The topic of reinforced autoclaved aerated concrete (RAAC) has become headline news in the UK recently. We are now seeing the closure of schools – but the impact of RAAC is being felt more widely.
The government inquiry this summer into the use of RAAC expanded from schools to cover all public buildings, with defective RAAC being identified in police stations in Scotland and contributing to the risk of collapse in NHS hospitals.
It seems inevitable that there would be further RAAC discoveries and media attention in light of the inquiry, which highlights the urgent need for guidance and for the identification and implementation of remedial measures where the material is used.
Concerns are compounded by the fact asbestos is sometimes present in buildings where RAAC is found. This complicates investigations and presents a risk of asbestos fibre release in the event of collapse.
The potential failure of a school roof, with no warning, in an environment meant to be safe for the educating children is an emotive subject. No wonder there is concern.
While there is also a political agenda given the wider debate on public spending, building surveyors should focus on the technical points, including what RAAC is and what appropriate action should be taken.
RAAC developed as lightweight concrete alternative
Trevor Rushton's book Investigating hazardous and deleterious materials and his Built Environment Journal article Identifying problematic RAAC planks give an in-depth description of RAAC.
Detailed descriptions of autoclaved aerated concrete (AAC) are also given by the Building Research Establishment (BRE) in its information paper (IP) 7/02.
However, in brief, AAC is a non-organic material whose raw materials are cement, blast furnace slag, pulverised fuel ash or silica flour, mixed with water and aluminium powder.
It has no coarse aggregate, but rather is formed from these fine aggregates and chemicals to create gas bubbles that increase its volume. It is cast in a mould and then cured at high temperature and pressure in an autoclave.
AAC is much lighter and softer than normal concrete and has lower thermal conductivity. It was originally developed in Sweden to form lightweight blocks with thermally insulating properties, and using this technology wide reinforced planks – RAAC planks – were also created.
These were mostly rectangular in shape, with chamfered edges. They were between 300mm and 750mm wide, although typically 600mm; between 100mm and 250mm thick; and up to 6m long.
RAAC is less dense than normal concrete and is not as strong in compression. The reinforcing steel does not bond very well with AAC, and so it needs to be bent into hooks or must contain crossbars to lock it into place.
As AAC does not provide the same passivity – that is, corrosion protection – as normal concrete, the steel also needed to be coated with latex or bitumen.
Uses concentrated in public sector
RAAC was introduced into the UK in the 1950s, but production stopped here in 1982 following concerns about its durability.
In 1999, the Standing Committee on Structural Safety (SCOSS) 12th report confirmed the life expectancy of RAAC was 30 years, based on earlier investigations by BRE, and it is now acknowledged that RAAC has a relatively short life.
RAAC elements are not thought to have been incorporated into buildings before the late 1950s or in significant quantities after 1980, although there are reports of it being imported into the country until around 1998. The planks that originally prompted concern were designed before 1980.
The planks have most commonly been used as lightweight roof decks, floors and internal partitions; however, use as an external wall material is also possible.
Advice varies on the type of buildings constructed with RAAC, but it is evident their predominant use was in the public sector, notably for schools and hospitals.
Nonetheless, there are reports of their presence in court buildings, prisons and police stations.
The Institution of Structural Engineers (IStructE), specifically Collaborative Reporting for Safer Structures UK (CROSS-UK) – which succeeded SCOSS and is managed by IStructE and the Institution of Civil Engineers – has reported that RAAC is present in buildings constructed as recently as 1998.
Some structural engineers are now suggesting it has been used in buildings as late as 2018, with galvanised steel reinforcement and later planks using stainless steel reinforcement.
As an example of a non-public sector application of the material, CROSS-UK also cites the case of defective RAAC planks in a 1975 shopping centre.
It has also been reported as present in the Houses of Parliament, and may have been used in community centres, libraries, and sports halls.
Importantly, these planks were available commercially and may be in many buildings. Buildings, of course, change use and ownership, so there will be instances of some in the public sector that are now privately owned.
That said, in my experience it is rare to see RAAC planks in commercial buildings, and the predominant concern relates to schools.
It is also very rare in residential buildings, being limited to some plant rooms on blocks of flats or some panelling.
Reports begin to identify defects
Laboratory testing in 1991 by BRE, reported in its IP 10/96, already observed that excessive deflections and cracking arose because of a degree of slippage in RAAC planks between the reinforcement and AAC.
It found that planks ultimately failed because of local crushing, horizontal cracking, and extensive cracking of the concrete near the supports, with considerable visual indications of distress before that failure.
BRE IP 10/96 also noted excessive deflections and cracking in RAAC planks, and found evidence of corrosion beginning in the reinforcement.
However, there was no evidence in 1996 to suggest planks posed a safety hazard to building users. Although no specific examples of deflection to floors had been reported, BRE did recommend inspection of RAAC components in floors.
In February 1999, SCOSS noted these concerns in its 12th report, advising owners of buildings that have pre-1980 RAAC plank roofs to arrange for such roofs to be inspected if this had not been done since 1994.
However, SCOSS added that, generally, the deterioration of these planks did not jeopardise structural safety.
In 2002, BRE published further information on the performance of roof planks and gave advice on inspection in IP 7/02, supporting earlier guidance but with firm recommendations for regular inspection of roof planks of this type.
The paper noted that RAAC panels were widely used in Europe, and a new European standard was due to be published.
However, in 2006's Investigating hazardous and deleterious materials, Rushton stated that the stability of panels had been called into question.
He added that RAAC planks are not thought of as a durable form of construction, and – as noted in the SCOSS 12th Report – from 1997 to 1999 reference to them in the relevant British and European standards was removed.
School alert advises plank replacement
More recently, a SCOSS alert was issued in May 2019 following the collapse of the roof at Singlewell Primary School in Gravesend, Kent, in July the previous year. Fortunately, this had happened on a Saturday when the school was empty.
The primary advice from the alert was: 'Pre-1980 RAAC planks are now past their expected service life and it is recommended that consideration is given to their replacement.'
The alert was for owners of schools and similar buildings constructed from the 1960s to 1980s with flat roofs. It stated that, in the 1980s, many RAAC roof planks had been installed but that many buildings where they had been used were subsequently demolished.
A review of several case studies revealed some primary deficiencies, including incorrect cover for the reinforcing steel, high span-to-depth ratio, insufficient provision of crossbars to anchor longitudinal steel, failure of the roof membrane waterproofing that protected the planks, and rapid worsening of local steel corrosion.
In terms of the 2018 failure, there was evidence of shear cracking adjacent to a support, and possible indications of steel reinforcement stopping short of that support.
The roof had recently been resurfaced, and it is possible that this may also have increased heat absorption and led to thermal movement in the steel causing planks to deflect, with the failure occurring during a period of hot weather.
Shearing, sagging and deterioration present concerns
As such, the most notable concern with RAAC planks is their potential for sudden shear failure.
If a building structure is going to fail, it is preferable that this happens by degrees so there are warning signs rather than immediate catastrophic failure.
Shear failure is caused by issues with the steel reinforcement, typically at the end of the planks.
However, there are other concerns leading to the deterioration of RAAC planks, potentially affecting their structural stability.
RAAC has a high elastic modulus compared to normal concrete, meaning planks are subject to greater deflection under similar loading conditions. As such, they are prone to sagging, particularly if:
- they are incorrectly detailed
- working loads have been underestimated
- elements are undersized
- reinforcement is inadequate.
Furthermore, where there is sagging there is potential for cracking and for steel reinforcement to be exposed to moisture, becoming susceptible to corrosion.
Water may also pond on a sagging roof, increasing the load and potentially leading to further moisture ingress as well as exposure of the steel reinforcement, corrosion and cracking.
The 2019 SCOSS alert states that more recent investigations have shown corrosion of reinforcement even where the bituminous coating appeared to be intact.
Rusting of embedded reinforcement that leads to cracking and spalling of the AAC cover is thus a concern, regardless of visible condition.
It does appear we are now in the next phase of the problem, with surveyors, engineers and the wider construction industry placing greater emphasis on removal or strengthening of RAAC planks, based on ageing panels and current concerns, rather than ongoing monitoring.
'The most notable concern with RAAC planks is their potential for sudden shear failure'
IStructE and DfE guidance
IStructE published guidance on the investigation and assessment of RAAC panels in March 2022 and further guidance in April 2023.
In August 2023, the Department for Education (DfE) published an RAAC identification guide, and guidance for responsible bodies and education settings with confirmed RAAC in their buildings earlier this month.
Identifying RAAC and assessing risk
Although issues relating to RAAC have recently risen in prominence, we had already seen commercial clients recommending that all buildings with a primary structure constructed between 1960 and 1990 should be surveyed to determine whether RAAC is present, following advice in the 2019 SCOSS alert.
However, there is no central register of buildings with RAAC roof planks – or floor or wall panels – and so identification depends on local knowledge and independent inspections.
Department for Education (DfE) guidance also outlines the following simple principles for identification purposes.
- Typically, panels are 600mm wide.
- There are distinctive, regularly spaced V-shaped grooves.
- Panels are white or light grey.
- When accessing records and drawings relating to the building, planks may be referred to by proprietary names such as Siporex, Durox, Celcon, Hebel, Xella or Ytong.
- The inside of the planks will also appear bubbly, like an Aero chocolate bar.
- There will be no visible stones – aggregate – in them.
RAAC panels are most often found in flat roofs, but can also be found in pitched roofs as CROSS has noted, as well as floors and walls.
As panels are very soft, if you press a screwdriver or nail into the surface you will be able to make an indentation. However, care should be taken when looking for RAAC in this way because applied surface finishes may contain asbestos.
If RAAC is discovered, a risk assessment must be carried out, considering the use of the building, the specific areas affected, and any deterioration of the structure.
The position of reinforcement can be identified by scanning using a cover meter and testing samples for carbonation; which, where present, can result in cracking.
If there is a doubt about structural adequacy then SCOSS recommended in the 2019 alert that replacement is considered.
'If RAAC is discovered, a risk assessment must be carried out, considering the use of the building, the specific areas affected, and any deterioration of the structure'
Appointing professionals to detect defects
The alert also advises that if the presence of RAAC is suspected then an appropriately experienced chartered structural engineer or chartered building surveyor should be appointed for identification and inspection work.
Their investigations will include but are not limited to:
- assessing cracking
- measuring deflections
- reviewing water penetration and corrosion
- using cover meters to assess regular spacing of reinforcement
- checking that such reinforcement extends to the end of planks and is properly supported
- taking and sending samples for laboratory testing.
This is in line with the procedures for inspection set out in BRE IP 10/96.
The SCOSS alert notes that key warning signs about the condition of RAAC and potential issues are as follows:
- significant cracking and disruption to planks at bearing ends
- planks have deflected for more than one-hundredth of their span
- bearing widths are less than 40mm
- the roof has been resurfaced; there is particular concern regarding increased loads and black surfaces leading to greater heat absorption and subsequent thermal movement
- the roof has leaked historically, or is currently leaking
- significant ponding of rainwater.
In our experience, structural engineers have recommended that RAAC planks either be removed or strengthened wherever they are found, regardless of condition, given the risk of sudden catastrophic failure and the fact that such planks are likely to have exceeded their design life.
However, subject to a full assessment of the risks, it may be possible to manage them with a more proportionate response.
Determining appropriate management or replacement
The 2019 SCOSS alert acknowledges that, in certain circumstances, retaining RAAC and carrying out regular inspections may be merited, with records kept on changes to condition.
This must be done in conjunction with proper management of the building to ensure there are no concerns with roof ponding, water ingress and cracking.
Also, where the roof has been resurfaced the correct falls must be in place so that rainwater can drain to outlets and does not remain in situ.
If, following assessment, clients or building owners decide that RAAC must be removed then this would involve the installation of a suitable replacement structure.
Possible replacements include profiled metal deck concrete slabs, precast concrete slabs, or cross-laminated timber (CLT) panels.
However, if removal and replacement is problematic and strengthening is being considered, it may be possible to retain the primary structure.
For instance, supporting beams could be retained to support the new floor infill or roof. In this case, timber joists or steel beams can be constructed underneath; alternatively, a replacement structure can be built over the top of RAAC elements.
Manufacturer Metsec has a lightweight steel system to remedy sagging in roof planks.
This involves installing lightweight arrangements of castellated and lattice steel beams directly under the RAAC planks for support, with special tubing inserted between the beams and soffit that is then inflated to jack up the deflected planks.
A non-shrink grout is then forced into the gap to keep the planks in position.
Remain attentive for RAAC and the need to replace
With specific concerns about RAAC having changed over time as planks exceed their service life, attitudes – and approaches – are changing, as they have with asbestos. There has therefore been a move away from retaining and managing problematic materials in situ.
We are seeing a transition from regular inspections and potential retention of RAAC planks, as recommended by BRE IP 10/96, to considering their replacement regardless of condition.
This seems a natural trend. First, as problematic materials age, their condition will worsen and risks of failure will increase; and second, replacement will relieve building owners and occupiers of the management obligations and liability associated with retaining such materials.
Surveyors should be aware of the issues posed by RAAC and be alive to the potential for such materials to be present in buildings constructed from 1950 to the late 1990s.
While use became less frequent from the early 1980s, actions should be taken to identify materials in all relevant buildings from the era.
RICS has a dedicated webpage on RAAC with FAQs and useful links. In addition, a free for members RAAC webinar on 20 September 2023 will be recorded and made available to members with an active subscription to RICS' CPD Support Pack.