|Autoclaved Aerated Concrete
Autoclaved Aerated Concrete
Autoclaved aerated concrete, or AAC, is made with fine aggregates,
cement, and an expansion agent that causes the fresh mixture to
rise like bread dough. In fact, this type of concrete contains 80%
air. In the factory where it is made, the material is molded and
cut into precisely dimensioned units.
blocks or panels of AAC are joined with thin bed mortar. Components
can be used for walls, floors, and roofs. The lightweight material
offers excellent sound and thermal insulation, and like all cement-based
materials, is strong and fire resistant. In order to be durable,
AAC requires some type of applied finish, such as a polymer-modified
stucco, natural or manufactured stone, or siding.
Key aspects of AAC, whether designing or building with it, are described
at the links below:
More information on AAC is available from the Autoclaved
Aerated Concrete Products Association. From codes and standards
development to education and marketing, AACPA supports the success
of autoclaved aerated concrete products and the mainstreaming of AAC
into North American construction.
combines insulation and structural capability in one material
for walls, floors, and roofs. Its light weight/cellular properties
make it easy to cut, shave, and shape, accept nails and screws
readily, and allow it to be routed to create chases for electrical
conduits and small-diameter plumbing runs. This gives it design
and construction flexibility, and the ability to make easy adjustments
in the field.
- Durability and dimensional stability. A cement-based material,
AAC resists water, rot, mold, mildew, and insects. Units are precisely
shaped and conform to tight tolerances.
- Fire resistance is excellent, with 8 in. thick AAC achieving
a 4-hr rating (actual performance exceeds that and meets test
requirements for up to 8 hr). And because it is noncombustible,
it will not burn or give off toxic fumes.
- The light weight means that R-values for AAC are comparable
to conventional frame walls, but they have higher thermal mass,
provide air tightness, and as just noted, are not combustible.
That light weight also gives a high sound reduction for privacy,
both from outside noises and from other rooms when used as interior
But the material does have some limitations. It is not as widely available
as most concrete products, though it can be shipped anywhere. If it
has to be shipped, its light weight is advantageous. Because it is
lower strength than most concrete products or systems, in load-bearing
applications, it must typically be reinforced. It also requires a
protective finish since the material is porous and would deteriorate
if left exposed.
blocks and panels are available. Blocks are stacked similarly to
conventional masonry, but with a thin-bed mortar, and panels are
stood vertically, spanning full-story heights. For structural needs,
grouted, reinforced cells and beams are placed within the wall section.
(Concave depressions along vertical edges can create a cylindrical
core between 2 adjacent panels.) For usual applications, a vertical
cell is placed at corners, on either side of openings, and at 6-
to 8-ft spacings along a wall. AAC averages about 37 pcf (lbs per
cu ft), so blocks can be placed by hand, but panels, because of
their size, usually require a small crane or other piece of equipment.
- Panels extend from floor to top of the wall:
- Height: up to 20 ft
- Width: 24 in.
- Thickness: 6, 8, 10, or 12 in. (4 in. thick interior partition
panels are available)
- Blocks are larger and lighter than traditional concrete masonry:
- Height: 8 in. typical
- Width: 24 in. long
- Thickness: 4, 6, 8, 10, and 12 in.
- A standard 8 x 8 x 24 in. unit weighs about 33 lbs.
- Specialty shapes:
- U-shaped bond beam or lintel blocks are available in thicknesses
of 8, 10, and 12 in.
- Tongue and groove blocks are available from some manufacturers,
and these connect to adjacent units without mortar at vertical
- Cored blocks available for creating vertical reinforced
Connections, and Finishes
Due to the similarity to traditional concrete masonry, AAC units
(block) can be easily installed by concrete masons. Sometimes, carpenters
get involved in installation. Panels are heavier due to their size
and require the use of a crane for placement. Manufacturers offer
training seminars, and it’s usually adequate to have 1 or
2 knowledgeable installers on small projects. Depending on the type
of finish selected, they can be directly adhered or mechanically
attached to the face of AAC.
course is laid and leveled. Blocks are stacked together with
thin-bed mortar in a running bond with a minimum of 6-in.
- Walls are plumbed, leveled, and squared with a rubber mallet.
- Openings and odd angles are cut with a handsaw or bandsaw.
- Reinforcement locations are determined, rebar placed, and
grouting occurs. Grout must be mechanically vibrated to consolidate
- Bond beams are placed at the top of the wall and can be
used for heavy-duty fixture attachment.
- Panels are placed one at a time, starting from the corner.
Panels are set into a layer of thin-bed mortar and vertical
rebar is attached to dowels extending up from the floor before
the adjacent panel is placed.
- A continuous bond beam is created at the top, either with
plywood and AAC material or with bond beam block.
- Openings can be precut or field cut.
- Roof frame/framing is connected to a conventional top plate
or hurricane straps embedded in the bond beam.
- Floor framing is attached with standard ledgers anchored
to the side of the AAC assembly adjacent to a bond beam.
- AAC floor systems bear directly on top of AAC walls.
- Larger structural steel members are set on weld plates or
bolt plates set into the bond beam.
finishes are made specifically for AAC. These polymer-modified
plasters seal against water intrusion yet allow moisture vapor
- Conventional siding materials are mechanically attached
to the wall face. Furring strips should be employed if back
ventilation of the siding material is desirable.
- Masonry veneers can be directly adhered to the wall face
or can be built as cavity walls. Direct-applied veneers are
typically lightweight materials like manufactured stone.
and Energy Considerations
AAC offers both material and performance aspects from a sustainability
perspective. On the material side, it can contain recycled materials
like fly ash and rebar, which may help contribute to credits in
LEED® or other green rating systems. Further, it incorporates
such a large quantity of air that it contains less raw material
per volume than many other building products.
From a performance perspective, the system leads to tight building
envelopes. This creates an energy efficient envelope and protects
against unwanted air losses. Physical testing demonstrates heating
and cooling savings of roughly 10% to 20% compared to conventional
frame construction. In consistently cold climates, the savings may
be somewhat less because this material has lower thermal mass than
other types of concrete. Depending on the location of manufacturing
relative to the project site, AAC may also contribute to local materials
credits in some green building rating systems.
and Physical Properties
First, several ingredients are blended into a slurry: cement, lime,
water, finely ground sand, and often, fly ash. An expansion agent
like aluminum powder is added and the fluid mixture is cast into
a large billet. As the slurry reacts with the expansion agent to
generate air bubbles, the mixture expands. After its initial set,
the resulting “cake” is wire cut into precisely sized
blocks or panels and then baked (autoclaved). The heat helps the
material to cure faster so that blocks and panels maintain their
dimensions. Reinforcement is placed within panels prior to curing.
This manufacturing process produces a lightweight, noncombustible
material with the following properties:
20 to 50 pcf (lb per cu ft)—this is light enough to float
- Compressive strength: 300 to 900 psi
- Allowable shear stress: 8 to 22 psi
- Thermal resistance: 0.8 to 1.25 per in. of thickness
- Sound transmission class (STC): 40 for 4 in. thickness; 45
for 8 in. thickness
Building Code Issues
After introduction as an appendix in the 2005 Masonry Standards
Joint Committee document, design of AAC masonry became more standardized
(see the 2008
MSJC Code). In the 2011
version of the MSJC, the information will appear in the main
body of the document and the appendix will be dropped. The code
addition of AAC as a structural masonry system removes a potential
hurdle for use of the material by designers and builders. With AAC’s
inclusion in the code, the International Building Code and
International Residential Code from the International Code
Council now reference MSJC for information related to autoclaved
In addition, the American Concrete Institute 523 committee published
a guide in 2009 entitled Guide for Design and Construction with
Autoclaved Aerated Concrete Panels (523.4R-09). Its purpose
is to present information that can help in the design, specification,
and construction of factory-reinforced panels of autoclaved aerated
Autoclaved Aerated Concrete
There is no trade association currently representing
the Autoclaved Aerated Concrete industry. Manufacturing of AAC
still exists in North America. We suggest you search the internet
for dealer representatives that can assist you with potential product
availability in your area.
A Tale of 3 Cities: The Residential
Versatility of AAC
benefits of using autoclaved aerated concrete
(AAC) are numerous. Perhaps in testament to the versatility of AAC,
the 3 residential projects described here are quite different—but
share a common theme of safety. A large single family home in a
forest, whose construction is being managed by the owner himself;
a modest single family home on a wooded site, designed by an architect
seeking environmentally sound and healthy living; and a large development
along the Louisiana Gulf Coast requiring superior weather resistance.
More on AAC residential projects.
The “Suite” Comfort of Concrete
guests in a Georgia hotel are sleeping better tonight thanks to
autoclaved aerated concrete (AAC). About an hour outside of Atlanta,
the site for the Forsyth, Georgia Comfort Suites, a slim lot adjacent
to an interstate, posed a few challenges. And high land costs are
making it increasingly common to build on sites that have inherent
challenges such as noise, unlevel terrain, or minimal set-backs.
So developers turned to a concrete system to help meet their needs
for delivering a quality project—in this case, a strong, quiet,
4-story structure near a heavily traveled highway. More
on the AAC hotel.
The listing of organizations and information resources constitutes
neither an endorsement nor recommendation by the Portland Cement
Association (PCA). PCA disclaims any and all responsibility for
the selection of organizations listed and the products they represent.
PCA also assumes no responsibility for errors and omissions in this