Autoclaved Aerated Concrete (AAC)

Builders in the U.S. can use an innovative concrete material that Scandinavians have built their homes with for decades. Autoclaved Aerated Concrete (AAC) is a precast structural product made with all-natural raw materials. In 1914, the Swedes discovered a mixture of cement, lime, water and sand that expands by adding aluminum powder. The material was further developed to what we know today as autoclaved aerated concrete (also called autoclaved cellular concrete).

It is an economical, sustainable, solid block that provides 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 two decades.

To manufacture AAC, 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. The reaction between aluminum and concrete causes microscopic hydrogen bubbles to form, expanding the concrete to about five times its original volume. After evaporation of the hydrogen, the now highly closed-cell, aerated concrete is cut to size and formed by steam-curing in a pressurized chamber (an autoclave). The result is a non-organic, non-toxic, airtight material that can be used for wall, floor, and roof panels, blocks, and lintels which according to the manufacturers, generate no pollutants or hazardous waste during the manufacturing process

AAC units are available in numerous shapes and sizes. Panels are available in thicknesses of between 8 inches to 12 inches, 24-inches in width, and lengths up to 20 feet. Blocks come 24”, 32”, and 48” inches long, between four to 16 inches thick, and eight inches high.

AAC features include structural capacity, thermal, fire, and acoustical resistance properties. With an R-value of approximately 1.25 per inch, dependent on density, AAC significantly outperforms conventional concrete block or poured concrete. Consistency in quality and color may be difficult to obtain in AAC made with fly ash. Unfinished exterior walls should be covered with an exterior cladding or parged with mortar when exposed to physical damage, dirt, and water, because atmospheric debris can collect in the open cells. If installed in high humidity environments, interior finishes with low vapor permeability, and exterior finishes with a high permeability are recommended.

Because of the thermal mass of AAC and its ability to store and release energy over time, AAC may be beneficial in climates where outdoor temperature fluctuates over a 24-hour period from above to below the indoor temperature conditioned air set point.
 

	The insulating properties of AAC, when compared with conventional concrete, make it an energy-efficient choice.
	AAC is recyclable.
	AAC is naturally resistant to fire, termites, and fungal decay.

 

AAC can be cut with conventional saws and tools. The lightweight, precast blocks are stacked like conventional concrete masonry units (CMU).

Panels are usually installed by crane. Reinforcement bar is placed in panel joints for continuity and diaphragm performance.
 

ACC walls that are installed as block units cost approximately $3 in 8” x 8” x 24”, or $2.30 per square foot. This is comparable in price to the cost of concrete masonry units (CMU). Labor may be slightly less because the lighter weight of the units allow faster installation, however, this efficiency does not inure to the builder under piecework contracts which are common to masonry trades.
 

Compared with conventional CMU, AAC will have lower energy costs because it has a greater thermal resistance (approximately R-10 vs less than R-1 in 8” thicknesses).
 

Prescriptive construction methods for AAC are not contained in model building codes. The codes, however, do not limit the use of alternative building materials. To comply, submit product ICC-ES report with architectural plans at plan review.
 

Hughes Construction: Lexington, North Carolina
 

The light weight of AAC makes shipping and handling more economical AAC block is installed with a thin-set mortar that limits leveling adjustments by masons. Block or panels are cored and filled with grout and reinforcement bar at stated intervals.

 

The material is easy to finish, too. Exterior walls can be painted, plastered, or finished with claddings like vinyl siding or fiber-cement siding. Interior surfaces can be plastered, sheet rocked, tiled, painted, or simply left exposed.
 

AAC is a green building material. It has little manufacturing impact, is recyclable, does not off gas and provides termite, water, fire, and sound resistance.