RAAC Panel Testing: What We’ve Learned from the Lab

RAAC (Reinforced Autoclaved Aerated Concrete), and the potential risks it poses, have been in the headlines in recent years. But while much of the national conversation has focused on identifying it and deciding what to do about it when it's found, less attention has been paid to the panels and how they actually function. How do they really perform? What causes them to fail? And how much of the current guidance is based on assumptions rather than evidence?

At Medway Council’s Gun Wharf building, RCS, collaborating with the Building Research Establishment (‘BRE’) and Loughborough University, set out to answer those questions, not in theory, but in the lab. What they found has added a critical layer of understanding to the wider RAAC conversation.

Pitched RAAC Panels, No Precedent

Gun Wharf is a large, Grade II-listed civic building in Kent, with a distinctive roof with pyramid structures made from trapezoidal RAAC panels. These panels were installed in the 1970s and were very much still in use when a routine survey uncovered them in 2023.

This discovery triggered a classification of High and Critical Risk in some areas. Parts of the building were evacuated immediately, leaving Medway’s front-line services in jeopardy, and the Ministry of Justice courts, also housed in the building, out of action.

But there was a problem: the current IStructE guidance on RAAC is based on flat roofs with square and rectangular panels. The configuration at Gun Wharf, where pitched, trapezoidal panels were concealed under tiles, had no direct precedent in the available literature and RAAC guidance. There was no relevant test data, no detailed guidance, and no performance history on record. To move forward with confidence, Medway Council needed more than visual and structural surveys; they needed hard data.

30 RAAC Panels, Fully Tested

RCS worked closely with the BRE to test 30 full-size RAAC panels removed from the roof at Gun Wharf for detailed laboratory testing. The aim was to establish a baseline of structural data for this specific configuration and to test the assumptions currently shaping RAAC risk management.

The BRE test programme tested the panels under strict laboratory conditions and included:

• Three and four-point bending tests
• Load capacity testing across multiple spans
• Testing under deliberately reduced bearing widths
• Compressive strength analysis
• Full panel reinforcement mapping

Rectangular and trapezoidal panels were tested, so that direct comparison could be made between panel types and installation conditions.

These panels hadn’t been tested like this before... The industry had theories, but we needed data.

Philip Holden - Principal Structural Engineer at BRE

Challenging the Assumptions

The results from BRE challenged several key assumptions that underpin current RAAC guidance.

1. Bearing width: an assumed weakness in RAAC panels was narrow bearing supports; any bearing less than 75mm is considered substandard and to present an unacceptable risk and remedial actions are recommended. BRE deliberately tested panels with reduced bearing lengths to reproduce worst-case scenario conditions in the lab. In fact, the load capacity decreased, but the panels performed far better than expected, even with significantly reduced bearing widths.
2. Panel geometry: trapezoidal panels are generally considered higher risk due to their irregular shape and the uncertainty of their reinforcement layout. But the panels removed from Gun Wharf, originally designed by ARUP in the 1970s, turned out to be bespoke, with additional reinforcement beyond baseline assumptions. “The trapezoidal panels actually performed better than their rectangular counterparts in many cases,” said Holden. “They contained more embedded steel and were structurally stronger as a result.”
3. The role of visible deterioration: The lab testing also showed that visible deterioration isn’t necessarily always a reliable indicator of panel weakness. Some panels with surface cracking actually performed well under load, while others with no visible issues failed earlier than expected. This confirmed what RCS had long advocated: that a visual survey alone isn’t enough to determine safety, especially in complex structures.

Building Confidence in the Structure

The test results clearly showed that certain areas of the building were performing better than anticipated and that, with appropriate remediation, reoccupation was a viable option. And it also gave the project team a way to prioritise interventions at Gun Wharf. Zones that performed worst in the lab were targeted for temporary works and load reduction. Others could be managed with enhanced inspection and monitoring.

This detailed testing regime helped shift the project from an initial emergency response into a science-led programme of selective intervention and strategic refurbishment, challenging long-held assumptions about RAAC and setting a precedent for similar buildings nationwide.

A Unique RAAC Resource

The dataset from testing the Gun Wharf panels is among the most detailed RAAC panel studies in the UK. It is already being used to inform other RCS projects and challenge generalised assumptions about RAAC performance in complex buildings. It has also highlighted the limitations of current standards and guidance. Where panels fall outside the flat-roof profile described in the IStructE guidance, testing may be the only way to avoid adopting an overly conservative approach to decision-making.

RAAC isn’t inherently unsafe. What’s previously been missing is the science to make case-by-case decisions. The Medway project shows what’s possible when research, data and practical expertise come together, and that collaboration, not fear, should drive the response.

Professor Chris Goodier - Loughborough University

Conclusion

RAAC risk cannot be managed on assumptions alone. As the Medway project shows, real understanding comes from testing and from being willing to challenge what we think we know. The more data we gather, the better the industry will be at making safe, proportionate, and defensible decisions. For buildings outside the norm, and there are many, that process starts in the lab.

If you have found RAAC in your building, or you think you may have RAAC, get in touch so we can talk you through the next steps.