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Reinforced Autoclaved Aerated Concrete (RAAC): Identification, Inspection and Dealing with it

Reinforced Autoclaved Aerated Concrete (RAAC) is a lightweight precast concrete that was widely used in the UK from the mid-1960s to the 1990s, particularly in public sector buildings such as schools and hospitals. It is made from a mixture of sand, cement, water and aluminium powder, which gives it a cellular structure that makes it much lighter than traditional concrete.

RAAC is generally a durable material, but it can be susceptible to moisture ingress and corrosion of the reinforcement. This can lead to cracking and other defects, which can compromise the structural integrity of the building.

Identifying RAAC

RAAC can be difficult to identify visually, as it looks similar to traditional concrete and other building materials. However, there are a few key characteristics that can be used to identify RAAC:

  • It is lightweight and porous, making it less dense than traditional concrete.
  • It has a distinct, honeycomb-like structure that can be seen when the material is cut or broken.
  • It has a smooth surface that is free from visible aggregate or coarse materials.

RAAC panels are typically 600mm wide and up to 6 metres long. They may have a chamfer along their edge, which creates a distinctive V-shaped groove every 600mm in the surface of the roof, floor or wall.

Guidance for Identification, Inspection and Dealing with RAAC

The UK Government has published guidance on the identification, inspection and dealing with RAAC. This guidance is intended for responsible bodies of state-funded education estates in England, but it can be used by all parties involved in the identification of RAAC in the relevant education estates.

The guidance recommends that all buildings constructed or modified between the 1950s and 1990s should be assessed for the presence of RAAC. This can be done by a qualified building surveyor or structural engineer.

If RAAC is identified, the guidance recommends that it should be regularly inspected for signs of deterioration. This includes checking for cracks, water penetration, deflection and corrosion of the reinforcement.

If any defects are found, the guidance recommends that a qualified building surveyor or structural engineer should be consulted to assess the severity of the problem and recommend appropriate remedial action.

Specific Issues with RAAC can include:

  • An inconsistent manufacturing process, particularly with regard to the level of concrete cover to reinforcement and the positioning of the end transverse reinforcement bar;
  • The bearing depth onto the support

The bearings are the primary method of catastrophic failure, which results from either a bearing of less than 75mm onto the support, or that the transverse bar is not placed onto the support itself, as it is too far away from the end of the concrete panel. So only the longitudinal steels sit on the bearing and this is less structurally sound.

The concrete cover issue mentioned above, also allows carbonation and water ingress to affect the steels more easily and can lead to some minor loss of strength if there is less concrete.

Role of Chartered Building Surveyors and Structural Engineers

Chartered Building Surveyors and Structural Engineers are qualified professionals who can provide expert advice on the identification, inspection and dealing with RAAC.

Chartered Building Surveyors can assess the condition of a building and identify any potential defects, including RAAC-related defects. They can also advise on appropriate remedial action.

Structural Engineers can assess the structural integrity of a building and design and supervise remedial works to RAAC.

If you are concerned that a building may contain RAAC, you should consult a Chartered Building Surveyor or Structural Engineer for advice.

Additional Information

In addition to the UK Government guidance, there are a number of other resources available on the identification, inspection and dealing with RAAC. These include:

  • The Building Research Establishment (BRE) has published a number of reports on RAAC, including a guide to the inspection and repair of RAAC roofs.
  • The Concrete Centre has a dedicated section on its website on RAAC, which includes information on identification, inspection and repair.
  • The Association of Consulting Structural Engineers (ACSE) has published a document on the inspection and repair of RAAC roofs.
  • The Department for Education has guidance which has been updated in September 2023 entitled, Reinforced autoclaved aerated concrete (RAAC): identification guidance

If you are responsible for a building that may contain RAAC, it is important to be aware of the risks and to take appropriate steps to identify, inspect and manage any RAAC-related defects.

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