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To read what manufacturers and distributors say about it, you'd think autoclaved aerated concrete (AAC) was some kind of new, space-age environmental miracle.
Although it certainly has some nifty properties, AAC isn't new and isn't miraculous--but it's certainly popular in Europe, and has been for decades; according to one source, it accounted for 60% of all new construction in Germany in 2006. It has enjoyed pretty flat market share (of near zero) here in the U.S., though, since it was first introduced in the 1990s.
Is there space for AAC in the U.S. market? Should the green building community be working to make space?
AAC is similar to other concrete types, except that it contains no aggregate; sand or fly ash is included, with aluminum powder added to react with one of these ingredients and "leaven" the concrete, creating tiny bubbles just like baking soda does when it reacts with the buttermilk in your muffin batter. (Your muffins are full of carbon dioxide bubbles, but AAC is full of hydrogen bubbles.)
[Note: Robert Riversong points out in comments that sand is aggregate, which I also thought when I started researching it, but after some more digging, my understanding is that the sand is used as a reactant and is therefore not considered aggregate in AAC. For more, see here.]
The concrete is poured into molds, left to rise, and then "baked" in an autoclave, which uses steam and pressure to complete the chemical reactions and speed up the curing process significantly--completing in hours rather than weeks. The resulting blocks are so full of bubbles that a block of the same size has about one-fifth the material required by regular concrete.
Like conventional concrete masonry units, AAC is sold in a variety of block shapes and sizes, but unlike conventional units, most don't have cores. They are porous and light, like muffins, but not hollow.
The main advantage of AAC when it was first developed in Sweden in the early 20th century was simple: it wasn't wood. It's still not wood, but in North America (unlike in Sweden at the time and in most of Europe now), wood is still plentiful and cheap.
Compared with conventional concrete, AAC still has advantages, though:
In a report written for UC–Davis (PDF), Stefan Schnitzler finds few disadvantages to AAC. Here are the two demerits on his list:
We would like to add a few drawbacks that we've found:
That said, AAC does appear to have significant advantages for applications where conventional concrete would normally be the best material--like in the American Southwest and in other climates where thermal mass can increase the "effective" or "mass-enhanced" R-value of the wall. Even then, its performance may still be outmatched by that of insulated concrete forms, depending on the needs of the client.
Unfortunately, much of the information we have on AAC performance in the U.S. comes from manufacturers. We'd like to hear some empirical evidence from the field.
Are you using AAC on any of your projects?
If you've used it, how did it perform? If not, what would it take for you to try it out?
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