Precast concrete

Precast concrete is poured into a reusable mould or ‘form’, cured in a controlled environment, and lifted into place. Precast concrete offers durable, flexible solutions for floors, walls and even roofs in every type of domestic construction from individual houses to multistorey apartments. Precast concrete’s high initial embodied energy can be offset by its long lifespan and high potential for reuse and relocation.

Common production methods include tilt-up (poured on site) and precast (poured off site and transported to site). Each method has advantages and disadvantages and choice is determined by site access, availability of local precasting facilities, required finishes, and design demands.

Photos: Silvester Fuller, DACA.

The advantages of precast concrete include:

• speed of construction
• reliable supply — usually made in purpose-built factories and not affected by weather
• high-level performance in thermal comfort, durability, acoustic separation, and resistance to fire and flood
• inherent strength and structural capacity able to meet engineering design standards for housing
• high flexibility in form, shape and available finishes
• ability to incorporate services such as electrical and plumbing in precast elements
• minimal waste; low wastage rates on site and most waste in the factory is recycled
• safer sites from less clutter
• ability to incorporate waste materials such as fly ash
• ease of deconstruction, reuse or recycling.

The disadvantages of precast concrete are that:

• each panel variation (especially openings, bracing inserts and lifting inserts) calls for complex, specialised engineering design
• it is often more expensive than alternative construction systems (however, this can be offset by reduced construction times, earlier access by following trades, and simplified finishing and services installation)
• building services (power, water and gas outlets; conduits and pipes) must be accurately cast in and are difficult to add or alter later; this requires detailed planning and layout at design stage when plumbing and electrical trades are not usually involved
• detailed accurate design and pre-pour placement of building services, roof connections and tie-down are essential
• panel connection and layout for lateral bracing requires detailed design
• erection requires specialised equipment and trades
• high-level site access and manoeuvring room for large floats and cranes free of overhead cables and trees is essential
• temporary bracing requires floor and wall inserts that have to be repaired later
• cast-in services are inaccessible and more difficult to upgrade
• it has high embodied energy.

Photo: Silvester Fuller, DACA

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.

Appearance

Precast concrete is available in an almost endless variety of shapes, colours, textures and finishes. 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).

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.

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.

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 3610.1 -2010 Formwork for concrete, and its Supplement 1, Blowhole and colour evaluation charts, apply to off-form concrete practices and finishes.

Structural capability

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.

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.

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.

If surface condensation or dew-point formation does occur, this may cause unsightly mildew and stains, but structural damage in precast concrete is unlikely. Insulation, ventilation and appropriate heating reduce this risk.

Thermal mass and insulation

Precast concrete can provide internally exposed thermal mass at a low cost without the need for insulation or other linings. At 2060 kJ/m³kelvin (K), dense concrete has the highest volumetric heat capacity of any commonly used housing material (for example, brick 1360kJ/m3K).

High insulation levels are readily achieved with precast concrete. 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 (refer to Insulation). This can easily be increased by selecting foam of 80mm or 100mm. An R value calculator for precast concrete is available at the National Precast Concrete Association of Australia.

Insulation design and detailing should avoid thermal bridging by ensuring continuous insulation between the internal and external skins in all external walls. Thermal bridging through continuous concrete ribs and edge beams, or failure to ensure all junctions have continuous insulation, are common design faults.

Proprietary composite or thermoplastic ties should be used between skins. If using steel ties, the effects of thermal bridging should be considered. Extruded or expanded foam insulation may be used.

Source: Composite Global Solutions

Sound insulation

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 Weighted Sound Reduction Index (Rw) rating required by the National Construction Code.

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 service 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.

Fire and pest resistance

Precast panels are highly fire resistant with zero flammability on both external and internal surfaces. The National Construction Code specifies the fire-resistance level required for structural adequacy, integrity and insulation for specific fire resistance periods (FRP). Australian Standard AS 3600-2018 Concrete structures, 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 with precast concrete, 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.

Environmental impacts

Lifecycle assessment

Lifecycle assessment by the Cement Concrete & Aggregates Australia concluded that there is no significant lifecycle difference in terms of energy use and greenhouse gas emissions between alternative construction systems used in 5 building types studied, including a detached house (CCAA 2001).

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 lifespan using standard assumptions about occupancy rates and operational energy use in accordance with Australian Standard AS/NZS ISO 14040:1998, Environmental management life cycle assessment — principles and framework. The study did not take into account the impact of climate change. Such consideration might indicate adverse outcomes for high-mass construction.

Eco-comparison websites can help you select options with low lifecycle environmental impact (see References and additional reading).

Embodied energy and emissions

The embodied energy of precast concrete is arguably its most significant environmental impact. However, this can be balanced with low energy use in a well-designed home. In addition, you can aim to design precast structures to be reused, extended or retrofitted to ensure that their initial embodied energy is written off over their long lifespan.

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 take 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 sent to 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 reduces waste.

Transport to site

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.

The 2 most common production methods — precast (poured offsite and transported to site) and tilt-up (poured on site) — each has advantages and disadvantages. Your choice should be determined by site access, availability of local precasting facilities, the standard of finishes and design requirements.

Design and detailing

Lifting is generally the most critical design consideration and dominates the design of the panel in most situations.

The design and planning phases of a precast or tilt-up project are among the most crucial for its success. The entire design and construction procedure for the building must be worked out in advance of the first pour and factored into the design of each panel including size (casting table or space and transportability), detailed set-out of services, lifting and temporary bracing inserts, and lifting procedures (erection sequence, crane positions and lateral bracing).

The building should be designed specifically for the construction method. Effective planning requires continuous involvement and interaction between every member of the design and construction team from design to completion.

Openings

Almost any shape or size of connection can be achieved with detailed engineering design. The greatest structural loads usually occur during lifting and transport.

Openings required for alteration and addition can often be simply added (subject to engineering advice) by cutting or removing existing panels.

Photo: Nic Bailey

Panel sizes

Panel area should be maximised where possible (20m² or larger) to reduce labour, truck movements and cranage costs. Always consider the available casting area onsite for tilt-up and casting table size for offsite.

The rule of thumb limit for flat lift-out of beds is 10 tonnes total weight, although some tilt tables and transport methods used by precast plants allow greater sizes. The possible weight and size always depend on crane size and area onsite.

The maximum height of panels transported vertically in ‘A’ frames is generally between 4.0m and 4.5m. Length is determined by trailer configurations and is generally a maximum of 12m. Transportation limitations are set by state road authorities and may differ between states.

Casting

Accurate positioning of services and inserts is critical. Cast-in services require that:

• all plumbing be silver soldered with minimum joins, or some other system with proven durability used
• services conduits be oversized and provide access points for future upgrading or maintenance
• the plan (and photographs) of the services layout be retained for reference to avoid potential future damage when drilling for attachments
• all plumbing and particularly hot water lines be lagged to allow for thermal expansion and movement, and prevent heat loss
• the window supplier be notified that windows are for precast openings and must conform to precise dimensions
• precast window formwork and templates be accurate, square and sized to the window supplier’s actual dimensions
• rebates or cast-in flashing details are well resolved
• adequate curing times be allowed for on-site pours
• the concrete strength be certified at pour and before lifting (slump test and test cylinders).

While dependent on the precast contractor, accuracy levels with precast systems are usually extremely high due to the quality of reusable formwork and workshop conditions.

A slab not perfectly level, or twisted or warped, can cause problems during tilt-up. Formwork frames must be absolutely square to ensure clean, tight corner junctions and plumb walls.

Off-site precasting

Off-site precasting is carried out in a controlled environment to avoid weather-related variables. This minimises waste and delivers the highest level of quality assurance in the range of finishes and dimensional accuracy. It also reduces on-site materials storage and waste.

Controlled pouring environments in the factory also reduce concrete curing times through the use of temperature control and advanced mix design, including low water to cement ratios. These controlled conditions can reduce the amount of cement required (the highest source of embodied energy) to achieve adequate strength to withstand transport and erection loads. Tilting casting tables are also beneficial, because the greatest structural loads for precast concrete occur during lifting, transport and erection.

Precasting allows the use of high-quality formwork including vibrating casting tables and accurate positioning of inserts (lifting points and structural steel), insulation, services (plumbing and electrical conduits), openings, flashings, and termite barriers. Similar levels of quality assurance are difficult or more expensive to achieve on site, particularly on smaller projects.

Tilt-up or on-site construction

Tilt-up wall panels are cast on a horizontal surface and require simply tilting to vertical in their final location. This definition is expanded slightly to include all on-site precasting where horizontally cast panels are lifted by crane and moved to their final location.

This is often a more practical solution on small sites or where transport access or site constraints preclude the use of off-site precast methods.

Precast panels are usually formed up on existing concrete floor slabs either as ‘stack casting’ (one on top of the other) or as individual slabs near their final point of erection.

Bond breaker compounds are used to ensure that stack cast slabs are separable after curing. Bond breaker failure (often due to poor application) can lead to considerable waste, cost and delay.

While tilt-up overcomes transport and other logistical problems, it is slower than precast because walls cannot be poured before or during floor construction. Floor space is often limited or shorter than the walls that surround it, necessitating additional joints.

The on-site process can restrict other building work while the concrete cures (it cures only when kept moist). Curing times are often reduced by using stronger mixes (more cement and therefore higher embodied energy) to allow concrete to achieve adequate strength in shorter periods under less controllable site conditions. Additionally, small imperfections in the finish or variations in slab level result in corresponding lower grade finishes or ‘twists’ in finished walls. In other words, quality assurance is more challenging on site.

Photo: Suntech Design

Transport

The erection sequence determines the order of transport. Careful planning of the erection sequence to minimise crane positions reduces transport and cranage costs.

Site access for trailers and cranes is a critical consideration at the design stage and may preclude the use of precast construction.

Check the delivery route to site before designing panels, and limit wall heights (including truck or float deck height) to fit under bridges, overhead cables and other height restrictions. Site manoeuvring requirements for trailers and cranes can reduce these allowances.

The build process

Specialist erection contractors and specialised lifting equipment are essential for precast work. Occupational health and safety (OH&S) risks can be extreme if lifting procedures, temporary bracing and panel connections are not professionally conducted.

Photo: Silvester Fuller, DACA

Footings

In residential construction, precast concrete generally uses similar footing systems to other high mass construction systems.

With appropriate engineering design, walls can act as beams, and span between piers or piles drilled or driven to material of adequate or equal bearing value on sites with reactive soils; this can reduce footing size and simplify construction. Precast flooring can span long distances with less need for supporting columns.

Joints and connections

Continuous welding of steel angle inserts is often used to join precast walls. The inserts are typically rebated and plaster set internally. Junctions are often visually expressed externally. Welded joints are problematic during deconstruction. For this reason, concealed or recessed bolted connections are preferable.

Lifting inserts should remain accessible and structurally sound for future relocation. A deconstruction schedule should be prepared that includes plans and photographs of each panel after erection and notes any damage to lifting inserts.

Other joint and connection considerations include:

• waterproofing and fire rating to the same specification as walls
• ensuring the continuity of insulation
• maintaining structural integrity as specified in the engineer’s drawings
• termite proofing.

Fixings

Fixings including bracing and lifting inserts (chosen from a wide range of commercially available, purpose-designed proprietary products) are accurately cast-in during pouring.

These are generally designed to be cast just below the slab surface and located with void formers or removable screw-in plugs with neoprene or other flexible antennae protruding to the surface of the slab so as not to interfere with trowelling.

Safety issues

Ensure adequate concrete strength is achieved before lifting and have a well-prepared site with pre-planned crane positions, wall erection sequence, and bracing layout. Use specialist experienced tradespeople for erection.

Ensure all bracing is on site and in good order before erection. Use only certified proprietary lifting eyes and temporary bracing. Check all brace inserts and lifting eyes before and during erection. Ensure corners and junctions are waterproof and structurally sound.

• Eco-comparison websites
  –    Australian National Life Cycle Inventory Database
  –    Building Products Information Rating
  –    Ecospecifier Global
  –    Environmental Product Declaration Australasia
  –    Global GreenTag
  –    Good Environmental Choice Australia
• Austral Precast
• Cement Concrete and Aggregates Australia.
• Cement and Concrete Association of Australia (2001). Concrete panel homes handbook: a guide to the design and detailing of concrete panel construction for housing, Publication No. T54. St Leonards, NSW.
• Ministry of Business, Innovation and Employment (2004). TM 34 tilt-up technical
  manual [online document], Government of New Zealand.
• National Precast Concrete Association Australia (2009). Precast concrete handbook, 2nd edn, Blackwood, South Australia.

Learn more

• Read Concrete slab floors to explore this option for your flooring
• Look at Thermal mass to understand how concrete can contribute to good passive design
• See Waste minimisation for more ideas on how to minimise waste during construction

Authors

Principal author: Chris Reardon 2013