Autoclaved aerated concrete
- Autoclaved aerated concrete (AAC) is concrete that has been manufactured to contain closed air pockets. AAC is one-fifth the weight of concrete.
- AAC is available as panels and blocks. AAC wall panels are typically used for cladding, but can also be loadbearing. AAC floor and roof panels are also available.
- AAC blocks can be used for loadbearing structures up to 3 storeys, and AAC panels can be used for cladding.
- AAC has medium thermal mass and provides very good thermal insulation.
- AAC is relatively easy to work with but can also be easily damaged. Specific construction techniques are required.
- AAC used externally should be coated with a vapour-permeable, water-resistant finish. Render to blockwork must be a proprietary mix compatible with the AAC substrate.
Understanding autoclaved aerated concrete
Autoclaved aerated concrete (AAC) is concrete that has been manufactured to contain closed air pockets. It comes in the form of panels or blocks. Lightweight and fairly energy-efficient, it is produced by adding a foaming agent to concrete in a mould, then wire-cutting blocks or panels from the resulting ‘cake’ and ‘cooking’ them with steam (autoclaving). The gas used to ‘foam’ the concrete during manufacture is hydrogen formed from the reaction of aluminium paste with alkaline elements in the cement.
AAC is durable and light, does not burn, is an excellent fire barrier and a good insulator, and is able to support quite large loads. The popularity of AAC in Australia has grown since the first production of AAC here in 1990, although the market remains dominated by one manufacturer. In Europe, AAC has a long history of development, having been in use since the 1920s.
Buildability, availability and cost
AAC is one-fifth the weight of concrete and comes in a variety of sizes. It is relatively easy to work with and can be cut and shaped with hand tools including woodworking tools. However, it requires careful and accurate placement – skilled trades and good supervision are essential.
AAC panels are easy to work with and faster to construct than blockwork, but must be handled carefully to avoid damage. Competent bricklayers or carpenters can work successfully with AAC blocks. Dimensional tolerances are very small when blockwork is laid with thin-bed mortar. Thick-bed mortar is more forgiving but is uncommon and not the industry preferred option. Very large blocks may require 2-handed lifting and be awkward to handle but can result in fewer joints and more rapid construction.
The construction process with AAC produces little waste as offcuts can be reused in wall construction. Good design that responds to the regime of standardised panel sizes encourages low-waste, resource-efficient AAC panel construction.
The cost of AAC is moderate. In Australia, AAC blockwork is competitive with other masonry construction and AAC panels are competitive with other cladding options.
Source: Autoclaved Aerated Concrete
AAC is light-coloured. It contains many small bubbles or voids that can be clearly seen when looked at closely. These air pockets contribute to the material’s insulating properties. There is no direct path for water to pass through the material; however, it can wick up moisture and an appropriate coating is required to prevent water penetration.
Photo: Maxiwall (© Big River Group)
The compressive strength of AAC is very good. Although it is one-fifth the density of normal concrete, it still has half the bearing strength, and loadbearing structures up to 3 storeys high can be safely erected with AAC blockwork.
The Australian Standards AS 3700-2018 Masonry structures and AS 5146.2:2018 Reinforced autoclaved aerated concrete design include provisions for AAC block and panel design. External AAC wall panels – which are not blockwork but are precast units – can provide loadbearing support in houses up to 2 storeys high. AAC panels and lintels contain integral steel reinforcement to ensure structural adequacy during installation and design life.
Increasingly, AAC is being used in Australia in its panel form as a cladding system rather than as a loadbearing wall. Entire building structures can be made in AAC from walls to floors and roofing with reinforced lintels, blocks and floor, wall and roofing panels available from the manufacturer.
AAC floor panels can be used to make non-loadbearing concrete floors that can be installed by carpenters.
Thermal mass and insulation
The thermal performance of AAC depends on the climate in which it is used. With its mixture of concrete and air pockets, AAC has a moderate overall level of thermal mass. It is most useful in warmer climates with high cooling needs. However, if used extensively (for example, for internal walls and flooring) AAC can be suitable for climates requiring higher levels of thermal mass.
AAC has very good thermal insulation qualities relative to other masonry, but generally needs additional insulation to comply with National Construction Code (NCC) requirements.
A 200mm thick AAC wall gives an R value rating of 1.43 with 5% moisture content by weight. With a 2 to 3mm texture coating and 10mm plasterboard internal lining, it achieves an R value of 1.75 (a cavity brick wall achieves 0.82). The NCC requires that external walls for all climates except alpine climate zones must achieve a minimum total R value of 2.8. For the most up-to-date requirements, check the National Construction Code.
To comply with NCC provisions for thermal performance, a 200mm AAC blockwork wall requires additional insulation.
Photo: Maxiwall (© Big River Group)
A texture-coated 100mm AAC veneer on a lightweight 70mm or 90mm frame filled with bulk insulation achieves a higher R rating than an otherwise equivalent brick veneer wall.
Relative to their thickness, AAC panels provide less insulation than AAC blockwork (for example, a 100mm blockwork AAC wall has a dry state R value of 0.86 and a 100mm AAC wall panel has a dry state R value of 0.68).
Photo: Paul Downton
With its closed air pockets, AAC can provide very good sound insulation. As with all masonry construction, care must be taken to avoid gaps and unfilled joints that can allow unwanted sound transmission. Combining the AAC wall with an insulated asymmetric cavity system gives excellent sound insulation properties.
Fire and pest resistance
AAC is inorganic, incombustible and does not explode; it is thus well suited for fire-rated applications. Depending on the application and the thickness of the blocks or panels, fire ratings up to 4 hours can be achieved. AAC does not harbour or encourage insects or other pests.
Durability and moisture resistance
The purposely lightweight nature of AAC makes it prone to impact damage, both during construction and once built.
The porous nature of AAC can allow moisture to penetrate to a depth but appropriate design (damp proof course layers and appropriate coating systems) can prevent this. When exposed to moisture, AAC does not easily degrade structurally however its thermal performance may suffer.
If the surface is protected to resist moisture penetration, AAC is not affected by harsh climatic conditions and does not degrade under normal atmospheric conditions. The level of maintenance required by the material varies with the type of finish applied.
A number of proprietary finishes (including acrylic polymer-based texture coatings) give durable, water-resistant coatings to AAC blockwork and panels. They need to be treated in a similar fashion with acrylic polymer-based coatings before tiling in wet areas such as showers. The manufacturer can advise on the appropriate coating system, surface preparation, and installation instructions to give good water-repellent properties.
Photo: Paul Downton
Toxicity and breathability
The aerated nature of AAC facilitates breathability. There are no toxic substances and no odour in the final product. However, AAC is a concrete product and calls for precautions similar to those for handling and cutting concrete products. Dust from AAC contains crystalline silica. These particles are small enough to penetrate deep into the lungs and can cause irreversible lung damage. It is advisable to wear personal protective equipment such as gloves, eye wear, and respiratory masks during cutting due to the fine dust produced by concrete products. If low-toxic, vapour-permeable coatings are used on the walls and care is taken not to trap moisture where it can condense, AAC may be an ideal material for homes for chemically sensitive people.
AAC has some significant environmental advantages over conventional construction materials, meeting longevity, insulation, and structural demands in one material. As an energy and material investment it can often be justified for buildings intended to have a long life.
Weight for weight, AAC has manufacturing, embodied energy, and greenhouse gas emission impacts similar to those of concrete. But based on volume, the impacts can be up to one-fifth that of concrete. AAC products or building solutions may therefore have lower embodied energy per square metre than a concrete alternative. In addition, AAC’s much higher insulation value reduces heating and cooling energy consumption.
Offcuts from construction can be returned to the manufacturer for recycling, or be sent out as concrete waste for reuse in aggregates; alternatively, the odd pieces can be used directly for making, for example, garden walls or landscape features.
Eco-comparison websites can help you select construction systems and materials with low lifecycle environmental impact (refer to References and additional reading).
Photo: Paul Downton
Using autoclaved aerated concrete
AAC is available in blocks of various sizes, or more commonly, as reinforced panels sold as part of a complete building system that includes floor and roof panels, and interior and exterior walls.
Wall panels are storey height, reinforced and mechanically fixed. AAC can also be used in panel form for floor and roof construction. AAC blocks are made to very exacting dimensions and are usually laid in thin-bed mortar that is applied with a toothed trowel, although more conventional thick-bed mortar can be used.
Design and detailing
All structural design should be prepared by a competent person, and may require preparation and approval by a qualified engineer. The AAC manufacturer can provide detailed technical advice that, when followed, should help to ensure successful use of the product.
Any AAC wall can be designed to be loadbearing.
AAC is soft enough to be cut with hand tools. Niches can be carved into thicker walls, corners can be chamfered or curved for visual effect and you can easily make channels for pipes and wires with an electric router.
Use appropriate dust reduction strategies with all carving and cutting, and wear appropriate personal protection equipment at all times.
Photo: Paul Downton
Exterior surface coating systems for AAC panels must meet the requirement of Australian Standard AS5146 Part 3 – 2.8.4 and be warranted by the manufacturer. AAC blockwork can accept cement render, but manufacturers recommend using a proprietary render mix compatible with the AAC material substrate, with lower strength than conventional renders. All renders should be vapour permeable (but water resistant) to achieve a healthy breathable construction. All external coating finishes should provide good UV resistance, be vapour permeable, and be proven suitable for AAC. Consult the manufacturer’s literature for further information on coatings.
The build process
All masonry construction has to comply with the NCC and relevant Australian Standards.
AAC panels can be used as a veneer cladding over timber or steel-framed construction. The standard panel size is 600mm wide by 75mm or 50mm thick with lengths ranging from 1200mm to 3000mm.
AAC blocks can be used in a similar manner to traditional masonry units such as bricks: they can be applied as a veneer in timber frame or serve as one or both skins in cavity wall construction. The standard block size is 200mm high by 600mm long. Block thickness can range from 50mm to 300mm but for residential construction the most common block widths used are 100mm, 150mm and 200mm.
AAC panel and block construction requires level footings designed for full or articulated masonry in accordance with Australian Standard AS 2870–2011 Residential slabs and footings. For blockwork, stiff footings are preferred because the wall structure of thin-bed mortar AAC acts as if it were a continuous material and cracking tends not to follow the mortar beds and joints as it does in traditional masonry walling. Thick-bed mortar AAC block walls act more like traditional masonry, but are not the preferred method for AAC.
Frames may be required for various structural reasons. Structural framing must comply with Australian Standard AS 1684 for timber frames and the National Association of Steel Framed Housing (NASH) standard for residential and low-rise steel frames. Earthquake provisions tend to require multistorey AAC structures to have a frame of steel or reinforcement to withstand potential earthquake loads that may induce strong, sharp horizontal forces.
Photo: Paul Downton
Joints and connections
AAC panels are joined using thin bed adhesive: a dry mixed product made from a blend of cement, graded aggregates, and performance additives. Joints should be 2 to 3mm thick and fully filled. AAC panels should be bedded in 10mm mortar at the base of the wall when constructed on a slab edge set-down.
AAC blocks can be laid with conventional thick-bed mortar (approximately 10mm), however the manufacturer’s approved option is a proprietary thin-bed mortar. With this method, the procedure of laying the blocks is more like gluing than conventional brickwork construction. This is why many traditionally trained bricklayers may need some time to adjust to this different method of working. In addition, bricklayers are used to lifting bricks with a single hand and AAC blocks often require 2-handed manipulation. Although this may appear a slower construction process than laying masonry units, an AAC block is equivalent to 5 or 6 standard bricks.
All masonry walls are required to have movement or expansion joints at specified intervals. For AAC construction, movement joints must be provided at 6m horizontal centres maximum (measured continuously around rigid corners). Refer to the manufacturer’s guidelines for further information.
AAC has low compression strength. The use of mechanical fasteners is not recommended, as repeated loading of the fastener can result in local crushing of the AAC and loosening of the fastener. Proprietary fasteners are specifically designed to accommodate the nature of the material by spreading the forces created by any given load, whether it is a beam, shelf or picture hook. Proprietary fixings for AAC come with extensive guidance in product literature. If you are not sure, consult the project engineer or fastener manufacturer for guidance.
References and additional reading
- Eco-comparison websites
- Aroni, S, RILEM Technical Committee 78-MCA & RILEM Technical Committee 51-ALC (1993). Autoclaved aerated concrete: properties, testing, and design: RILEM recommended practice, 1st edn, E & FN Spon, London; New York.
- Comité Euro-International du Béton, Bave G (1978). Autoclaved aerated concrete: CEB manual of design and technology. Construction Press, Lancaster, United Kingdom.
- Hebel, Technical manuals.
- Lawson B (1996). Building materials, energy and the environment: towards ecologically sustainable development, Royal Australian Institute of Architects, Canberra.
- Robati M, Oldfield P, Akbar Nezhad A and Carmichael D (2018). Embodied carbon and capital cost impact of current value engineering practices: a case study, CRC for Low Carbon Living, Sydney.
- Safe Work Australia (2020). Crystalline silica and silicosis.
- Staines A (1993). Australian house building the easy Hebel way, 2nd edn, Pinedale Press, Caloundra, Queensland.
- Read Construction systems to get ideas about the different building systems for your home
- Look at Insulation for more ideas on how to slow heat and coolth transfer through your home
- To see how to reduce the amount of building waste from your site going to landfill, explore Waste minimisation
Principal author: Paul Downton 2013