REQUIREMENTS OF FORMWORK – QUALITY, SAFETY & ECONOMY
In the selection of materials for formwork, the three general principles of quality, safety and economy must be paramount. Material quality can ensure safety, and significantly contributes to the achievement of economy. Formwork failure can result in loss of life and always causes catastrophic financial loss.
Some general guidelines can be given for form face and framing materials, and for the associated components. These can be covered under the headings of
The material strength must be adequate to resist the forces anticipated. This is not only a structural design requirement, but also an essential safety aspect.
The structural movement under load must be small and predictable. These deformations and deflections can be a significant part of the total deviations in the formed concrete surface. When the formwork designer is planning the formwork system, decisions must be made on the total deviation that will be acceptable, and to what extent workmanship errors and structural deformation will each contribute to this. To ensure that the total deviations do not exceed the tolerances, the material stiffness and the workmanship accuracy must be consistent.
The forms must be built to ensure that the damaged form, although unserviceable, does not generate falling debris. It follows, that the way in which the formwork materials fails, will determine this. To comply with this important safety aspect, materials exhibiting ductile failure are far superior to those that fail in a precipitate and brittle manner.
In the interests of economy, and the achievement of quality concrete product at each reuse of the formwork, its materials must be durable. Formwork is almost always built and used out in the open. Between re-uses, its materials and components are commonly stored out in the weather. Ideally, framing, components and formface materials should be resistant to the ravages of the environment. They should have a slow rate of deterioration under the effects of sun, wind and rain. Their resistance to deterioration can be enhanced by proper care and maintenance. Material durability is not only important for the achievement of good quality concrete surface finishes, but also to ensuring that formwork structures are always safe.
In the assembly of formwork, most individual members and components are moved into position by hand. This occurs even when the completed formwork assembly is so heavy that it can only be moved and positioned by crane. Ideally, for efficiency and economy, framing members, formwork components and formface materials, should be sized such that their weight is within the lifting ability of one form worker. If the weight exceeds that which can be carried by two personnel, crane handling is called for. The next level of formwork weight restrictions is set by the lifting limitations of the on-site crane.
For economy, it should be possible to assemble formwork with the minimum of fitting and cutting of materials. Consistency of size of materials, plywood sheets and framing members, is important to this aim. The accuracy of plywood sheets and the sizing of timbers for consistent dimensions are discussed later in this chapter.
The materials of the formwork must not be incompatible with either the fluid concrete or the hardened concrete. At the formface the constituents of the form materials must not react with the hydrating cement of the concrete. For example, some timbers contain wood sugars that break down the cement. After the concrete hardens some timbers, such as eucalypts, can severely stain the concrete. When water runs over this timber and onto the concrete, dark brown stains usually results.
Extremes of heat and cold present problems in the choice of form materials and their protection. The rate of setting of concrete and subsequent strength gain is slowed by low temperatures, and if the water in the mix becomes frozen, the formation of ice will destroy the chemical bonding within the concrete matrix. In situation where concrete has to be placed at low temperatures, aggregate storage bins and mixing water can be heated to produce warm concrete that will not cool during the initial setting period, while its own internal heat builds up.
For all forms, the placing of the fluid concrete, particularly with crushed rock aggregate, can cause some abrasive damage to the formface at first use and each reuse. When the concrete has hardened, the forms are stripped, and this can contribute to surface damage. Abrasion will occur if the forms are permitted to slide on the concrete face.
Secondly moisture absorption at the formface must be minimised. Moisture loss from the concrete into the formface causes hydration staining of the concrete, with severely darkened surface patches. For high quality concrete surfaces, where colour control is specified, this is totally unacceptable. For any concrete surface, hydration staining means poor cement hydration, weak concrete and low surface durability.