Unmasking RAAC: Understanding Reinforced Autoclaved Aerated Concrete

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RAAC Defined

RAAC stands for Reinforced Autoclaved Aerated Concrete, a type of lightweight concrete that was developed in the 1920s as a lighter alternative to traditional concrete mixes. It gained popularity in the UK construction industry being used between the 1950s and into the 1990s.

It was usually used to form reinforced concrete panels in roof structures but also elsewhere, such as, but not limited to, floors, walls and pitched roofs. However, since the mid-1980s it was found to have structural weaknesses which could lead to it failing.

RAAC’s Rise and Fall

RAAC products were introduced into the UK market due to their lightweight properties, fire resistance, and thermal performance. They were widely used in commercial and public sector buildings, including schools, colleges, and hospitals. This does not mean they were never used in any other buildings and we have come across RAAC panels in office buildings and blocks of flats.

However, their undoubted benefits came at a cost.

RAAC’s lightweight nature compromises its structural strength, making it significantly more prone to deflection and failure compared to traditional reinforced concrete.

Problems often arise because of poor construction of the panels, where the reinforcement does not reach the end of the panel. Problems also arise because of poor roof maintenance which can allow water into the structure, which causes the reinforcement and the concrete, to deteriorate. Over loading can also be an issue, with additional plant and equipment placed on roofs which might overload the structure. By the 1980s, RAAC roof planks installed in the 1960s began to exhibit problems.

Source: DfE, IStruct / BBC

Recent Developments – RAAC Roof Failures

In 2018, a school in Essex experienced a partial collapse of its flat roof which was made from RAAC roof planks. This incident, which fortunately occurred on a weekend when the building was unoccupied, prompted the Local Government Association (LGA) and the Department for Education (DfE) to issue alerts to all school building owners. The alerts recommended assessing assets with RAAC roofs, performing risk assessments and taking necessary remedial actions.

The Standing Committee on Structural Safety (SCOSS) issued an alert in 2019, indicating that RAAC roof plank failures were not limited to schools but also affected various types of buildings. Hospitals, such as West Suffolk Hospital, were identified as having serious structural concerns related to RAAC roof planks. The government allocated £100 million for urgent remediation work at affected hospitals.

Identifying a RAAC Roof

There is no central register of buildings with RAAC roof planks, so a good start in identifying whether RAAC panels were used in the construction, is to do a desk top exercise.

Armed with the necessary documents, assuming they are available, the date of construction can be determined and if ‘as built drawings’ are available they may contain details of the method of construction used. The latter would still need to be confirmed because materials are often substituted during construction. Beyond this, physical inspections are crucial.

RACC panels have an open structure, rather like and an Aero chocolate bar. They are friable and you can easily dig into one with a chisel or screw driver. The panels usually measure 600mm x 2.4m long and when viewed from the underside, the edges are usually chamfered, creating a ‘V’ at joints.

Common evidence for failure is deflection, often visible on the roof surface or the underside of ceilings. Cracking along the edges of the panels can be a sign of failure and knowledge of poor roof maintenance and water leaks should ring alarm bells.

However, roofs and ceilings are usually covered over and cannot always be easily inspected.

How to Tackle RAAC Issues

If a RAAC roof is confirmed, a detailed condition report should be obtained by a Structural Engineer or other qualified building professional. They will be able to make recommendations if RAAC is found. In the worst cases this may require the complete replacement of the panels but in other cases repair or propping may be enough. And on other occasions it may be enough simply to monitor the structure.

How Can Maguire Brothers Help? 

We work closely with a number of building professionals to help them identify RAAC panels used in roofs. This usually involves site visits and technical support services to help assess RAAC and address any other roofing challenges.

A useful tool is core sampling. This involves cutting a ‘core’ though the roof waterproofing to determine the condition of the roof coverings and how the roof is built up. We don’t take core samples through the roof deck as this may weaken it but we are able to check what it is formed from.

The results of our findings will be included in a condition report to help clients make informed decisions about remediation.

In summary, RAAC, despite its benefits, poses significant structural risks in the long term. Regular assessments, roof maintenance and, when necessary, replacement or repair/support of RAAC roof planks will be crucial for building safety and durability.

Contact us for expertise and services to help you address these concerns effectively.

This article, as with all other articles we produce, is for guidance purposes only. It does not constitute formal advice and should not be relied upon as such. For bespoke, unbiased advice relating to your commercial roofing project please contact us and we would be pleased to assist.

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