formwork

Formwork is the temporary structure built to contain and shape fresh concrete until it hardens enough to support itself. It consists of a mold face, typically made from plywood or timber boards, backed by a support frame that prevents the wet concrete, which exerts significant lateral pressure, from bursting outward. Once the concrete has cured to sufficient strength, the formwork is stripped away, leaving behind the hardened concrete structure with the surface profile defined by the mold's inner face.

The support structure behind the mold face carries the load of plastic concrete. For walls and columns, this usually means timber props, steel props, or adjustable metal shores arranged in a grid pattern. For slabs and beams, the formwork comprises soffit boards supported on beams or joists resting on props at the base. The spacing of props depends on concrete depth, pour rate, and ambient temperature, all of which affect the pressure the concrete exerts during setting.

Formwork design must account for the hydrostatic pressure of the concrete, which at full depth of a tall wall can exceed 50 kilopascals at the base. Incorrect bracing or premature removal causes blowout, where concrete pushes through the mold, rupturing the face and wasting material. Conversely, formwork must be removed at the right age: too early risks structural failure; too late wastes resources and delays the next pour.

Variants and Surface Finishes

Formwork materials range from rough sawn softwood planks, which leave a coarse rusticated texture, to plywood with phenolic facing, which produces a smoother, more uniform surface suitable for architectural concrete. Steel formwork systems, assembled from modular panels, allow rapid reuse on repetitive work but carry higher capital cost. Some projects use slip forming, in which the mold rises continuously as concrete sets, ideal for tall structures like chimneys or shafts.

The term encompasses not just the mold itself but the entire assembly: the sheathing, stiffening members, props, connections, and ancillary items like spreader bars and tie rods. In bridge and complex civil work, formwork is often the single largest cost component and the critical path item. Removal and reuse of formwork over many pours amortizes its cost; timber formwork can typically be reused five to ten times before degradation makes replacement economic.

Poor formwork practice creates defects: air voids and honeycomb from inadequate concrete consolidation or vibration patterns dictated by form geometry; surface blemishes from form oil migration; dimensional inaccuracy from insufficient stiffness or settlement of props. Modern practice emphasizes early planning of formwork strategy in design, consideration of concrete release agents, and careful sequencing to balance speed, cost, and quality.