RAAC three years on: What have we learned about remediation strategies?

When reinforced autoclaved aerated concrete (RAAC) became headline news in autumn 2023, attention understandably focused on identifying affected buildings and ensuring people remained safe. Today, the immediate crisis has subsided, but thousands of asset owners, estates teams and designers are still dealing with the practical realities of assessment, risk management and remediation.

For building designers, the challenge is rarely as straightforward as determining whether RAAC is present and then replacing it. Every building presents a different combination of condition, loading, operational requirements and estate constraints, meaning successful remediation depends on understanding the specific risks before deciding on an intervention.

RAAC was widely used between the late 1940s and mid-1990s as a lightweight alternative to traditional precast concrete systems. Commonly found in roofs, floors and wall panels, particularly within public sector estates such as schools and hospitals, it offered a fast and economical construction solution at a time when speed and material efficiency were major priorities.

Unlike traditional concrete, however, RAAC has very different structural characteristics. Its low density and aerated composition mean it behaves differently under load, while factors such as water ingress, reinforcement corrosion, inadequate end bearings, previous alterations and changing building use can all influence its long-term performance. As buildings age, understanding those factors becomes increasingly important.

The issue gained prominence as industry concern grew about the condition of ageing RAAC panels and several significant investigations into their performance. A revised position issued by the Department for Education in August 2023 led to widespread reviews of school buildings and brought the issue into public view, but the engineering questions surrounding assessment and remediation remain relevant today.

One of the most important lessons from the past three years is that remediation decisions should be driven by evidence rather than assumptions. Before recommending any intervention, designers need a clear understanding of whether RAAC is present, where it sits within the structural load path, what condition it is in, and how the building is being used.

That requires more than a visual inspection. While screening surveys can help identify potential areas of concern, some defects are not immediately visible. Reinforcement corrosion, hidden support arrangements and long-term structural deflections may only become apparent through intrusive investigations and detailed engineering assessments.

Assessment should also consider the defects and characteristics known to influence performance. These include inadequate end bearings, poorly anchored reinforcement, cut panels, water ingress, excessive deflection, cracking and historic modifications such as service penetrations. While none of these issues automatically indicate imminent failure, understanding their presence and severity is critical when determining risk.

Many organisations now use a structured red, amber and green classification process to support decision-making. Lower-risk panels may be suitable for ongoing monitoring and management, while higher-risk situations could require immediate restrictions on access, temporary propping or more substantial structural intervention. The key point is that identical-looking panels can require very different responses depending on the evidence gathered during assessment.

For many building owners, particularly those responsible for hospitals, schools and other critical public facilities, the greatest challenge is balancing safety with operational continuity. Closing an entire facility is rarely practical, meaning remediation strategies often need to be developed alongside detailed plans for maintaining day-to-day operations.

Immediate mitigation measures may include enhanced inspections, load restrictions, management of water ingress, localised access controls and increased monitoring of panel condition. In some circumstances, emergency propping may be necessary to reduce immediate risk while a longer-term strategy is developed.

Successful delivery depends on close collaboration between structural engineers, designers, estates teams, contractors and building users. Regular coordination allows technical findings to be translated into practical actions, while ensuring decisions are informed by operational requirements as well as engineering considerations.

This was particularly important during Mott MacDonald’s development of remediation works for a large operational hospital with RAAC across sections of its roof and wall panels. Several affected areas were within live clinical environments where relocating services would have been difficult and disruptive, so the design had to accommodate restricted access, existing services and construction within constrained ceiling voids while allowing patient-facing departments to continue operating.

Where major works are required, phasing becomes particularly important. Temporary relocations, out-of-hours working and carefully sequenced construction programmes can help minimise disruption. However, in particularly sensitive areas, the solution itself may also need to be adapted to reduce installation time, manual handling and the need for disruptive construction activities.

There is no universal solution for RAAC remediation, which is why building designers should be cautious about approaching projects with a preferred material or system already in mind. The most appropriate intervention will depend on the condition of the panels, the operational requirements of the building, programme constraints, whole-life cost considerations and long-term estate objectives.

In some cases, monitoring and management may remain the most proportionate approach. Elsewhere, structural interventions such as enhanced bearing arrangements, positive support systems, passive fail-safe measures or replacement of individual panels may be required. A fail-safe system, for example, can provide independent support beneath the RAAC so that, if a panel cracks or loses bearing capacity, it remains supported and does not fall into an occupied area below.

At the more extensive end of the spectrum, complete roof replacement may offer the most effective long-term outcome.

The choice of support or replacement structure can also have significant implications for programme, structural loading, carbon, fire performance and installation complexity. Familiarity with a particular material should not determine the outcome: designers need to consider how each option will be installed, how much additional load the existing structure can accommodate and how the work will affect the continuing operation of the building.

Design work for the hospital referenced earlier illustrates how those considerations can shape the eventual solution. As already outlined, the hospital had RAAC across sections of its roof and wall panels, with particular concerns around deteriorating condition and inadequate end bearings, the points at which panels are supported by the surrounding structure.

Following investigation, the project team considered several approaches, including hot-rolled steel, cold-formed steel framing, proprietary channel systems and timber. Rather than attempting to strengthen the RAAC directly, the adopted approach provided an independent fail-safe support system beneath the panels. If a panel were to crack or lose bearing capacity, the new structure would support it and reduce the risk of material falling into occupied areas below.

Timber was selected because its relatively low weight limited the additional load placed on the existing structure and reduced the likelihood that the primary frame would also require strengthening. It could be cut and adapted on site around existing services and within constrained ceiling voids, reducing the reliance on bespoke fabrication and disruptive hot works. Its lower weight also supported safer manual handling and quicker installation, while offering an embodied carbon advantage over the steel options considered.

The system has now been installed across several areas, providing independent support beneath the affected panels while allowing those parts of the hospital to remain in use.

The significance of the example lies not in presenting timber as a universal answer, but in demonstrating how the choice of solution should follow from the evidence and the building’s constraints. In this case, the combination of restricted access, live clinical operations, structural loading and programme requirements favoured a lightweight and adaptable system that could be installed with limited disruption.

As understanding of RAAC continues to evolve, the industry has an opportunity to move beyond reactive responses and adopt a more strategic approach to managing ageing estates. Better asset data, earlier investigations and stronger multidisciplinary collaboration can all help organisations make informed decisions before defects become critical issues.

Good practice today involves viewing RAAC as part of a wider building and estate strategy rather than as a standalone structural problem. When remediation works are being planned, there is often an opportunity to coordinate wider improvements, address related maintenance issues and improve the overall performance of the asset.

Three years on from the public buildings crisis, perhaps the most important lesson is that successful RAAC remediation starts with understanding the evidence. Once designers understand the condition of the structure, the needs of the building and the constraints of the wider estate, the most appropriate solution usually becomes much clearer.

Laura Jones is a senior structural engineer at Mott MacDonald.