formboard
A board or slab that serves as a mold for poured concrete.
formboard: the mold that shapes wet concrete
A formboard is a rigid panel, typically made of plywood, lumber, or steel, that acts as a temporary containing surface for freshly poured concrete. It holds the wet material in place until hydration sets it solid, then is removed to reveal the finished concrete surface. In small jobs it may be a single thickness of board; in heavy construction it forms part of a complete formwork system with bracing, ties, and supports capable of withstanding the lateral pressure of concrete during placement.
Plywood is the most common material for formboards in building work because it is affordable, lightweight, and cuts easily. The face that touches concrete is usually the smooth side; rough or damaged surfaces transfer texture to the finished concrete. Board thickness ranges from 1/2 inch for light loads to 1.5 inches for walls taking significant hydrostatic pressure. Steel formboards are used where reuse justifies the cost or where very smooth or sharp-edged results are required.
Common variants and applications
Formboards appear as wall panels, deck supports, or beam sides. Edge beams use formboards as soffit panels underneath. Slabs often rest on formboards laid flat across joists or subgrade. Column formwork involves stacked circular or square steel forms, but these still rely on formboard principles: containment until set.
Concrete pressure against formwork increases with depth and with the rate of pour. A 4-foot-deep wall of concrete exerts roughly 480 pounds per square foot of lateral force at the base. Formboards must not deflect excessively or the concrete surface will sag, creating an unacceptable finish. Bracing, walers, and ties resist this pressure. Once concrete reaches initial set, usually within 24 hours in moderate conditions, the formboards can be stripped and reused.
Common failures include inadequate bracing leading to bulging or blow-outs, insufficient drainage of bleed water causing surface defects, and premature removal while concrete is still gaining strength. The board surface also matters for aesthetic work: oil or release agent applied before pouring prevents concrete bond and aids stripping, but must be applied evenly or it leaves marks.