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RAAC – a ticking time bomb

RAAC – a ticking time bomb

RAAC – a ticking time bomb

What is RAAC?

Reinforced autoclaved aerated concrete (RAAC) is an aerated concrete with no coarse aggregate.  In cross-section its appearance is similar to that of a chocolate Aero bar.  It is very lightweight and much more malleable than traditional concrete. A screwdriver or drill will easily penetrate RAAC.  Indeed, a common test for RAAC when inspecting a building, is to push a sharp instrument into the concrete.  If the concrete can be penetrated, then it is likely to be RAAC rather than traditional concrete.

Unsurprisingly the structural behaviour of RAAC differs significantly from traditional reinforced concrete.  While it is lightweight, it has much less strength and hardness than traditional concrete and it is much more porous.

The use of RAAC

Up until the 1990s, and going back as far as the 1950s, RAAC was in widespread use in the building industry.  Due to the lightweight nature of RAAC it was commonly used in the formation of flat roof structures.  However, this made the installed material, which was often hidden behind ceiling panels, very difficult to survey and maintain.

What is known about the properties of RAAC?

In May 2019, following the sudden failure of a roof structure in an Essex school, the Standing Committee on Structural Safety (SCOSS) issued a Structural Safety Report on the failure of RAAC planks.  The Report noted that concerns with RAAC planks included:

  • “Rusting of embedded reinforcement leading to cracking and spalling of the AAC cover;
  • Cracking, of varying degrees of severity, thought to be associated with moisture and temperature related movements in the planks;
  • Excessive deflections due to creep;
  • Floor and roof planks tending to act independently, rather than as a single structural entity.

The Report went on to note that tell-tale signs of RAAC failure include:

  • “ponding of rainwater, with the potential increase in the imposed loading,
  • distress to certain types of waterproof membrane and associated finishes, and
  • water penetration sufficient to promote corrosion of the embedded reinforcement.”

In conclusion the Report advised that, in buildings where RAAC components have been identified, the owner/building manager should take the following steps:

  • “Conduct a risk assessment. The use of space beneath a roof will affect the risk assessment e.g. a classroom will be a higher risk than a store. If there is doubt about the structural adequacy of the planks and/or there is evidence of water ingress, then it is recommended that consideration is given to their replacement. The use of the space beneath the roof may need to be discontinued until the roof has been strengthened or replaced.
  • Consider the long-term plan for the RAAC roof. In some cases, the life span of the roof will have come to an end and replacement will be necessary. In other cases, it may be felt that regular inspection is merited and that records are kept so that the significance of any changes in behaviour can be readily assessed;
  • Check with maintenance staff, facilities managers, contractors and others who have access to the building to ask about roof ponding, roof leaks, cracks on the underside of flat roofs or other signs of deterioration;
  • Check with the same people about re-surfacing that may have taken place as this could affect the load on a roof. This includes checking if a levelling compound was used to recreate the roof fall prior to replacing waterproofing;
  • Check the colour of the roof surfacing – if it is black then this may indicate enhanced sensitivity to thermal effects;
  • Ensure that all staff know to report any leaks, cracks and or other potential defect issues;
  • If there are sudden changes such as audible cracking sounds or greatly increased water ingress, or observable deflection, then the area should be immediately closed off. This would apply to any form of structure;
  • Any such observations could be warning signs and should merit expert attention from an appropriately experienced Chartered Structural Engineer or Chartered Building Surveyor.”

In September 2022, the Office of Government Property issued an RAAC Safety Briefing Note regarding the dangers posed by RAAC (due to its propensity for sudden collapse with little or no warning), and the challenge of identifying the use of RAAC in buildings (because RAAC planks look the same as traditional precast concrete, and are commonly hidden above false ceilings).  The Note stated, in notably stark terms, that “RAAC is now life-expired and liable to collapse”.

In response to the challenge of identifying the use of RAAC, the Institution of Structural Engineers issued a Report to support its members with the investigation and assessment of RAAC.  One of the main risk factors identified by the Report was the poor bearing capacity of RAAC panels.  Materials with poorer bearing characteristics require shearing loads to be spread over a larger area.  The Report notes that codes of practice associated with the design of RAAC from the 1950s to the 1980s recommended minimum end bearings of only 45mm for roof panels and 60mm for floor panels.  The Report takes a much more cautious approach, and recommends a minimum bearing length of 75mm. Accordingly, any bearing less than 75mm should now be considered substandard and present an unacceptable risk to panels from shear failure, requiring immediate remedial action to be taken.

Conclusion and Legal Implications

Most people find it astounding that concerns regarding the use of RAAC in buildings have been known about for over 30 years.

The Government’s inquiry, which was initially only focused on school buildings, has now become a UK government-wide inquiry into the use of RAAC in public buildings in light of fears that the wider public estate, including schools, universities, hospitals, offices, and leisure facilities, could be in danger of collapse.  In March 2021 that portfolio was valued at £158bn, with some of the estate having been sold to the private sector.

Matt Byatt, the president of the Institution of Structural Engineers, has noted that the problem is likely to be far wider than initially thought, and has expressed frustration that the Government knew that many of its buildings were beyond their serviceable life, noting that it is the responsibility of owners and building managers to ensure that their buildings are safe.

Although the Government inquiry should be able to identify affected public sector properties, the extent and cost of safeguarding measures and remedial works remains unknown.  In contrast the prevalence of RAAC in the private sector is likely to remain hidden for some time, the obvious fear being that issues will only be discovered after catastrophic disasters.

Owners have an ongoing responsibility for repairs to the structure of the building under the defective Premises Act 1972, and hold a duty of care towards their tenants and any third parties who might be injured by their failure to maintain or repair the building.  Owners of buildings built between the 1950s and 1990s would therefore be well advised to commission a survey of the structure of the buildings and to properly investigate any RAAC found.

Persons involved in the surveying or the renovation or reconstruction of buildings constructed between the 1950s and 1990s may also face increased risks if RAAC is present in the structure and remains unidentified.  Similarly, organisations involved in the professional maintenance of buildings may find themselves in the firing line if losses are exacerbated because of remedial action not being taken sooner.

Considering that the widespread use of RAAC came to an end in the 1990s, it is probable that the limitation period for most claims relating to the original design and construction has now expired, even when applying the new 30-year period under the Defective Premises Act 1972, meaning that the original designers and contractors (along with their professional indemnity insurers) are likely to be exempt from those claims.

Speak to our specialist Construction and Engineering lawyers

Paul Brampton is the Head of Construction and Engineering at IBB Law LLP and has a degree in Civil and Structural Engineering.

If you have any questions about this blog, please speak to one of our construction team on 03456 381381 or email construction@ibblaw.co.uk.

Alternatively, contact Paul Brampton on 01895207276 or paul.brampton@ibblaw.co.uk.