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CROSS Safety Report

RAAC assessment undertaken by persons not suitably experienced

Report ID: 1224 Published: 21 August 2023 Region: CROSS-UK


Overview

A reinforced aerated autoclaved concrete (RAAC) assessment was carried out by persons who did not appear to have appropriate experience, potentially putting building users at risk of harm.

Key Learning Outcomes

For building owners, managers, surveyors, and other persons responsible for the safety of buildings:

  • Understand that building users could be at significant risk of harm if incompetent assessments of RAAC are relied upon
  • If RAAC is suspected, an assessment should be made by a Chartered Structural or Chartered Civil 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
  • Risk assessments should have input from an engineer with appropriate knowledge and experience of RAAC
  • The Department for Education (DfE) publication, Reinforced autoclaved aerated concrete: estates guidance, contains advice useful when appointing an engineer to assess RAAC

For civil and structural engineers:

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This report is about a reinforced aerated autoclaved concrete (RAAC) assessment carried out by persons who did not appear to have appropriate experience which, potentially, put building users at risk from RAAC plank failures. The reporter has been involved in undertaking RAAC surveys on buildings across England. Prior to each survey, the reporter's firm undertakes a desk study to gather available information about the building, including plans, previous surveys, condition reports, and any other relevant information that can be found.

In one such desk study, they came across a report that contained the structural calculations used to establish the capacity of existing RAAC panels. The calculations, supposedly based on Eurocodes, used a proprietary software package and the parameters entered treated the RAAC as if it was normal structural concrete. This demonstrated a fundamental misunderstanding of the difference between RAAC and normal structural concrete, how RAAC would have been designed originally, and how it should be assessed.

Demonstrated a fundamental misunderstanding of the difference between RAAC and normal structural concrete

The reporter believes the fact that the assessment author did not realise the calculations were not suitable suggests that they were not an engineer. The inclusion of calculations in an assessment report could suggest, to a layperson, including many building owners, a thorough approach to the RAAC assessment. However, a competent engineer would understand the calculations were inappropriate and made grossly optimistic assumptions about the properties of the RAAC.

The calculations were inappropriate and contained grossly optimistic assumptions about the properties of the RAAC

The calculations used a compressive strength of 37 N/mm2 whereas autoclaved aerated concrete (AAC) might more typically have a compressive strength of circa 3N/mm2. Significant other differences between RAAC and normal concrete were also not taken into account. Furthermore, the reporter states the assessment did not consider other RAAC specific risks such as brittle failure arising from narrow bearings. Despite these failings, the assessment concluded the RAAC planks were fit for purpose. The reporter considers that if the owners of the building relied on the report, they potentially had an inaccurate picture of the building’s safety and, consequently, a large number of people could have been put at risk of harm.

A large number of people could have been put at risk of harm

Fundamentally, the reporter considers the issue to be that the survey and assessment report appear to have been undertaken by someone without appropriate engineering knowledge and RAAC assessment experience. The reporter’s view is that the assessment of RAAC, in terms of risks and suitability for continued occupation, should only be carried out by a suitably experienced Chartered Structural Engineer.

The reporter concludes that any client procuring a RAAC assessment should follow the guidance provided by the IStructE and ensure they only appoint suitably qualified and experienced individuals or organisations.

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This worrying report refers to an assessment that may well have been assumed to be competent but as shown by the reporter this was not the case. Building users could be at significant risk of harm if incompetent assessments of RAAC are relied upon. All persons responsible for buildings where RAAC is present, must understand that the assessment of such elements should only be undertaken by engineers with appropriate knowledge and experience.

Why are engineers with knowledge of RAAC required?

There is a risk of structural failure of RAAC planks. Failure can be gradual or sudden and, 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.

Failure could potentially result in death or injury

As the reporter says, AAC is very 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. The unit weight and compressive strength of AAC varies 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.

May have only about 10% to 20% of the compressive strength of everyday structural concrete

As RAAC is a very different material from structural concretes, engineers undertaking assessments of buildings containing RAAC must have appropriate knowledge and experience of how RAAC elements can behave. Without this understanding of the material, very serious potential structural failures which should have been averted, may be missed.

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, the Local Government Association and the Department for Education contacted all school building owners about the collapse of a plank in a school, and the SCOSS Safety Alert, Failure of reinforced autoclaved aerated concrete (RAAC) planks, was issued in May 2019.

Appoint an engineer with appropriate knowledge and experience of RAAC

Many buildings are overseen or managed by construction professionals such as surveyors, architects and engineers. However, it is also the case that the management of buildings can be the responsibility of persons who do not have any significant experience of buildings or construction. Regardless, all persons responsible for the management and safety of buildings should be made aware of the significant concerns surrounding RAAC planks and panels.

All persons responsible for the management and safety of buildings should be made aware of the significant concerns surrounding RAAC planks and panels

Where there is a concern that a building may contain RAAC elements, the responsible body or person should ensure an engineer with appropriate knowledge and experience of RAAC undertakes the required assessment. The DfE publication, Reinforced autoclaved aerated concrete: estates guidance, published in 2022, provides advice to responsible bodies in education settings on the process of assessing, investigating and developing a RAAC management and remediation strategy. This guidance may be very helpful to responsible bodies and persons across other sectors in both the public and private sector. The guidance includes matters to be considered when appointing an engineer who has the necessary knowledge and experience of RAAC.

The IStructE has published 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 SCOSS 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.'  

The IStructE also published Reinforced Autoclaved Aerated Concrete (RAAC) Investigation and Assessment – Further Guidance in 2023, which provides further advice on the critical risk factors associated with RAAC panel construction. It includes a proposed approach to the classification of these risk factors and how these may impact the proposed remediation and management of RAAC.

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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