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RAAC - what is it, and why is it a problem now?

21 September 2023

In the United Kingdom, the use of Reinforced Autoclaved Aerated Concrete (RAAC) faces a myriad of contemporary challenges, shedding light on the urgent need for a comprehensive analysis of its issues and potential solutions. Rob Glynn, Executive Adjuster in DWF's Claims Management & Adjusting team, provides some insight.  

The first time I encountered RAAC was in 2004 as a structural engineer, during a school refurbishment project. At the time we relied on Building Research Establishment guidance from 2002, which provided comprehensive details of how to identify defects and deal with them. It was clear even then that the RAAC roof planks had outlasted their intended design life and the inherent defects known to exist meant that their continued use would result in a significant risk of collapse.

So what is it, and why is it a problem now?

RAAC is a way of using concrete for walls and roofing that is lightweight due to 80% air content. Primarily developed to provide thermal insulation in walls, which it does very well. Then adapted to span between walls as a roof panel, with the inclusion of light reinforcement. Reinforcement and normal concrete work well together, but aerated concrete and reinforcement do not have a durable synergy.

Early use of RAAC after WWII was based on manufacturer's information and borrowing analysis methods from 'normal' concrete design, which was a tenuous analogue. Finally, a European standard was published in 2013, but by then thousands of buildings included RAAC within their fabric.

The reality is that buildings where RAAC has been used are past their intended design life. Unless refurbishment work led to the discovery of RAAC in buildings, building owners will hopefully be checking now. 

The problem with a nationwide panic (that seems to be underway now), is that the resources do not exist in sufficient numbers. We are still short of fire engineers to assess cladding defects and we may now struggle to find enough structural engineers to assess RAAC. 

You should beware of new companies setting themselves up to take advantage of the requirement to survey buildings. It is important to remember that the most critical RAAC elements are in flat roofs, which may have asbestos content in ceilings, so an asbestos survey may well be required first.

Comparisons to the cladding crisis are relevant and we may see the Building Safety Fund being extended to include RAAC remediation. Property insurers will be looking to control this new risk and bracing for block claims being made before current policy periods expire.

The next question that will be forming in everyone's minds is, are there any other latent defects that we need to worry about? RAAC is a type of 'system' building and there are many others. The second half of the 20th century saw a divergence from traditional construction and a use of new technologies. Another example of this is the lightweight steel truss framing that borrowed techniques from building Spitfire planes. Such methods allowed rapid building of much needed facilities. However, the design life of system buildings was likely to be around 30 years, which has certainly expired now.

To find out more about the points raised in this article please contact Rob Glynn.

Further Reading