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Autoclaved Aerated Concrete Builders in this country now can take advantage of an innovative concrete material that Europeans have time-tested and adopted. Autoclaved Aerated Concrete (AAC) is a pre-cast, manufactured building stone made of all-natural raw materials. It is an economical, environmentally friendly, cellular, lightweight but structural material that features thermal and acoustic insulation as well as fire and termite resistance. AAC is available in a variety of forms, ranging from wall and roof panels to blocks and lintels. Although it has been a popular building material in Europe for over 50 years, AAC has only been introduced to the U.S. in the past few years. Details In 1914 the Swedes discovered a mixture of cement, lime, water and sand that expands by adding aluminum powder. A material like wood but without the disadvantages of combustibility, decay, and termite damage, the material was further developed to what we know today as autoclaved aerated concrete (also called autoclaved cellular concrete ACC). Portland cement is mixed with lime, silica sand, or recycled fly ash (a byproduct from coal-burning power plants), water, and aluminum powder or paste and poured into a mold. Steel bars or mesh can also be placed into the mold for reinforcing. The reaction between aluminum and concrete causes microscopic hydrogen bubbles to form, expanding the original concrete volume about five times. After evaporation of the hydrogen, the now highly closed-cell, aerated concrete is cut to size and form and steam-cured in a pressurized chamber (an autoclave). The result is a non-organic, non-toxic, airtight material that can be used in non- or load-bearing exterior or interior wall, floor, and roof panels, blocks, and lintels. According to the manufacturers, the production process generates no pollutants or hazardous waste. There is a variety of precast AAC units. Panels are available in thicknesses of 3" to 16", 24" wide, spanning up to 20'. Blocks come 24"L x 3"-12"W x 8"H. The cellular AAC not only features structural capabilities, but also excellent thermal, fire, and acoustical properties. Depending on the density, AAC has a compressive strength of 300 to 900 pounds per square inch, which makes it a structural material for low-rise buildings. With an R-value of 1.25 per inch, AAC does not replace glass fiber insulation, but it significantly outperforms conventional concrete at R-0.05. According to one manufacturer, an 8" AAC wall exceeds the ANSI/ASHRAE 90.2-1993 energy requirements of single family homes in the Orlando, FL area. AAC 4" wall panels have a fire rating of 4 hours (ASTM E-119) and acoustically perform with an STC rating of 41. Installation AAC is used similarly to common types of masonry or prefabricated building panels, however there are some significant differences. Its light weight makes shipping and handling more economical. Instead of commonly used 8"x16" concrete blocks (CMU), AAC can be laid by one mason in blocks up to 12"x 24" or more, cutting labor costs. The solid AAC blocks are laid with a thin-set mortar and can be sawn, drilled, routed, and nailed with common tools. The material is easy to finish, too. Exterior walls can be painted, plastered, and finished with a veneer or as specified by the manufacturer. Interior surfaces can be plastered, sheet rocked, tiled, painted, or simply left exposed. Benefits/Costs AAC is a good ecological choice in terms of manufacturing, construction, recycability, and indoor air quality issues. By altering the mixture proportions the manufacturer can manipulate insulation values and compressive strength, which makes AAC more versatile. Because it is lightweight and workable, AAC saves construction time, waste, and energy. Properly installed, higher initial material costs can be compensated by lower labor, finish, maintenance, and energy costs. As a base price $3-$4/sqft is common. Limitations Quality and color consistencies may be harder to obtain in AAC with fly ash. Untreated exterior walls should be finished when exposed to physical damage, dirt, and water, which can collect in the open pores. If installed in high humidity environments, interior finishes with low vapor permeability, and exterior finishes with a high permeability are recommended. As far as the conductivity of AAC is concerned, the heat transfer delay makes the material most favorable in climate zones where over a 24-hour period outdoor temperatures change from above to below the desired indoor temperature, which limits its applications in warmer climates, unless it is installed with insulation. Code/Regulatory According to the manufacturers, ASTM, UL and ULC approvals have been or are being issued. The National Evaluation Service (NES) has reviewed and accepted AAC products, and has issued National Evaluation Reports (NER). On February 1, 2003, America's four building-product evaluation services officially combined their operations under the International Code Council. The four "legacy" evaluation services that came together to form ICC-ES were the National Evaluation Service, Inc.; BOCAI Evaluation Services; ICBO Evaluation Service, Inc.; and SBCCI Public Service Testing and Evaluation Services, Inc. Information on these NERs can be obtained by contacting ICC at their web site. Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. Curtesy of the U.S. Department of Housing and Urban Development (HUD/FHA),
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