Historically, a keystone is the stone at the top of an arch which holds other stones in place. Today a keystone may be defined as the concrete structure atop a door or window or other opening or even, in some cases, to interlocking concrete blocks used for retaining walls. Pier caps also can have different meanings. They can be decorative tops on concrete posts or the tops of concrete piers used to support floor joints and beams in post and beam construction. Whatever the definition or use, keystones and piers can be made of concrete.
Build forms for basic concrete keystones over doors, windows and other openings from wood. The most basic forms are 2-by-6-inch planks nailed into a rectangular three-sided box open at the top. The form is coated with a vegetable oil or similar sealant to prevent concrete from sticking and then filled with concrete. The open surface is finished with a trowel and finishing tools to give it a very smooth, polished look.
Vary the keystones for openings by using different form materials. Bend thin wood around wooden supports into curves to make rounded or arched keystones. Make one side of a form with elaborate wood moulding, which will leave that impression in the concrete. Angle the ends of the form to make diagonal keystones or build a triangle into the top of the form for decoration.
Make basic keystone blocks by building 2-by-6 or 2-by-8 rectangular forms, either square or in common dimensions, such as 4-inch depths. Angle one end to provide an interlocking effect when blocks are placed in a wall or structure.
Make a basic pier using cardboard forms, which can be bought in varying sizes at building supply stores. Cut to any length with a carbide-toothed blade in a circular saw. Set the forms upright, place reinforcing bars inside and fill the forms with concrete. Strip the cardboard forms off once the concrete has set.
Add a pier cap by pouring concrete in a different size form, then mortaring it on top of the pier. Vary the cap by making a square wooden form, using varying sizes of wood to create a decorative effect. Make a more elaborate cap by building a form of heavy moulding to form ridges in the concrete cap. Mortar these caps to the concrete post or pier.
Build a cap for a support pier by placing a metal Simpson bracket in the top of the concrete after it is poured into a form, either a round cardboard one or one built of wood with sloping sides. Insert upright reinforcing bars before the concrete is poured. Attach a Simpson bracket to rebar or insert it into the concrete after it is poured and before it sets