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Concrete formwork is an essential element of commercial construction. Formwork is used to shape and support concrete structures until the concrete attains sufficient strength to support its own weight.
In addition to foundations and walls, formwork is used to create virtually every component of a building including: core walls, columns, staircases, beams, suspended slabs, chimneys, and more.
Formwork molds are typically constructed from wood, steel, aluminum and/or other prefabricated material:
Today’s formwork systems are mostly modular, designed for speed, efficiency, and increased accuracy. Pre-fabricated modules minimize construction waste and include enhanced health and safety features. The two major advantages of pre-fabricated formwork systems, compared to traditional timber formwork, are 1) speed of construction and 2) lower life-cycle costs. Minimal on-site skilled labor is needed to erect and strip pre-fabricated formwork, and modular steel or aluminum formwork is almost indestructible—capable of being used hundreds of times depending on care and application.
Formwork is one of the most important factors in determining the success of a construction project in terms of speed, quality, cost, and worker safety. Formwork can account for up to 35-40% of the total cost of concrete construction, which includes formwork material, fabrication labor, erection, and removal.
Regardless of material, formwork should meet the following requirements:
Formwork failure during concrete construction usually occurs when the concrete is being poured. Some unexpected event causes one portion of the formwork to fail, thereby overloading or misaligning the entire formwork structure until it ultimately collapses. One or more of the following can cause formwork failure:
1) Lack of inspection/attention during formwork placement and construction.Many failures occur due to lack of inspection, or the inspector/crew is inexperienced or unqualified.
2) Inadequate design. Most failures due to design flaws are related to lateral forces and the temporary structure’s stability. The lack of a bracing system to deal with lateral forces, like wind and construction loads, causes the formwork system to collapse when an excessive load is applied. Also, as formwork is reused, its capacity to hold a load over time is reduced. Unfortunately, the formwork designer often omits the safety factor and calculates the load using original capacity data. The design of formwork should be approved by a licensed engineer before installation.
3) Defective components. Some cases of formwork system failure have been the result of the improper maintenance of formwork components, which then become defective after being reused several times. The capacity of these formwork components has been reduced due to corrosion and damages, yet is seldom taken into consideration during the erection.
4) Improper connections. Formwork components are sometimes inadequately connected to enable easier and faster dismantling. But lack of proper connection can result in progressive collapses. Insufficient bolts, nails or splicing, poor weld quality, and faulty wedges can readily compromise formwork integrity. Incredibly, sometimes there is no connection at all between two components.
5) Premature removal. Untimely removal of formwork prior to proper concrete curing usually happens because workers are in a hurry to reuse the form quickly because of tight scheduling requirements or budget pressures.
6) Improper shoring. Inadequate shoring is a significant cause of formwork failure, where impact loads from concrete debris and other effects trigger the collapse of vertical shores during concreting. Additionally, shoring must be installed to provide a continuous load path from the formwork to the foundation or other structural component capable to supporting the formwork and new concrete.
7) Insufficient foundation. Many formwork foundations fail to transfer the load to the ground, or are positioned on weak subsoil. These foundations are often constructed from sill plates, concrete pads, and piles, which can cause differential settlement of formwork and overloading of shores, ultimately resulting in collapse. In addition, insufficient foundation capacity can reduce the carrying capacity of the formwork.
To safeguard against formwork failure and the potential for worker injuries, the following preventative measures should be addressed at the three critical formwork stages:
1) Formwork Erecting Stage
2) Concrete Pouring Stage
3) Formwork Stripping Stage