Someone involved in recent RAAC surveys on buildings across England found that previous surveys assumed, out of ignorance, that RAAC has the same structural properties as structural concrete.

It is not known how many buildings had previously been considered safe based on faulty calculations before the error was picked up.

It would appear to help explain, however, the sudden panic at the Department for Education in August that led to the closure of school buildings just before the start of the new school year.

The issue was reported to Collaborative Reporting for Safer Structures (CROSS), the anonymous reporting service previous called the Standing Committee on Structural Safety.

The CROSS report, filed just a week before the government shut school buildings, details how the whistleblower has been involved in undertaking RAAC surveys on buildings across England. Prior to each survey, their firm undertakes a desk study to gather any available information about the building, including plans, previous surveys and condition reports.

In one such desk study, they came across a report that contained the structural calculations used to establish the capacity of existing RAAC panels. The calculations, supposedly based on Eurocodes, used a proprietary software package and the parameters entered treated the RAAC as if it was normal structural concrete, CROSS reports.

The calculations in the earlier survey used a compressive strength of 37 N/mm² whereas autoclaved aerated concrete (AAC) might more typically have a compressive strength of circa 3N/mm². Significant other differences between RAAC and normal concrete were also not taken into account, including risks such as brittle failure arising from narrow bearings.

“This demonstrated a fundamental misunderstanding of the difference between RAAC and normal structural concrete, how RAAC would have been designed originally, and how it should be assessed,” it says.

The point is made that any building owner relying on the initial report, potentially had an inaccurate picture of the building’s safety.

The full report can be found at www.cross-safety.org.

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