A government plan to deal with reinforced autoclaved aerated concrete is needed, but engineers also have a role to play

Steve McSorley is director at Perega

Present within many hospitals, schools and municipal buildings, it’s a massive headache for both asset owners and occupants. That’s all before we’ve got to the private sector, where swathes of unidentified and unrecorded RAAC potentially exists.

Often you don’t know when this material will fail, and its widespread use in roofing means it poses a serious, spontaneous and potentially fatal, hazard. Structurally, it’s a ticking time bomb if left unaddressed. The best practice approach is to either support it or remove and replace it entirely, however, this is not without its challenges.

Perhaps we should start with a little context. RAAC was a building material used between the early 1960s and mid-1990s, particularly due to its relatively lightweight and low price point. This made it attractive for tightly budgeted public sector buildings where it was mainly used for roofs, floors, and walls.

In terms of its composition, RAAC is a precast product containing aerated cementitious materials with no aggregate coating the steel reinforcement. It differs considerably from traditional reinforced concrete (RC), not least that it has a far higher porosity and that's where part of the problem lies.

Fundamentally it’s extremely susceptible to moisture penetration and, as a result, corrosion of the reinforcement, far from ideal when used on a roof or loadbearing floor. Even worse, smooth rebar was used for the reinforcement, meaning the aerated concrete does not bond to the steel, so the transfer of forces between the cementitious material and the reinforcement relies wholly on transverse reinforcement. In traditional RC the alkaline cement bonds to the bars, creating a protective environment. In RAAC, there is no protection. So the presence of moisture and oxygen within the cement means the reinforcement is vulnerable to corrosion and a reduction in strength.

Compounding the situation is the way it has been installed, especially around the supports of the RAAC planks. The standard practice was to use a minimum 50mm bearing on supporting elements, however new guidance from the Institution of Structural Engineers published in April 2023 recommends a minimum of 75mm, bringing it into line with other precast concrete elements.

Furthermore, as RAAC panels rely on a transverse reinforcement bar to transfer the forces from the RAAC matrix into the main reinforcement, the last transverse bar should be beyond the bearing point. Unfortunately, that’s often not the case because of the way the cast planks were cut in the works, this leaves them at higher risk of shear failure which, unlike failure in bending, can be sudden and without the benefit of warning signs such as excessive deflection.

A recent example which springs to mind is Hinchingbrooke Hospital in Cambridgeshire, where disclosed documents found the hospital prone to serious structural failures. With the likelihood of RAAC panels collapsing being high, urgent action was required to rectify the matter and remove all panels in question. It’s certainly not an isolated case, with multiple facilities across the UK vulnerable to this issue.

The solution is in theory at least, fortunately, a relatively simple, and cost-effective one, especially in the case of roof replacement. It can be achieved by completely removing those panels and planks and replacing them with materials, such as metal decking. Delivering greater value, this exercise often prompts a complete retrofit of these spaces at the same time, guaranteeing the building is holistically safe.

While the methods to deal with this issue are recognised and relatively straightforward, a Government plan of action to deal with RAAC in the least disruptive way is needed. I’ll be interested to see what comes out of the current departmental identification drive. To undertake the required work the areas being worked on need to be emptied, and this is particularly difficult in an institution like a working hospital where the impact on delivery of essential healthcare will be seriously affected and creating decant space is costly and challenging.

Going further, some buildings might be extremely vulnerable to shear failure, yet engineers will potentially be unable to fully identify the risk of failure, due to the nature of the fabric, without causing the affected area to fail. In these instances, you have to initially at least, use non-destructive investigation methods and assess the subsequent risk.

We as an industry also have a responsibility to communicate this to the private sector, which is, I believe, lagging behind the public sector in addressing this issue due to a lack of awareness and understanding.

I suppose the small silver lining of the situation is that it represents a welcome opportunity for the structural engineering community nationwide. Of course, there are the commercial benefits brought by a pipeline of work, but I also think it offers us a chance to demonstrate, through actions, our professional commitment to safeguarding people within the built environment.

There have not been many instances of RAAC failure as yet, but why take the risk? To its credit, the NHS has already committed to an eradication policy and I expect that others will follow suit. It's now up to structural engineers to show that, whilst this is a big problem, it can be resolved to achieve better quality and safer buildings.

• Steve McSorley is a director at Perega

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