IStructE statement & FAQs: Reinforced Autoclaved Aerated Concrete (RAAC)

Author: IStructE

Date published

25 October 2023

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IStructE statement & FAQs: Reinforced Autoclaved Aerated Concrete (RAAC)

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Author

IStructE

Date published

25 October 2023

Author

IStructE

Information on Reinforced Autoclaved Aerated Concrete (RAAC)

RAAC is a building material used in some buildings to form roof planks, wall panels, and sometimes floor planks, between the mid-1950s and mid-1990s.

 
If properly designed, manufactured, in good condition and with good bearing, RAAC installations are considered safe. However, the panels can creep and deflect over time, and this can be exacerbated by water penetration. A more recent incident indicated that if they have insufficient bearing and their structural integrity is compromised, they can fracture and collapse with little or no warning.  
 
We have issued guidance about RAAC to enable building managers and their consultants to manage the situation, established a RAAC working study group.

Our advice, alongside that of Collaborative Reporting for Safer Structures UK (CROSS-UK), is that if a building owner or manager has a building from this period and is unsure of the form of construction, they should carry out an inspection and a risk assessment.

If RAAC planks are present, their structural condition will need to be determined by a Chartered or Incorporated Structural Engineer. The IStructE has provided guidance on this investigation and assessment:

Subject to the Chartered or Incorporated Structural Engineer’s findings, a process of ongoing monitoring and/or remedial propping or strengthening works may be needed. In some instances, it may be necessary to remove or replace RAAC planks.

We understand the government decision, as publicised on the 31 August, to advise school building operators to carefully manage areas where RAAC is present whilst these further actions are planned.

We continue to work proactively with stakeholders to help them understand and deal with the issue. We also support the continued inspection of buildings to determine the extent of RAAC within the public realm and manage the issues. 

 

About RAAC and concrete 

RAAC is a highly aerated, lightweight, concrete based material, with different material properties to conventional concrete. It was typically used in precast panels in walls, roofs and sometimes floors. Problems associated with older forms of the construction include high deflection, corrosion and spalling, and, where there is a low-end bearing, the possibility of sudden collapse due to cracking.

However, traditional concrete is a highly reliable material with high compressive strength, that when combined with steel reinforcement to become ‘reinforced concrete’, has the ability to form some of the world’s biggest and heaviest loaded structures ranging from high-rise buildings to bridges, dams, and nuclear power stations. 

- ends - 
 

Q & A's


 

1. What is RAAC?

RAAC is a building material used in some buildings to form roof planks, wall panels, and sometimes floor planks, between the mid-1950s and mid-1990s. 
 
It is a highly aerated, lightweight, concrete-based material, with different material properties than conventional concrete. As such the properties of the material and structural behaviour differ significantly from ‘traditional’ reinforced concrete.
 
RAAC is predominantly found as precast panels in roofs (commonly flat roofs but sometimes pitched) and occasionally in floors and walls. Panels forming flat roof structures are often difficult to access and are therefore difficult to survey, maintain, and replace (due in part to sub-ceiling structures, including services, suspended ceilings, finishes, and external roof membranes).
 
 

2.  Why was RAAC originally specified?

RAAC was seen as a lightweight, robust, and, crucially, cost-effective material.  
 
This made it particularly attractive for public sector buildings where it could be quickly manufactured and installed, to bring critical infrastructure (eg schools and hospitals) online sooner.  
 
It also possessed strong thermal performance quality, which created further economies within the heating and ventilation of these buildings.  

3. What is the lifespan of RAAC? The media and some professionals have stated it only has a 30 year lifespan.

We are aware of these statements and they are misleading. We believe they relate to correspondence exchange in The Structural Engineer, (which is the IStructE magazine) in 1995, there is no specific data that we can point to that supports the stated 30-year lifespan.

If manufactured correctly, installed correctly, and appropriately maintained (for example no overloading and managing water ingress) throughout its in-use life then RAAC should perform comparably with similar materials.

The lifespan of RAACs will depend on how it was manufactured, constructed, and maintained, including whether there has been any water ingress, as this can add to the dead weight, induce corrosion of the steel reinforcement, and weaken the strength of the AAC.

The risk of sudden failure can stem from RAAC panels with inadequate bearing (support) length. It is therefore important that the bearings of the RAAC panels are inspected and measured.
 

4. Why has RAAC become a risk within the built environment? 

RAAC has been found to creep and deflect over time, particularly if thinner units were installed.

The panels are porous and if they are subjected to water penetration they can deflect further. Water penetration can also cause the reinforcement to corrode, compromising the material and causing it to spall and break apart.

As mentioned earlier, it has also been found that if panels have insufficient bearing and their structural integrity is compromised, they can collapse with little or no warning. This may occur where the internal reinforcement stopped short of the load-bearing point. This is difficult to detect without intrusive investigation.

5. Why is RAAC an issue now?

It came to public and media attention when the Department for Education (DfE) decided to close schools before the start of term in late August 2023 in a change of policy guidance. This is because part of a RAAC roof panel in a school unexpectedly spalled on 24 August when workmen were drilling into it to install light fittings. DfE was also notified of two other examples of RAAC plank failures. These discoveries led to a swift reappraisal of their perception of the potential risk posed by uninspected RAAC panels. (see reports from select committee hearings https://www.building.co.uk/news/collapse-that-sparked-raac-u-turn-caused-by-bearing-failure-says-dfe-official/5125257.article)

There have been warnings about the problems of RAAC for some time. In the 1990s, there were other concerns raised relating to structural deficiencies in RAAC by both the Building Research Establishment and SCOSS (the Standing Committee on Structural Safety).

In December 2018, the DfE and the Local Government Association (LGA) made building owners aware of a recent RAAC roof panel failure in a property constructed using RAAC. In May 2019, SCOSS raised an alert to emphasise the potential risks from such construction, highlighting the failure of a RAAC panel roof construction within an operational school. This collapse was sudden, with no warning signs.
 

 

6. Does this mean all the concrete in the building is at risk?

No, traditional concrete is a highly reliable material with high compressive strength.  When combined with steel reinforcement it becomes ‘reinforced concrete’.  
 
It has the ability to form some of the world’s biggest and heaviest-loaded structures ranging from high-rise buildings to bridges, dams, and nuclear power stations.   

 

7. The DfE stated their decision was based on failures reported to them possibly not covered by IStructE guidance. What is the current IStructE's position?

According to the Education Committee “Oral evidence: unsafe concrete in education settings, HC 1817,2 Tue 19th Sept”, three failures were reported to the DfE over the summer of 2023. Information was passed to the IStructE study group for them to assess against the guidance we published in February 2022 and April 2023.

These three failures were: a roof panel end bearing failing, where the panel remained in place on its supporting steelwork, failures of sections of a roof panel in another school when builders' work was being carried out, and the failure of a roof panel in end bearing/shear in a commercial setting, where the panel fall had been arrested by machinery below.

The study group assessed these failures and concluded that the incidence and risk regarding all three were covered by the existing IStructE guidance and the current guidance therefore remains valid. We have informed the relevant authorities.

8. What guidance is IStructE providing to support the identification, removal or strengthening of existing RAAC?

Our RAAC website page includes advice and guidance about this material and how it should be managed.  
 
This page explains the key actions to support building owners/ managers, including suggested steps that building owners/managers who are responsible for the safety of buildings should follow. This enables building owners/ managers to address any issues relating to RAAC that may be present in their structures. These steps are Identification, Assessment, and Solutions.  
 
IStructE has issued guidance about RAAC to enable building managers and their consultants to manage the situation. 
 
Following failures in reinforced autoclaved aerated concrete (RAAC) planks/panels, SCOSS issued an alert along with reports of failures to CROSS.  
 
This led to the Institution establishing a RAAC working study group, comprised of senior members who are experts in RAAC, to help provide up-to-the-minute consultancy, academic and industry and expertise on this issue.  
 
We also continue to liaise with relevant government departments closely. And we will keep our guidance under constant review. 
 
Additionally, we are looking to develop additional support, such as holding technical lectures and we are also developing RAAC training/CPD for all our members, and looking to update our online RAAC register so that members can self-declare their RAAC expertise. However, please note that any competent IStructE Chartered and Incorporated Members will have experience in assessing structures, using their engineering judgement, and providing solutions for different types of buildings. 
 
We have an open study group, where we aim to keep members up to date with our guidance. You do not need to be a member of the Institution in order to join the study group - it is open to all. 

 

9. What measures should be taken if we suspect, or are unsure, that RAAC is present within our building?

Our advice, alongside that of Collaborative Reporting for Safer Structures UK (CROSS-UK), is that if a building owner or manager has a building from the affected period and is unsure of the form of construction, they should carry out an inspection and a risk assessment.

If RAAC planks are present, their structural condition will need to be determined by a Chartered or Incorporated Structural Engineer.
 
The IStructE has provided guidance on this investigation and assessment: 

Subject to the Chartered or Incorporated Structural Engineer’s findings, a process of ongoing monitoring and/or remedial propping or strengthening works may be needed. In some instances, it may be necessary to remove or replace RAAC planks. 

10. Why is it important to work with an IStructE Chartered or Incorporated Engineer?


IStructE professional recognition is a mark of quality and should always be sought when looking to engage a structural engineer to address issues as serious as this one.

In order to achieve this award our members have had to demonstrate their competence. Furthermore, all IStructE Chartered and Incorporated have to abide by our Code of Conduct.
 

11. Are there enough skilled engineers to assess and survey buildings suspected or with RAAC?

Because there is no register of RAAC in buildings, we do not know how many are affected. Therefore, we do not know how many structural engineers or surveyors will be needed to do this work.  
 
We are developing additional support, such as holding technical lectures and RAAC training/CPD for all our members, and looking to update our RAAC register so that members can self-declare their RAAC expertise. However, please note that any competent IStructE Chartered and Incorporated Members will have experience in assessing structures, using their engineering judgement, and providing solutions for different types of buildings and should be able to identify and assess RAAC.
 
 

12. Do we know how much RAAC is in the public estate and also in the private sector or people’s home?

No, we don’t know how prevalent RAAC is, because there is no RAAC register in the UK. 
 
As far as we are aware to date, RAAC has only been associated with two estates that used a trade-marked type of RAAC construction called Siporex – at Basildon, Essex, and a site in Scotland. But landlords need to be vigilant.

There was some local authority housing built in the 1960s and 1970s with flat roofs using RAAC, some of which would have been demolished, and it is also in student residences. Buildings from the 1960s and 1970s that have been converted into housing could contain RAAC, though, again, this is unlikely to be widespread.

In September the Social Housing Regulator wrote to those organisations (mainly housing associations and local authorities) that come under its aegis pointing out that it “expects landlords to ensure that they have a good understanding of their homes, including building safety issues and whether homes contain RAAC  components and the risk to tenant safety arising from these; that you develop proportionate mitigation and remediation plans where required, and seek suitably qualified advice where necessary."
 

13. Is RAAC still being used?

RAAC is still manufactured and installed in many parts of the world including Germany, North America, Australia, China, India, Mexico and Japan, and others.

It can be an appropriate construction material if properly designed, manufactured, installed, and maintained. As far as we know, use in the UK declined in the 1990s.
 

14. Who do I contact for further information on this subject?

For further technical information please contact [email protected]

Please note that IStructE is not an advisory body and cannot answer questions related to individual projects. For project assistance refer to our “Find an Engineer” portal. For general advice, refer to our technical guidance.

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