What is RAAC and why is it a risk?

In September 2023, a large number of schools and other public buildings in the UK were closed because it was assessed that load bearing floors and flat roofs made of RAAC planks were in imminent danger of collapse.

Between 2018 and 2023, a combination of monitoring and remediation kept RAAC out of the headlines, but now there is a growing acceptance that, ultimately, removal is the only sure way to avert a catastrophic structural failure.

There is no central register of buildings with RAAC components. Most at risk are school and hospital buildings dating from late 50s to early 90s. Some public libraries, police and fire stations are also affected. It is thought that commercial and residential buildings from this era are less at risk, though not immune.

What is RAAC?

First produced in Sweden, Autoclaved Aerated Concrete (AAC) was introduced to the UK in the late 1950s, primarily in the form of lightweight, grey or white coloured thermal blocks.

They are made of cement and various powdered ingredients, which are then mixed with water and aluminum powder. A bubbling chemical reaction ensues, the mixture expands fivefold, and the resulting honeycombed concrete is cured at a very high temperature in an autoclave.

RAAC is a steel reinforced version of AAC which, by 1960, was commonly used in the form of planks (up to 6 meters long) in the construction of flat roofs, floors and wall panels. Amid fears for the longevity of RAAC, production in the UK was stopped in 1982, though imported versions enabled continued use of the material until the early 90s.

Why the building trade chose RAAC

  • Lightweight, soft, and easy to handle and cut
  • Economical due to the fivefold expansion caused by chemical reactions in the production process
  • Extremely good insulation properties due to the honeycomb like structure
  • Seemingly ideal for applications where it was not subject to heavy loading

Why the building trade was wrong

As time went by, it became clear RAAC had a number of defects which far outweighed its benefits:

  • It does not bond well with its steel reinforcement rod and is subject to shearing when under pressure
  • With its limited load bearing capability, in several cases installation of a new roof covering heavier than the original has been enough to precipitate failure
  • It is susceptible to excessive expansion and contraction (much greater than that of its steel reinforcement) as temperatures rise and fall. Dark, heat absorbent roof coverings are likely triggers for failure
  • It is much more water absorbent than conventional concrete. A leaking roof, a flooded floor, any ingress of water will both degrade the concrete and corrode the steel reinforcement.

As these shortcomings became apparent, modifications to RAAC were made to address them. Lateral spurs were added to the reinforcement rods to improve bonding and minimize the risk of shearing. Additionally, the reinforcement was treated or coated to overcome the water induced corrosion problem.

Undoubtedly the product in 1980 was much better than in the late 50s, but the consensus of the various building regulatory bodies was still that RAAC, even when used in appropriate fashion, had a shelf life of no more than 30 years.

In other words, any buildings still with RAAC floors and flat roofs today are beyond “Use By Date” and therefore potentially dangerous.

Is RAAC in my building?

RAAC is most likely to be found in larger buildings constructed between the late 1950s and 1990, and in earlier buildings that underwent major structural changes during that period.

Both public and private sectors are affected, though it is thought the public sector is at greatest risk. Private housing, apart from apartment blocks, is probably unaffected.

RAAC is relatively easy to identify (light grey or white in colour and soft enough to scrape by hand) but not always so easy to access. Signs that it is in poor condition include visible sagging over a span, deflection (from the horizontal) in the planks, and cracking (most often close to the supporting wall).

What to do?

The degree of danger is not so easy to assess. If you suspect you have RAAC in your building, call in an expert to inspect and advise. Building surveyors such as Tate Surveying Services and structural engineers are the best qualified.

If an issue is identified, there are sadly no easy solutions or cheap fixes. The choices are:

  • Retro fit extra reinforcement at all structurally critical points
  • Replace the RAAC components with a safe alternative system
  • Demolish the affected parts (or all) of the building
  • A combination of the above

Again, Tate Surveying Services (or your structural engineer) can advise on the feasibility and likely cost of the various solutions. It may be, for example, the building is structurally so complex that replacement of RAAC is more expensive than partial demolition and rebuild. And retro fitting will not stop further deterioration of the RAAC, and consequently the need for further ongoing maintenance.

Get in touch

To discuss your RAAC problems in further detail, speak to our team today by calling 01273 031646.