¶  Aerated Concrete

Definition
A lightweight, precast concrete hardened in molds and cured in an autoclave.                 

Description
When aluminum powder is added to concrete during manufacture, the end product expands dramatically. This "foamed" concrete is then hardened in a mold and cured in a pressurized steam chamber (autoclave). The result is a lightweight, precast building material with uniform structure. Autoclaved aerated concrete (AAC) is used extensively in residential, commercial and industrial buildings throughout Europe.

AAC blocks measure 250 mm by 635 mm and are manufactured in thicknesses of 100, 200 and 25 mm. A 250 mm block weighs about 21 kg, significantly less than a standard concrete block with voids. The blocks are laid with thin-set mortar applied with a toothed trowel. AAC is also produced in reinforced and unreinforced building panels, which are sold as part floor and roof panels and exterior walls.

Although a familiar product in Europe, AAC is a recent product in North America. Some manufacturers in the US are considering the use of flyash in their manufacturing processes, if their plants are located close to sources of flyash. Flyash would replace the sand in the concrete mix.

One advantage of AAC is its higher insulation value. A 200 mm block wall has an R-10 value, which is not better than an insulated wood stud wall. However, the greater mass of the AAC wall means that heat transfer through the wall is delayed. This is an advantage in climates where the outdoor temperature fluctuates above and below the desired indoor temperature. In cold climates, the thermal lag is of little advantage. The AAC wall may also lower summertime cooling costs somewhat.

Buildings made from AAC are less prone to air leakage than buildings made from wood stud construction. At a reference pressure of 50 Pa, a house built with AAC was found to be 2.6 air changes per hour, tighter than typical wood or steel frame assemblies.

AAC is promoted as an alternative to interior concrete partition walls because of its superior acoustic properties and resource efficiency (it embodies one-fifth of the amount of concrete than a conventional concrete wall). AAC is a relatively inert material, and if care is taken in construction, it may prove to be better for occupant health than wood-stud construction. In addition, there is much less construction site waste generated from AAC than from conventional construction concrete products. A Canadian firm, TekBlok, manufactures AAC blocks that are used for interior walls. These walls are left unfinished, or are finished with paint, wallpaper or veneer. Because they are not fixed to the building structure, these walls are easily moved, and thus increase the adaptability of buildings.                                                  

Benefits

provides greater insulation

 increases sound absorption

 conserves cement

 protects against insect damage

 Limitations

 limited to lowrise structures or non-load-bearing walls in highrise structures

 must be protected from exterior weather by stucco or siding

Application
The density of AAC (about one-fifth that of conventional concrete) limits its strength, so it is suitable only for lowrise structures, or in partition walls and curtain walls in highrise structures.

AAC must be protected from the exterior environment. It should be finished on the exterior with plaster, acrylics, bricks or cedar shingles. In cold climates, AAC walls will need to be insulated. Interior walls can be finished with wallboard, tile, or water-based acrylic paint with high vapour-permeability. Utilities are installed through channels routed directly into the blocks.

If AAC is used to reduce contamination of indoor air, then care must be taken to select non-toxic sealants and to avoid trapping moisture where it can condense.

Experience
Hundreds of homes, primarily in the southern US, have been built using AAC. In this climate, AAC increased comfort levels and lowered energy costs for the owners. The performance of AAC in cold climates and commercial buildings is not well documented.

Cost
At least one US manufacturer claims that the houses constructed with exterior AAC walls cost 1 to 5% more than a conventional wood frame houses.

(Source :http://www.advancedbuildings.org/_frames/fr_t_building_dyn_buffer_zone.htm)