Permanent formwork has just been added to the Responsible Building Materials credit in Green Star as a recognition that the use of materials in permanent, or stay-in-place (SIP) formwork is on the rise.
The growing use of this product has the potential to change significantly the way walls are erected, with flow on consequences in the environmental, health and safety aspects of construction. It’s therefore timely to take a closer look at this product and at some of the reasons behind its adoption.
Concrete has been the material of choice for wall construction for decades. Temporary formwork has traditionally been used to construct poured concrete walls where its function is to shape and support the concrete until it attains sufficient strength to carry its own weight; after which point the formwork is removed.
Timber, plywood and sometimes steel have been the most commonly used materials for formwork. Besides materials, a significant aspect of temporary formwork is that it is relatively labour intensive. Because of its material and labour requirements, the cost of formwork itself can represent 40 to 60 per cent of the total cost of the concrete structure. The potential to reduce both material and labour costs has, over the past 10 years or so, led to innovation in formwork systems. Australia has been a key player in this development, establishing world-leading patents and material formulations.
Glass-reinforced plastic, fibre-reinforced plastic, thermoplastics (PVC, polystyrene), fibre cement and metals are examples of materials used to pre-fabricate formwork for permanent systems where the materials remain in place for the life of the building. The components can be interconnected to create formwork that can be erected with a high level of efficiency.
Innovative Australian companies are producing modular SIP systems with proprietary components such that different brands of formwork have different connecting moulds and systems. These new systems can be used to replace load-bearing conventional precast concrete, tilt-up or masonry block walls and are today used for basements, stair and lift shafts, blade columns, irrigation tanks, retaining walls and other areas, and in buildings 30 storeys high.
Temporary formwork has been used for as long as poured concrete walls have existed. It is a well-established product which the building industry as a whole knows inside-out. So what factors are leading to this rise in the use of permanent, or stay-in-place, systems?
Although concrete is now the material of choice for construction, it is not devoid of problems. One of the reasons for it to be used is its high compression strength. However, concrete usually requires reinforcement with rebar because of its poor tensile strength. It is also prone to cracking under sustained pressure, such as in basements and retaining walls. Because of its porous nature, it can develop 'concrete cancer' (usually caused by water penetration). The extent of this issue can be ascertained by the amount of industry discussion, particularly in Australia. Strata Community Australia has warned of an "epidemic" of concrete cancer in Australia, in part due to a major proportion of our property assets being in coastal locations; the exposure to salt and water accelerating the rate at which steel reinforcements might rust.
Using SIP formwork helps overcome these problems as the concrete is sealed or protected from the environment by the formwork for the life of the building. The connectors in the latest SIP formwork systems facilitate concrete flow and consolidation and there’s usually no need to vibrate the concrete.
In the case of thermoplastic SIP formwork, the interlocking system of the modules completely seals the joints and the polymer effectively acts as a waterproofing membrane, a feature that can be particularly beneficial for basement walls. Some systems allow a reduction in the quantity of cement required, and an increase in the amount of fly ash or aggregates used as a substitute without raising concerns of concrete cancer.
Besides these technical aspects of permanent formwork systems, they have been shown to have the potential to reduce the environmental impacts as well as health and safety risks on construction sites. Some of the latest SIP formwork systems reduce the quantity of steel reinforcement required, therefore playing a significant part in reducing materials consumption and, importantly, the embodied energy of the built form.
In addition, there’s no need for stripping SIP formwork, which together with their light weight and ease of installation can improve health and safety on site and reduce injury risk. However, because of their light weight, some modular systems can be more susceptible to wind load during installation and before fixing; they may need bracing before the concrete pour.
It’s also critical to ensure that any modifications to a SIP formwork system’s mould design or the use of a combination of two systems’ components is checked and signed off by suitably qualified people because of the proprietary, and therefore differing, nature of systems in the market.
The lack of familiarity with these construction systems and a traditional conservativeness in the building industry is likely to slow the rate at which they might replace temporary formwork. However, those companies that have started to use SIP formwork cite cost savings, construction time savings and ease of installation as key attractions to the systems. The decision to use permanent over traditional formwork or masonry work can also be driven by site requirements, particularly where there are water issues and retaining wall requirements; and project design requirements such as multi-storey underground basements.
From an environmental and safety perspective, these systems have been shown to benefit projects through transport energy savings, significant reductions in embodied energy of materials, better site safety and reduced long-term maintenance. Some of these systems have also been certified to meet earthquake and bushfire requirements and can therefore contribute to the need in our region for a more resilient built environment.
GBCA’s inclusion of permanent formwork in its Responsible Material Credit recognises its rising use in the building industry, but these construction systems may also represent opportunities to rethink the way we build, the quantities and types of materials we use, the ability to improve housing affordability and the durability of our built environment.