More than 100 schools in England have been told to close at least some part of their buildings because they contain unsafe concrete.
Reinforced autoclaved aerated concrete (Raac) was widely used in schools and other buildings such as hospitals between the 1950s and 1990s.
Last week, the government said schools should close after a beam collapsed at an educational setting.
Ministers are under pressure to officially name the schools affected, despite more than 70 already being identified in the media.
Questions also remain over how to pay for repairs to affected schools, with the total bill expected to run into the billions.
On Monday, prime minister Rishi Sunak - who has been forced to defend himself over accusations he failed to fully fund a school rebuilding programme while chancellor - insisted that extra money will be made available to schools outside their existing budgets, something hinted at by chancellor Jeremy Hunt at the weekend.
Watch: Rishi Sunak defends himself over school building funding accusations
However, it has been reported that Treasury sources indicate the money would come from the Department for Education’s (DfE) existing capital budget.
Yahoo News UK examines the options available to structural engineers when they find Raac at a school:
What is Raac?
Reinforced autoclaved aerated concrete (Raac) is a lightweight material that was mostly used in flat roofings, but also in floors and walls, between the 1950s and 1990s.
It has a bubbly, porous appearance and was a cheaper alternative to standard concrete but it has a limited life of 30 years.
Raac is not as strong to standard concrete and must be coated with another material to protect it against corrosion.
However, Raac is susceptible to cracking because of moisture as its bubbles can allow water to enter it. And if a roof leaks, the material used to reinforce Raac can also degrade.
Raac can also lead to sagging panels and water ponding on a roof, making leaks more likely and adding extra loads on its panels.
- What is Raac concrete? The concrete that could cause English schools to collapse (Northern Echo, 4 mins)
How to identify Raac
The Department for Education (DfE) has issued guidance on how to identify Raac on flat roofs and in pitched roofs, floors or walls.
It said: "RAAC panels are light-grey or white in appearance, the underside of the panels will appear smooth. The inside of the planks will appear bubbly, often described as looking like an Aero bar.
"Unlike traditional concrete, there will not be visible stones (aggregate) in the panels."
The department said Raac panels are "very soft" and that an indentation can be made if you press a screwdriver into the surface.
After identifying that a building contains Raac, the department advises the appointment of a building surveyor or structural engineer.
- Construction expert discusses how to fix problem of ageing concrete in schools (The Independent, 1 min)
How can Raac be fixed?
According to advice published in April by the Institution of Structural Engineers, there are at least five options available when tackling Raac.
These remediation strategies include:
- Adding secondary supports or beams at the end bearing to provide an increased effective bearing length.
- Adding positive remedial supports to actively take the loading from the panels. This could include new timber or lightweight structures to support the panels directly.
- Passive fail safe supports to mitigate catastrophic failure of the panels if a panel was to fail. This could be a secondary structure designed to support the panels.
- Removal of individual panels and replacement with an alternative lightweight solution.
- Entire roof replacement.
Raac expert Chris Goodier, a professor of construction engineering and materials at Loughborough University, said: "This might mean a complete replacement of the roof, which takes time and can be costly, or it might mean some form of failsafe or structural strengthening, which involve installing, for example, some steel or timber brackets.”
The Department for Health and Social Care has announced that the seven NHS hospitals most affected by Raac will be replaced by 2030.
In a statement last week, it described how Raac will be tackled in hospitals, saying: "The approach will differ depending on the assessment of risk but these include secondary supports or beams for the highest risk to inspection regimes for lower risk areas, as well as plans for limiting operational loads, such as no-walk zones on Raac roofs and maintaining roof drainage, restricting new or removal of existing equipment or reducing humidity."
- Which NHS hospitals have Raac concrete? (National World, 2 mins)
What do roofers say about Raac?
Roofing companies generally advise that Raac should be fully removed.
On its website, waterproofing group Tremco says: "Many structural deck types can be repaired when cracks or other deterioration is found without the need to replace the whole supporting structure.
"However, due to the nature of Raac, with potentially no warning signs before failures and buildings exceeding the structural deck’s lifespan, the safest solution would be to fully remove it.
"When it comes to Raac, even if no damage is found, it is still advisable to completely remove the fragile deck eliminating any future risk."
Waterproofing company Garland UK said: "If a Raac roof deck has indeed been found, then a structural engineer could be commissioned to report on the condition of the deck and, if found to be in good structural condition, they may deem it possible to overlay with a new membrane.
"However, it is our strong advice that considering the life span of Raac roof planks has now been exceeded, this is unlikely to be feasible or safe. Garland UK would not recommend retaining or working on existing Raac decks.
"If a Raac roof is discovered, we will always recommend to our clients that the Raac roof should be completely removed and replaced with a suitable timber or metal deck, keeping your building safe and protected for years to come."
Watch: Chancellor says government 'will spend what it takes' to fix schools concrete problem