Autoclaved Aerated Concrete (AAC)
Autoclaved Aerated Concrete (AAC) is a lightweight and versatile building material that has gained popularity for its unique properties. Comprised of quartz sand, calcined calcium sulfate, lime, cement, water, and aluminum powder, AAC undergoes a curing process in an autoclave where heat and pressure create a solid, foam-like structure. Invented in the mid-1920s, AAC offers a combination of structural strength, insulation, and resistance to fire and mold. Its flexibility allows for various forms such as blocks, wall panels, floor and roof panels, cladding panels, and lintels.
AAC finds applications in both interior and exterior construction projects. It can be painted or coated with stucco or plaster compounds to protect against weather elements, or covered with siding materials like veneer brick or vinyl. One of the advantages of AAC is its ease of installation, making it a preferred choice for builders. Additionally, AAC materials can be shaped on-site using standard power tools with carbon steel cutters, providing further convenience.
It's important to note that the reinforced steel version of AAC, known as RAAC, has a limited lifespan of approximately 30 to 40 years. Beyond this timeframe, RAAC is susceptible to structural instability without any visible signs of deterioration or warnings. Therefore, it's crucial to consider this aspect when using RAAC in construction projects.
Published Guidance
The recent release of the "Reinforced Autoclaved Aerated Concrete: Guidance for Responsible Bodies and Education Settings with Confirmed RAAC" document by the Department for Education has triggered a significant concern in England's educational landscape. It is anticipated that approximately 150 schools across the country may face closure due to potential structural safety issues associated with this type of construction material. The need for this document was escalated following an on-site failure of specific RAAC components, previously believed to have a low risk of deterioration. Subsequent research, including a study commissioned by the NHS through Loughborough University (which incorporated petrographic findings from Petrolab) and risk factor guidance from the IStructE’s RAAC study group, has further contributed to current knowledge and understanding of the associated risks. However, it's worth noting that failures linked to RAAC materials have been identified and investigated as early as the 1990s in the UK. This early wave of failures led to the cessation of RAAC material production in the 1990s, prompting extensive research and investigations by organizations like BRE and SCOSS.
Reinforced Autoclaved Aerated Concrete (RAAC) is a specific form of precast autoclaved aerated concrete (AAC) primarily produced during the 1950s-1980s to provide lightweight closed cell components. It was widely used in constructing precast planks, flat roofs, and walls within public sector buildings during that period. In modern applications, precast AAC masonry blocks continue to be popular construction materials across several regions. RAAC primarily consists of AAC plank material requiring embedded longitudinal and transverse steel reinforcement to enhance flexural and tensile strength. The composition of the cementitious component is best described as an aerated mortar material, incorporating blends of cement, lime, silica flour (microsilica), gypsum, blast-furnace slag, pulverized fuel ash, a small amount of finely graded sand, and an entraining agent like aluminium powder. The AAC is cured using a high-pressure steam curing process known as autoclaving for a period of 6-12 hours. The aluminium powder and lime react with water during mixing and casting to form calcium aluminate and liberate hydrogen, resulting in the abundant air entrained voids throughout the binder structure, giving AAC its characteristic "Aero-bar" texture and very low to low density.
Following mixing, the material undergoes autoclaving, a process that introduces higher temperature and pressure into the system, causing further microstructural changes in the formed cement hydrates. This results in a mixed cement paste of poorly crystalline calcium-silicate-hydrate paste (C-S-H) and coarser tobermorite paste, largely dependent on pozzolans.
One of the major durability concerns associated with RAAC stems from its high void content and susceptibility to moisture ingress and carbonation. This makes it ineffective in providing protection to the embedded reinforcement, relying instead on its coating system to inhibit corrosion. Carbonation of the poorly crystalline C-S-H and tobermorite paste can lead to deterioration, including loss of strength, loss of alkalinity, increased deflection potential, carbonation shrinkage, and susceptibility to external processes like thaumasite and gypsum-form sulphate attack. These processes, while not individually leading to failure, can contribute to the loss of strength and durability of the RAAC material as a whole. Further research is needed on historically utilized systems, which are now operating well beyond their original 30-year service life expectancy. While much of the examined plank material appears to be relatively sound, it's being used far beyond its intended service life, and factors such as microfracturing and evidence of cyclical moisture ingress suggest that the plank material may no longer be suitable for its purpose.
In addition to petrographic concerns, physical issues related to panel bearings, anchorage, cracking, moisture ingress, excess loading, and high deflections are identified as the highest risk factors associated with structural collapse and failure. These issues are detailed in the IStructE guidance documents.
When it comes to examining RAAC materials, analysis, identification, and management are best conducted by suitably experienced and qualified onsite structural engineer-led investigation. Petrography plays a crucial role in materials identification and comparative examinations to identify evidence of moisture ingress and active deterioration, particularly in load-bearing planks.
In conclusion, the release of the "Reinforced Autoclaved Aerated Concrete: Guidance for Responsible Bodies and Education Settings with Confirmed RAAC" document by the Department for Education has highlighted critical safety concerns associated with RAAC and its potential impact on the structural integrity of numerous schools in England. The composition, durability issues, and material analyses related to RAAC emphasize the need for thorough investigation and proactive measures to ensure the safety and longevity of structures built using this material. Ongoing research and advancements in construction materials technology are essential to addressing the challenges posed by aging structures and developing safer alternatives for the future.
History
The development of AAC dates back to the mid-1920s when Swedish architect and inventor Dr. Johan Axel Eriksson collaborated with Professor Henrik Kreüger at the Royal Institute of Technology. They perfected the process and obtained a patent in 1924. Production commenced in Yxhult, Sweden, in 1929, giving rise to "Yxhults Ånghärdade Gasbetong," the world's first registered brand of building materials known as Ytong. Another brand, "Siporex," was established in Sweden in 1939 and currently holds licenses and operates plants in 35 locations globally. Hebel, a renowned international brand of cellular concrete, traces its origins back to Josef Hebel, the founder and technician from Memmingen, Germany. The first Hebel plant was established in Germany in 1943.
Initially, Ytong AAC in Sweden was manufactured using alum shale, which contained combustible carbon beneficial for the production process. Unfortunately, the slate deposits used in Ytong also contained trace amounts of natural uranium, resulting in the emission of radioactive radon gas within buildings. In 1972, the Swedish Radiation Safety Authority highlighted the unsuitability of a construction material emitting radon gas, leading to the cessation of alum slate usage in Ytong production by 1975. Ytong responded by developing new formulations that excluded alum slate. The revised compositions, comprising quartz sand, calcined gypsum, lime, cement, water, and aluminum powder, resulted in a new type of aerated concrete free from radon exposure. Today, this white autoclaved aerated concrete is produced using state-of-the-art technology and similar formulations adopted by manufacturers worldwide.
In 1978, the Swedish team at Siporex Sweden established the Siporex Factory in the Kingdom of Saudi Arabia, operating as the "Lightweight Construction Company – Siporex – LCC SIPOREX." For over 40 years, this factory has met the demand for aerated concrete in the Middle East, Africa, and Japan. Nowadays, numerous companies produce aerated concrete globally, particularly in Europe and Asia. While AAC production has slowed down in Europe, Asia experiences significant growth due to high demand for housing and commercial spaces. China has emerged as the largest market for aircrete, with several hundred factories. AAC manufacturing and consumption are substantial in China, Central Asia, India, and the Middle East.
Benefits of Using AAC:
Improved Thermal Efficiency: AAC provides enhanced thermal insulation, resulting in energy savings and increased comfort for building occupants.
Superior Fire Resistance: The porous structure of AAC offers excellent fire resistance, enhancing safety compared to traditional concrete materials.
Workability and Minimized Waste Generation: AAC's workability allows for precise cutting and shaping, minimizing solid waste during construction, contributing to a cleaner and more efficient building process.
Resource Efficiency: AAC's manufacturing process utilizes resources efficiently, requiring fewer raw materials and generating less waste compared to other construction materials.
Lightweight and Easy Handling: AAC's lightweight nature facilitates easier handling during transportation and construction, leading to cost and energy savings and improved seismic safety.
Risks of Using AAC:
Installation during Rainy Weather: AAC can crack during installation, especially in rainy weather, necessitating precautions like using dry blocks and appropriate mortar.
Brittle Nature: AAC blocks are more brittle and prone to cracking than clay bricks, requiring careful handling during construction.
Attachments: Due to brittleness, special, larger diameter wall plugs and longer screws may be needed for attaching fixtures, potentially incurring additional costs.
Insulation Requirements: In some regions, AAC alone may not meet insulation standards, necessitating additional insulation layers and impacting wall thickness.
Limited Lifespan for RAAC: The reinforced version of AAC (RAAC) has a limited lifespan and can become structurally unstable after 30 to 40 years, posing a risk to long-term structural integrity.
In 2023, five hospitals in England that were built with RAAC between the 1960s and 1980s were deemed at risk of collapse due to deteriorating concrete infrastructure. Consequently, the hospitals needed to be rebuilt.
It is essential to consider these limitations and challenges associated with AAC, particularly RAAC, when planning and implementing construction projects to ensure the safety and longevity of the structures.