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CROSS Safety Report

Construction period of RAAC planks

Report ID: 954 Published: 20 February 2023 Region: CROSS-UK


Overview

A reporter says that there are structural issues with reinforced autoclaved aerated concrete (RAAC) panels installed in a building as late as 1998. This date is significantly later than previous reports suggest as the last significant use of RAAC elements in the UK.

Key Learning Outcomes

For building owners, managers, surveyors, and other property professionals:

  • If RAAC is suspected, an assessment should be made by a chartered structural engineer familiar with the investigation and assessment of reinforced concrete structures
  • If RAAC is confirmed a risk assessment of the building and its use is advised
  • CROSS Theme Page Structural safety of reinforced autoclaved aerated concrete (RAAC) planks provides further RAAC information

For civil and structural engineers:

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A reporter has found structural issues with RAAC panels that were installed as late as 1998. The reporter does not know if this was an isolated example but considered it important to report their findings since this date is much later than that shown in previous reports confirming when RAAC was still in use.

Expert Panel Comments

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CROSS published report 1125 Reinforced aerated autoclaved concrete planks found on pitched roof of 1990s hospital building in August 2022. This report dealt with a hospital building constructed in the early 1990s which contained RAAC elements, albeit that no condition concerns were reported.

This new report of a building with RAAC panels installed as late as 1998 which show structural issues should be noted, as it potentially extends the period of use of RAAC. The fact that the report indicated structural issues with the RAAC should be further noted since this younger RAAC might be expected to be less likely to show deterioration.

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 (2023), 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, examples of buildings constructed in the 1990s and found to contain RAAC elements, potentially increases the scope of buildings that could require screening. Those persons responsible for buildings that could potentially be affected by RAAC concerns are advised to take this new information into account. To this end, it would appear helpful if relevant Government Departments updated those persons responsible for public buildings, as to the types and ages of buildings that could contain RAAC.

persons responsible for buildings that could potentially be affected by RAAC concerns are advised to take this new information into account

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.

In the 1990s there were instances of failure of RAAC roof planks installed during the mid-1960s and a number of such installations were subsequently demolished. In 2018 a report was received 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.

Current RAAC guidance

The Institution of Structural Engineers published updated guidance Reinforced Autoclaved Aerated Concrete (RAAC) panels: Investigation and assessment that provides identification and remediation solutions for RAAC planks in 2022. 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, including Report 1125 Reinforced aerated autoclaved concrete planks found on pitched roof of 1990s hospital building which contains a summary of historical design standards for RAAC elements. The CROSS Theme Page Structural safety of reinforced autoclaved aerated concrete (RAAC) planks provides a collation of all RAAC information published by CROSS.

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