Sunday September 3rd 2023
An image of RAAC (not in Midlothian)
Given the current focus on Reinforced Autoclaved Aerated Concrete (RAAC) in school buildings it was timely that last week Midlothian Council presented a report to councillors on RAAC in Midlothian schools.
The report advised members of the survey work carried out to understand the process of assessing, investigating and managing any presence of RAAC panels in floors, walls, eaves and roofs (pitched and flat), of council buildings which followed an alert by the UK Government’s Department for Education drawing attention to their advice.
A Council Officer gave a verbal update at the meeting. Seventeen school buildings were identified as having being built within the period when RAAC could have been used in their construction. The council’s Estates Team conducted initial studies and assessment followed up by the appointment of qualified specialists to undertake site visits and detailed surveys.
In all of the 17 school buildings no RAAC was suspected or found. The council are now extending the scope of the work beyond the education estate to other council owned buildings and a further report will be presented.
In March of this year the Brunton Theatre was closed by East Lothian Council as ‘crumbling concrete’ found in roof.
You can watch the Midlothian Council meeting on the video below
What is RAAC?
Reinforced Autoclaved Aerated Concrete (RAAC) is a lightweight form of concrete. The Standing Committee on Structural Safety (SCOSS) has noted that: ‘Although called “concrete”, (RAAC) is very different from traditional concrete and, because of the way in which it was made, much weaker. The useful life of such panels has been estimated to be around 30 years’ (SCOSS Alert, May 2019).
RAAC ‘panels’ were precast offsite and used for flat and pitched roofs, eaves, floors and walls within building construction. RAAC was used in UK schools, colleges and other building construction from the 1950s until the mid-1990s and may be found in any educational or ancillary building that was either built or modified in this time period. RAAC panels can span between isolated beam supports (steel or concrete) or onto masonry walls (brickwork or blockwork).
The potential risks from such construction and highlighted the failure of a RAAC panel roof construction within an operational school. This collapse was sudden as RAAC has the following embedded systemic problems:
– Panels have low compressive strength, being around 10-20% of traditional concrete, meaning the shear and bending strength is reduced. This strength is further impacted by water saturation.
– It is very porous and highly permeable. This means that the steel reinforcement within the panels is less well protected against corrosion ‘rusting’ than steel reinforcement in traditional concrete.
– The reinforcement within RAAC panels is less well bonded to the surrounding concrete. The dominant connection is via
secondary reinforcement (transverse reinforcement).
– It is aerated (looks ‘bubbly’) and contains no ‘coarse’ aggregate, therefore it is less dense than traditional concrete; being around a third of the weight.
– RAAC has reduced ’stiffness’ characteristics resulting in high displacements, deflections and sagging.
– The bearing of planks is often insufficient, by comparison to modern standards, which presents a significant risk.
– There was limited quality control during manufacture and installation meaning there is a high degree of variability between panels.
– RAAC panels can span between isolated beam supports or onto masonry walls
It is recognised that RAAC panels have material and construction deficiencies making them less robust than traditional concrete. This increases the risk of structural failure, which can be gradual or sudden with no warning. Sudden failure of RAAC panels in roofs, eaves, floors, walls and cladding systems would be dangerous, and the consequences could be serious.
In the 1990s, several bodies recognised structural deficiencies apparent in RAAC panels, that the performance was poor with cracking, excessive displacements and durability all being raised as concerns.
In the mid-1990s, the Building Research Establishment (BRE) undertook a number of inspections of school roofs, reporting the findings within BRE Information Paper IP10/96. The concerns were also raised within the 1997 Standing Committee on Structural Safety (SCOSS) report. The report recommended that school owners should identify and inspect RAAC panel construction to determine deterioration and put in place management strategies.
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