Reinforced Autoclaved Aerated Concrete (RAAC) is a ‘bubbly’ lightweight form of reinforced concrete that is manufactured with extremely fine aggregates meaning its structurally and physically different from traditional higher-density concretes.
Although categorised as concrete, RAAC has a cellular-type construction that is formed by a chemical reaction during the curing process when foaming gases are emitted creating a honeycomb-like structure. The resulting precast planks are then heat and pressure-treated (Autoclaved) to provide additional strength. This manufacturing process was cost-effective and created an incredibly lightweight building material that can be thermally efficient and fire resistant but has significantly lower structural capacity than most structural grade concretes making it susceptible to fracture or complete failure over time.
Often found in public sector buildings, especially schools and colleges between the 1950s and 1990s.
RAAC was specified in multiple building applications such as flat or pitched roof structural decking, supporting floor and wall constructions, and eaves designs.
In September 2022, the Office of Government Property sent a ‘Safety Briefing Notice’ to all Property Leaders, confirming the dangers of RAAC, stating that ‘RAAC is now life-expired and liable to collapse’.
The problems and associated risks;
For a long time, it has been considered that visual deterioration can be seen before RAAC failures. However, in more recent experience, roofs have failed with no warning signs. This suggests that the problems and risks associated with RAAC could be more serious than once thought. Considerably so when many building owners are not aware that it is present in their building.
“Tens of 1000s of these structural panels exist across a broad cross-section of buildings, many constructed in the 1960s and 70s, and many are showing signs of wear and tear and deterioration. The vast majority form the roof of the structure, usually flat, and hence are difficult to access, survey, maintain and replace.” - Professor Goodier
RAAC can be identified by its characteristic appearance and properties. It has a porous structure with a cellular or honeycomb-like appearance, which is visible when the material is cut or broken. The surface of RAAC is often rough or textured, and it may have visible holes or pores. RAAC is also applied in precast planks that typically have chamfered edges making it easy to identify.
To confirm whether a building material is RAAC, follow the flowchart created by the Department for Education (DFE). Though, we recommend you consult with a professional or a manufacturer to ensure accurate identification immediately.
If you are concerned about the performance or safety of a RAAC structure, there are several warning signs that you should look out for:
If you notice any of these warning signs, it is important to consult with a qualified professional for an assessment and recommendations for remediation measures. Proper maintenance and monitoring of RAAC structures can help to ensure their long-term performance and safety.
Suspected fragile roof decks must be assessed in accordance with HSG 33 health and safety guidance publication. Further details can be found at https://www.hse.gov.uk/pubns/books/hsg33.htm.
Many structural deck types can be repaired when cracks or other deterioration is found without the need to replace the whole supporting structure. However, due to the nature of RAAC, with potentially no warning signs before failures and buildings exceeding the structural deck’s lifespan, the safest solution would be to fully remove it.
At Tremco, for all roofs, a free no-obligation roof survey is conducted to determine and assess the build-up and what condition the roof is in inclusive of the deck (stage 2 of the DFE flow chart). If RAAC is potentially present, a structural engineer should also be appointed to confirm this, and a holistic roof condition report created. This report, will identify any properties constructed using RAAC and summarise the potential risks.
If the findings demonstrate damage, in any capacity, Tremco will strongly advise against retaining or working on existing RAAC decks due to safety concerns and exceeded typical lifespan of around 30 years. Instead, it’s recommended to completely remove and replace the RAAC deck with a suitable timber or profile metal deck solution. When it comes to RAAC, even if no damage is found, it is still advisable to completely remove the fragile deck eliminating any future risk.
Once removed, a new Tremco roof system can be incorporated, with thermal efficiency upgrades and compliance with Broof(t4) classification in line with fire safety.
Tremco manages each aspect of roof refurbishment from initial assessment/survey, through to design, supply, install and inspections throughout.
Tremco Trusted Partners are available with extensive expertise and training to ensure that the removal and replacement work is completed safely and to the highest professional standard. Additionally, the company offers a Single-Point Warranty of up to 20 or 25 years to provide further assurance to our clients.
Step-by-step how-to video for conducting field adhesion testing of liquid applied products on flat roofs.
Highlighting the extensive independent fire testing Tremco have commissioned delivering total peace of mind to specifiers and building owners.
The estate department’s building was leaking. Tremco delivered a refurbishment overlay through Trusted Partner AAC Waterproofing, installing the durable AlphaGuard PUMA liquid applied system to weatherproof the building for years to come.