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:
- Conventional formwork is often built on-site using timber and plywood or moisture-resistant particleboard. Plywood is the most common material used for the concrete facing panel. It is easily cut to shape, and can be reused many times if properly maintained. Although easy and inexpensive to produce, this formwork method is time-consuming for larger commercial projects and the plywood facing has a relatively short lifespan.
- Steel formwork is stronger, more durable, and has a longer life than wood. Unlike plywood, steel does not absorb moisture from concrete so it doesn’t shrink or warp. Steel forms can be installed and dismantled with greater ease and speed, too. They are frequently used for large projects and considered most suitable for circular or curved structures.
- Aluminum is often used in pre-fabricated formwork that is put together on-site. Aluminum is strong and light so it can be quickly and accurately assembled. Aluminum formwork is also easy to clean after removal, enabling faster construction cycles, ease of handling, and repeated used without losing quality.
- Glass reinforced plastics (GRP) and vacuum formed plastics are used when complicated concrete shapes are required, such as waffle floors. Although vacuum formed plastics will always need support, GRP can be fabricated with integral structural components making it self-supporting. Like steel, plastic formwork can be reused many times, as long as care is taken not to scour the surface.
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 Best Practices
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:
- Strong enough to support the weight of concrete during pouring and vibration as well as any other incidental loads, including workers and equipment.
- Rigidly constructed and efficiently propped and braced both horizontally and vertically to retain shape.
- Sufficiently tight joints to prevent leakage.
- Permit removal of various parts in desired sequences without damaging concrete.
- Be set accurately to the desired line, and levels should have planar surface.
- Handled safely and easily using the equipment available.
- Sufficiently stable in all weather conditions—should not warp or get distorted when exposed to the elements.
- Rest on firm, secure base or foundation.
Formwork Failures and Prevention
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
- Make sure the formwork is designed by a competent, qualified person experienced in the design of the formwork type being utilized and the design can support the expected dynamic and static loads. If the as-installed formwork does not comply with the original design, amend the formwork to comply with the design, or confirm that the designer inspects the formwork and verifies the revised formwork design won’t compromise structural integrity.
- If proprietary formwork systems are used, ensure they are assembled in accordance with the manufacturers’ recommendations.
- For custom formwork designs, whether combining different formwork systems or using proprietary systems outside the manufacturers’ recommendations, verify that the design is completed by an experienced formwork design engineer.
- Inspect formwork components before use, replacing or repairing defective components prior to use.
- Before the concrete is poured (and other trades gain access to the work site), the erected formwork should be inspected by a qualified person to verify it has been erected in accordance with the formwork design. The person should document the inspection and sign-off the formwork as ready for use.
2) Concrete Pouring Stage
- Ensure the structural integrity of the formwork has been verified before starting the concrete pour.
- Create an appropriate boundary zone to prevent workers from accessing the area under the formwork during the concrete pour and maintain the zone until the concrete reaches adequate strength.
- Monitor the formwork during the concrete pour to identify any early signs of failure. Access areas under the formwork should be prohibited unless a risk assessment has been undertaken to determine it is safe to do so.
- Ensure the formwork is not overloaded during the concrete pouring operation.
3) Formwork Stripping Stage
- Minimum curing time specified in the formwork design must be reached prior to formwork removal, or receipt of appropriate certification following concrete specimen testing.