shutter

A shutter is a rigid panel, frame, or system that holds freshly poured concrete in place while it sets and hardens. Once the concrete reaches sufficient strength, typically after 7 to 14 days depending on thickness and weather, the shutter is removed, leaving behind the finished concrete surface. Shutters are not permanent; they are formwork engineered to be dismantled and often reused on subsequent pours.

Shutters are manufactured from several materials. Timber shutters, made from softwood or plywood, remain the most common on smaller projects due to low cost and ease of cutting to size. Steel shutters offer superior rigidity and durability for high-rise work and repetitive casting. Aluminum shutters are lighter and corrosion-resistant. Plastic-faced plywood combines dimensional stability with surface finish quality. The choice depends on concrete grade, pour height, surface finish required, and reuse frequency.

Shutter systems and stripping

Shutters must support hydrostatic pressure exerted by wet concrete, which increases with depth and temperature. Props, ties, and bracing resist this lateral force. Vertical shutters on walls carry less load than those supporting horizontal slabs or deep beam sections. The concrete surface quality depends heavily on shutter face texture and release agent application; poor release preparation causes concrete to stick, damaging both the shutter and the finished surface when removal occurs.

Stripping is the process of removing shutters. Early stripping saves cost but risks surface damage or structural failure if concrete has not cured sufficiently. Delayed stripping improves concrete quality but ties up expensive formwork. Weather affects timing: cold temperatures slow hydration and postpone safe stripping. Improper stripping technique, especially prying with force, breaks edges or creates blow-holes and surface voids where concrete failed to fill the shutter completely.

The term "shutter" derives from older timber boarding used to shield or enclose spaces. In industrial trades, shuttering refers to the entire formwork system, and shutters specifically denote the face panels. On exposed concrete finishes such as those used in architectural work, shutter marks remain visible unless the surface is ground or treated, making shutter selection and condition critical to final appearance.