It clearly hasn’t just been found out about this type of construction. Has there been an incident that hasn’t been reported? Or someone suddenly decided to create issues that would probably be ok for many years with regular inspection?
The media reports suggest that while the deterioration has been a known issue for several years, it was the (relatively) recent sudden and total failure of a beam which was showing no signs of deterioration that set the cat among the pigeons.
It wasn't the failure of a beam, as RAAC isn't strong enough to make beams from. The beam failure issues were with
pre-stressed beams made with High Alumina Cement (HAC) in the 1970s. RAAC was developed as a way to make lightweight, low-load-bearing panels for flat roofs and walls.
RAAC is concrete with no aggregate that is mixed with a foaming agent that is then cured with pressurised steam in an autoclave that produces a foam-like cross section.
As it is full or air bubbles it has very good insulation properties and is lightweight which meant longer spans or less supporting columns could be specified as there was less weight to support, and the lack of aggregate made it very cheap to produce, but that lack of aggregate leaves it very weak, with the steel reinforcement making up most of it's strength (in regular reinforced concrete the steel handles the tension loads and the concrete handles the compressive loads).
Unfortunately, it's foam-like consistency also leaves it prone to moisture absorption and retention. When it was originally developed it was intended the reinforcing used should either be made from stainless steel, or carbon steel that was galvanised or had some other rust protection system applied. Unfortunately, to reduce cost, standard 'black bar' steel (the same that is used in standard concrete) was routinely used instead, in the hope that the waterproofing systems that would be applied on top of the panels should stop any moisture ingress (it ended up also being used to make walls and celings of humid areas like swimming pools, changing rooms and kitchens, with no waterproofing applied on the inner surfaces, however).
When moisture does get into RAAC it has two effects - it increases the loading on the panel from it's own weight, and causes the carbon steel reinforcement (which is providing most of the strength) to corrode. As it corrodes, it then causes the concrete cover to spall off, weakening the panel further and exposing the steel, increasing the rate of corrosion.
Problems with it first became an issue in the early 80s, as it had been used for roofing panels that did not have enough strength to support themselves over the spans they were intended to cover, or had incorrectly placed or missing steel reniforcement. When these issues came to light, inspections were done and the affected buildings either had the panels replaced, or had additional support columns installed. There were however plenty of others that seemed not to have any issues where nothing was done, and no subsequent inspections were made. In the 40 years since, these panels have degraded to the point of being life expired, mostly due to moisture ingress due to failures of the waterproofing systems that were applied on top of them,
The Building Research Establishment
published a paper on the end-of-life isues with RAAC roof planks designed before 1980 in 1996. In 1999 the Standihg Committee On Structural Safety (SCOSS) issued warnings about RAAC panels being life expired, and the BRE issued
test results on expired panels and revised design guidance for the use of RAAC in 2002, but successive governments have bascially kicked the can down the road about doing anything about it, presumably in the hope the buildings would get replaced before they fail (preferably by some nice fat PFI contracts to their chums in the building industry, directorships all round etc.).
The issue finally came to a head in 2018 after
a primary school roof collapsed in Sutton near St Helens due to failure of the RAAC panels (luckily it was early in the morning before it opened). As a result fo the investigation into that incident, and issues subsequently discovered with spalling of RAAC panels at a retail premises, SCOSS commissioned further research and issued
further warnings and calls for urgent inspections in 2019.
The NHS, MoD and DfE all then began inspection programmes, the results of which they seem to have been trying to keep quiet until now. It first appeared in the media in March this year in an
ITV news report. In June this year it was reported that a
number of schools in Newcastle had structural issues as aa result of RAAC inspections.
![_130985800_therectangularroofpanelsintheschoolslibrarycontainraac[1].jpg](https://www.railforums.co.uk/data/attachments/137/137672-c41294397f736572001ad1d61dbbe6fc.jpg "_130985800_therectangularroofpanelsintheschoolslibrarycontainraac[1].jpg")
