In some European countries 60% of new home construction uses autoclaved aerated concrete (AAC) blocks or panels for exterior wall construction. AAC is also a common building material in the Middle East, Far East, Australia, and South America, but most homeowners, builders, and concrete contractors in the United States have never heard of it. David Napier, director of marketing for TruStone America, Providence, R.I., says that AAC is one of the world's most produced building materials after concrete. Finally, AAC is beginning to catch on in the United States, where there are now three AAC manufacturing plants with several more planned. This represents a serious commitment since the cost of a plant to produce AAC blocks and panels is between $30 and $40 million.

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Blocks for constructing walls are solid except for holes to place vertical reinforcement. They are filled with high-strength grout afterwards. Workers place thinset mortar with a notched trowel to join blocks together.

AAC was invented in Sweden in the 1920s by architect, Johan Axel Ericksson, who was looking for an alternative to wood products, which were scarce after World War I. AAC is manufactured by placing silica in a ball mill and reducing it to a fine powder. The ground silica is mixed with water to form a slurry. Then limestone powder, portland cement, and a small amount of aluminum powder are added, and the mix is quickly cast into a mold. Within seconds the aluminum reacts with the lime and cement, initiating a chemical reaction that releases hydrogen gas. The gas forms bubbles up to 1/32 inch in diameter, causing the mixture to rise like a loaf of bread. The result is a material that is about 80% voids by volume.

After the mixture partially sets, it is still soft enough to be wire-cut into its final shapes as blocks or panels. The pieces are then placed in a 400º F steam-heated autoclave oven, pressurized at 13 atmospheres. Autoclaving converts the material to Tobermorite, a naturally occurring mineral found in limestone deposits whose crystal structure has some properties similar to those found in glass. When the product emerges, 8 to 12 hours later, it has all of its finished properties. AAC can bear loads up to 1100 pounds per square inch, yet its weight is 1/5 that of concrete.

THE BENEFITS OF BUILDING WITH AAC

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Autoclaved aerated concrete is manufactured as blocks or panels. Shown here are panels being placed for residential walls.

Unlike concrete masonry units, AAC blocks are solid, with no molded core holes. Standard blocks are 8 inches high, 24 inches long, and 4 to 12 inches thick. An 8x8x24-inch block weighs only 35 pounds, so it's easier to handle than a conventional concrete block. AAC can easily be tooled also, and even cut, drilled, and shaped with woodworking tools. Napier says that there is no other material commercially available that can match AAC for fire resistance. Four inches of AAC has a 4-hour fire rating, making it ideal in commercial buildings for encasing steel columns, surrounding elevator shafts, and for other fire-stopping requirements.

One important reason owners choose AAC for home construction is to save money on energy. Napier refers to it as “structural insulation,” and claims that an 8-inch-thick AAC wall is more energy efficient than a 6-inch stud wall with R-19 insulation. The energy efficiency of a building product is determined by its R-value, thermal efficiency, and the effect of thermal mass. The R-value of a material is a measure of its resistance to conductive heat transfer, which is the energy that moves from molecule to molecule. The R-value of a typical 8-inch-thick AAC wall is R-10; a 10-inch wall is R-12.5, and a 12-inch wall is R-15.

But the R-value of AAC is only one of the ways it saves energy. As with a concrete wall, the mass of an AAC wall stores thermal energy when the ambient temperature is higher than the wall temperature. That energy is released when the ambient temperature drops below the wall temperature. This moderating effect can result in significant savings, especially in climates where temperatures vary widely during a 24-hour period. And, in a typical wood frame house, exterior air moving through the wall can account for up to 30% of the heating or cooling costs. Napier says that TruStone had the air-leakage rate of an AAC wall assembly tested, resulting in a leakage rate of 0.002 ft 3/min/ft2 at an air pressure of 1.57 lbs/ft2, which is significantly lower than for gypsum wallboard. Air infiltration around windows and doors can also be a significant factor in the thermal efficiency of a home.

Other reasons people like living in AAC homes include:

  • They are quieter because AAC walls have good sound-insulation properties
  • AAC homes are wind- and water-resistant, and rodents or termites can't make homes or tunnels in the walls (the soft walls can even stop bullets and shrapnel).
  • The cost and length of time to build AAC shells can be significantly less than for wood-frame construction.