A number of our clients have been concerned about reports recently regarding reinforced Autoclaved Aerated Concrete or RAAC in schools. And, understandably, asking us whether their buildings are likely to contain it too.
RAAC has been in the national news in conjunction with the urgent closure of schools found to contain this type of concrete as there is a real risk of sudden collapse, putting both staff and pupils in danger. A small number of schools have suffered catastrophic failure of their roof structures and it was fortunate there were no serious injuries or fatalities in these cases.
What is RAAC
If you imagine normal dense concrete to be like the most dense plain chocolate bar which you can barely break with your fingers or teeth. Then in comparison, RAAC is like a thin wafer of honeycombed chocolate, similar to an aero or crunchy bar. It is much lighter and more easily broken into pieces under certain conditions.
RAAC was widely used by the construction industry from 1950 to 1995 mostly in flat roofing, less frequently in floors and walls. In particular, RAAC was chosen for the construction of schools as it was a cheaper alternative to standard concrete, quicker to produce and easier to install making it more cost-effective. And it was much lighter and provided better thermal insulation. It was also used in the construction of other public building projects including hospitals, as the units were factory made, making them ideal for larger buildings with repetitive footprints.
We are not aware of RAAC’s use in domestic residential construction but this cannot be completely ruled out in large Local Authority blocks built during this period. The Institution of Structural Engineers said in a report to the Government that RAAC was so different to traditional concrete that “it is perhaps unfortunate that the term concrete has been retained for these aerated products”.
Where is RAAC most commonly found?
RAAC is mostly found on flat roofs, but it can also be found in pitched roofs and in floors and walls.
How long does RAAC last?
The life expectancy of RAAC can be as low as 30 years, which is roughly half the theoretical design life of buildings built using dense concrete, although both normally last longer. The failure of reinforced concrete tends to be gradual whereas with RAAC it can happen suddenly, which is one of the main issues as there can be little or no warning prior to building collapse. This is why the Government has made the decision to close those schools which contain RAAC for safety reasons until the appropriate action can be taken.