#2: Material: Autoclaved Aerated Concrete
Also know as ACC, is a 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).
The popularity of AAC in Australia has grown since its introduction here 20 years ago, although the market remains dominated by one manufacturer, Hebel. In Europe AAC has a long history of development, having been in use for more than 70 years. It has a moderate embodied energy content and performs well as thermal and sound insulation, due to the aerated structure of the material and its unique combination of thermal insulation and thermal mass. It is light, does not burn, is an excellent fire barrier, and is able to support quite large loads. It is relatively easy to work with and can be cut and shaped with hand tools including woodworking tools.
AAC relatively easy to work with, and can be cut and shaped with hand tools including woodworking tools.
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. Wall panels are storey height, reinforced and mechanically fixed. AAC can also be used in panel form for floor and roof construction. It has a long life and does not produce toxic gases after it has been put in place.
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 (see Construction systems).
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.
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 & 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 & 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.