Whitefox MD Darren Frias-Robles looks at Reinforced Autoclaved Aerated Concrete structures, which hit the news in recent months with three UK schools experiencing sudden roof collapses.

RAAC (Reinforced Autoclaved Aerated Concrete) is a form of lightweight concrete structure that was often used to form panels or planks. These were often used in flat roofs but also in some floor and wall panel construction from the mid-1950s until as late as the 2000’s.

RAAC was used in various building types, including schools, hospitals, theatres, sports halls, public toilets, and a range of other non-domestic buildings.

Unfortunately, it has since been identified that RAAC can fail suddenly and without warning, particularly if it has been damaged by water ingress causing corrosion of the reinforcement.

In May 2019, the Standing Committee on Structural Safety (SCOSS) issued a safety alert identifying the risk of sudden collapse of RAAC structures, following the collapse of a school roof in 2018. It reported that the 2018 collapse was ‘sudden with very little noticeable warning’.

Following the issue of this alert, there has been heightened awareness of the risk of buildings built with this material. This safety alert that public sector stakeholders are continuing to action highlights the need for a clear understanding of the form of construction and the need to identify if RAAC construction has been used and if so, its condition and any residual risk that may be present.

Failures of RAAC

1 – Shear Failure

Samples of RAAC planks have been identified that have a lack of reinforcement extending to the ends of the planks where they bear onto supporting walls or other supporting structure. Sudden shear failure can occur where the planks ‘snap’ suddenly where reinforcement is lacking, resulting in immediate total failure.

RAAC planks should be inspected carefully including use of a cover meter to detect the position of the steel reinforcement to confirm it extends fully throughout the length of the RAAC planks.

2 – Water damage

The SCOSS alert identified evidence of water-related damage and spalling to the roof of a commercial building and identified other examples of corroded steel reinforcement to RAAC planks resulting from water ingress.

RAAC has a sponge like aerated structure which holds water around the embedded steelwork reinforcement causing it to corrode. As the reinforcement deteriorates, it rusts and expands thereby cracking and spalling the concrete around it.

3 – Excess Deflection & Sagging

RAAC planks are prone to deflection and sagging. This is because they are less rigid than traditional reinforced concrete planks. Typical indications for this form of failure include transverse cracking and cracking and distortion around the end bearings.

4 – Carbonation

Carbonation is a chemical reaction that occurs in concrete where the Carbon Dioxide in the air causes calcium carbonate to form which corrodes the passive oxide layer to the steel reinforcement which then leaves it exposed to corrosion.

The aerated nature of the concrete is such that there is a lack of coverage to the reinforcement and far greater opportunity for carbonation to occur. This results in the steel rusting, expanding and causing the concrete to blow and spall from the planks.

Advice on RAAC from the Royal Institution of Chartered Surveyors (RICS)

The RICS has provided guidance to try and provide some clarity on the subject. Owners and managers of buildings should in the first instance check their record drawings and building manuals to see if there is evidence of RAAC used in construction.

Product names including Durox, Celcon, Hebel and Ytong could indicate the use of RAAC. Local authority Building Control can in some instances assist in this as they may have archive record drawings of public buildings.

If there is a concern that the building could contain elements constructed from RAAC, an inspection should be undertaken an RICS chartered building surveyor or chartered structural engineer.

Where RAAC is identified, immediate measures should be put in place to manage the risk (e.g. temporary propping and support to floors and roofs).  Any remedial works can then be planned appropriate to the risk.

How remediation works are delivered will vary considerably depending on building type and use. For example replacement of a RAAC flat roof may include the opportunity for further improvement works or change in design that could include planning and building regulations requirements.

The cost of works too will vary dependent on size and complexity of the works and any required temporary propping or waterproofing works.

What is important is understanding first whether RAAC has been used in the construction of a building. If it has, then its condition can be assessed. Risk of failure can then also be ascertained and either managed through a careful programme of planned preventative maintenance or repair / replacement as necessary.

Whitefox – Chartered Surveyors’ Building Surveying team provide support and advice to our clients on a wide range of building defect issues including concrete defects analysis and repair.

If you are responsible for the management of a commercial building and have any concerns, please do contact us should you wish to obtain any further advice on this subject.  www.whitefoxgroup.co.uk  E: [email protected]

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