

Ingenuity is the key to successful forming. There are no hard and fast rules. Think about what you are trying to achieve, and then choose materials that meet those needs.