RAAC explained – and the concrete plan of action you should take Thursday, 14th September 2023

RAAC explained – and the concrete plan of action you should take

By Andrew Wright, Structural Engineer and Director, Ingleton Wood

Teachers, parents, pupils and communities came together admirably to overcome a diverse set of difficult challenges during the pandemic and have been rightly applauded for catching up on educational standards and accelerating sustainability measures over the past two years.

What they really didn't need was another potential crisis to resolve

But then came ‘Reinforced Autoclaved Aerated Concrete’ (RAAC) that the public and probably most teachers hadn’t heard of until the concluding days of the summer holiday. Of course, structural engineers have been only too aware of its existence and serious shortcomings for decades.

The front-page announcement was that the potentially dangerous RAAC building material, which dates to Britain’s post-war building boom and is scornfully known across the built environment as the ‘Aero bar’, had been identified in over 150 schools across the UK.

The abrupt disruption has provoked scenes reminiscent of lockdown and prompted many questions, while urgent remedial action continues to take place.

With a proven track record of supporting those responsible to identify and commit to a programme of remedial work for RAAC and similar building safety issues, this explainer article is designed to help you navigate the next steps if your building is confirmed or suspected of having RAAC material.

What is RAAC

Invented in the 1930s, Reinforced Autoclaved Aerated Concrete (RAAC) is a lightweight, ‘bubbly’ form of concrete that was commonly used in the construction of thousands of buildings from the 1950s until the mid-1990s, such as schools, hospitals, courts, leisure centres and council offices.

As a cheaper building material, RAAC was in high demand following the Second World War when budgets were limited for the nation's mass rebuilding programme.

Where is RAAC found in buildings

RAAC is usually used as precast panels in roofs, particularly flat roofs, and occasionally in floors and walls.

It is most renowned for being used in flat roofs, such as classroom buildings, because of the simple installation method of placing multiple panels. It can be found in any building built or modified during the second half of the 20th century.

How serious are the safety risks of RAAC

In the 1990s, it was discovered that RAAC panels were exhibiting several defects such as cracking and spalling. At the time it was thought to be caused by poor reinforcement detailing and installation, defective workmanship, and possible thermal effects caused by roof finishes.

As well as its porous nature having the potential to absorb water and making the structure heavier, RAAC panels are vulnerable to large units being placed on top like water tanks. Subsequent felt roof repairs or overlays of the existing finishes can also increase the load on the panels and possibly change the manner in which they perform. Water ingress and subsequent retention in the voids can lead to corrosion of the steel reinforcement within it.

This puts the affected roof or wall at serious risk of complete collapse, with a sudden structural failure at the plank bearings being a significant concern.

This can happen without warning, especially if unmonitored or unmanaged over time.

In 2018, the roof of a school in Kent did just that – collapsed without warning. Fortunately, it was on a Saturday and no-one was injured.

The incident triggered a series of RAAC safety warnings, thousands of surveys, and sustained engagement between government and headteachers, that has led us to this current situation.

For now, most of the current publications on RAAC concern public buildings, particularly the education sector. In terms of how widespread, there are over 22,000 schools and colleges in the UK and less than 1% are known to be affected currently.

However, privately owned properties in the commercial and residential sectors may not be immune. The situation facing the sectors is currently unknown but prudent action now would involve the commission of initial inspections and/or risk assessments to establish the potential scope and scale.

What schools and building owners need to do now

Our main advice for headteachers and others responsible for building safety comes in two phases.

First, you need to employ a qualified building surveyor or qualified building engineer to conduct an initial appraisal at your site, whether you suspect you have RAAC or if you simply don’t know and need reassurance either way.

In my experience, the tiny hairline cracks that often appear close to the supports, and which are the hallmarks of RAAC, can be difficult to spot by the naked eye. This is why only qualified and experienced experts in their field should conduct this painstaking work.

Our Practice has been providing this type of assistance to many schools following the Department of Education’s instructions for schools to close areas affected by RAAC.

Second, if RAAC is subsequently suspected or confirmed as being present, you should employ a chartered structural engineer to obtain a more detailed understanding of your unique situation and provide a bespoke programme of recommended remedial work.

How Ingleton Wood can support your next steps

Like RAAC building material, Ingleton Wood can trace its origins back to the 1930s.

And with decades of expertise, our in-house structural engineers and building surveyors can deliver an effective integrated service to risk assess, investigate and remediate RAAC issues for you.

Committing to a concrete plan of action like this can be both complicated and costly. However, prevention is better than cure.