Autoclaved aerated concrete (AAC) is strong, lightweight and a good insulator.
AAC has been approved for a variety of commercial and residential construction. It is also know as aircrete or cellular concrete.
AAC also has many of the advantages of concrete, such as fire resistance and durability.
What is AAC?
AAC is produced by adding a foaming agent to aerate the concrete as it sets, cutting blocks or slabs from the resulting 'cake' with wires, and finally curing it with steam in an autoclave.
AAC is available in New Zealand in the form of blocks, and panels for walls, floors and roofs. The panels contain reinforcing steel. It integrates structure, insulation and fireproofing in one material.
AAC blocks are usually 'glued' together with thin-bed mortar or used as a cladding panel on horizontal battens over timber framing. AAC needs to be plastered and painted to be weathertight.
AAC has been around for 75 years but has only recently started to be popular outside Europe.
Key features of AAC
AAC is more expensive then conventional concrete on a volume basis because AAC blocks and panels are imported from Australia or Asia. It is also more expensive than timber framing.
AAC is lightweight - depending on the exact product, about one-fifth of the weight of conventional concrete.
Ease of construction
Using AAC panels can speed construction time. However, AAC blocks need to be laid by a skilled block layer. A badly laid wall can have reduced strength.
Even though it is lightweight, AAC is strong enough to carry relatively large loads, limited mainly by the light timber or steel structure they are fastened to. Using incorrect fixings (for example, expansion anchors) can cause it to crumble.
Durability and weathertightness
AAC buildings are durable and low maintenance, though they are not as strong as conventional concrete. AAC can be damaged by hitting it with a hammer. But minor damage is easily repaired.
AAC will allow moisture to penetrate, but it can be made water-resistant with plaster and paint.
AAC is relatively good at withstanding earthquakes because of its light weight. It needs to be reinforced with steel.
AAC is fire resistant. It will not ignite in a fire, though it can melt at extremely high temperatures.
AAC is fire resistant.
Is it compatible with passive design?
AAC is a very reasonably good thermal insulator when used as blocks. However, in cladding panels, it has more limited insulation value and it may not be enough to satisfy Building Code requirements for colder parts of New Zealand. AAC also provides good sound insulation.
It is not effective at storing heat - standard concrete is much more effective as thermal mass.
With AAC, doors and windows can be placed where needed to make maximum use of the sun's warmth and breezes for cooling.
Is it right for my site?
Topography and site impact
AAC is more suitable for level sites. Excavating a site will have an impact on its flora and fauna, and on stormwater runoff. Excavation can also increase the likelihood of erosion and soil instability.
For warmer regions of New Zealand, AAC can supply the minimum required by the Building Code for thermal insulation for exterior walls. Extra insulation is needed to reduce running costs over the life-time of the building.
From Smarter Homes
- Timber construction
- Light steel frame construction
- Concrete construction
- Insulated concrete formwork
- Earth construction
- Straw bale construction
- Construction site practice
- Exterior design
- Passive heating
- Passive cooling
- The Building Act
- Leaky buildings; this includes information about building for weathertightness
From other sites
You can find more information about AAC construction at the Cement and Concrete Association of New Zealand website.
Most home construction needs a building consent from a building consent authority (usually your local council). The Local Government website has links to local council websites.
You can buy New Zealand Standards relating to various construction systems from the Standards New Zealand website.
You can buy BRANZ publications about construction systems from the BRANZ website (click on the link to the BRANZ shop).