Wednesday, 25 March 2015

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.

  • 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.

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