RAAC Attack: Don’t Panic Edition

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In 1924 in the city of Yxhult, Sweden, architect and inventor Dr. Johan Axel Eriksson working with Professor Henrik Kreüger at the Royal Institute of Technology patented RAAC: reinforced aerated autoclave concrete. Their organisation, Yxhults Ånghärdade Gasbetong became the first registered building materials brand in the world. Later, cellular concrete brand, Hebel, opened it’s first plant in Germany in 1943. Today Hebel continues to manufacture and sell RAAC, their website claiming to be Australia’s leading manufacturer of autoclaved aerated concrete.

Hebel is a strong yet lightweight building product that is available in panels or blocks. It is kind to the environment, quick to build with and better to live in.

Almost 100 years after RAAC was patented, UK kids rejoice as the government announced to extend their long summer holiday. Of course that’s not how they worded it. At the start of September 2023 the UK government announced around 100 schools wouldn’t be able to reopen following the long summer holiday until safety measures have been implemented to address the risks of building collapse. RAAC was used in their construction.

The hysteria quickly spread beyond the UK. Governments are seeking status updates from relevant departments responsible for portfolios of public sector properties. Do we have this? Are we at risk? How quickly do we need to act?

Time for some much needed context.

Older RAAC is mainly found in roofs, occasionally in floors and walls, in public sector buildings like schools, hospitals, airports, housing blocks, theatres and other public buildings.

Reinforced: it contains steel reinforcement bars to provide strength against tension.

Autoclaved: It’s made in an autoclave, a machine used to carry out industrial and scientific processes requiring elevated temperature and pressure.

Aerated: Instead of relying on heavy coarse aggregate (the bigger stones that fill out concrete), it contains a high volume of air. Small air bubbles make up the body of the concrete, a cellular structure offering strength whilst remaining lightweight.

Concrete: a building material made from cement mix and water.

If you ever needed any insight into why our industry uses so many acronyms, here it is. RAAC.

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RAAC had some really great performance characteristics. It was really great at resisting fire and mould. It offered good thermal and acoustic insulation. It didn’t spall or delaminate. Hand tools could be used to manually drive mechanical fixings into it. Eventually RAAC fell out of favour in the 1980’s and soon identified as deleterious by the British Research Establishment (BRE). Deleterious is anything that does not perform as expected and/or poses a risk to health. Think lead paint, asbestos, high alumina cement, plasterglass ceiling panels, calcium silicate bricks, woodwool slabs, hollow pot floors and Aluminum Combustible Panels (ACP). Note that we have a good awareness of many of these materials, their properties, and how to manage them insitu where they are undamaged, undisturbed and considered low risk given their overall context.

In 1994 the UK Dept of Education asked BRE to inspect two flat roofs of schools in Essex reported to exhibit ponding water and excessive cracking to their undersides or soffits. Their inspections found excessive deflections caused water to pond.

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BRE published research papers in December 1996 which described excessive deflections and cracking had been identified in a number of RAAC roof planks and highlighted “excessive” issues in a significant number of older buildings using the material. The useful life of RAAC was considered to be 30 years, so this was consistent in especially older constructions, where RAAC use was popular between 1940s and 1960s.

Just like in today’s modern concrete, the steel reinforcement bar in RAAC becomes vulnerable to corrosion. However, due to it’s lightweight form, the expansion of the corroding steel is enough to suddenly compromise it without any warning; collapsing under a load it can no longer carry. RAAC remained out the public eye until confronting building collapse events occurred:

  • 2018 The roof of Singlewell Primary School in Gravesend, Kent, collapsed over a weekend because of RAAC failing.

  • 2021 A ceiling collapses at Rosemead Preparatory School in Dulwich, South East London – a teacher and 15 year three pupils were injured.

  • June 2023: The UK National Audit Office (NAO) published the Condition of school buildings report which identified 572 schools which might have RAAC present.

  • July 2023: Another collapse in a school building – previously not thought to have been at risk – is reported.

  • September 2023: The government announced 100 schools will be contacted before the new term begins

So even though this isn’t a new issue, following a long period of apparent inaction and a fear that nothing is being done, it has received a flurry of media attention and created a sense of urgency. Public pressure from the likes of school groups, activists, lobbyists, are causing politicians to react.

The status quo around this material hasn’t really changed. We know it exists, we know how to identify it, assess it, and what recommendations to make, just as we do for all other deleterious materials. Due to the media attention, we find ourselves fielding questions on it. As always, context is important. The Hebel I mentioned before manufactures and sells RAAC today. Their website shows how one of their products is as appropriate for corridor walls. We take into account the manufacturer’s intent and appropriateness for the use of their product and if that reflects how it is actually being used. More likely than not, it is being used responsibly and as intended.

The key is there’s no need for alarm or to suddenly shut anything down without careful investigation. We follow the same process we’ve always followed: when a risk is identified, we run it to ground. If the media attention prompts some managers to be proactive to rule out if this is an issue for them, that’s positive. We’re here to help.

BRE Information Paper 10/96 contains helpful detail for building surveyors on how to identify along with suggest maintenance measures. This was updated in 2002 to Information Paper 7/2002. Other resources include the European Standard EN 12602:2008+A1:2013, and update of which is due in 2023.

CJLM