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Autoclaved Aerated Concrete

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Autoclaved Aerated Concrete
This report investigates the Autoclaved Aerated Concrete (AAC). AAC, or known as Autoclave Cellular Concrete (ACC) is an innovative concrete material that used for all types of structures. In this paper, its history (development), manufacturing process, some typical attributes and advantages, properties and limitations would be interpreted; in order to give an idea why it is so popular to be building material nowadays.
AAC is a precast building single material made with all-natural raw materials, and it is also a lightweight product to provide structure, insulation, and fire and termite resistance. AAC is available in many configurations include blocks, wall panels, floor slabs and roof panels, and lintels. It is a very popular construction material due to its features of sustainable, economical, efficient and relative environment- friendly.
In 1914, the Swedes discovered a mixture of cement, lime, water and sand that expands by adding aluminum powder. This kind of material was then developing, till mid-1920s, Dr. Axel Eriksson, an architect working with Professor Henrik Kreüger at the Royal Institute of Technology was invented AAC. It went into Swedish production in 1929 in a factory in Hällabrottet, and became quite popular. In the 1940s the trade mark Ytong was introduced, but often referred to as “blue concrete”. This version of Ytong was produced from alum slate, which due to its combustible carbon content was beneficial to use in the production process. However, in 1975, the production ceased; the slate deposits used for Ytong contains uranium which makes the material emit radioactive gas. Ytong was then manufactured innovated AAC without uranium by used of other raw materials. Since 1980, AAC has been used successfully thoughout most of the world and new production pants are being built in the US, China, Eastern Europe, India and Australia.

Figure 1 Detail view of an AAC block
Manufacturing Process
Autoclaved Aerated Concrete (AAC) is an inert, nontoxic substance that has manufacturing process. Figure 2 below shows the manufacturing process of AAC.

Figure 2 Manufacturing Process of AAC
Unlike the other concrete applications, AAC has no aggregate larger than sand is used. To manufacturing AAC, the basic raw materials are lime, Quartz sand and/or Portland cement, and the later one is used as a binding agent. Aluminum powder is used in5%-8% by volume (depending on the pre-specified density) and water, even a large proportion of a silica-rich material-usually sand or fly ash. Once raw materials are mixed into slurry and poured into molds, a series of chemical reactions take place. Since the aluminum powder react with calcium hydroxide and water to create millions of H2 gas bubbles leading the material expand its volume by twice times. During a setting time of 30 minutes-4 hour, the H2 is replaced by air. Hence, cut it to the pre specified shape of blocks is panels and moved into to autoclave chamber for a period of 8-14hours. This gives AAC its strength, rigidity, and dimensional stability. The final products are usually shrink wrapped in plastic and delivery to construction sites.
Attributes and Advantages of AAC
--AAC is only one fifth the weight of concrete, and it would float on water. As mentioned before, AAC is full of small voids so that it is lightweight; hence save delivery even construction time, manpower, and energy consumed by equipment used during construction are affected. Since AAC are precast, in addition, precisely dimensioned elements of AAC allow rapid construction.

Figure 3 AAC is floating on water
--AAC could provide very good sound insulation, with its closed air pockets. As with all masonry construction, care must be taken to avoid unfilled gaps that can allow unwanted sound transmission. Combining the AAC wall with an insulated asymmetric cavity system will provide a wall with excellent sound insulation property.
--Good fire and vermin resistance of AAC. AAC is inorganic and non-combustible with a UL fire rating up to 4 hours for a 4 inch non-load bearing wall. If it is exposed to fire situation, AAC emit non-toxic gases. AAC does not encourage vermin (e.g. termite) and will not rot or decay.
--AAC has unique thermal properties because of its cellular structure. AAC has a moderate overall level of thermal mass performance. It combines high heat loss resistance for a masonry material with excellent thermal inertia resulting in an overall ‘mass enhanced R-value’. AAC is also performing outstanding on gentle thermal motion in variable climates, especially in area that experience large day-night temperature swings.
--Toxicity and Breathability of AAC. There are no toxic substances and no odour in the final product. If low-toxic, vapor permeable coatings are used on the walls and care is taken not to trap moisture where it can condense, AAC may be an ideal material for homes for the chemically sensitive.
--Durability and moisture resistance of AAC. Generally speaking, a structure that does not need major repairs and renovation every 20 years or so, as many wood products require, can save money, inconvenience, energy and other resources. AAC has proven to be a very durable material. It will not rot, wrap, corrode, or otherwise decompose. AAC provides an extreme low maintenance building, saving considerable time and money. In despite of damaging, the repair measure is simple. If installed in high humidity environments, interior finishes with low permeability, and exterior finishes with a high permeability are recommended.
Environmental Impacts
AAC had been invented more than 70 years, one of the most important being its very low environmental impact. AAC has manufacturing embodied energy and green house emission impacts similar to other concrete, since its volume is one fifth of others. AAC products may have lower embodied energy per m2 than a concrete alternative. Its high resource efficiency gives it low environmental impact in all phases of its life cycle.
Figure 4 illustrates energy consumption of AAC compare with other traditional construction materials; AAC could be reduced by 50%.

Figure 4 Comparison of materials for energy consumption
AAC Strength
--Compressive Strength- The compressive strength of AAC increases while density increasing and it decreases with an increase in moisture content of AAC.

Figure 5 Relationship between compressive strength and moisture content
In addition, compressive strength also decreases with time of exposure as proven.
--Tensile Strength-The tensile strength of AAC is usually 1/4 to 1/6 of the compressive strength.
--Shear Strength-The shear rupture strength, could be assumed as 25-30% of the compressive strength. The value for pure punching shear can be taken as direct tension, if no information about shear strength.
Limitations and Design Considerations
Although AAC has plenty of advantages, its limitations also exist. The previous literature reported by Tietz, he mentioned: AAC chips easily; the face tends to get damaged easily; dowels are required between units; the anchorage of units should beware of pullout, especially for dynamic loads, etc. In these cases, suitable design considerations are required. AAC structural units need full-scale tests, such as dynamic repeated loads tests. Results of testing AAC indicate that the fields of application of this material must be chosen properly and carefully.
By investigating the above aspects of Autoclaved Aerated Concrete (AAC), it found that AAC provides significant environmental and other benefits for the builder and the building owner. The short and long-term effect of using AAC compared tom any other building materials results in lower energy consumption, reduced operating costs, greater safety and comfort, and more trouble-free building, and these attributes provide us a friendly environment.
1. WIKIPEDIA, Aerated autoclaved concrete, Viewed 12 October 2009, 2. ToolBase Services, Autoclaved Aerated Concrete (AAC), Viewed 12 October 2009, 3. Understanding Cement, Autoclaved aerated concrete(AAC, Aircrete), Viewed 12 October 2009, 4., Production, Viewed 14 October 2009, 5. William V. Abbate, Precast Autoclaved Aerated Concrete, Hebel USA, 6. AerBlock, AAC Attributes, Viewed 14 October 2009, 7. Technical Manual, Autoclaved Aerated Concrete, Viewed 14 October 2009, 8. PCA, Cement & Concrete Basics, Autoclaved Cellular Concrete, Viewed 15 October 2009, 9. Frank J. Powell, Stanley L. Matthews 1987,Thermal insulation materials and systems, ASTM, Baltimore…...

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