Precast concrete offers durable, flexible solutions for floors, walls and even roofs in every type of domestic construction from individual cottages to multi-storey apartments. Concrete’s high initial embodied energy can be offset by its extended life cycle (up to 100 years) and high potential for reuse and relocation.
An almost endless variety of shapes, colours, textures and finishes is available for precast concrete. It can be moulded and shaped to suit almost any design or form.
Simple surface treatments are:
• rebating and grooving
• surface coatings
• cement-based renders.
Other more complex treatments can also be used, such as:
• form liners
• oxide colouring
• applied finishes such as etching, grit-blasting, honing, polishing and exposed aggregate (off-site only where highly alkaline wash-down water and sediment can be recycled to avoid pollution of waterways).
Minor variations in finish between design intent or specification and the actual outcome are common. Assessment of samples and prototypes is highly advisable. Established, professional precast companies usually deliver quality assured finishes to the standard specified.
Complex or detailed methods and finishes require a high degree of quality control and are often best handled under the controlled conditions of an off-site precast factory.
The detailing of joints, inserts, openings and fixings is critical. Typically, each precast panel is connected to the next or other building structure with metal components and anchors.
Joints between panels are usually filled with sealant to allow for expansion and contraction. Internal joints can be concealed behind abutting walls or, more usually, are set using typical plasterboard methods. Careful detailing is required to achieve a stable, watertight and visually pleasing result.
Australian Standard AS 3610-1995, Formwork for concrete, and its Supplement 1, Blowhole and colour evaluation charts, apply to off-form concrete practices and finishes.
Structural precast concrete panels are a strong, durable and versatile building material, particularly suited to Australia’s harsh climatic extremes and requiring minimal maintenance. They can be engineered to meet the structural needs of every type of domestic construction.
Concrete is flood and fire resistant and doesn’t shrink, rot or distort. It gains strength as it ages and in structural terms is ideally suited to the unpredictable conditions associated with climate change.
The inherent structural properties of precast walls mean that they do not require additional bracing to resist racking loads and that simple design of cross walls and junctions can provide adequate lateral bracing. Tie-down inserts to resist cyclonic winds are simple to design, install and connect.
Precast finishes are highly impact resistant, withstand wear and tear, and require minimal repairs and maintenance.
Precast delivers high standard internal finishes off the form (mould providing internally exposed thermal mass at a low cost without the need for insulation or other linings.
At 2060 kJ/m2Kelvin (K), dense concrete has the highest volumetric heat capacity of any commonly used housing material (e.g. brick 1360 kJ/m2K). (see Thermal mass)
High insulation levels are simply achieved.
Insulated, 220mm-thick, precast sandwich wall panels (70mm concrete externally, 50mm XPS foam, 100mm concrete internally) commonly used in residential construction achieve a total R-value of 2.42. This can easily be increased by selecting foam of 80mm or 100mm.
The National Precast Concrete Association, in conjunction with Adjunct Assoc. Prof Terrance Williamson from the University of Adelaide, has developed a mass enhanced R-value calculator. The calculator recognises the thermal mass benefits of the insulated precast concrete of sandwich panels and offers Building Code of Australia (BCA) compliance.
The R-value calculator and the supporting industry standard are available on the website of the National Precast Concrete Association of Australia, https://nationalprecast.com.au.
Insulation design and detailing should avoid thermal bridging by ensuring continuous insulation between the internal and external skins in all external walls. Bridging through continuous concrete ribs and edge beams or failure to ensure all junctions have continuous insulation is a common design fault.
Precast concrete provides one of the highest levels of acoustic separation of any common housing construction system for both internal and external walls.
Solid construction performs well above the minimum Rw45 rating required by the BCA. Single element walls and floors of solid construction (such as a 150mm thick concrete wall panel) have Rw ratings as high as 55.
Joints and openings must be detailed properly to maintain sound ratings. Precast concrete houses have fewer joints, and dimensional accuracy allows for the snug fitting of acoustically sound windows and doors.
Because services outlets are cast into wall panels, sound transmission weakness around switches and power outlets is avoided and transmission through cavities or air spaces is prevented.
Precast panels are highly fire resistant with zero flammability on both external and internal surfaces. The BCA specifies the fire resistance level required in various applications in terms of the fire resistance periods (FRP) for structural adequacy, integrity and insulation.
AS 3600-2009, Concrete structures (Section 5, Design for fire resistance), gives methods for determining the various FRPs for concrete walls. Precast concrete panels must comply with the following requirements:
• Adequacy: to the same level of FRP as structural insulation while satisfying AS 3600, Clause 5.7.4.
• Integrity: must attain the same level of FRP as structural insulation.
• Insulation: effective concrete wall thickness of 80mm achieves a 60 minute FRP and a solid 150mm thick panel achieves a 180 minute FRP.
Joints between panels must also meet the appropriate FRPs. Certified data on the performance of proprietary sealants should be provided by the sealant manufacturer.
The use of woven stainless steel mesh or other termite barriers is strongly advised, to prevent termites accessing the roof structure via the insulation core. These barriers are placed across the insulation between external concrete skins at the base of the wall.
Durability and moisture resistance
Dense precast concrete is generally not subject to rising damp or structural damage from condensation or dew-point formation. However, condensation problems can arise in any building due to temperature, humidity and ventilation conditions that encourage dampness and mildew.
No type of construction is immune but precast concrete is rarely affected by internal or interstitial condensation (i.e. from water vapour passing through the material).
Surface condensation or dew-point formation is more common and, while often causing unsightly mildew and stains, structural damage in precast concrete is unlikely.
Insulation, ventilation and appropriate heating reduce this risk.
The life cycle assessment by the Cement Concrete & Aggregates Australia (CCAA) concluded that there is no significant life cycle difference in terms of energy use and carbon emissions between alternative construction systems used in each of five building types studied (including a detached house).
Construction systems examined included:
• timber floor, timber frame with cladding and plasterboard, steel roof
• slab-on-ground, brick veneer with terracotta tiles
• slab-on-ground, double brick, rendered internally, cement tiles
• slab-on-ground, tilt-up walls lined with plasterboard and battens, cement tiles
• slab-on-ground, tilt-up walls, cement tiles.
The study was conducted over a 50, 75 and 100 year life span using standard assumptions about occupancy rates and operational energy use in accordance with AS/NZS ISO 14040:1998, Environmental management life cycle assessment — principles and framework (CCAA 2001).
The study did not take into account the likely impact of climate change over life cycle. Such consideration might indicate adverse outcomes for high mass construction. These aspects of life cycle assessment are examined in Thermal mass and Construction systems.
As with other common high mass construction systems such as brick, the embodied energy of precast concrete is arguably its most significant environmental impact (see Embodied energy). Detailed analysis of the life cycle benefits of high mass construction is recommended for each application.
The life expectancy of concrete structures is more than 100 years. Such a life expectancy requires that precast structures be designed to be reused, extended or retrofitted to ensure that their initial embodied energy is amortised over their life span. Design for deconstruction can also capitalise on the capacity of precast concrete to deliver the best life cycle embodied energy outcomes of all high mass construction systems. (see Construction systems; Thermal mass)
The use of recycled materials such as fly ash or recycled aggregate in precast concrete reduces embodied energy while improving strength and durability.
Precast products using calcium-based Portland cement release carbon dioxide as they cure. Emerging cement technology (not yet commercially available) using magnesite-based Portland cement alternatives actually takes in carbon dioxide as it cures.
Other Portland cement substitutes with low embodied energy are currently available.
Reuse and recycling
Careful design and detailing of precast structures increases their potential for relocation and reuse. Alternatively, they can be simply renovated internally, conserving resources and reducing waste and landfill.
Careful design of precast panels for disassembly can facilitate reuse or recycling when the building reaches the end of its useful life. Precast concrete elements can be crushed and reused as aggregate for new concrete or for road bases or construction fill, creating economic and environmental savings.
Recycled aggregate and steel can and are commonly used in precast concrete. Designers should specify their inclusion to ensure best outcomes. Waste materials (such as slag and fly ash) that would otherwise be used in landfill should also be specified and used. These methods can reduce cost while adding to concrete strength.
Controlled mix environments and curing options can overcome water absorption problems associated with the use of recycled aggregate. Stormwater and rainwater can be used in the precast concrete mix design, thereby reducing mains water consumption and allowing highly alkaline wastewater to be recycled into the mix.
Precast concrete methods favour the reuse of formwork, and off-site manufacturing allows most manufacturing waste to be recycled. Precise quantification and dimensioning further reduce waste.
Materials used by precast manufacturers are usually sourced locally for cost reasons and transport limitations. Precast elements are mostly locally manufactured and transported short distances to sites. This often reduces the transport component of embodied energy relative to other high mass systems.
Buildability, availability and cost
Subject to workable site access and the availability of quality local precasting works or tilt-up contractors, precast concrete construction can provide practical solutions for most housing applications.
For individual homes, costs can range in the upper end of high mass solutions. Reduced construction times and need for on-site trades can offset this to some extent. Economies of scale and repetition can further reduce costs.