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School concrete crisis: What methods will be used to fix crumbling Raac buildings?

The process to repair schools that are affected by Raac will be complicated, lengthy and expensive.

Remedial work being carried out at Mayflower Primary School in Leicester, which has been affected with sub standard reinforced autoclaved aerated concrete (Raac). Picture date: Monday September 4, 2023. (Photo by Jacob King/PA Images via Getty Images)
Remedial work being carried out at Mayflower Primary School in Leicester, which contains Raac. (PA)

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.

(Department for Education)
Raac has a bubbly texture that leaves it prone to moisture. (Department for Education)

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."

(Department for Education)
A drawing of a Raac panel illustrating how to identify it. (Department for Education)

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.

LONDON, ENGLAND - SEPTEMBER 01: Scaffolding is seen outside classrooms as repair work continues at Hornsey School for Girls on September 01, 2023 in London, England. The school has rushed to reinforce a number of classroom ceilings after being notified of the potential collapse of structures using a certain type of aerated concrete. Over 100 schools across England have been told that they need to close buildings or implement emergency safety measures due to the use of reinforced autoclaved aerated concrete (RAAC), a lightweight but less durable type of concrete used extensively in the latter half of the 20th century. (Photo by Leon Neal/Getty Images)
Scaffolding is seen outside classrooms as repair work continues at Hornsey School for Girls, which has Raac in its building. (Getty Images)
A taped off section inside Parks Primary School in Leicester which has been affected with sub standard reinforced autoclaved aerated concrete (Raac). More than 100 schools, nurseries and colleges in England have been told by the Government to close classrooms and other buildings that contain an aerated concrete that is prone to collapse. Picture date: Friday September 1, 2023. (Photo by Jacob King/PA Images via Getty Images)
A taped off section inside Parks Primary School in Leicester which has been affected with sub standard reinforced autoclaved aerated concrete, or Raac. (PA)

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.

Workmen at Abbey Lane Primary School in Sheffield, which has been affected with sub standard reinforced autoclaved aerated concrete (RAAC). More than 100 schools, nurseries and colleges in England have been told by the Government to close classrooms and other buildings that contain an aerated concrete that is prone to collapse. Picture date: Friday September 1, 2023. (Photo by Danny Lawson/PA Images via Getty Images)
Workmen at Abbey Lane Primary School in Sheffield, which has been affected with sub standard reinforced autoclaved aerated concrete, or Raac. (PA)

"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