Reinforced autoclaved aerated concrete( RAAC) is a featherlight material that was used substantially in flat roofing, but also in bottoms and walls, between the 1950s and 1990s.
It’s a cheaper volition to standard concrete, is hastily to produce and easier to install. It’s aerated, or” gamesome”, like an Aero chocolate bar.
But it’s lower durable and has a lifetime of around 30 times.
Its structural geste
differs significantly from traditional corroborated concrete.
also, it’s susceptible to structural failure when exposed to humidity. The bubbles can allow water to enter the material.
still, any rebar buttressing RAAC can also decay, rust and weaken, If that happens.
Because of this, RAAC is frequently carpeted with another material, similar as bitumen on roofing panels. But this material can also degrade.
The Standing Committee on Structural Safety( SCOSS) noted that” Although called’ concrete’, RAAC is veritably different from traditional concrete and, because of the way in which it was made, much weaker.”
According to Loughborough University, there are knockouts of thousands of these structural panels formerly in use and” numerous are showing signs of wear and tear and gash and deterioration”.
The Health and Safety Executive says RAAC is now beyond its lifetime and may” collapse with little or no notice”.
The differences to traditional concrete- and the pitfalls of RAAC use- were outlined as far back as 1961, around the time when Britain first began to use the material.
The Institution for Structural Engineers said in a report, seen by the BBC’s Newsnight, that RAAC was so different to traditional concrete that” it is maybe unfortunate that the term concrete has been retained for these aerated products”.
The report set up that short- term exposure to humidity reduced strength by about 13, while long- term exposure to” weakened air” reduced it by 40.
The implicit safety issues of geriatric RAAC were first reported in the 1980s and 1990s, when roof collapses led to structures being demolished.
A 1996 government- funded report by the Building Research Establishment had set up RAAC panels cracking in a casing development, and cracks and bends in panels installed in seminaries.
It said that while there were no immediate safety pitfalls, any RAAC panels in visually poor condition should be audited every time.
It recommended examinations every five times for those in good condition.
A report by the same body in 2002 came to three new conclusions
- Material used to cover RAAC will have veritably presumably come compromised in panels over 20 times old.
- Crucially, that erosion can do without visual suggestion that the panel was in poor condition- ie, there was a the threat of collapse without advising in panels over 20 times old.
- Some panels were shy and didn’t meet regulations when they were installed.
The government has known since 1994 that some public sector structures contain potentially compromised RAAC- and has been covering their condition since 2018. Professor Chris Goodier from Loughborough University, who has studied the problem for the NHS, said” We’ve a veritably old structure stock in this country right back to the puritanical period and artificial revolution. RAAC is one of numerous accoutrements that hasn’t been looked duly over the decade.” This is a particular problem for seminaries which occasionally have just one structure director, untrained in structural engineering. New guidance was issued in 2021 and 2022 about how to manage RAAC, and the DfE transferred out a questionnaire last time to” all responsible bodies”, asking them to give information about the use of RAAC in seminaries across the country. seminaries Minister Nick Gibb said the expert advice had been that if RAAC wasn’t in a critical condition, it was safe to continue to use the structure.
But over the summer a RAAC ray that had preliminarily been considered low threat collapsed, he said. That led to those seminaries linked with RAAC now all being labelled potentially dangerous and closed or incompletely closed at short notice.