Autoclaved aerated concrete

Stacked blocks of AAC (wrapped in foil)

Autoclaved aerated concrete (AAC), also known as autoclaved cellular concrete (ACC) or autoclaved lightweight concrete (ALC), was invented in the mid-1920s by the Swedish architect and inventor Johan Axel Eriksson. It is a lightweight, precast building material that simultaneously provides structure, insulation, and fire and mold resistance. AAC products include blocks, wall panels, floor and roof panels, and lintels.

It has been refined into a highly thermally insulating concrete-based material used for both internal and external construction. Besides AAC's insulating capability, one of its advantages in construction is its quick and easy installation, for the material can be routed, sanded, and cut to size on site using standard carbon steel bandsaws, hand saws, and drills.

Even though regular cement mortar can be used, 98% of the buildings erected with AAC materials use thin bed mortar, which comes to deployment in a thickness of ⅛ inch. This varies according to national building codes and creates solid and compact building members. AAC material can be coated with a stucco compound or plaster against the elements. Siding materials such as brick or vinyl siding can also be used to cover the outside of AAC materials.

AAC has been produced for more than 70 years, and it offers considerable advantages over other construction materials, one of the most important being its very low environmental impact.

• AAC’s excellent thermal efficiency makes a major contribution to environmental protection by sharply reducing the need for heating and cooling in buildings.
• AAC’s easy workability allows accurate cutting, which minimizes the generation of solid waste during use. Unlike other building materials, AAC can eliminate the need to be used in combination with insulation products, which increase the environmental impact and cost of construction.
• AAC’s high resource efficiency gives it low environmental impact in all phases of its life cycle, from processing of raw materials to the disposal of AAC waste.
• AAC’s light weight also saves energy in transportation. The fact that AAC is up to five times lighter than concrete leads to significant reductions in CO₂ emissions during transportation. In addition, many AAC manufacturers apply the principle of producing as near to their consumer markets as possible to reduce the need for transportation.

Raw materials

Unlike most other concrete applications, AAC is produced using no aggregate larger than sand. Quartz sand, lime, and/or cement and water are used as a binding agent. Aluminum powder is used at a rate of 0.05%–0.08% by volume (depending on the pre-specified density). When AAC is mixed and cast in forms, several chemical reactions take place that give AAC its light weight (20% of the weight of concrete) and thermal properties. Aluminum powder reacts with calcium hydroxide and water to form hydrogen. The hydrogen gas foams and doubles the volume of the raw mix (creating gas bubbles up to 3mm (⅛ inch) in diameter). At the end of the foaming process, the hydrogen escapes into the atmosphere and is replaced by air.

When the forms are removed from the material, it is solid but still soft. It is then cut into either blocks or panels, and placed in an autoclave chamber for 12 hours. During this steam pressure hardening process, when the temperature reaches 190° Celsius (374° Fahrenheit) and the pressure reaches 8 to 12 bars, quartz sand reacts with calcium hydroxide to form calcium silica hydrate, which accounts for AAC's high strength and other unique properties. After the autoclaving process, the material is ready for immediate use on the construction site. Depending on its density, up to 80% of the volume of an AAC block is air. AAC's low density also accounts for its low structural compression strength. It can carry loads of up to 8 MPa (1,200 PSI), approximately only about 10% of the compressive strength of regular concrete.

Since 1980, there has been a worldwide increase in the use of AAC materials. New production plants are being built in the USA, Eastern Europe, Israel, China, Bahrain, India, and Australia. AAC is increasingly used by developers, architects, and home builders.

History

The material was perfected in the mid-1920s by Dr. Johan Axel Eriksson, an architect working with Professor Henrik Kreüger at the Royal Institute of Technology. It went into production in Sweden in 1929 in a factory in Hällabrottet and became very popular. In the 1940s, the trade mark Ytong was introduced, but it was often referred to as "blue concrete" in Sweden due to its blueish tinge. This version of Ytong was produced from alum slate, whose combustible carbon content made it beneficial to use in the production process. The competing concrete brand Siporex used other raw materials. However, the slate deposits used for Ytong also contain uranium, which makes the material give off small amounts of radioactive radon gas to the surrounding air. In 1972, the Swedish Radiation Safety Authority pointed out the unsuitability of a radon-emitting construction material, and the use of alum slate in the production of Ytong ceased in 1975. Ytong produced after 1975 has used raw materials without the uranium content.

See also

• Concrete masonry unit

References

• Autoclaved Aerated Concrete
• Masonry Magazine, June 2008
• Green Building
• The Building of Syon Abbey
• Toolbase.org

External links

• Portland Cement Association's information on construction with AEC/ACC
• Aircrete Products Association