Autoclaved Aerated Concrete

Autoclaved Aerated Concrete

Autoclaved aerated concrete (AAC) is a product made with fine aggregates, cement, and an expansion agent that causes the fresh mixture to rise like bread dough. In fact, this type of concrete contains 80 percent air. In the factory where it is made, the material is molded and cut into precisely dimensioned units.

Cured blocks or panels of autoclaved aerated concrete are joined with thin bed mortar. Components can be used for walls, floors, and roofs. The lightweight material offers excellent sound and thermal insulation, and like all cement-based materials, is strong and fire resistant. In order to be durable, AAC requires some type of applied finish, such as a polymer-modified stucco, natural or manufactured stone, or siding.

Key aspects of AAC, whether designing or building with it, are described below:

Advantages

Autoclaved aerated concrete combines insulation and structural capability in one material for walls, floors, and roofs. Its light weight/cellular properties make it easy to cut, shave, and shape, accept nails and screws readily, and allow it to be routed to create chases for electrical conduits and smaller-diameter plumbing runs. This gives it design and construction flexibility, and the ability to make easy adjustments in the field.

Durability and dimensional stability. A cement-based material, AAC resists water, rot, mold, mildew, and insects. Units are precisely shaped and conform to tight tolerances.

Fire resistance is excellent, with eight inch thick AAC achieving a four-hour rating (actual performance exceeds that and meets test requirements for up to eight hours). And because it is noncombustible, it will not burn or give off toxic fumes.

The light weight means that R-values for AAC are comparable to conventional frame walls, but they have higher thermal mass, provide air tightness, and as just noted, are not combustible. That light weight also gives a high sound reduction for privacy, both from outside noises and from other rooms when used as interior partition walls.

Sizes

Both blocks and panels are available. Blocks are stacked similarly to conventional masonry, but with a thin-bed mortar, and panels are stood vertically, spanning full-story heights. For structural needs, grouted, reinforced cells and beams are placed within the wall section.

Installation, Connections, and Finishes

Due to the similarity to traditional concrete masonry, autoclaved aerated concrete units (block) can be easily installed by concrete masons. Sometimes, carpenters get involved in installation. Panels are heavier due to their size and require the use of a crane for placement. Depending on the type of finish selected, they can be directly adhered or mechanically attached to the face of AAC.

Sustainability and Energy Considerations

Autoclaved aerated concrete offers both material and performance aspects from a sustainability perspective. On the material side, it can contain recycled materials like fly ash and rebar, which may help contribute to credits in LEED® or other green rating systems. Further, it incorporates such a large quantity of air that it contains less raw material per volume than many other building products. From a performance perspective, the system leads to tight building envelopes. This creates an energy efficient envelope and protects against unwanted air losses. Physical testing demonstrates heating and cooling savings of roughly 10 to 20 percent compared to conventional frame construction. In consistently cold climates, the savings may be somewhat less because this material has lower thermal mass than other types of concrete. Depending on the location of manufacturing relative to the project site, AAC may also contribute to local materials credits in some green building rating systems.