The president of the Institution of Structural Engineers has given the BBC a quick rundown of what RAAC (pronounced: rack) actually is.

"It has a C for concrete", says Matthew Byatt, "don't get it confused with concrete. Concrete is a very thick, dense material. RAAC is 70% air, it was developed with steel reinforcements which is the R (in RAAC)".

Byatt says the material has deteriorated over the years and suffers from creep deflection, "which effectively means it sags. And the more it sags, the more load goes into it and then it sags more".

Byatt says the best way to fix the problem is replace it. "Long term, it should be taken out of the building environment. It's passed its serviceable life.

"Short term, we know most of the failures commonly occur at the supports - like where it connects to walls - so the most common way of mitigating is to increase those supports and make sure it has more of a bearing under the supported edges of the panels."

Image source, .

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One parent has sent us a letter from their child's primary school, Willowbrook Mead in Leicester, saying it will be closing its building on Friday and Monday so that classrooms and furniture can be rearranged.

It goes on to say that it will reopen for the youngest pupils on Tuesday, but year 3 and above will move to online lessons.

"Throughout the pandemic, the community at Willowbrook rose to the challenge and supported each other and the school, showing great spirit, patience, and comradery.

"We are once again appealing to you all to support us and your children through a challenging time ahead," the letter says.

The letter also says children eligible for free school meals will still be provided for.

James Kelly
BBC News

Kenneth Hope’s 11-year-old daughter was due to begin her secondary education at Ferryhill School in Ferryhill, County Durham, on 5 September.

But this afternoon the family received an email informing them the school would not be open until 11 September.

Kenneth, who has six children, is not happy about the short notice which has put them in a "difficult position".

"My daughter just wants to get there, she’s been a bit apprehensive about starting secondary school," he says.

He is also not pleased that more Covid pandemic-style remote learning is being proposed as an interim solution.

Nathan Standley
Education reporter

Image source, Ed Lawrence

We’ve been outside Crossflatts Primary School in Bingley this afternoon speaking to parents about what this announcement means for them.

Two schools in this area have had to close classrooms to keep pupils safe.

Fiona told us her son Seamus is about to go into year 5 – but the presence of RAAC in his classroom means he’ll be taught in the school’s on-site nursery instead.

“Our head’s brilliant and on top of everything, so they already have things in place,” she says.

“Obviously it’s going to be disruptive, but for the children we’ll work around it – but it is a little concerning that it’s so close to going back to school.

“But these things happen don’t they? I’d rather my child not be in school than the school fall down around him.”

Emma Owen
Live reporter

What we know:

The government has told more than 100 schools, colleges and nurseries in England to close some buildings.

It is worried about buildings made with a particular type of concrete that is prone to collapse.

The buildings need to be made safe before people are allowed back in.

Schools will let parents know , externaldirectly if their children's schools are affected.

What we don't know:

Which schools are affected - the government is not publishing a list.

Whether it's nursery, primary or secondary - again, there's no list.

What the timeframe is - the government hasn't said when the concrete concerned needs to be replaced.

Whether schools will have to delay the start of term.

Read more in our explainer about the school closures here.

Shadow education secretary Bridget Phillipson accused the government of "still not being up front" over its decision to shut school buildings in England.

Philipson called on the government to "come clean to parents" and publish the full list of schools affected by the announcement.

Teaching union NASUWT says today's announcement highlights "more than a decade of wilful under-investment" in schools.

And according to its general secretary, Dr Patrick Roach, the government has avoided a major disaster by "mere luck rather than judgment".

He says pupils going back to school next week should not be facing the prospect of having their education disrupted because of "a lack of investment and foresight".

"It is a disgrace that, despite ministers' promises to the contrary, there are any schools in such poor condition and state of repair and in danger of collapse."

We've put together this graphic to give you a better idea of what reinforced autoclaved aerated concrete (RAAC) is and what it looks like.

It is a cheaper alternative to standard building material but it has a more limited lifespan.

Image source, .

The Local Government Association (LGA) has issued a statement in response to Westminster's decision to close down more than 100 school buildings across England.

The LGA said it had been warning of the risks of RAAC in schools since 2018 and called on the government to "urgently establish a taskforce, including with the LGA and councils, to ensure the safety of both pupils and staff".

The statement also calls for financial support from the government to support those educational settings which have had to close.

"Councils will now work fast to ensure children’s education is disrupted as little as possible," the statement concludes.

If you're just joining us, here's a quick summary of what we know about the school closures:

• The government issued orders to close over 100 schools after RAAC - a concrete that's prone to collapse - was found in buildings
• This lightweight "bubbly" form of concrete used widely between the 1950s and mid-1990s
• 156 schools are known to have RAAC. Of those, 52 already have safety mitigations in place
• The 104 that don't, need to find spaces to relocate to before term starts next week
• We don't yet know which schools are affected but the schools in question will inform parents
• The education secretary says parents shouldn't be worried, and that child safety is her priority

Image source, .

A short while ago, we heard from Education Secretary Gillian Keegan, who urged parents not to worry about the school closures.

"We've been working on this a lot in great detail. We will identify the schools, we will have the teams of people and we'll be making sure that we minimise this as much as possible," she said.

Image source, Ed Lawrence

Shahzad Ismail said he was "devastated" after his son's school in Bingley, West Yorkshire, informed him it would be shut over concrete safety concerns.

"Even the head teacher that sent that letter, you could tell from the letter how emotionally stressed out she was because it's going to widely affect a lot of children," he told BBC News.

A lift and a changing room in the school, used by Ismail's son who has special needs, is likely to be affected, he said.

"I'm just waiting to go back to school on Monday and see what areas are actually available for him," he added.

He said works to improve the school's structure will take about two years to complete.

Ismail said he doesn't know how the children are going to adjust.

"It's going to be a challenge for them," he added.

Asked about how long disruption to learning might take at affected schools, Education Secretary Gillian Keegan says it will vary.

"How long will depend on the case worker. but most of them will be starting the work straight away," she says.

"On the whole, many of these things can be mitigated quite quickly," she says. "But each case is individual."

Mitigation measures include things like relocating students and propping up buildings.

The education secretary is explaining why the Department for Education has intervened now, and says there are bodies responsible for the safety of school buildings and "review these things on a daily basis".

"Because we've been learning more and more about RAAC, we've identified that we are a bit more concerned about some of these schools," Gillian Keegan says.

"So we are doing something that we don’t normally do as a department of education: We are assigning a caseworker and getting surveyors in."

Education Secretary Gillian Keegan has been walking us through the process for identifying reinforced autoclaved aerated concrete (RAAC), the concrete prone to collapse, in schools.

She says it only affects buildings from a certain period of time, which we know to be the mid-1950s until the mid-1990s.

Schools were sent surveys and questionnaires, and engineers were sent out to scope the schools that wrote back confirming buildings from that time period.

Does that mean that there could be some left unidentified?

"We don't have all of the questionnaires, we only have those that have been responded to," says Keegan in response.

"We've been trying to get schools to respond to us for a long time now," she says, offering a plea for schools that haven't responded to do so.

Education Secretary Gillian Keegan has been speaking about what some of the safety mitigation strategies will look like.

She says depending on where the RAAC is on the school site, some children will be moved to another part of the building, in some of the buildings the roofs will be propped up, and some will have a temporary classroom.

Each of the 104 schools identified today will get an individual case worker, Keegan says.

"So we've got a number of different things we've been working on to minimise the disruption as much as possible for parents and for children," she says.

Education Secretary Gillian Keegan has just been speaking on the school closures.

She says most parents need not be worried at all - there are more than 20,000 schools in England, and just over 150 have RAAC present.

"We are working to minimise this a much as possible. The priority for me is your children's safety and that's why we are taking these precautionary measures," she says.

Keegan says engineers have been combing school sites looking for RAAC and over the summer, "a couple of cases have given us cause for concern".

"We need to take the cautious approach," she says.

Short on time and need to know about school concrete scare?

BBC reporter Vanessa Clarke explains in just over a minute all you need to know about school closures over safety fears.

Dozens of schools, nurseries and colleges ordered by the government to close will need to "either fully or partially relocate" while safety measures are put in place, the Department for Education (DfE) says.

The DfE issued guidance to 104 facilities in England, stating they should find emergency or temporary accommodation for the "first few weeks", until buildings are made safe with structural supports.

It recommends finding space in nearby schools or community centres, or an "empty local office building".

Remote education, popularised during the pandemic, should only be considered as a "last resort and for a short period", the guidance states.

About 52 of the 156 educational settings identified as containing the concrete have taken protective steps already this year.