4 Frequent Questions About Autoclaved Aerated Concrete

6 October 2016
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Autoclaved aerated concrete--more commonly referred to as AAC--is a building material with a wealth of incredible benefits. Yet many people remain unfamiliar with even the basics of AAC. Whether you work in the construction industry, or are merely a curious amateur, if you would like to familiarize yourself with some fundamental information about AAC, read on. This article will provide answers to four commonly asked questions.

What exactly is AAC?

The basic ingredients of AAC are much the same as those used in any type of concrete:

  • aggregate (usually either gravel or quartz sand)
  • calcined gypsum
  • lime
  • cement 

What differentiates AAC from the garden variety of concrete is the fact that it contains a small proportion of aluminum powder. This powder is responsible for generating AAC's unique qualities. You see, when the aluminum powder comes in contact with water, it generates hydrogen gas. As this gas percolates through the wet concrete, it leads to the formation of small, regular pockets of air.

The air bubbles produced by the reaction of aluminum powder stay trapped inside of the concrete even after it has thoroughly dried. This greatly reduces the density of the resulting concrete. By adjusting the proportion of aluminum powder used in a given batch of AAC, a wide range of densities can be achieved.

What form does AAC come in?

AAC, unlike other concrete products, is not poured or formed at a particular job site. Rather it comes in the form of pre-cast units--either blocks or panels. Blocks are installed in much the same way as bricks, cinderblocks, or other forms of masonry. After a thin-bed mortar is applied, a block is laid down atop it, then another layer of mortar, and so on.

To create these uniform pieces, an AAC manufacturer first pours the concrete into large molds. Once the AAC has dried, the molds are removed, allowing the concrete to be cut down into units of the appropriate size. These pieces are then fully hardened inside an autoclave for several hours.

What are the benefits of AAC?

Perhaps the most immediate benefit of AAC is its incredibly light weight. This is a direct result of the fact that it contains such a high proportion of air--around 80 percent! All of those air bubbles also have the advantage of making AAC easier than traditional concrete to cut, drill, and shape. This makes the routing of such things as plumbing or electrical fixtures much simpler than it would be otherwise.

Furthermore, the incredible quantity of air inside of an AAC block greatly boosts its power of insulation. In other words, walls constructed out of AAC allow a much smaller amount of heat loss. The insulating capabilities of any building material are expressed in terms of its R-value; the higher the R-value, the better the insulation. AAC has an R-value of right around 1.0, whereas conventional concrete has a significantly lower R-value of 0.2.

How is AAC installed?

Both the block and panel forms of AAC are installed in a manner similar to that of bricklaying. A row of AAC blocks are first laid in place, the leveled, and then a thin layer of mortar is applied to the top, before putting the next row in place. Lengths of rebar may be installed vertically to help reinforce the wall at key locations.