RAAC – WHAT IS IT AND WHAT ARE THE DANGERS?

Reinforced autoclaved aerated concrete (RAAC) is a lightweight form of concrete, with an estimated useful life of around 30 years. RAAC has been used in hospitals, schools and other public sector building construction since the late 1950s/early 1960s up until the mid-1990s.

In 2019, the Standing Committee on Structural Safety published a safety alert ‘Failure of Reinforced Autoclaved Aerated Concrete (RAAC) Planks’, which identified concerns about the structural safety of this form of construction. In February 2022, the Institution of Structural Engineers (IStructE) published supporting guidance titled Reinforced Autoclaved Aerated Concrete Panels – Investigation and Assessment. This guidance provided further information for the assessment of RAAC panels.

For several months, HMPPS has had a full programme of work in hand to establish whether RAAC is present in the prison estate. Where RAAC is found or expected to be present, HMPPS has committed to commission additional structural surveys to identify what mitigations might be necessary in each case. HMPPS has also committed to write to Governors and Prison Group Directors where they find evidence of RAAC at their site.

Guidance produced by the IStructE in February 2022 provided advice on the form and scope of surveys to be adopted for RAAC panel installations. This identified the need to survey the panels for: • Measurement of deflections • Recording of cracks and defects • Recording evidence of water leaks • Hammer tap testing for signs of debonding concrete • Recordings of panels cut after manufacture • Recording of any alteration or penetration through panels after construction.

In addition, intrusive investigations may be required, which will result in damage to panels. The location and extent of the trial areas should be carefully selected by the engineer. Any such investigations should be kept to the minimum size, given the disruptive nature of any works that may impact on panel structural capacity. The engineer should assess the condition and capacity of panels ahead of the investigation works and consider the need for temporary propping or support. Investigations should be undertaken using hand tools with small diameter, nonpercussive drilling only, if needed. Investigations may include localised drilling to estimate depth of bearing, opening using hammer and chisel. All trial holes should be made good with a suitable proprietary repair mortar and all waterproofing or protective finishes made good to prevent further degradation of the panels.

Locations for intrusive investigation should provide a representative sample from around the building or structure. This should include any variation in span or support arrangements. The number of locations selected needs to be sufficient to gain an understanding of the original design intent for the panels and the range of manufacturing or construction installation tolerances.

RAAC presents a number of risks associated with the original construction form including the materials used, design intent, manufacturing control and construction/installation control. Further risks are presented through the in-service conditions including uncontrolled modifications, changes in loading regime, poor maintenance, and ageing.

Survey information should be used to assess a risk classification for the panels/building. A RAG (red, amber, green) risk-rating approach has been proposed. Red risks have been split into high risk and critical risk. The application of qualified and experienced engineering judgement is required to assess when a critical risk exists. Critical risks may occur where multiple risks exist for example major cracking and adverse loading conditions. The use of the building may also be a factor in the assessment. Depending on condition, critical risk areas may need immediate action.

Red – Critical

Requires urgent remedial works that may include taking out of use or temporary propping to allow the safe use of a building. Depending on the extent, this may be part or all of the building. Combined with awareness campaign for occupants, including exclusion zones.

Red – High

Requires remedial action as soon as possible. Combined with awareness campaign for occupants, which may include exclusion zones, signage, loading restrictions and the need to report changes of condition, for example, water leaks, debris, change in loading, and so on.

Amber – Medium

Requires inspection and assessment on a regular basis, for example, annually. Combined with awareness campaign for occupants, which may include signage, loading restrictions and the need to report changes of condition, for example, water leaks, debris, and so on.

Green – Low

Requires inspection and assessment occasionally, say three-year period depending on condition. Combined with awareness campaign for occupants, which may include signage, loading restrictions and the need to report changes of condition, for example, water leaks, debris, and so on.

The POA Health and Safety Committee continues to engage with the employer to protect your health and safety in relation to RAAC. In addition, it is liaising with the TUC and has joined it in asking the government to urgently publish a national risk register for all public buildings. l

Mick Pimblett
Assistant General Secretary
POA Health and safety Committee