Autoclaved aerated Concrete (AAC)
FOUNDATIONS: MATERIALS ENCYcLOPEDIA
Applications for this system
Ratings Chart for AAC Foundation
The ratings chart shows comparative performance in each criteria category. Click on the tabs below for detailed analysis of each criteria.
- HOW THE SYSTEM WORKS
- ENVIRONMENTAL IMPACTS
- EMBODIED CARBON
- ENERGY EFFICIENCY
- MATERIAL COSTS
- LABOUR INPUT
- SKILL LEVEL REQUIRED
- SOURCING & AVAILABILITY
- CODE COMPLIANCE
- INDOOR AIR QUALITY
- FUTURE DEVELOPMENT
Autoclaved aerated concrete blocks (AAC, sometimes called “autoclaved cellular concrete” or “autoclaved lightweight concrete”) are cement-based blocks that are light enough to have reasonable thermal properties while maintaining enough strength to be structural. Unlike typical concrete, quartz sand is the only aggregate and the cement is mixed with aluminum powder, causing a chemical reaction that creates hydrogen. These hydrogen bubbles aerate the mix, creating up to 80 percent void space in the mix. The mixes can be designed to have specific densities for particular uses. The formed, wet mix is steam-pressure hardened (autoclaved) for up to twelve hours, giving it its structural properties.
The combination of structural and insulative properties is highly desirable, as most foundation materials require a separate insulation element to achieve thermal resistance.
Cast off-site using the process described above, the blocks are laid on a poured concrete footing. A thin-bed mortar (similar to tile thin-set) is then used to bond them together, with joints staggered between courses. At the top of the wall, a special U-shaped block is laid, which becomes the form for a concrete and rebar bond beam around wall. The wall is then waterproofed in accordance with site conditions and local code requirements.
Environmental Impact Rating
Harvesting — Moderate
Limestone and aggregate are mechanically extracted from quarries and can have moderate to high impacts on habitat destruction and ground and surface water contamination and flow. AAC blocks use a much lower quantity of material than other masonry units, with air displacing up to 80 percent of the total volume of the blocks.
Manufacturing — High
The creation of portland cement is a high-intensity process, with limestone being heated in kilns to very high sustained temperatures. Fuels used include natural gas, oil, coal and landfill waste, in large quantities, with impacts including significant air pollution and very significant greenhouse gas emissions.
The autoclaving process used to make AAC subjects the blocks to temperatures of 190 °C (375∞F) and pressures of 8–12 bars for up to 12 hours, with impacts including high fossil fuel use and air pollution.
Waste and trimmings at the factory are immediately recycled into new blocks, resulting in little to no waste generation from manufacturing.
The polymer-modified mortar contains petrochemicals that have numerous impacts.
Transportation — Moderate to High
Sample building uses 3,880 – 7,760 kg of AAC
5.82 – 11.64 MJ per km by 15 ton truck
3.65 – 7.3 MJ per km by 35 ton truck
Sample building uses 7,800 kg of concrete
11.7 MJ per km by 15 ton truck
7.3 MJ per km by 35 ton truck
Blocks are manufactured at a small number of centralized plants and shipped across the continent. They are a high-volume material and impacts will rise proportionally with distance traveled.
Waste: Low to moderate
Biodegradable/Compostable — AAC offcuts.
Recyclable — Metal reinforcing bar.
Landfill — Bags or pails for adhesives and mortars.
Chart of Embodied energy & carbon
Energy Efficiency: moderate to high
AAC blocks are among the few foundation options that offer reasonable thermal performance without the addition of an insulation layer. The blocks range from R-0.8 to R-1.25 per inch, depending on the density. Typical block widths for foundations will range from 8–12 inches, resulting in R-values of 6.5–15.
Narrow AAC blocks can be used to make double wythe foundations, allowing any additional insulation to be placed between the blocks. This arrangement makes possible the use of a wider range of insulations, some of which have lower environmental impacts and costs than conventional foam insulations.
Material costs: moderate to very high
The ingredients of AAC blocks are not high-cost materials, but the manufacturing energy adds significant cost. If the blocks were in wider production, economies of scale would likely reduce the price significantly, but as a specialty product they are currently very high.
The fact that an AAC foundation may not require any further insulation can make it much more competitive with other options when full cost analysis is performed.
Labour Input: moderate to high
Cast off-site, AAC blocks do not require any manufacturing labor on-site. The placement and mortaring of delivered blocks, as well as coring and installation of required rebar, constitutes the labor input. A poured concrete footing beneath the blocks is common practice.
Skill level required for homeowners: moderate
AAC blocks are typically laid using a thin bed mortar that makes the process more like gluing the blocks together than setting them in a mortar bed as with conventional masonry. This method can lend itself more easily to inexperienced builders than many other foundation materials.
Sourcing & availability: moderate to difficult
AAC is not very common in North America, with only a handful of plants producing blocks for the whole continent. Availability will vary regionally, with the blocks being a special order item in areas further from production facilities.
Durability: High to Very High
This material has been in use for over seventy years, and by most reports and studies it has shown itself to be very durable. The steam curing process brings the cement to a full cure in the factory, so it is not susceptible to the same types of potentially problematic site conditions (water quality, temperature, soil conditions) that can alter the curing of site-poured concrete.
The porous nature of the material means water can penetrate the blocks, and freeze/thaw damage can occur. The blocks are typically protected in the same way as other masonry foundations, by parging and/or waterproofing membranes. Where adequately protected, freeze/thaw issues have not proven problematic.
Acceptance of AAC will vary regionally. In some areas it is an acceptable solution, and is used structurally for buildings up to six stories in height. Manufacturers typically have very thorough studies and reports that can be used to show code equivalency where the material is less common.
Indoor air quality: n/a
AAC will have little direct impact on indoor air quality. The greatest impact will be in helping to keep the floors and walls of the building dry and preventing IAQ issues. However, AAC is also used for wall and floor systems, and has excellent indoor air quality ratings in those applications.
Resources for further research
Winter, Nicholas B. Understanding Cement: An Introduction to Cement Production, Cement Hydration and Deleterious Processes in Concrete. Woodbridge, Suffolk: WHD Microanalysis Consultants, 2009. Print.
King, Bruce. Making Better Concrete: Guidelines to Using Fly Ash for Higher Quality, Eco-Friendly Structures. San Rafael, CA: Green Building, 2005. Print.
AAC is widely used in Europe where masonry units make up a much larger proportion of buildings. It is likely to see growing market share in North America, as its combination of structural strength and insulative value lowers material and labor costs and improves performance, especially in temperate climates where no additional insulation is required. The reduction in raw material consumption also work in favor of this material as it lightens the transportation burden to and from the factory.
Tips for a successful AAC foundation
AAC manufacturers provide installation instructions for their blocks, according to published standards, and are not detailed here.