Formwork

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What is formwork?

'Formwork' describes a provisional mold used to shape concrete until it solidifies into the desired configuration. Traditional formwork often uses timber shuttering, yet alternatives like steel, fiberglass-reinforced plastics, and plywood are also common.

The most prevalent form of formwork is shuttering, assembled on-site using timber and plywood needing to be water-resistant. Shuttering is relatively straightforward to create, though can be labor-intensive for larger or more intricate structures. This temporary formwork is generally preferred when labor expenses are lower than the costs of reusable formwork made from steel or plastic.

All varieties of formwork, including shuttering, require falsework support. Falsework consists of poles, stabilizers, and other elements that keep the formwork secure as the concrete dries. For additional details, see Falsework.

When the concrete has reached sufficient strength, the formwork can be removed. For vertical formwork, a minimum strength value of 5 N/mm2 is recommended to avoid concrete damage during removal.

Ensuring high-quality workmanship and meticulous inspection are crucial for achieving a superior appearance and standard in the final concrete structure, especially where the concrete will be visible.

edit]Formwork specifications

When choosing formwork, the concrete type and pour temperature are crucial as they affect the pressure exerted. The sides of the formwork must resist the hydrostatic pressure of wet concrete, which decreases over several hours based on setting and curing rates. The formwork base or soffit must withstand both the initial dead load of wet concrete and the subsequent load of dry set concrete.

Formwork must be:

• Strong enough to endure dead and live loads.
• Capable of retaining shape through efficient horizontal and vertical bracing.
• Designed to prevent leakage of cement grout, with sealed joints.
• Removable in sections without damaging the concrete.
• Suitable for recycling.
• Accurately set to the desired line.
• Lightweight as possible.
• Resistant to warping and deformation.
• Placed on a firm foundation.

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Different varieties of formwork

Various types of formwork are outlined below.

edit]Beam formwork structures

To form beams, formwork consists of a three-sided box supported, propped in the correct position, and to the desired level. The removal time for the formwork varies with air temperature, humidity, and curing rate. Typical striking times are provided for air temperatures between 7-16 °C:

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Column formwork solutions

Column formwork comprises a vertical mold of the desired column shape and size. Horizontal steel or timber clamps (yokes) are used at even spacing for batch filling and varying centers for single-pour filling to minimize formwork material thickness.

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The head of the column formwork can support beam formwork, although this provides good top lateral restraint, it complicates the formwork design.

Alternatively, one can cast the column up to the underside of beams. Later, a collar of formwork can be attached around the cast column to finalize casting and support incoming beam formwork.

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Eco-friendly Plastic formwork

Reusable plastic formwork is employed for rapid concrete pours. Comprising interlocking panels or modular systems, it suits simpler concrete structures but lacks the versatility of timber formwork due to prefabrication needs. This makes it ideal for low-cost, repetitive projects like mass housing.

Permanent plastic formwork generally consists of prefabricated fiber-reinforced plastic assembled on-site for piers and columns. It remains in place, offering permanent axial and shear reinforcement while protecting against environmental damage.

Proprietary systems using a series of tubes and ties support vertical formwork as concrete cures.

edit]Associated articles on Designing Buildings

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External links

Introduction to System Formwork & why the adoption ...