Reinforced aerated autoclaved concrete planks found on pitched roof of 1990s hospital building

Autoclaved aerated concrete (AAC) is different from normal dense concrete. It has no coarse aggregate and is made in factories using fine aggregate, chemicals to create gas bubbles, and heat to cure the compound. It is relatively weak with a low capacity for developing a bond with embedded reinforcement. It has been used in two main forms of structural elements; lightweight masonry blocks and lightweight structural units (such as roof planks, wall, and floor units). The unit weight and compressive strength of AAC vary greatly depending upon constituents and manufacturing process, but typically, AAC might weigh about 20% to 30% of normal structural concrete and may have only about 10% to 20% of the compressive strength of everyday structural concrete. It was used extensively because of its perceived advantages.

In the 1990s there were instances of failure of RAAC roof planks installed during the mid-1960s and a proportion of such installations were subsequently demolished. In 2018 a report was received via the Local Government Association about the collapse of a plank in a school and the SCOSS Alert: Failure of reinforced autoclaved aerated concrete (RAAC) planks was issued in May 2019.

There is a risk of structural failure of RAAC planks. Failure can be gradual or sudden, if sudden, there is no warning. Structural failure can be caused by several mechanisms and it is now recognised that RAAC is considerably less robust than structural concrete and ages much less well. Because RAAC planks were most commonly used in roofs, sudden failure can be dangerous and could potentially result in death or injury. It should however be noted that, at present, reported failures of RAAC are few and far between.

RAAC elements were not thought to have been incorporated into buildings before the late 1950s or in significant quantity after 1980, however, this case of a building constructed in the early 1990s and confirmed as containing RAAC elements, albeit that no condition concerns were reported, potentially increases the scope of buildings that could require screening for RAAC concerns. Those persons responsible for buildings that could potentially be affected by RAAC concerns are advised to take this new information into account.

The finding of RAAC being incorporated into a later building should however not be totally unexpected. Whilst it is not known how widely RAAC was used, RAAC has continued to be available as a construction option in the UK to the present day, even though some problems were identified in the UK in the 1990s. The problems found in the 1990s were in RAAC that had been designed pre-1980. Design standards, however, continued to allow the use of RAAC. The design of RAAC was covered in British Standard 8110 from 1985 to 2001 whilst the EU harmonised standard EN 12602 has existed from 2008 to date.

The planks originally promoting concern were designed pre-1980. Any planks designed say from the mid-1980s may have been designed and/or manufactured differently from earlier designed and manufactured units. Those manufactured after say 2010, when the European standard was available and those manufactured after 2013, which would have legally required to have been CE marked to EN 12602, may also perform differently from earlier designed and manufactured units. CROSS is not aware of any evidence as to how later designed and manufactured units may perform.

The Institution of Structural Engineers has published updated guidance Reinforced Autoclaved Aerated Concrete (RAAC) panels: Investigation and assessment that provides identification and remediation solutions for RAAC planks. This guidance is recommended as essential reading when considering RAAC induced risk. The conclusions within the guidance state: 'Assessments of buildings with RAAC panels are recommended to include a balance of risks for the continued use of the building against the benefit of strengthening or replacement of the panels. The assessment should include a robust risk assessment and include consideration to the on-going monitoring and future management of the RAAC panels. The failure of the panels which resulted in the SCOSS¹ Alert was a sudden failure and could be an indication that it was due to a brittle shear failure at or close to the bearing. Based on this a cautious approach to the assessment of RAAC panels is recommended and assessments should only be undertaken by a Chartered Structural Engineer with experience in the investigation and assessment of reinforced concrete structures.'

In addition to the Alert Failure of reinforced autoclaved aerated concrete (RAAC) planks published in May 2019, CROSS has published a number of reports concerning RAAC. The CROSS Theme Page Structural safety of reinforced autoclaved aerated concrete (RAAC) planks provides a collation of all RAAC information published by CROSS. 

For all those with experience of RAAC planks, CROSS asks you to consider confidentially sharing your experience with CROSS for others to learn from.