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What is RAAC (Reinforced Autoclaved Aerated Concrete) and how can we help?

5th September 2023

Reinforced Autoclaved Aerated Concrete (RAAC) is a specific type of construction material.

Its been in the news recently because of the governments decision to shut a number of school buildings known to contain it. This is not a new issue and has been heavily researched by the BRE & IStructE.

Autoclaved Aerated Concrete (AAC) is a lightweight, precast building material that provides excellent thermal insulation, fire resistance, and acoustic insulation properties. It’s made from a mixture of sand, cement, lime, water, and a small amount of aluminium powder. The mixture creates many tiny air pockets in the material, making it lightweight and insulating.

When “reinforced”, AAC (RAAC) will typically contain steel bars or mesh to improve its structural strength, making it suitable for more extensive structural applications, such as floor and roof panels.

RAAC however has encountered problems especially in older buildings. Over time, the reinforcement can corrode, leading to potential structural issues. As such, it’s crucial to ensure the material’s proper inspection and maintenance when used in construction.

raac concrete

RAAC (concrete)

Reinforced Autoclaved Aerated Concrete (RAAC) has been used in construction for several decades, especially in Europe. However, certain problems and concerns have emerged over time:

  • Corrosion of Reinforcement: The steel reinforcement in RAAC can corrode over time. Corrosion can lead to a reduction in the cross-sectional area of the reinforcement, weakening its tensile strength and ultimately leading to structural failures.
  • Moisture Intrusion: If the concrete is exposed to water or moisture for prolonged periods, it can lead to increased corrosion of the reinforcement and also reduce the material’s durability.
  •  Reduced Lifespan: Due to issues like corrosion and moisture susceptibility, the lifespan of RAAC might be less compared to other reinforced concrete materials.
  • Limited Knowledge and Experience: As RAAC is less commonly used in some parts of the world, there may be limited expertise in its installation, maintenance, and repair, leading to potential mistakes during its application.
  •  Spalling: When the embedded reinforcement corrodes, it can lead to spalling, where parts of the concrete surface break away, revealing the corroding reinforcement.
  • Potential Structural Failure: In the worst-case scenarios, the corrosion of the reinforcement and the degradation of the concrete can lead to catastrophic structural failures. There have been incidents, especially in older buildings, where RAAC roof planks have collapsed due to these issues.
  • Challenging Repairs: Repairing or retrofitting RAAC structures can be challenging due to the unique nature of the material and the potential widespread issues of corrosion.
  • Lack of Awareness: Some building owners and managers might be unaware of the potential risks associated with older RAAC structures. This lack of awareness can lead to missed inspections or maintenance, increasing the risk of problems.

Given these concerns, many professionals recommend regular inspections and maintenance for buildings with RAAC to ensure safety and prolong the material’s lifespan. At WBC we are able to provide RAACS inspections to help diagnose if the building has RAACS. We can also put in place ways to help manage its presence and ensure your compliance with H&S law. We have trained RICS surveyors and IStructE structural engineers that can help. If you would like to discuss this with us please contact us here.

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