Using precast concrete allows the industry to incorporate all the positives of production line manufacturing into the forming of concrete modules that link together on-site to construct a variety of structures. These structures can range from simple box- like warehouses to the new desalination plant on the Gold Coast, and to complex structures like the John Curtin School of Medical Research in Canberra or the Berry Sports and Recreation Centre in the NSW Southern Highlands.
The precast industry is also set to benefit from recent reforms to the Building Code of Australia which increases the thermal performance requirements of buildings. Increasingly, innovative precast concrete sandwich panels, with an inner foam panel set between two layers of concrete, are becoming a popular walling material. Such panels provide insulation and improved fire protection.
The executive officer of the National Precast Concrete Association, Sarah Bachmann, says that better surface finishes and strengths are assured with precast, as precast modules are generally factory-made on steel beds in controlled factory conditions.
“Factory staff oversee mould setup, reinforcement placing, casting in of services and concrete mixing and placement, so achieving quality assurance is easier than with on-site pours,” Bachmann says. “Quality testing is also regularly undertaken to ensure high standards.”
Production generally occurs in casting beds, so all parties are aware of the finished product’s appearance before any concrete is placed. Also, the manufacturer knows how much concrete will be needed, thereby minimising wastage. Furthermore, moulds are generally used at least 50 times.
In-factory operating eliminates weather problems. Precasting also allows design flexibility as moulds can be formed into any shape and can include specific curves, grooves or inserts. Different textures such as grit blasting, polishing or acid etching can also be achieved in the factory.
There is a rising use of form or mould liners which allow a myriad of patterns in the precast. Combining patterns with staining gives endless possibilities, and staining is maintenance free. Patterns can even mimic a brick or stone wall, but can be more efficient. Such technological developments produce surfaces that allow increased aesthetic options, which are durable and low maintenance.
As precasters use efficient, high-quality concrete mixes, longer and stronger spans are made at high strengths with a longer life expectancy than on-site concrete poured on site. Precasting is also more time efficient. Manufacturing of the precast can start before the building site is even ready, thereby telescoping schedules. Just-in-time delivery of the prefabricated sections allows them to arrive and be dropped in on cue for rapid construction; with reports of building time cut by up to 75 per cent.
“It is possible for the walls of a house or the whole floor of a commercial development to be erected in one day,” Bachmann says.
Another precast innovation is using the high thermal mass of hollowcore flooring (made with longitudinal conduits) as a way of floor heating or cooling. Hot or cold air is passed through the cores of the flooring system.
A bigger role looms for precast concrete in housing and other buildings in fire prone areas, in the aftermath of the Victorian bushfires. Precast concrete is fire resistant and easily meets the AS3600 fire standard — a 170 mm wall has a four-hour fire rating.
Combined with sensible vegetation clearing, ember-proofing, double glazing, fire shutters and sprinkler systems, precast concrete can be a logical choice for walls.
The environment
Low maintenance durability makes precast concrete a green choice for construction as it is durable and low maintenance. Precast panels can incorporate recycled supplementary cementitious material such as slag or fly ash, as well as recycled aggregate and recycled water. Factory waste is minimal and is mostly recycled by the precaster. There are also on-site benefits of less pollution, noise and debris.
It also has a high thermal mass. If designed correctly, a precast building can require less heating and cooling, thereby minimising energy usage. These benefits are enhanced when precast sandwich panels are used, as most of the thermal mass (typically 150 mm) is on the inside of the building, where it is needed.
An external concrete layer, or whythe, (typically 75 mm) provides durability and robustness. The two skins sandwich a layer of insulating foam and are held together by ties that do not conduct heat from one wythe to the other, which has the potential to give better insulation than brick veneer.
Daniel Chen of Girotto Precast, which has factories in Victoria, NSW and Queensland, says precasting gives improved aesthetics and allows elaborate architectural features to be included that could not be done on-site.
“Often, workers are constricted on a building site by the limited space. If you bring in precast wall sections, all you do is prop them until they are held together by the roof or the concrete slab of the next floor. Site operations are much simpler and safer as there is less clutter. Occupational health and safety problems are minimised,” he says.
Chen says that over the past five years there has been greater industry acceptance of the use of precast in the construction of warehouses, apartments and shopping centres, plus a conscious move toward ‘greener concrete’. “Other environmental benefits include initiatives to reduce energy use in manufacturing, less noise, lower air pollution and the incorporation of recycled materials, such as fly ash from coal-fired power stations,” Chen says.
The future of precast
Peter Webb, marketing manager of Hanson Precast, says a major development in recent years has been the automation of factory production lines for precasting.
Precasting tables used in producing the flat grey panels that are the backbone of the industry had formerly all been stationary. However, now there are two automated plants in Sydney, and one each in Queensland and WA, where the tables now move through the plants on short motorised posts. He says Boral and Hanson lead the field in automation.
“All companies involved see such processes as benefiting from a growth in demand from the housing market for concrete wall panels to replace bricks,” Webb says.
The move over the past decade by brick makers to produce common bricks that are painted or stained has seen a marked decline in the production of faced bricks. This trend has opened the way for more efficient construction using precast concrete walling, which can also be painted or stained, instead of bricks.
The big innovation in precast concrete in the next five years will be the rise of staining and mould liners. Staining technologies, introduced into Australia by Canadian based company Nawkaw, allow the colouring of concrete to meet consumer needs and offer a 25-year guarantee. Mould liners, brought to Australia by the German company Reckli, allow slabs to have the appearance of brickwork, slate or stones.
Incorporating these techniques into precast concrete walling will allow them to replace bricks in some facets of building. The increasing problem of insufficient trades people will also encourage the use of precast for walls.
