Autoclaved aerated concrete, or AAC, is concrete that has been manufactured to contain lots of closed air pockets. 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).
Autoclaved aerated concrete is light coloured. It contains many small voids (similar to those in aerated chocolate bars) that can be clearly seen when looked at closely. The gas used to ‘foam’ the concrete during manufacture is hydrogen formed from the reaction of aluminium paste with alkaline elements in the cement. These air pockets contribute to the material’s insulating properties. Unlike masonry, 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.
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 three storeys high can be safely erected with AAC blockwork. 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.
The Australian Standard AS 3700-2011, Masonry structures, includes provisions for AAC block design. External AAC wall panels — which are not blockwork but are precast units — can provide loadbearing support in houses up to two storeys high. AAC panels and lintels contain integral steel reinforcement to ensure structural adequacy during installation and design life
AAC floor panels can be used to make non-loadbearing concrete floors that can be installed by carpenters.
The thermal mass performance of AAC is dependent 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 performance. Its use for internal walls and flooring can provide significant thermal mass. The temperature moderating thermal mass is most useful in climates with high cooling needs.
AAC has very good thermal insulation qualities relative to other masonry but generally needs additional insulation to comply with Building Code of Australia (BCA) requirements.
A 200mm thick AAC wall gives an R-value rating of 1.43 with 5% moisture content by weight. With a 2–3mm texture coating and 10mm plasterboard internal lining it achieves an R rating of 1.75 (a cavity brick wall achieves 0.82). The BCA requires that external walls in most climate zones must achieve a minimum total R-value of 2.8.
To comply with building code provisions for thermal performance, a 200mm AAC blockwork wall requires additional insulation.
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, e.g. 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.
Loadbearing, insulating and capable of being sculpted, AAC has enormous potential as an environmentally responsible choice of building material.
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 a wall excellent sound insulation properties (see Noise control).
Fire and vermin 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 four hours can be achieved. AAC does not harbour or encourage vermin.
Durability and moisture resistance
The purposely lightweight nature of AAC makes it prone to impact damage. With the surface protected to resist moisture penetration it 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.
The porous nature of AAC can allow moisture to penetrate to a depth but appropriate design (damp proof course layers and appropriate coating systems) prevents this happening. AAC does not easily degrade structurally when exposed to moisture, but its thermal performance may suffer.
A number of proprietary finishes (including acrylic polymer based texture coatings) give durable and 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.
Plasticised, thin coat finishes are common, but here a non-plasticised thick coat render (10mm approximately) was used. Some variation in the amount of show-through of the blockwork pattern can be seen in this example, which also illustrates the use of glass blocks as well as more conventional windows.
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. 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 the chemically sensitive.
Weight for weight, AAC has manufacturing, embodied energy and greenhouse gas emission impacts similar to those of concrete, but can be up to one-quarter to one-fifth that of concrete based on volume. AAC products or building solutions may 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. AAC has some significant environmental advantages over conventional construction materials, addressing 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 (see Material use).
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.
Buildability, availability and cost
Although AAC is relatively easy to work, is one-fifth the weight of concrete, comes in a variety of sizes and is easily carved, cut and sculpted, it nevertheless requires careful and accurate placement: skilled trades and good supervision are essential. Competent bricklayers or carpenters can work successfully with AAC but 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 block sizes may require two-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 blockwork 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 to high. In Australia, AAC is competitive with other masonry construction but more expensive than timber frame. Lack of competition in the marketplace makes consumers highly dependent on one manufacturer.