Autoclaved aerated concrete: product offers tremendous business opportunities for the masonry contractor

Masonry Construction, March, 2006

Autoclaved aerated concrete was patented for manufacturing in 1924 and has become a common building material in Europe and Asia. There are more than 300 AAC manufacturing facilities on six continents.

Unfortunately, many architects, designers, and specifiers in this country are not aware of the technology or its many benefits. In reality, AAC offers a tremendous opportunity for the masonry contractor to increase his business opportunities. Other trades are actively pursuing this work, and the masonry contractor must become proactive and fight for his share of this growing market.

AAC can match almost any design and detail. It is equally applicable in commercial, industrial, institutional, and residential buildings. The product can be used for interior partition walls, load-bearing walls, back-up walls, firewalls, column wraps, shafts, and numerous other applications.

AAC provides the building owner with numerous advantages, including lightweight, longevity, non-combustible, energy efficient, mold resistant, quick and easy installation, and cost competitive. In addition, AAC is an environmentally-friendly building material and many of its attributes are recognized by the LEED Green Building Rating System.

The following list of frequently asked questions about AAC--broken into major subject categories--serves as a primer for additional information on the product. The material was adapted from a Technology Brief developed by the International Masonry Institute.

Manufacturing

What is AAC?

Autoclaved aerated concrete is a lightweight cellular concrete formed by a chemical reaction between finely ground aggregate, lime, Portland cement, aluminum powder, and water. Raw AAC mix is a slurry, which is introduced in large molds, allowed to initially set, cut or sliced into shapes, and then cured in a high pressure steam autoclave. This type of curing method hydrates 100% of the cementitious materials, which results in high early strengths.

Are AAC masonry units reinforced at the factory?

Units to be laid in mortar do not receive reinforcement. Units used to construct a reinforced masonry wall will receive rebar set in grout in the field. AAC panels are reinforced internally in the factory, but are also field reinforced between units to provide a unitized floor, roof, or wall panelized system. Factory reinforced lintels also are available.

Material Properties

How "green" is AAC?

Approximately 80% of AAC's volume is attributed to residual air voids and pockets. Twelve cubic feet of finished AAC material can be produced from 3 cubic feet of raw material slurry. AAC can incorporate fly ash as an aggregate component. AAC consumes about 1/3 the Btus required to produce clay masonry units. This, in turn, further reduces C[O.sub.2] and other greenhouse gasses. AAC's high "R" values (approx R = 1 per inch thickness) are attributed to the many air voids and internal pockets.

AAC utilizes 100% of its material waste in the manufacture of units and panels. Following final curing and cutting, any residual AAC can be ground up and used in oil dry, cat litter, or new slurry mixes. On the jobsite, any piece of AAC that is greater than 4 inches may be used in wall construction. The 1/8-inch thick joints better utilize mortar materials.

AAC manufacturing produces a naturally occurring mineral found in limestone deposits, called Tobermorite. The environment does not need to "learn" how to assimilate this product because it is already found in nature.

Codes & Standards

What codes and standards govern AAC in the U.S.?

Material standards are governed by ASTM, while design requirements and workmanship standards are regulated by the 2005 edition of the MSJC (ACI-530 & 530.1) Code & Specifications. The International Code Council (ICC) has approved ICC-ER6062 for AAC, which provides for the use of AAC supplied by Autoclaved Aerated Concrete Products Association (AACPA) members to jurisdictions covered by ICC. ICC-ES reports generally establish a path for a material or system to be promulgated into the International Codes.

What ASTM standards govern AAC?

There are currently three ASTM standards for AAC--ASTM C-1386 for material properties, ASTM C-1452 for reinforced AAC panels, and ASTM C-1555 for AAC masonry units.

What building codes govern the design and use of AAC?

AAC is included in the 2005 MS JC Code. The IBC references the MS JC Code for the design, construction, specification, and inspection of masonry products and systems.

If AAC is not currently included as part of the IBC, how can I get it approved?

Several third party organizations, such as NES and ICC-ES, evaluate products and systems not specifically addressed in the current code. The resulting report will lay the basis of approval by the local code enforcement agency. Chapter 1, the administrative section of the IBC, provides for these approvals. Currently, the ICC-ER Report 6062 for AAC allows for just such approvals.

Moisture and Air Resistance

Can the exterior of an AAC wall be sealed?

Yes, but the coating should be water-resistant, not waterproof. The coating must allow the walls to breathe. Clear coatings, such as silane and siloxane, may be used to improve the water-resistant properties of AAC, but should not be considered a substitute for an exterior wythe of masonry, a cladding system, or a decorative breathable coating such as portland cement-based stucco.