Information on Reinforced Autoclaved Aerated Concrete (RAAC)

The LGA would like to draw member organisations' attention to a recent building component failure involving a property constructed using Reinforced Autoclaved Aerated Concrete (RAAC).


The LGA is contacting its members to draw your attention to a recent building component failure involving a property constructed using RAAC and the suggested steps that you need to take as a responsible body to:

  1. identify any properties constructed using RAAC and validate the potential risk appropriately
  2. consider and monitor the possible impact of reduced maintenance regimes on the condition of your property portfolio, in particular where RAAC  is used. 

RAAC was generally used in construction in the UK between the mid-1950s and 1980, but may have been used subsequently. [1]

It may be found in a variety of types of buildings including, but not confined to, schools.

In the 1990s and again in 2002, the Building Research Establishment described the difficulties in service with RAAC roof planks and commented on cases of excessive and progressive deflections in service, associated with wide spread hairline cracking of the soffit of the planks.

From tests carried out by the BRE the report concluded that RAAC planks gave adequate warning through visual deterioration before failing. However, an RAAC roof has now failed within 48 hours of cracks first appearing and it is therefore necessary to reconsider maintenance and inspection regimes.

Inspections by the local authority have revealed some issues suggesting that there were defects in the RAAC planks that coupled with severe weather conditions giving rise to the mechanism of long term creep, and in addition economies in maintenance may be impacting on the performance of the concrete planks generally. On inspection, these factors combined with the panel defects; which included the longitudinal reinforced steel bar being of an inadequate length, very high span/depth ratios, and a poor aggregate mix, resulting in shearing of the planks and collapse. Subsequent checks by the Authority to their other buildings have not as yet revealed any significant performance issues with further RAAC properties, but this is still to be concluded.

Figure 1: Differential deflection of the planks
Figure 2: Shear crack through the beam

Given that the recent failure did not conform to the expectations derived from BRE’s investigations and given that many RAAC buildings are now at least 38 years old, the LGA and DfE now advise that members and responsible school bodies take the following steps as a reassurance measure to confirm the safety of RAAC construction:

  • Ensure that the condition of all their buildings are regularly monitored, taking a risk-based approach that gives due deliberation to the use of the building with consideration given to the possible impact of reduced maintenance.
  • Ensure they have identified any RAAC property in their portfolio
  • Ensure that RAAC properties are regularly inspected by a structural engineer including using a cover meter to check the provision of traverse and longitudinal reinforcement, note deflections, check the panels in the vicinity of the support, the width of the support bearing, cracking, water penetration and signs of reinforcement corrosion and any inconsistencies between panels. The frequency of subsequent inspections should be determined by the structural engineer conducting the initial inspection.
  • Adopt good roof maintenance practices: in particular:
    • ensure water outlets are clear and are at such a level that allows free drainage of water from roof areas.
    • if the internal surface of the planks is to be decorated, use paint which allows moisture vapour to pass through it. Protect external surfaces with a coating which provides an effective barrier against the transmission of liquid water.
    • where appropriate, reduce the dead load on roofs by removing chippings and replacing them with an appropriate solar reflecting coating
    • ensure that all waterproof membranes are maintained in good condition
    • keep records of deflections of RAAC planks and inspect the construction regularly.
  • ensure that those responsible for the day-to-day management of any RAAC building:
    • Know that RAAC is used in the building and where it is used
    • Check regularly for visual signs of cracks, water penetration, deflection to soffits and ponding to roofs
    • Ensure that all staff know to report any cracks and or other identified potential defect issues
    • Are instructed to immediately close off any part of the building where cracks or other material defects appear pending further checks

References

SCOSS Reinforced autoclaved aerated concrete

BRE IP 10/96 Reinforced autoclaved aerated concrete planks designed before 1980

BRE Report 445 2002  Reinforced autoclaved aerated concrete panels - Review of behaviour and developments in assessment and design 

If you have any questions about the above please contact Charles.Loft@local.gov.uk

[1] BRE Report 445 2002  Reinforced autoclaved aerated concrete panels - Review of behaviour and developments in assessment and design (page 15) identifies three categories of RAAC:

  • RAAC panels designed before 1980- in summary panels were tested and found to be safe but there was a concern that the span-effective depth ration was in the order of 28, was inadequate and did not comply with CP110 where an expected value would be less than 20.
  • RAAC panels constructed after 1980 but before creation of prEN12602:2000 design guidance
  • RAAC panels constructed to prEN12602:2000 design guidance. In summary in relation to this guidance the BRE advised:
    • panels constructed to this guidance had smaller span-depth ratio`s than previous
    • limited testing shows in-service performance likely to be satisfactory but would be prudent to monitor their actual performance after a number of years in service.

(NB: Panels is the BRE description but this is the same as planks).