This week's newsletter looks at the recent warnings about reinforced autoclaved aerated concrete (RAAC), which was in heavy use over the 1950-90s, but is now the subject of safety warnings across the UK, with hundreds of buildings at risk.
RAAC was a convenient building material used in the second half of the 20th century as it was cheaper than traditional concrete to produce, quicker to use and easier to construct with. It was also lighter than traditional concrete with its aerated bubbles. However, the downside is that its lifespan was only ever considered to be for around 30 years. Furthermore, RAAC panels and structural elements made with the material appear to be easily compromised by exposure to moisture and water making them prone to crumbling.
The fact that this construction material was used to build schools and other public buildings, such as hospitals and colleges, has become a concern as many of these buildings have not been adequately maintained and the projected lifetime of RAAC in most buildings has now expired.
"RAAC roof planks were last installed in the 1980s and have now exceeded their 30-year life span. If discovered, we would always recommend their complete removal and replacement with a new roof deck to ensure the continued safety of your building," said Ben Whitemore, technical product manager of roofing specialists Garland UK.
RAAC technology
RAAC products are formed by first mixing cement, lime, extremely fine sand or pulverised fuel ash and calcined gypsum. Aluminium powder and water are then added to form a concrete slurry which is cast in a mould containing the steel reinforcement. The aluminium powder chemically reacts with the lime and water to produce small bubbles of hydrogen gas, which causes the mixture to foam, more than doubling its volume. Finally, the mould is removed, and the product is cured at both high temperature and pressure in an autoclave for around 12h, which causes the sand and lime to fuse into a form of calcium silicate hydrate crystal, increasing the strength of the finished product.
What's happening now?
A sudden roof collapse at a primary school in Essex, England in 2018 brought the RAAC issue to light and now a total of 20 hospitals at 18 NHS trusts around the UK have been identified as suffering from the blight. Education Secretary Gillian Keegan closed more than 100 UK schools and colleges after the findings in the first week of September 2023 and many theatres have also been forced to close.
While the exact scale of the problem is still being assessed, the cost of repair or demolition is expected to be considerable. “Builders could be overwhelmed with repair work to schools, hospitals, theatres, and other unsafe public buildings due to failing RAAC panels. In September 2023, the government revealed 156 schools contained reinforced autoclave aerated concrete, RAAC, of which 104 required urgent action with 52 already repaired,” said Hank Zarihs Associates.
Continued risk
While RAAC use was phased out in the 1990s, many buildings using this construction method remain and need to be checked annually for deterioration, a process that has been ongoing since the 1990s. There is a continued risk of RAAC failure in buildings. RAAC's use has mainly been cited in the UK, but it was also very popular in Ireland, Australia, New Zealand and South Africa from the 1950s up to the 1980s.
The implications for the evolving range of low-carbon cements, such as those using calcined clays, new supplementary cementitious materials and recycled materials, in cement production could be far reaching. While the ASTM and CEN standards take significant time to become accepted for commercial use, new cements don't have the credentials of cements that have been used for decades. Continued testing will be required for cements that replace ordinary Portland cement (OPC) for many decades. Public authorities, architects, construction and insurance companies need to be able to assess the strengths and weaknesses of different building materials for structural failure. RAAC is a lesson in ensuring that building materials are not used past their expected lifetime and that the products are fit for the purpose they were designed for.
