Molds or Forms for Concrete Structures

Terminology

• Making concrete blocks or a concrete wall requires using a form to keep the concrete mix in place. The form doesn't need to be very detailed, just strong enough to resist the weight of wet concrete. Sinks, counter tops and similar items for interior use are cast in the same way, but the forms that give them their shape are called molds. A mold is really just another name for small and detailed form-work.

Molds

• You can make your own molds from materials such as polystyrene foam and plastic glass, sculpting the material into the desired shape then adding an epoxy shell. This process works well for custom molds that have an irregular shape, but it is time-consuming and requires some technical skill. Alternatively, manufacturers often buy molds from companies that specialize in making them. Counter-top molds are often made from melamine, and sink molds are often made from fiberglass or similar materials.

Forms

• The type of form used depends on project size. If a project is small, then most builders just use wood nailed to posts or stakes to keep wet concrete in place. Larger projects may require the use of sturdy metal forms to keep the wet concrete stationary. Forms for even larger projects such as high-rise foundations necessitate gigantic forms made from good-quality steel. Architects also may make use of form liners, which are attached to the inside of the form-work and leave an imprint on the concrete.

Considerations

• Detaching concrete from molds made of rubber or steel may require the use of a release agent. Releasing concrete from molds made of plastic glass and melamine, however, generally does not necessitate a release agent, and those kinds of molds provide a very smooth surface.

  If appearance is important, the mold or form should be watertight so that water does not leak from the concrete. Insulating concrete form-work is another option that incorporates polystyrene foam form-work into the finished concrete, thereby providing built-in insulation.