Reinforced Autoclaved Aerated Concrete (RAAC) has been used in building structures in the UK and Europe since the late 1950s, most commonly in flat roof construction but occasionally in pitched roofs, floors and wall panels in both loadbearing and non-loadbearing arrangements. They were produced in a controlled factory environment and supplied as precast elements.
RAAC differs from traditional concrete insofar as it has no coarse aggregate being formed from a cementitious mortar which is aerated (think of a ‘Aero’ chocolate bar), so is lighter than ‘normal’ concrete. It bears a resemblance to Aerated Concrete building blocks such as Celcon or Durox.
RAAC Panels are distinguishable from traditional reinforced concrete members in a number of ways. The Autoclaved Aerated Concrete (AAC) material being aerated has the benefit of being considerably lighter than traditional concrete. Typically, AAC has a density of 600-800kg/m³ compared to 2400kg/m³ for traditional concrete. This aerated nature and reduced density results in reduced compressive strength (2-5N/mm2), increased permeability and increased creep leading to unfavourable deflections in the long term.
Potential Risks of RAAC Panels
In the 1990s structural deficiencies with the material became apparent, the products being the subject of several reports published by the Building Research Establishment (BRE) and in May 2019 the Standing Committee on Structural Safety (SCOSS) issued an alert after being notified of the failure of roof panels in a school. The alert showed some visual evidence of a shear failure close to the support of the RAAC panels. This failure mode was different from those discussed in earlier BRE papers and the failure suggested that there may be a risk of sudden structural failure of RAAC panels.
The SCOSS alert and previous BRE papers indicated that the following key defects in RAAC panels can include;
It is clear that all of the above defects can be an early warning of, or lead to, structural collapse.
The failure mechanism is often triggered by water ingress from, for example, a leaking flat roof, which can lead to degradation of the RAAC and corrosion of the embedded reinforcing.
It is therefore vital that all asset owners and stakeholders are aware of the potential risks associated with RAAC units and allow for regular inspection and, if necessary, remedial works should any be present within their estate.
It is vital that structurally deficient RAAC elements (primarily planks) are remediated to ensure structural robustness and a safe environment for all building occupants.
Once the presence of RAAC elements has been established, condition surveys should be conducted by a suitably qualified Structural Engineer, allowing for the assessment of all RAAC elements within a building. Depending on the findings of the condition surveys, the Structural Engineer may recommend further monitoring inspections, remediation, strengthening or replacement of RAAC panels. These can include:
- Emergency propping when panels are deemed to be in a severe condition.
- Enhanced end bearing, to mitigate against known deficiencies or unknown/unproven end bearing conditions.
- Positive remedial supports, to actively take the loading from the panels.
- Passive, fail safe supports, to mitigate catastrophic failure of the panels if a panel was to fail.
- Removal of individual panels and replacement with an alternative lightweight solution.
- Entire roof replacement to remove the ongoing management liabilities.
- Periodic monitoring of the panels for their remaining service life.
AKSWard has been instructed to conduct condition surveys within buildings where RAAC elements have been identified and have provided remediation solutions to ensure that our clients and their occupants are kept safe from potential structural failure.
What action should you take?
It is now understood that RAAC planks were widely used in Schools and Hospitals and the first course of action is to undertake a document search followed by a visual inspection to establish if the units are present.
If you suspect or have concerns that your buildings may have RAAC elements then please do contact AKSWard so that we can carry out an initial risk assessment and if necessary offer advice on monitoring or remedial solutions.
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