What is AAC?

Autoclaved aerated concrete (AAC) is an economical material used to produce a variety of building products, ranging from precast wall and roof panels to masonry blocks and lintels. AAC was invented in Sweden in 1923 and has been a popular building material in Europe for more than 50 years. However, AAC didn’t become commercially available in North America until around 1990 and the North American construction industry has been slow to embrace this innovation in concrete technology.

Close-up view of a sample of autoclaved aerated concrete
Photo by Marco Bernardini

The raw materials that go into AAC are a mixture of Portland cement, lime, silica sand, water, and aluminum powder. Fly ash (a byproduct of coal-burning powerplants) is sometimes also added, which provides some environmental benefits by reducing the amount of Portland cement required in the mix (Portland cement production results in significant carbon dioxide emissions) and keeping some fly ash out of landfills. The fresh concrete mix is then poured into a mold. Chemicals reactions between the aluminum and the hydrated cement cause many microscopic hydrogen gas bubbles to form in the fresh concrete and the mix expands to approximately five times its original volume. The hydrogen gas dissipates to the atmosphere, leaving behind a highly aerated concrete. The concrete is given just enough time to solidify and gain enough strength to hold its shape. Then, the aerated concrete is cut to the desired size and shape and is placed in a pressurized chamber, called an autoclave, where the concrete is steam-cured. Steam-curing helps the concrete gain strength more rapidly and more uniformly through the thickness relative to air curing.

AAC is available in a number of shapes and sizes. Panels are 600 mm (24”) wide, typically 200 mm to 300 mm (8” to 12”) thick, and up to 6100 mm (20’) long. Blocks are 200 mm (8”) high and are available in lengths of 600, 800, and 1200 mm (24”, 32”, and 48”) and thicknesses between 100 to 400 mm (4” to 16”).

The porous internal structure of AAC makes AAC much lighter than traditional solid concrete (AAC floats in water!) while still maintaining good fire resistance and noise attenuation properties. Reduced weight of the structure might permit some cost savings on the foundation construction. In seismic regions, reduced weight has the additional benefit of smaller earthquake forces. A traditional 200 mm (8”) thick hollow concrete masonry wall weighs about 215 kg/m² (44 lbs/ft²), has a Sound Transmission Class of about 48 and a 2-hour fire rating, while a wall of the same thickness constructed using solid AAC blocks would weigh about 122 kg/m² (25 lbs/ft²), have a Sound Transmission Class of about 45, and a 4-hour fire rating. Thermal resistance is also significantly improved in AAC. Traditional solid concrete has a thermal resistance of about RSI-0.49 to RSI-0.69 per metre (R-0.07 to R-0.10 per inch), compared to about RSI-5.55 to RSI-8.67 per metre (R-0.80 to R-1.25 per inch) for AAC.

However, there are some drawbacks to consider. AAC isn’t as strong as traditional concrete, so it might not be suitable where there are significant structural loads to carry. The specified compressive strength of hollow concrete block masonry is typically about 7 to 18 MPa, compared to about 2 to 6 MPa in AAC masonry. AAC is generally not suitable for exterior exposures unless it is protected with an exterior cladding or parging because AAC is more susceptible to impact damage, freeze-thaw damage, and moisture intrusion. That said, there are many examples of successful use of AAC masonry in the building envelopes, such as the buildings shown below:

Hualapai Head Start, Arizona

Trotwood Middle School, Ohio

In short, AAC is essentially a kind of concrete foam. It's lightweight, better for the environment, and has good noise attenuation, fire resistance, and thermal insulation properties.

References

• Bernardini, M. Aerated autoclaved concrete – close-up view. Personal photograph.
• CSA. (2004). CSA Standard S304.1-04: Design of Masonry Structures. Canadian Standards Association, Mississauga, ON.
• Klingner, R. E. Using Autoclaved Aerated Concrete Correctly. Masonry Magazine, June 2008.
• MSJC. (2013). Building Code Requirements and Specification for Masonry Structures, Containing TMS 402-13/ACI 530-13/ASCE 5-13, TMS 602-13/ACI 530.1-13/ASCE 6-13, and Companion Commentaries. Masonry Standard Joint Committee.
• van Boggelen, W. (2014). History of Autoclaved Aerated Concrete: The short story of a long lasting building material. http://goo.gl/KuyYL8

Masonry Terminology

Construction terminology can be confusing, especially where masonry is concerned. Many terms unique to masonry construction can leave laypeople scratching their heads. In this post, we make sense of some of the terminology used within the masonry industry.

Masonry is simply an assemblage of modular units, which are usually either solid bricks or hollow blocks. Units are typically bound together with mortar made from sand, water, and cement. Other options for assembling the units also exist, such as dry-stacked masonry, where no binder is used, interlocking masonry, where the units fit together like puzzle pieces to provide mechanical connection to each other, and glued masonry, where units are bound together with a thin layer of adhesive.

A layer of a wall that is one unit thick is called a wythe, though the term leaf is also sometimes used. A row of units along the length of the wall is known as a course. Horizontal mortar joints are called bed joints and vertical mortar joints are called head joints. 

Masonry units can be laid in six different orientations, each of which has been given a name to differentiate them: stretcher, header, rowlock, soldier, sailor, and shiner. Stretchers are laid with the long, narrow side facing out and the long edge horizontal. Soldiers are also laid with the long, narrow side facing out, but the long edge is vertical. Headers are laid on their broad side, like stretchers, but have the small face exposed. Rowlocks also have the small face exposed but are laid on the long, narrow side.  Sailors and shiners are laid with the broad side exposed and the long edge vertical for sailors or the long edge horizontal for shiners.

Hollow blocks are typically laid as stretchers, occasionally as headers, and should never be laid as sailors or shiners. Solid bricks are also typically laid as stretchers, but headers, rowlocks, and soldiers are also pretty common, especially in older buildings. Sailors and shiners are rare in solid brick masonry, but have been used in the past to produce some interesting brickwork patterns. Masonry is strongest when loaded as a stretcher or header, so rowlocks, soldiers, sailors, and shiners are typically used for decorative purposes where the strength demand is low.

After the bricks have been laid, the mortar joints have to be finished. Mortar joints can be finished in several different ways to produce different aesthetics, as illustrated below.

The most common joints are concave joints (occasionally called bucket handle joints), which provide a finished look without negatively affecting the overall strength or ability to manage moisture. Flush joints work well for masonry that will be coated with parging or plaster because flush joints aren’t as likely to show through the coating as it ages. Raked joints are made by raking out some of the mortar before it hardens. This emphasizes the edges of the units and can create a good aesthetic. However, raked joints are generally weaker and more susceptible to moisture intrusion than most other joints. Extruded joints, also known as weeping joints and occasionally called skintled joints, are really just unfinished joints. The extrusion is formed from mortar that’s squeezed out of the joint when the brick is laid in place. Some people like extruded joints, claiming they give the brickwork a rustic look, while others feel extruded joints just look messy and unprofessional.

We looked at some of the different terminology used within the masonry industry, including the six orientations of a brick and nine different ways to finish mortar joints. In a follow-up post we will take a closer look at some of the many different patterns masonry can be constructed in.

References

BIA. (1975). Technical Note 2: Glossary of Terms Relating to Brick Masonry. Brick Industry Association, Reston, VA.

Brunskill, R. W. (1997). Brick Building in Britain. Gollancz, London, UK.

Hatzinikolas, M. A. and Korany, Y. (2005). Masonry Design for Engineers and Architects. Canadian Masonry Publications, Edmonton, AB.

Lloyd, N. (1925). A History of English Brickwork. Antique Collectors' Club, Woodbridge, UK.